Turning Fins



What kind of lathe are you using? Can you run backwards without causing the chuck to unscrew? Or, can you hold the workpiece in a collet?
Anyway, what I discovered in my long saga with chattering cutoff tools is that one big danger is self-feeding of tool into work. A very rigid machine limits this, but not all machines are all that rigid. One alternative is to turn the tool upsidedown and spin the work backwards, so the cutting forces instead tend to pull the tool out of the work. This will prevent grabbing due to self-feeding.
The other issue is to shape the cutting tool so it's max width at the cutting edge and tapers as one goes away from the cutting edge. In other words, the tool becomes slightly necked, prevent cutting on the sidewalls of the groove. Make sure that everything is square and locked, so the tool feeds straight into the groove without translating into either sidewall.
And ensure that the groove is flooded with cutting fluid. A brush will not do.
Now, I'm assuming that your lathe is tight. If not (and my lathe was not tight at first), you must first fix the lathe. I must say one stark moment was when I found that I couldn't make a 0.0625" wide slot in mild steel using my 1500# lathe. I knew that this cannot be right, and started the investigation. Everything that could be loose was loose. Search past RCM postings for "Clausing 5914 chatter" and "Trepanning and Parting Off" in the subject for the long sad saga.
Joe Gwinn
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Joseph Gwinn wrote:

Had a similar moment with my little 9" Logan, trying to part off a 1" rod. It finally dawned on me that something was shifting, so I started at the headstock mounts and worked my way through every joint up to the tool, cleaning, shimming, re-torquing. Took an evening, but it's still tight years later.
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Also a neglected machine.
More saga at "Clausing 5914 - Chatter and Self-Feeding Summary" and at "Clausing 5914 - Test Report Requirements". And so on. That lathe generated a lot of stories.
Joe Gwinn
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While these may well be idiotic ideas, I'll offer them anyway.
1. Have you considered simply using a hacksaw while the part is spinning?
2. (A variant) Have you considered soldering the spine of a piece of hacksaw blade to a tool blank?
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wrote:

I've actually made those things before.... for guys racing model boats!
Anyway, what points to the problem is the chatter. How or with what did it "bind?"
What is loose? How big a lathe? How are you holding the part? Was the cutter properly lined up with the axis of the lathe? Proper clearances all around? Was the carriage locked? How much lost motion is in the compound rest (or was it locked)? What sort of tool holder? How close to center was the tool? Lotsa questions!
Unless this is a really tiny lathe you really shouldn't have any chatter with a .050 wide grooving tool.
--

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Once you get the chatter taken care of, you might be interested in this tip from people who make small engines with narrow fins: Make a disk that just fits between an adjacent pair of fins, and split it so you can place it in the last slot you cut. Tape should hold it in place.
When you're cutting fins with a slotting tool the previously-cut fin can get pushed out of shape by side force from the cutting tool. The filler should prevent the problem.
I have tried this but, frankly, I don't know if I would have had a problem without it, because I used the trick on the first cylinder I tried cutting. That experiment had some other problems but chatter wasn't among them. I haven't had a lot of chatter problems with cutoff or slotting. 'Lucky, I guess.
-- Ed Huntress
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Shouldn't be a major problem. Make sure you have side relief on the tool. 2 degrees per side would be ideal, but one will do if it's consistent. That gives the back of the tool being 10 mils thinner than the front for 1 degree. Make sure the edge is perfectly square and sharp, with no more than 2 degrees of front relief (that'll help to stop it digging in). No top rake at all. you want a tool that can be fed very controllably and won't tend to dig in.
When mounting the tool, use a DTI or dial test gauge on the side to ensure that the tool is straight. The change in reading as the cross slide is moved should be the same for both sides of the tool.
Lock the saddle, nip up the top slide gib screws and ensure that the cross slide gib screws aren't slack.
If the cylinder has already been bored, then mount it against a shoulder on an arbor turned to a firm fit, drilled and tapped for a clamping screw and washer and then not removed from the chuck. Turn the clamping washer before doing the arbor!
You should be able to turn at normal turning speeds, just keep the feed under control if the tool post/top slide are a bit flexible.
Mark Rand RTFM
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Did you grind the nose so it isn't trying to push left or right?
Diamond hone it sharp. On center. Positive rake.
Minimize the stick out as in no extra tool protruding from holder.
Not sure this is a good idea but can you grind a valley in top of parting tool to fold your chip for clearance? I'd like to hear comments on that idea if it is good or bad.
Slitting blade, arbor, rotary table and mill also comes to mind.
Wes
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Tim Wescott wrote:

keeping all overhangs to a minimum, buth tool and workpiece, are good things to do. Possibly going to an even narrower grooving tool, and alternating taking a little off each side of the groove would work better.
You probably need to understand what was going wrong. Was the work too flexible, or the tool? It depends to some extent on the rigidity of the lathe, too. When I used Atlas/Craftsman lathes, any cutoff job was a nerve-rattling exercise, and bind-ups were VERY common. Now that I have a 3500 Lb Sheldon 15" lathe, I rarely have any problem with cutoff or grooving operations that would have been unthinkable on the Atlas. So, the total rigidity of the machine must be part of the difference.
Jon
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Tim Wescott wrote:

