Noob Lathe Q: Turning between centers



We lose 10 points each time we restart a part, but I'm taking the class to learn--not necessarily to fluff my GPA. I may very well do another wheel simply for the hell of it. Either that or when I get to make my own project up after finishing this one I'll do something with a wheel on it.

So what does the C2/C5 rating refer to? Grain size? Carbide vs. "other" content? Is there some sort of system to choose which grade where, or is it just down to C2 for some stuff, C5 for others, all other grades not made anymore? Searching on google got me a whole lot of marketing, but not much else.

Sounds like bicycle parts: cheap, durable, light--pick two.

These inserts get used on any damn thing that finds its way onto the lathes. Poor things probably had to cut stone at some point. In fact, I had to put a steel center into the chuck and take a light cut off of it so the mandrel would run true immediately before I machined the wheel. So, yeah, it got "steeled" first. Would the steel or aluminum--not both rule also apply to HSS bits, or are they OK with it since they can be sharpened? I wound up leaving the outside of my flywheel .003" oversize (we have a tolerance of .005" over/under) so I could file/sand off the uglyness. Alas, there is no lubricant. I did try painting my wheel with a film of oil for one cut, but it didn't appear to help any, so I didn't mess with it any more. Got a bit stinky anyhow. My lathe actually had two carbides sitting at it. One was undamaged, but the other had an impressive little heap of metal welded firmly to the cutting edge. But that's really nothing compared to the guy who took a .050" pass using a carbide insert cutter in the mill, but ran it backwards and way too fast. It took off .050, but as a smear instead of as chips. Ah, school.
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wrote: snip--

I've been out of the shop so long that I am no longer knowledgeable where carbides are concerned. It's particularly bad considering they've made great strides with carbide, too, so I can only assume that it may not be as critical as it once was. I'm not convinced you'd find many machinists that really understood the differences in how they're put together, though. When you go back to my time in the shop (primarily brazed carbides), constituents of the carbide were altered to accomplish the performance required. Carbide went from hard to strong, and from edge wear resistant to crater resistant. As you moved from one feature towards another, you gained certain qualities, but lost others. It could be that today it's just not as important. Dunno.

Your chance of accomplishing good cuts is way down. It would be especially true if the carbide is intended for machining steel, which I would imagine it is.

HSS is not as critical as to how it's applied, but when it is a concern, one selects tool steel high in cobalt, which offers tougher conditions at elevated temperatures. Any HSS will work for aluminum, especially if you understand chip breakers and rake angles. You can create tools that will peel it off faster than you can imagine. Lubrication is very important, however. It doesn't take much, something as simple as brush application works fine. Kerosene is the lubricant of choice for aluminum, but almost anything is better than nothing. I've used Stoddard solvent that isn't real clean with great success, but if you'd like things to smell nice, consider a small can of WD-40. It appears to be nothing more than solvent with a little wax dissolved in it, along with a perfume. It works fine for aluminum.
It was common practice to have a small can on the machine with an acid brush in it. To keep the can from getting blown about by the air hose (they're use extensively in production shops, believe it or not) you'd place a piece of stock in the bottom. I've always had a can of kerosene and sulfur based oil at my lathe and mill.

With the inserts you described, it's no wonder. Once you find chip welding on an insert, for all practical purposes, it's no longer a good insert. The welding generally occurs because of tip flaking or cratering, with the chips welding into the rough edge or surface. There's usually no way in hell it will cut well once that happens.

Carbide is so weak in tensile that a tool that backs up ever so slightly is usually ruined. Running a cutter backwards is a sure recipe for destruction. You must have witnessed a hell of a lot of sparking unless he was machining aluminum.
You'll come to realize that not everyone in your class will have the same dedication to learning as you may have. I took a welding class several years ago. One of the students was a druggie, and wasn't interested in the least in learning anything. He took advantage of circumstances for reasons best known to him. Didn't make sense to me, but it apparently did to him. Sigh! He finally quit showing up-----
Harold
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I'll do some more digging on carbides and grades. If I find anything juicy I'll post it in RCM. Seems like something that would be nice to know.

