Turning 4140 half hard

The 4140 should be softer as you get deeper since it is a oil hardening metal. I would double check your tool height. I would guess that you might be above center and as you get smaller in diameter the tool is rubbing rather than cutting.
I have also tried a cobalt Coolant is not that important. It is important in production since it allows you to cut faster and heavier because it is ..... coolant... to cool the tool and material. It does supply some lubrication. If you want you could use some cutting oil but it will smoke up your shop. Old time machinists will follow the smell to your doorstep :)
There are many different grades of carbide, and each grade is designed for a particular metal. Using the wrong grade of carbide will show up in premature edge failure. The radius of the point matters too. The smaller the radius, the less push into the piece. It is easier to hold size with a tool that the cutting edge is perpendicular to the shaft and has a fine point.
The text seam to imply that I could Yup
Your feed rate may be too small. For roughing I use about .012 feedrate. 4140 cuts better when you get under it a little. By that I mean that a cutting depth of less than .010 will give you a poor finish. Also too small of feed rate will do the same thing. The faster the rpm the better the finish.
John

First of all 4140 HALF HARD is a strange specification given that 4140 can be hardened up to about RC 60. The first thing to do is try to determine how hard the stuff really is. Try filing a corner. Grab it in the vise and have at it with both hands. If you can file the edge then use the corner of the file, or a hack saw, and try to file/cut a groove all the way across the end of the shaft. If you can file/hacksaw all the way across the material then you can cut it.
It sounds as though you are trying to face the end of the bar. Try it this way. With the lathe off run the cutting edge of the tool bit up against the end of the bar. Is there clearance under the cutting edge? If not grind some. If so continue. With the tip of the tool positioned about the center of the bar imagine a line drawn perpendicular to the lath ways. Does the end of the tool bit have clearance? If not grind some.
If the tool bit looks all right then position the tool bit with the tip on vertical center with the work piece. Stick your 6 inch scale between the tool bit and the work and raise or lower the tool bit until the scale is vertical.
Finally, set the tool bit with the tip angled about 15 degrees toward the work, set the speed to the slowest and feed to say, .005/rev. Spot the end of the work with a center drill and stick the end on the tool bit in the hole, estimate about a 0.010 cut. Lock the carriage and hit the feed lever.
Watch the chips. If they stay white then you are going good. If the turn yellow or blue then slow the RPM even more.
This should let you face the bar.
Now, if you are successful then you can experiment with cutting speeds (RPM) and chip thickness (Feed Speed). Generally the fastest method is to set the cutting speed to that recommended for the material you are turning and juggle the cutting speed until you get the right color chips.
For the moment forget coolant. You don't need it yet.
Bruce in Bangkok (brucepaigeatgmaildotcom)

I double checked the steel. The correct terminology is Pre-Hardened 4140 Alloy. It is hardened to 262-321 Brinell (approximately 26-32 RC)
I will play with this. I have been trying to error on the high side, since this is an eyeball setting and I figured my not so stiff Logan leth would push it down.
Which grade is recomended for 4140 Pre-Hardened. Where can I find this out. The Enco and MSC catalogs are very vauge on this.
Should I even be screwing arround with carbide at this point or should I use HSS.
Unfortunatly at the moment my feed is not working. I have been going everything by hand. Is this a major problem.
Thanks for the reply

Yes I clarified that in the last response it is caller 4140 Pre-Hardened to ~ 30RC
Thanks the last response also seemed to feal I had the tip too high. I like the 6" scale trick for checking this. Thats the kind of stuff you dont find in text.
Unfortunatly power feed is down for the moment Thanks for the reply

Yeah. I saw that and figured the poster had 4140 pre hardened.
Half Hard is usually a term used to describe brass that has been cold worked.
Sharp tools, rigid set-up.
Cheers Trevor Jones

Trying to err to the high side?
You were trying to get it wrong from the start? :-)
Try to get it on center or a tiny bit below. At least then, the clearance under the cutting edge will not rub for sure.
Errors due to being below center really don't start to become as much of an issue until you get into really small diameters or turning fairly small tapers. You ca work out the actual errors with a little trig, but once you do, I doubt that you have metrology equipment to measure the errors or discern them from errors due to wear or flex in your machine. You have to get a fair ways below center on most of the taper sizes we are like to turn before you get out of whack enough to effect the usefulness of the part.
Going high and relying on the lathe to flex is really not helping things at all. If you start out high and it flexes, the tool has to dig in deeper, making it worse. If you are low and the lathe flexes, it reduces the chip load somewhat. Better to be a bit low than high if you have a choice.
A consistant cut is a good cut!
You know how to check center height with a straight slip of metal?
Gently pinch a slip of sheet stock (an Al. shim, or a small ruler) between the work and the cutting tip. If you are on center, the piece will be straight up and down. Too high, the slip will have the top leaning away from the tool, too low, the bottom will be closer to the tool. Dead easy, fast, and pretty accurate.
Cheers Trevor Jones