Oh, one other thing for parting or grooving tools. Grind a slight groove in the top of the tool, so that the chip is forced to curl inward. What you need is the left and right edges of the tool to be higher than the middle, so there is a little bit of a valley coming out towards you. This curling inwards makes the chip narrower than the groove, eliminating the binding in the groove. When you get it right, the chips come off rolled up, and there is a clear barrel-shape to them.
Jon
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This is the way that commercial cutoff inserts are made, and it works fine with those wider cutoff/grooving inserts. I haven't had any luck trying to do it with very narrow ones because the chips just jam against each other.
Have you tried it with narrow grooving tools? Tim says this one is only 0.050" wide.
-- Ed Huntress
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Sounds like your tool isn't properly ground, Tim.
Did you provide side relief, both top to bottom and front to back? If the tool touches anywhere but at the tip, the point of the cut, you can expect terrible results.
Did you provide a small amount of positive rake? It often helps the cut, and provides additional relief when the tool is properly ground.
Did you try parting dry? That's a mistake unless you're machining 2024, and even then it's not the smartest thing you can do.
As for creating the fins, you'd be well served to use a long travel indicator to index the cuts. Measure the width of the tool, then determine how thick you want the fins to be. Make that offset after each one is machined.
if your tool cuts well, you should end up with a clean cut on both faces of the fin. If you find it doesn't, you might explore a tool that is slightly undersized, then face each side after you've taken the groove to depth.
Unless you have a pygmy lathe (not trying to be rude), you shouldn't have any problems with this little project. The grind of the tool is everything.
Harold
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On Sat, 27 Dec 2008 05:04:29 +0000, Harold and Susan Vordos wrote:

Having read all the posts, I suspect that my problems are, in order:
* I let the tool get dull -- it worked for some cuts, then got worse and worse and ... * Not enough positive rake, possibly exacerbating the sharpness. I knew this, but I was lazy. * Heat. Coolant would be a real pain, but I can try flooding it with lubricant, and going slow.
There may be other problems, but when next I get the chance I'll go remake the tool and give it a try.
--
Tim Wescott
Control systems and communications consulting
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snip---
That is likely a function of surface speed. The tool should have held up for the amount of cutting involved. Try slowing down ever so slightly. If the chips came off anything but a pale yellow or silver metal color, you ran too fast.

As you increase rake, the tool will tend to peel the metal instead of push it off. That lowers cutting pressure and often results in a cooler cut----although with the reduced mass at the point of cut, the result of the angle of rake, the tool may heat just as quickly as it did previously. Pressure will be lower, however, and that's a good thing.

Cooling is important, but not nearly as important as lubrication. It's virtually impossible to part dry---although a few metals will cooperate. Leaded steels and brass tend to part dry fairly well, as does 2024 aluminum, but you're never sorry for adding lubrication. If it doesn't do anything else, it helps chip evacuation, although there's far more involved. You limit or eliminate chip welding, and cut cutting pressure in ways that are hard to believe. You need not flood the cut----an acid brush (don't use a paint brush---it doesn't hold a supply of oil the way an acid brush does) held in the groove while cutting will make a remarkable improvement, plus the reduced friction will yield less heat. Overall, you'll see an improvement you can't believe. Give it a go. You'll be pleasantly surprised. If you don't have any of the 1,1,1, trichloroethane, try sulfur based cutting oil. It stinks, but it does a decent job.
One more consideration. The material you're using can make a huge difference. If you're using mild steel (C1018 up through any of the 1020 materials, hot or cold rolled), you can expect grief. You might consider making the cylinder out of Stressproof (1144). It machines beautifully, especially with HSS. It also has great properties.

Be certain to have relief properly ground.
It would have been interesting to hear about the lathe you're using. Did you make mention and I missed it?
Small machines are not the best scenario for parting, although with a narrow tool you should achieve excellent results. If you find you're having trouble, use a narrower tool, plunge the centerline of each groove, then, using a long travel indicator, step over and face each side. With a sharp tool you should be able to do so without problems, assuming the fins are reasonably thick.
How about a report on the project when you're finished? I'd be interested in hearing the outcome. Sounds like a good one to pursue.
Harold
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On Sat, 27 Dec 2008 08:55:14 +0000, Harold and Susan Vordos wrote:

Sorry -- it's a Smithy "3" in one (actually 2 + excuse in one, but the excuse is the milling section; the lathe seems to be pretty good). Other than the compound, which seems to have some taper in the dovetail, I can get good control of the rigidity by snugging down the jib screws.
--
Tim Wescott
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Tim, you can check the rigidity of the compound/top slide to see if it is firmly supporting a cutoff/parting/grooving tool with overhang, particularly since the compound is located near the edge of the cross slide on a Smithy.
By placing a length of stock or a bar-shaped tool about 12 inches long in the toolholder (or turret), and applying light to moderate vertical finger pressure at the end of the bar, any movement that is seen in the compound would be an indication that the cutting tool has vertical movement onder the load of cutting. Besides the cutting tool being able to dip down while cutting, any sideways twist will very likely break a small cutting tool of that size.
If you're compound is set parallel to the spindle axis, you may want to try resetting it to be perpendicular to the spindle axis, or somewhere in-between.
I realize that supporting such a short/small part with the tailstock center on a Smithy is probably not possible. Attempting to use the extension for the tailstock ram will probably not be worthwhile, as the overhang is too great for the area of the base of the tailstock.
I agree with the suggestions of placing the cylinder on an expanding arbor, this would be a great improvement over trying to chuck a hollow part.
If you have a big endmill holder (and a drawbar) that fits the spindle taper MT4, it should be a good substitute for using a chuck. By turning a snug-fitting stem to put in the big endmill holder, a stub (the diameter that would hold your cylinder) could be turned for it to mount on. If you wanted to go further, you could drill the stub, tap for a 1/8" pipe plug or set screw, then slit the stub so it would expand inside your cylindrical part. Otherwise an end screw and a washer would probably suffice (with the motor running in the forward direction).
--
WB
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On Mon, 29 Dec 2008 10:43:43 -0500, "Wild_Bill"

This may be considered by some as silly, but have you considered mounting something like a Dremal with a saw blade on your cross-slide and using the saw to slice out the fin areas while you turn the lathe chuck VERY slowly, by hand even? That should relieve any problems with digging in and will allow you much more control over the whole process. Just a thought.
Jim
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SNIP---------------->

Actually, what will work better - I've done it to make very fine features in ebony, is to use one of those high speed carving tools - mine is 200,000 rpm - it's like a cheapened dental drill - put a straight sided bur in it, set the lathe to turn medium slowly and then slowly advance the tool (running, of course) into the work to the desired depth. I cut some fins in ebony that were about .020 thick with this method - it puts almost no load on the material. you could use a ball ended burr if you wanted a rounded bottom to your grove - get the burrs free from your dentist - that's what I do.
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I agree with all of your recommendations, Jim. And it's reasonable that if "I do this" works well, then that's definitely the way to go. Some of the other folks with different machines will need to adopt their own "I do this" procedures for whatever works best for them.
The parting tool I've been using is 1/8" wide x 1/2" high HSS, the tapered cross section type. The tool holder I use also holds the blade at an (upward) angle, but I haven't measured it. I was concerned that a chip breaker ground into the tip would be too much top/back relief, but it works well. The chip breaker might not be the ideal tool profile for power cross feed, but it's definitely been beneficial for manual cross feed. The chip breaker tip has worked well for all of my mild steel and aluminum parting operations up to about 1.5" diameter, and smaller diameters of stainless steel (just haven't had any need to part/cutoff larger SS yet). The very slight front relief made a dramatic improvement, completely eliminating any digging/grabbing in the workpiece. Of course variable spindle speed makes it easy to find a good relationship between spindle speed and manual cross feed rate.
For larger diameters, I use the band saw to cut sections, since the cutoff blade would be extended too far to complete larger diameters, unless the larger diameter would have an axial hole, then the parting/cutoff blade would probably work well. The problem with large diameter stock is that for parting, the tailstock center should never be use to complete the parting, so the section of large round stock would need to be short so that the parting groove would be near the lathe chuck.
Nearly any parting operation will go smoother if the workpiece is to have an axial hole in it, and the hole is drilled before the parting/cutoff operation.
The tapered type parting/cutoff blades provide good side clearances, but it's important to mount the blade as perfectly vertical as it can be. Some blade holders aren't machined properly to set the blade in the vertical position, so the operator needs to be aware of this.
I haven't used the T-shaped parting blades, but I've read good comments about their performance.
--
WB
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Tim Wescott wrote:

I would guess that you ground the parting tool from 1/4 square? If so your asking a lot for the tool to plunge .3" deep even if ground very well.
For a one off groove I'll use the compound set parallel to the ways and feed 1x to 1.5x the tool width then pull out, move the compound over and plunge again, etc.
Obviously, the tool must be ground narrower than the finished groove for you to do it this way.
Most folks part with the tool on or slightly below center with somewhere between "0" up to half the usual rake used for a given material. Side clearance is crucial for deep plunging, but less so for "pecking".
You've got lots of time, the part is precious.
Matt
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