I'm sure it is too. Would an aluminum-cutting setup involve only a different insert, or also a different insert holder. IIRC, the holders out there have a negative rake (angles down towards the work, which I believe is called negative) and a negative side-rake. (down towards the headstock) Are the rake angles a function of the holders, or are all holders angles this way, expecting the lip on the edge of the carbide to determine back/side rake?

Rake angles I think I've got a handle on. At least enough to get started. As I understand it a chip breaker is just a little groove some ways back from the cutting edge, correct?

Is diesel close enough to kerosene to work? I can get a little container of diesel easy, but kerosene will be a hassle. I'm in texas, so our diesel is as sulphury as law allows. (:

Good to know. Thanks.

It was aluminum. Now I can't wait for someone to do it with a steel piece so I can watch the sparks! (: Before I ever use a cutter out of the "studentized" cabinet I always check it first. I think I know which one he used because it's covered in rubbing marks and two of the three inserts are missing the tips. The teacher says he's going to leave it in the cabinet for the rest of the semester so people will learn to look at their tools before using them.

Heh, over in my welding class two weeks back some guy showed up for the first time since the beginning of the semester. The teacher simply told him up-front that he hasn't been there, his excuse (broke down truck) was bullshit, and he would do nothing more than waste his and the teacher's time and a whole bunch of electricity. Kicked him out on the spot. I like that teacher! In my machining class we've gone from 12 down to seven or six students. Probably going to be five by the end of the semester. Welding is worse, started with 11, down to six who show up at all, and it'll likely be three of us by the end. OTOH, my other classes are grad-level and our flaky guys are the ones who've missed as many as three days. (:
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In article <DoNotSpamthegoat4-87661B.09280206032005

Diesel will work, or you can buy a jug of lamp oil at Wally World for a few bucks. It's just deodorized kerosene, which is the major component of WD-40. You can even get it with a pleasant floral scent (yuk).
Ned Simmons
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wrote:

other
where
as
though.
resistant
you
Agreed. Very few seem to grasp the significance of choosing the proper grade. I'd welcome anything you can uncover.

There are holders that determine rake angle, such as the negative rake types you're apparently using, but there are inserts with grinds that can alter the rake angle to some degree. It's conceivable to have negative rake holders that have positive rake inserts, but they're usually somewhat timid in performance. The edges are relatively fragile and easily chipped, so they're usually used for light cuts only.
I'm shaking my head in disbelief as I read you're being taught to use a negative rake holder with aluminum, however. You couldn't come up with any worse combination than you have. Even for roughing, positive rake is a better choice in aluminum. It requires far less horse power. The material, being quite soft and low in tensile, machines very easily. There is little benefit in using negative rake, and it goes from a bad idea to a horrible one for finishing. Negative rake tools don't cut at the tip, but rely instead on what is known as a false cutting edge. The cut occurs back of the tip, where cratering is generated. When you try to take light cuts, the tip comes into play and isn't up to the task. Long ago, it was recommended that a slight chamfer (.005") be stoned on the tip to discourage chipping, prolonging the useful life of the insert. You can imagine how poorly it would perform when taking a light cut. It's not a good idea to use negative rake for finishing, not even in steel, although if the cuts are heavy enough, it works fine. You really should be using positive rake, and a sharp tool, with no chip welding of any kind.
Positive rake tools generally have a cutting edge on one side of the insert only, so a triangular insert has only three cutting tips, not six.

or
one
you
real
consider a

Yes, that's correct, but it's a good idea to incorporate a chip breaker as your rake angle. It takes a little grinding experience to get good at it, but it really pays benefits. A simple rake angle improves machining ability, but creates horrible chip problems. When you couple the chip breaker with the rake angle, all that goes away. If nothing else, you gain considerable safety, not generating long strings that can wrap around your spinning object.

brush
(they're
piece
based
I own a couple Dodge diesels, and hate the smell of the fuel. I've never tried it, but I can't imagine it wouldn't work. I used Stoddard in place of kerosene to get away from the smell, which is similar to diesel. If you have a (Stoddard) solvent container for cleaning parts, just dip a little out as needed. I've done that for years. Sulfur is very much a part of lubricating when machining. One of the best oils for machining steel is sulfur based cutting oil. The stinky stuff.