The previous posts are all on track. 32 Rc for carbide should be now problem. What ever the actual hardness is, if you cut it with a HSS saw the carbide should cut without any problem. I suspect something major, like too deep a cut, built up edge , or as mentioned by other people no clearance due to improper tool height. Remember- turn below, bore above, center that is, about .005 to .010 is fine.

Trevor Jones wrote in news:_Kx9i.6379$vT6.5455 @edtnps90:
Why not just measure the tool height from the cross slide with a scale? The spindle centerline to cross slide height should be a known amount (from the machine specs).

Inserts are a great mystery in some respects. Each manufacturer has their own numbers for grades of carbide. One Kennametal grade for 4140 would be KC850. It is a good all around insert grade for carbon and alloy steels. You will probably want to use a positive rake insert, the reason being it takes less power to cut metal than with a negative rake insert and your machine may not be stiff enough to do a good job with the negative rake insert. The second letter of the insert code tells the rake of the insert TPG is a positive rake insert, TNG is a negative rake insert.
I learned machining on an old lathe with no power feed many years ago. You just have to get the feel and the machine will tell you how fast to turn the crank. If you are using carbide you can look at the chip coming off and tell pretty much how fast you should turn. With inserts, you want the chip to break, that is the chip should look like a chip, not stringy long pieces. Ideally you increase the feed until the chips come off in little pieces. If you lathe is not tight enough you may not be able to feed it that fast.

Well, for one, I don't have to bend over the machine to try to get an accurate read on a scale to check my tool height this way. If I can find the scale. Or if the machine I am on, has had a scale made up for it.
My method works on any machine, not just the one that precisely matches the specs sheet.
There are thousands of different designs for height setting gauges. Their main purpose seems to me, to have been to create a little busy-work for the maker.
Same goes for the often suggested "use the tailstock center". When you have to change out a tool, and there's a 6 foot long, 200 pound chunk sitting on the center, it ain't available, but a slip of shim or a small pocket ruler always is.
Cheers Trevor Jones

You probably could but as every machinist I ever saw carried a 6 inch steel scale in his shirt pocket it was quicker and easiest to just hold the scale up against the work and crank the tool bit in against it.
Actually you wouldn't know the distance from the cross slide to the spindle center. You came in and they said "why don't you take that machine in the corner and turn out about 20 of these". Never got to see the machine specifications at all.
Bruce in Bangkok (brucepaigeatgmaildotcom)

I have no intention of starting a war but what is this fetish with carbide tooling? Certainly I understand its use in production settings and certainly there are occasions when one is machining extremely hard materials when it is needed but in 90-something percent of the cases HHS works perfectly well.
If you use carbide you are always scratching around trying to find the right shape insert, or the correct grade.With HSS you just step over to the grinder and make what you want., if it doesn't cut quite right just grind a bit off the top, or whatever.
As far as speed of stock removal it has been my experience that HHS would remove about as many lbs. of material per shift as carbide. . When I was an apprentice boy the main advantage of carbide was seen as it ability to stay sharp longer particularly in a production shop where the machine operators had to call the foreman to change tooling -- but in a home workshop? Bruce in Bangkok (brucepaigeatgmaildotcom)

It last so much longe and is found everywhere dirt cheap. Unless it's an interrupted cut I alway will choose it first.
Certainly I understand its use in production settings I never do. If you are content with HSS you certainly don't need to worry about grade. TPG does 90% of my work.
With HSS you just step over Same thing with carbide on a diamond wheel.
Up till now it seemed to be a matter of personal preference but now you are dreaming.