have
film
undamaged,
cratering,
no
By now you grasp the importance of not using negative rake, hopefully. You have enough material left to make one good pass and end up with a beautiful surface. Don't blow it screwing around with the negative rake inserts. If, by chance, you do, chalk it up to learning. You'll remember the screw up far better than you would good success. Making mistakes is very much a part of the learning process, especially on machines. You'll come to realize that not everything is as it appears. Often you'll dial a cut and the machine takes more or less than the amount you "requested". A good machinist knows how to get around that problem. A loser rarely ever figures it out.

the
reasons
him.
By the time we're old enough to attend such classes, we're expected to behave as adults. People like him (and the druggie that was in my class) have a way of disrupting the learning of those that are serious. I like that teacher, too.
Harold
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says...

That one would work fine. You don't really need a chipbreaker for what you are doing. I respect harold a lot but he's pretty compulsive about doing things the 100% right way. For small depths of cut you won't see any real detriment to not having a chipbreaker except maybe the chip coming off will mark the finish to some slight degree.
Try grinding a turning tool that is similar to the sherline one, but don't put any back rake on it - use only three grinds to form 1) front clearance, 2) side rake, and 3) side clearance. It will look like the one on the far right side of this photo:
<
http://www.metalworking.com/DropBox/_2000_retired_files/Tp3.jpg
and in that shot the tool would be cutting *away* from the viewer.
Another view of the same tool, where it would be cutting towards the viewer and slightly to the right:
<
http://www.metalworking.com/DropBox/_2000_retired_files/Tp2.jpg
And the final view from underneath:
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http://www.metalworking.com/DropBox/_2000_retired_files/Tp1.jpg
From the photos you can see it's pretty similar to your sherline diagram, but the grind for the top surface is parallel to the long axis of the tool, there's no 'back' angle. The edge thus formed between the top surface and the side grind is nearly exactly along the original edge of the side and top of the square cross-section tool. What this means is you can resharpen this kind of tool by simply grinding its *front* face, and the height of the cutting edge does not change.
It's only three grinds on the HSS tool blank - if you do the front, and side, but no top grind to form side rake, you've made a brass turning tool.
I've found that back rake has never been essential for the stuff I do, at work or at home. So I just leave it out.
If you *wanted* to form a chipbreaker in that kind of tool, the idea would be to run a narrow groove alongside the cutting edge, on the top surface. This allows the chips to flow off at a larger angle than the real rake angle, and they eventually strike the grooves far edge and snap.
When I took a shop class for the first time, there was an entire four hour class period devoted to trying to get HSS tools ground properly. Apparently in some classes the instructor simply hands out 1/4 CRS blanks for the students to practice on.
One final caveat for you: pedestal grinders look pretty innocuous. They're not. While you are grinding tool blanks by hand, the blank will become hot, and your hand will become tired. You will be applying a fair amount of pressure to the tool blank to force it against the wheel.
The wheel will remove flesh faster than you can imagine - I personally ground a pretty good-sized divot out of my thumb when I slipped while grinding a lathe tool. Didn't bleed much at first but that thing throbbed for weeks afterwards. Take your time and think of the wheel surface the same way you would if you were grinding your tool on the roadway, out of an open car door. I bet it's about the same SFPM.
Jim
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Good point. But somewhere along the line I'm going to try to force the issue and find a cut that'll require a chip breaker, just so I have a chance to play with the idea. I think perhaps steel would do it. So far even with the carbides it's been more prone to looooong curls of chip. If you know a way to force long chips that would mandate a breaker I'd like to know it too.

While I'm thinking of it... As I understand things, if I do change the height of the cutting edge the only real drawback is that I need to compensate and move the tool up until it's level with the centerline. Correct? The lathe I've been using has a quick-change post that lets me set the height, and I have a feeler gauge set that's unreadable now, but would make dandy shims if I'm ever using a non-adjustable tool holder.

Cool! I'll grind the other end of my tool that way and see how it works out since I have to redo a brass bushing. Why a flat top on a brass tool? To keep it from pulling itself in? Are there other materials that would want zero side rake? Is there perhaps a list somewhere of common materials and appropriate cutter geometry?

Is there a standard distance back I should go, or just look and see where the chips are hitting the top of the tool and put it there?

I've had my share of grinder mishaps. My personal best--or worst--is getting a bolt I was grinding a flat onto get pulled between the grinding wheel and rest, taking my finger with it. It finally took enough flesh off the side of one finger that I could slide my hand out sideways without getting snagged again. Imagination be damned! I have first-hand (heh) experience! Whole thing took maybe two seconds. Since then I've had lots and lots of respect for even wee little grinders. If the rest has a gap big enough for my finger I either adjust the rest, remove it entirely, or find another grinder. Seems that accident only took skin--healed up fine and really doesn't stand out above the other 5 billion scars my hands sport. My second-best experience was grinding something, shutting off the wheel, and having it explode after I got about five feet away. That one had a vibration somewhere below operating speed, so my guess is that the wheel popped when it hit that speed/vibration/harmonic while winding down. So now I also stand clear while the wheels are spinning up and spinning down and won't touch (or walk past) a machine that shakes while running. While I grind I tend to keep my head to the side a bit--just in case.
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B.B.

True on all accounts. The problem here is if you wait until you actually need a proper chip breaker, you've missed a golden opportunity to get it straight, from the start.
Chip breakers, once again, are load sensitive. Jim is right on the money. A chip breaker that works beautifully for a roughing cut is likely to fall flat on its face when taking finish cuts. At best, you'd profit by the positive rake, but not much else. That's not a problem when you're running parts in volume, because you generally go through a roughing sequence before any of the parts get finished. That, too, is a part of learning to machine properly. Your tools are ground and setup appropriately. My point in getting you to try chip breakers is to learn about them and have them firmly entrenched in your mind, so when the need arises you don't have to go through the learning curve.

Simple. Proper positive rake. Light feed. No chip breaker. You can generate all the strings you desire. That's the point of my information. The use of chip breakers is a package deal. Lots of things have to come together.
Understand that some aluminums cut without making strings. It's the nature of the material. Amongst them are 2024 and 7075, each of which will gladly cut string free under the right conditions, including the absence of a proper chip breaker. 6061 is another story, and is generally more than happy to generate strings, although generally very easy to control.
snip-

Regardless of the holding system, that's an ongoing problem. I use a square tool block (my choice) so I shim all my tools. You get used to having a small box of shims handy and it takes almost no time to set up your tools. You get to the point where you can pick up a tool and know what shim is required to hit center. I use anything for shims, including strapping material, which comes in a myriad of thicknesses. Old feeler gages are fine, too, just expensive. And then there's always shim stock!

You can go full circle on this issue and stick to making finishing cuts with chip welded negative rake inserts. I strongly recommend you *don't* avoid the learning curve. Learn to do it right, even when it takes more time. You can always back off when it's not important, but without the experience, you'll have nothing to use as a reference when you're facing problems. The real benefit of learning the little things is when a tough job comes along, you may be the only one equipped to deal with it. I gained the respect of my peers because of my anal approach to machining. No one wanted to emulate what I did, but they sure as hell understood why I had success when they did not. Remember, often the difference between one who can and one who can't is nothing more than how they apply what they know. You see that on a daily basis in a commercial shop.

That's too simplified, and may or may not work. If you look at chip breakers that are formed on negative rake inserts, you'll notice they are a shallow radius which begins at the theoretical false cutting edge of the insert. It's assumed you're going to take a particular depth of cut and a given amount of feed, so they factor that in when creating the inserts. The whole idea is to encourage the chip to start rolling, not to stub into a wall. That raises cutting pressure, often breaking your tool from chip stalk up.
A chip breaker that doesn't function has no value. Be certain to learn to incorporate rake when you attempt chip breakers. It's particularly important when using HSS, so you can keep temperatures low at the cut, and reduce cutting pressure as well.
There's no shortcut, you have to start at the bottom and experiment with each material, slowly accumulating enough information in your head that it will become useful. You'll know when you've hit a winner. The chips curl and break nicely, and the tool cuts without much noise. Watch the chip color (when machining steel) so you don't produce anything above the slightest hint of yellow. That usually spells an early death for your tool unless you have very high cobalt, or are lucky enough to be using Stellite. Tools of this nature generally respond very well to lubrication, too. Helps keep the temperature down and prolong the tool's life.