That's a damned clever idea, Trevor. I tried it, it works as described.
Thanks, I've spent way too many times regrinding tool bits when the bit was fine, the idiot who placed it in position wasn't.
Mike Eberlein
Trevor J> jr>

I have feelings on both sides of this particular fence.
Pro for the HSS: -few jobs that it won't do -easy resharpening -Cheap! -relatively forgiving
Pro for Carbide: -hard materials -replace an indexed tip after a boo boo -no grinding (not counting the brazed carbide bits) -last a long time (in good hands)
When I was starting out with machine tools, every time I touched a carbide cutter I broke it. Talk about a recipe for cash-strapped frustration! As my experience grew, so did the lifespan of my tool tips.
At anywhere from a couple bucks each on WAY up, inserts for tools are expensive, and I would recommend a learner stay away from them. So should guys with lathes with limited horsepower and slipping drive belts.
With all that has been written over the years about tool grinding and grinding jigs, it's no surprise that the average new hobbiest is scared spitless of grinding HSS tools. Most seem to think that the angles and clearance amounts are a set-in-stone bunch of numbers that MUST be exactly followed or something bad will happen, when the truth is, they are a decent guideline based on what has worked for others in the past. Plus or minus a degree or four or five, in most cases, won't stop a tool from working.
I find now, I tend to set up a couple tools in holders and they stay in until I do something dumb, or need to mount up tools for different jobs.
I like carbide on my Myford, because I can pretty much run anything I want to cut with the lathe going flat out, and not worry about frying the tip of the insert. I have burnt the ends off a couple HSS tools doing the same. I use some heavier tooling at work, mainly on a couple 15" Colchesters, and we have a decent if boring selection of positive and negative rake toolholders and inserts, but we still use an awful lot of HSS bits for a lot of the small work we do, too.
I think a lot of guys see all the pictures of industrial setups (and hobby pictures) and see carbide tools in use and think that they HAVE to have them to accomplish anything.
Cheers Trevor Jones

Depends on what the shop does, too.
In raw production, carbide has it all over HSS.On the machines that can drive the HP through the tool or spindle. In a shop where setup and onesie-twosie work are the norm, there is not much advantage either way.
In my opinion, anyway.
Cheers Trevor Jones

According to Bruce :
[ ... ]
Sure.
Also true.
Well ... this home workshop has a bed turret for the lathe, and sometimes engages in light production, with both tooling in the turret and in the quick-change toolpost on the carriage.
And also a smaller CNC lathe with a quick-change toolpost as well.
In both cases, there is a benefit to carbide insert tools where if a corner goes dull, you simply rotate the insert and continue working with the same cross-feed dial settings, or with the same CNC program. If you have to sharpen a HSS tool bit, you change its length. So you have to re-zero the dials (or change the tool offsets in the CNC program) to get repeat performance.
So -- they *do* serve useful functions in a home shop from time to time.
And, I tend to keep the toolpost set up square to the cross-slide, and use the Aloris BXA-16N holder which holds two inserts, one for turning and the other for facing at the same height, so I just swap the tool holder to the second dovetail on the toolpost and keep on working. I swap out for other tools when I am after finish, or threading, or parting, but much of what I do with the toolpost and carriage can be done with the two inserts on that one holder.
Enjoy, DoN.

Gentlemen
Thank you for the many replies. Using the steel scale trick greatly improved the results. I was able to successfully face the piece although it tends to consume all three corners of one insert.
I think another problem I have is that currently my setup is a 1 1/2 inch shaft sticking 6" out of a chuck because it wont go in any further. At times the lath starts vibrating enough to walk across the floor. I think once I trim the part and work closer to the chuck the vibration problem will go away.
As far as the carbide verses HSS. I just consumed about 3 inserts at about $7.00 each. They are size 212 TCGT inserts made of C2 carbide non coated. Since I seam to have the same problems with carbide as I do with HSS, I am going to do back to HSS for a while. Im not afraid of sharpening a tool for what I have to do.
As one person said you don t have to get the tool just perfect. Thats probably true. I have taken drill bits and sharpened them into lathe tools. My problems seam to be with anything more than mild steel.
By the way there is also Cobalt as a tool steel. Your thoughts on Cobalt.
Tomorrow I will once again try parting the shaft using a Cobalt parting tool. I also had dismal failure with this previously, but maybe the steel scale trick will bail me out.
Thank You all John Roncallo
John R> Gentlemen

Well, let me give you an example. We had a job once where were making wood planer heads out of some 12 inch line shafting that the customer had.
The planer heads were a shaft with a 2 inch bearing journel on each end and a center section about 8 inches in diameter. We had a lathe that would take the 5 foot sections of 12" shaft between centers and we were roughing the shafts out, from 12 inches down to 8 inches.
I don't remember the lathe size but using (if I remember correctly) 1/2 inch HHS bits we were able to take a cut heavy enough that the lathe would blow a fuse if we either increased the speed or tried a little heavier cut. I don't think that using carbide would have let us work that machine any harder.
Bruce in Bangkok (brucepaigeatgmaildotcom)