My policy is to grind the breaker and take a trial cut. Again, the cut required determines everything. Maybe start short and shallow and see how it behaves. You can always go wider and deeper, but can't reverse the process without wasting a good amount of tool. Once you see what the chip's doing, you can make the appropriate adjustment. This process can really pay benefits when you're boring deep holes. With the proper tool configuration, you can go deep and not generate any strings, which generally spell tool death when they wrap around the bar. If your bar has enough clearance, your chips will come out as short crumbles and drop neatly at the mouth of your part. Lots depends on bar clearance, but I think you get the idea.

To start, that's not a bad idea. The only real problem is the CRS doesn't grind worth a damn. The concept remains unchanged, though.
One thing I don't use is a tool rest. Never. Not for grinding tool bits. They get in the way and restrict your ability. You likely won't be able to do that at school, but keep it in mind for home use. Have a special grinder that you use exclusively for sharpening toolbits, and have it mounted such that you can stand erect and grind at a comfortable level. It takes some getting used to, but once you master it, you'll never go back to a conventional pedestal grinder for grinding toolbits. Knowing how to properly dress the wheel becomes quite important when you grind this way. It must run smooth and true, for it becomes your reference point.
Harold
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Something I never did. LOL. Might be time to investigate, eh? I seem to recall teenut saying something about the chipbreaker groove on a HSS tool could be done with a tiny cutoff wheel and a die grinder.

6061 is like turning silly putty. 2024T6 is more like steel than aluminum. I love that stuff. Too bad you can't weld it.

This is one reason I avoid back rake - because it means that sharpening the tool requires resetting center height.

Toolrests are OK for roughing stuff out, but I agree that 90 percent of the lathe tool grinding I do is offhand, no rest. You have to be able to get the angles right. Good lighting is vital. My grinder is tucked away in a far corner of my shop, to keep grit off the machines. The lighting sucks there, it's one more of those roundtuit items.
Jim
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Harold and Susan Vordos says...

firmly
It's one of those things that you can go a life time without and swear it makes no difference.. Bottom line is how much you expect from your machine, your setup, your ability. I've always carried a definite edge over others because I cared enough to learn early on. You're likely to find someone that has the opinion that it doesn't matter. I think it does. I may sound like an ass, but I can tell you all I need to know about a guy's ability on a machine by looking at his lathe tools. When they look like they've been gnawed out by a gopher, I rarely expect anything of substance to come from the person, and I'm rarely disappointed.

I don't claim to be teenut, but I sure agree with him in this instance. However, I don't limit myself to that. I can hand grind the vast majority of my chip breakers using the corner of the wheel. Only when I get involved with tricky form tools do I usually turn to anything else.

nature
gladly
It is a pleasure. During the period that we built the missile, and shortly thereafter, it was the only choice of aircraft quality material we used. My first introduction to 7075 was when I had already started my business and had bid on some helmet gun sights for helicopters. Needless to say, I was impressed. While you claim 2024 machines like steel, 7075 really does, and emulates it in tensile strength. I liken it to leaded steel in how it machines.
One of my favorite memories of being self employed was building a wave guide for Univac. They never came in large quantities, sometimes as many as four or five, but more often than not, one at a time. They were made from 2024 T351, a real pleasure to machine. Inside the guide a double ended plastic component that came to a .005" flat end (wedge shaped) was installed, held in place by four pins, also made of the same plastic. The job brought out the best of my ability, with it's tight tolerance and exacting demands for generating the proper configuration of the plastic insert.

your
shim
strapping
Not a very good reason, Jim. I'm the first to step forward and suggest that you can machine with or without rake, with or without a chip breaker, but the best work comes from the proper combination of each. It really jumps out at you when you have more than a couple parts to make. You've done yourself a disservice, something you'll slowly come to understand. I guarantee you, your tools don't cut nearly as well as they can. Chip control is as important as chip generating.

to
grinder
such
some
Chuckle! All in good time, eh Jim? Just don't let your entire life time slip away, putting up with the bad setup as it is. It's almost impossible for me to properly sharpen tools right now, what with our living in the shop. My (rough) grinding section , roughly 30 feet from my precision machines, happens to be the cloths drying station while we're living here, so my grinder isn't properly set up. Of all the things I can't use as desired, I miss it more than any of them. Especially the diamond wheel, for touching up brazed carbide tools. I'm still a strong believer in them. Old habits die hard.
Harold
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Ah Haa! So this is why you don't like insert tools - you can't use that as a metric any more! :^)
Jim
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Harold and Susan Vordos says...

machine,
others
substance
Chuckle!
Actually, I do use them, Jim. A lot more than I admit to. My favorite is the RB negative rake holder that cuts positive. The inserts are a modified triangle and have chip breakers diamond ground such that the tool cuts positive. I also have heads for boring bars that use the same type, but smaller, insert. They are likely the finest insert tools I've ever owned. They were particularly valuable to me when I used to run the metering pumps for a blood analyzing machine for the pharmaceutical industry, where I ran hundreds of components with a half thou tolerance. I also have insert tooling for my tracer, which, just like running a CNC, really demands such tooling.
One thing that turns me off is anyone that turns to insert tooling and refuses to learn to grind either HSS or brazed tools. Until such time one knows and understands cutting geometry and how it applies to success, they're kidding themselves. Any damned hack can load parts in a machine and apply a tool provided by others. It takes skill and talent to do it the hard way, and it's all very much a part of becoming a machinist in the true sense of the word. To substitute the learning curve with insert tools is a huge disservice to the operator.
I realize that my philosophy may not fit today's circumstances, what with CNC machines having replaced manual machines almost entirely, but for the most part, we home shop types don't have the luxury of CNC's, so shortcutting the learning process becomes a huge stumbling block.
Harold
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But the flip side of this is that one can be skilled and adept at grinding tools by hand, and still have *zero* experience or skills at NC programming. I had pretty much zero there when I first went to work at that night production job.
Most of the guys there didn't even know you *could* make cutting tools by hand! But they knew G-code stuff inside and out. I learned enough in my couple of years there to keep me from being a menace to myself!
Jim
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OK. Maybe you guys can tell me how to dress a new 10 inch grinding wheel!
I have a baldor 10 inch grinder and I installed new name brand wheels from MSC. I have two different wheel dressors (0-10) (10+) and I still cannot get the wheels round. I even tried using a diamond point and ruined it.
So, How do you dress a new 10 inch grinding wheel ??
chuck
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way.
it.
Nothing you're saying makes a lot of sense, Chuck. Can you fill us in on what you're doing, and how? The very notion that you ruined a diamond point tells me something is woefully wrong.
Please clarify your wheel dressers, too. I have no clue what the designations mean. (0-10) and (10+). I can only assume you're making a reference to the size of the dresser as it applies to the wheel size.
If you are trying to shape and true your wheels with star type dressers, you're likely to not like what I have to say about them. Before expounding, I'd like to comment that there is no better way to prepare a wheel to cut than with such a dresser, but they present some serious problems in their application. For one, they're difficult to apply such that you can get a wheel running smoothly. A bouncing wheel is pretty much useless for sharpening tools. There are better ways to true wheels, which we'll discuss as quickly as I understand more about your problem.
For sharpening your tool bits it is important that you have aluminum oxide wheels, *never* silicon carbide, be it green or black.
Tell us more about your setup and lets talk. (Sorry I didn't get back to your email last weekend. I came down with a killer cold and spent two days in bed. Life sucks!)
Harold
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I have a baldor 10 inch grinder with new norton grinding wheels. (one course and one fine) purchased from MSC.
I have two Enco grinding wheel dressers. One is a #0 and is rated for wheels 0-10 inchs. The other is a #1 and is rated for wheels 10-16 inchs. They are not called star dressers in the catalog but I think they are star dressers.
I also have a 1 inch square Silicon carbide wheel dresser.
I cannot make the grinding wheels round using any of the above tools. I can remove grit from the wheel but it still does not make it round. At low speed I can see noticable runout.
I even tried to use a diamond point intended for the surface grinder to make them concentric and it didn't seem to work very well. I was holding it by hand.
How do I dress the wheels to make them round?
Since the wheels are not round, they are difficult to use for anything except rough grinding.
hope this is clearer?
chuck
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If they have segmented pieces that spin, that's what they are. Jim likes 'em, and even ol' teenut used to extol their virtues, but I'll put my grinding experience up against their opinions any time anyone wants to have a discussion. As I said, there is no better way to prepare a wheel for cutting. By their nature, they leave very sharp grain when properly applied. Problem is, it's hard to get a wheel running true. In order for them to function properly, they require a given load, otherwise all you do is grind away the components. They dress by hammering the wheel, breaking it into fine particles. Assuming you can get the wheel running dead true, you can then bear down adequately for the dresser to do a good job, but you can waste a lot of wheel getting there, and it's not all that hard to get the wheel to start bouncing again while you're using it. As I said, I avoid them totally. Don't even own one, nor do I care to. If I had a large 3 phase grinder for grinding on weldments, I'd likely have a different attitude. They're great for that application.

That's the tool I thought I'd have to explain to you.
You can't bear down with them if you want a wheel to come true. If you have a work rest, this is one place where it can pay benefits. You place the dressing stick on the rest, picking a sharp corner, lightly touching the grinding wheel, which should be at operating speed. The highs only will make contact with the stick and slowly be lowered until you finally have full contact all around. If you crowd the dressing, you tend to encourage the wheel to shrink, but not get corrected. From this you should understand that it takes a light hand. You can't horse getting the wheel running true. Once it is running true you can aggressively remove more wheel with reasonable results, though. Remember: A light touch.
Assuming your wheel is out a considerable amount, and you'd like to true it up with a diamond first, that works fine, too, but you have to use the same principle, lightly applying the diamond so only the highs get removed. As quickly as you have the wheel running true, you should change to your dressing stick. Diamond dressed wheels don't lend themselves well at all to offhand grinding because the surface is way too smooth. Exactly the opposite from what a star dresser does. A light touchup with a dressing stick will usually prepare the wheel for quite good grinding performance. Avoid the use of the little dressing sticks that are 1/4" x 1/2" x 3" in size, and are one solid piece. They are boron carbide, and while they're even harder than the silicon carbide dressing sticks, unless they are very sharp, they tend to dull the grain excessively, so your wheel cuts quite hot----very similar to a wheel prepared with a diamond, in fact.

You may see some real strange conditions as wheels spin up and down. Don't worry too much about that, just make sure that when your wheel is up to speed, it runs true. Often the imbalance of larger wheels will cause some vibration that makes them look like they're running out a lot more than they really are. If you find your wheels vibrate excessively, it's not impossible to have ended up with wheels that are not well balanced. Precision grinders have provisions for balancing the wheels, but pedestal grinders usually do not.
I trust you know to use the proper flanges, and to always run the blotters. You risk fracturing wheels when you mount them without. Hopefully you also understand how to "ring" a wheel to insure it's not cracked when you mount it.

As I said, there's nothing wrong with the concept, but it requires a light touch to be effective. Assuming you still have the diamond at your disposal, you might try again, making sure the nuts on the spindles are well snugged. I failed to mention the possibility of your wheels moving about on the arbor. You want the nuts tight, but not excessively. It's desirable to learn to use a little less pressure when tightening fasteners on grinding equipment. Unless something goes very wrong, there's usually not all that much pressure involved in grinding.

Hopefully something I've said will be useful. Why don't you give it a go, then let us know.

You don't have to say anything to convince me. I'm the first to say that a grinding wheel that doesn't run true is worthless.

I think so, although I'm really surprised that you aren't experiencing success. Are the bearings good in the motor?
I hope you realize that what the problem could be might jump right out at me, or others, but you may not recognize it, so you could have overlooked something that might be important. That's why I asked for you to describe what you're doing, and how you're going about doing it.
Harold
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Yep, that sounds like a star dresser to me.
I've never had a wheel that ran out, right from the box, so I can't say how poorly they would true one. They are pretty good for getting the wheel opened up again, and they're pretty good at getting a flat surface once it gets grooved or marked.
I've tried the SiC dresser stick that you suggested. I may have been using too hard a touch on it, given your comments above.
As I said, I've never had a wheel that did not serve for at least reasonable grinding, right out of the box. What are the chances that there's just something wrong with the wheel(s) he mounted?
Jim
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New wheels come with cheap plastic centers so its difficult to get them mounted concentric in the first place. Might not be a problem with small wheels but it doesn't take much on a 10 inch wheel to make the whole grinder vibrate. I think that lead use to be the prefered material for adapting the large hole in the grinding wheel to the smaller grinder shaft. Should I replace the cheap plastic spacers with something better?
I am using the standard flanges and blotters supplied with the wheels. I did not tighten the wheels very hard because they ususally tighten themselves up pretty tight. A 1.5HP motor accelerates pretty rapidly and the 10 inch wheels have a lot of mass so I suspect the motor tightened them up very tight by now.
I suspect I'm using too much pressure on the dressers and not the light touch that harold recommends.
This grinder was purchased used and the original 10 inch wheels were worn down to about 7 inchs. It ran fine with the old wheels.
Perhaps I should remove one wheel to reduce the vibration and dress the remaining wheel. After I get it running smoothly I can then add the second wheel and dress it.
chuck
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snip------

If you find the plastic permits the wheel to move about too much, that wouldn't be a bad idea, but use good sense. There are issues with the spacers that are critical, which explains why they're sloppy. They may not work perfectly that way, but they're safe. If you make your spacers too tight, and the bore of the wheel isn't at a perfect right angle to the wheel face, it's possible to fracture the wheel when you tighten the nuts. The snug fit would limit the wheel's ability to square with the flanges, which it will do, either by cracking the wheel, or compressing what ever is in the way.

On the outside chance that isn't happening, check how tightly they are held. If the nut had crud in the threads, it's possible it's not turning freely, so it feels tight but it isn't. Long shot, yes, but worth a look. Be certain that the nut runs freely, all the way until it hits the flange, otherwise you have no clue if you're tightening the wheel, or fighting the dirty thread. Your flanges should be imprinting the blotters. If you see signs of the wheel sliding about, with smudges on the blotters instead of imprints, you're not tight enough. The blotter should appear clean, but compressed.

It has been my experience that once a wheel starts bouncing, it encourages itself. Especially if you have no fixed reference, when you try to reduce the runout, all you do is make the wheel smaller. If you think you can simply hold the dresser of choice against the wheel and improve it, you'll be disappointed endlessly. You have to work smart at reducing the runout, which means to take it off a little at a time, such that the bounce of the wheel isn't transmitted into the dressing device. A light touch usually solves that riddle, along with taking a narrow approach. Don't use the flat face of a dressing stick, but a corner instead, where the amount in contact is minimized. Once you get a spot running true, you can usually move out and improve the immediate areas. It's not rocket science, but it does require a little thought. Surely you've already noticed how the bouncing wheel sets up a pattern that tends to repeat. You have to break the cycle.

Unless you ended up with some very poor quality wheels, there's no reason to not expect new wheels to run smoothly once dressed. If that's what it takes, then that's what you should do. Just be certain that all your efforts are when the wheels are at operating speed, and don't discount the possibility that the wheel sides may not be parallel. That would certainly yield a heavy side, and would be corrected only by dressing the sides of the wheel. I often dress the sides with a diamond, strictly for balance, right down to the blotter. It often improves how a wheel runs.
Harold
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