Please recommend turning inserts for my lathe

J

javaduke

Registered
Registered
Joined
Jul 24, 2013
Messages
43
Like many others (I'm sure :) I want to get the best possible finish on my parts, but here's what I'm struggling with right now. My lathe is PM-1130 and I primarily work with mild steel and 4140 steel, up to about 2" in diameter, just making small parts, nothing fancy. With my lathe I got a box of holders and inserts sold by Precision Matthew, and they are CMMT coated and uncoated. I spin my lathe at about 400 RPM, give or take a few, and my feed is .001 - .002, and normally I'm taking .025-ish roughing cuts and .005 finishing cuts. That being said, the surface finish these CMMT inserts leave is pretty bad and I spend a lot of time with emery cloth trying to get rid of these deep grooves. So I'm wondering if there are inserts better suited for my needs. What would you machining gurus recommend?
Many thanks in advance!
 
those inserts should work fine - increase your speed/feed/DOC until you're getting nice brown C's and 9's coming off. That should give you a good smooth shiny finish. Don't try and take a shallow finish pass, instead work out how much you're taking off each pass and aim to hit dimension on the final pass. Eg. to remove 0.08" off the radius, take two 0.03" cuts, measure (say it's 0.059" removed total), then dial in your finish cut to suit (eg. 0.021" in this case).

eg. turning a 3.5" piece of stressproof (4140?) on my SB heavy nice, I was able to take a repeatable cut at 0.02" depth, 500rpm or so and feed as fast as the lathe would let me. Gave a lovely finish with a CCMT insert.
 
I'll give it a try. Forgot to mention, the chips are not breaking well, I'm getting very long spirals, this probably adds to the poor finish. So clearly something wrong with my approach.
I tried increasing the feed, up to .005 but it makes it progressively worse, so far the slowest feeds produce better finish (but still not good).
 
typically my cuts go from ribbons->very long spirals->long spirals->short spirals->C's as I increase the feed for a given DOC and speed. I'd suggest just playing around with all three variables until you find what works. I'd certainly up your RPM some though, 6-800rpm would be more appropriate for that size stock.
 
Typically for the 4130/4140 turning with carbide inserts, you would want a SFM in the 400-600 range for that size lathe, and usually both a higher feed IPR in the 0.008-0.012 range, if not higher. Part of that depends on the nose radius of the cutter and chip breaker. So for 2" rod, 400 SFM is around 800 RPM, with a feed in the 0.009 range and DOC in the 0.050", you can't really do a finish cut of 0.005" with a 32.51 insert unless using a ground edge (rule of thumb is 1/2 the nose radius). The quality/type of insert may also be an issue, I primarily use Iscar CCMT 32.51 IC907 inserts and they have worked well in a wide range of materials.

This was some larger diameter 4140, mid way I went from an IPR of 0.004 to 0.009 if I recall, and you can see the difference in surface finish.
20190308_154155.jpg
 
If I were peeling a rusty, mill-scaled skin off of a part like that, I'd be using M2 or even a tougher but softer steel in the Mushet or Rex-aaa range, or maybe T7, T-8, or T-15 tungsten steel with increased wear resistance. I would not choose a carbide cutter for rough surface cuts, but that's my opinion and the opinions of the old men who wrote the books on tool steel.

Other than being able to change a carbide insert like a cartridge, I don't really see the appeal. I use them less than 10% of the time. I will never understand how HSS fell out of "popular" culture. I guess grinding facets is difficult?

I can buy 20 lbs at least of good, mixed species tool steel bits for what one box of Iscar or Kennametal inserts costs. Maybe I missed the boat on Chinese carbide too, but I don't own, use, or want any.

Chip formation is something that happens after the cut, as far as lathes go. Chip breakers break chips. Some engineers in the past have done a great disservice to machinists everywhere by making a science out of chip breaking. It takes a lot of math to design a breaker to clear a certain chip during a given process envelope, but it's not critical in any way on a manual lathe. The only point to having one at all is to get the dangerous mess clear of the mechanicals. Any type of chip breaker will do that just fine without calculations. All I do is grind a step on the tip of the tool behind the cutting edge without changing geometry at all, and no more wicked stringies.
 
javaduke said:
Like many others (I'm sure :) I want to get the best possible finish on my parts, but here's what I'm struggling with right now. My lathe is PM-1130 and I primarily work with mild steel and 4140 steel, up to about 2" in diameter, just making small parts, nothing fancy. With my lathe I got a box of holders and inserts sold by Precision Matthew, and they are CMMT coated and uncoated. I spin my lathe at about 400 RPM, give or take a few, and my feed is .001 - .002, and normally I'm taking .025-ish roughing cuts and .005 finishing cuts. That being said, the surface finish these CMMT inserts leave is pretty bad and I spend a lot of time with emery cloth trying to get rid of these deep grooves. So I'm wondering if there are inserts better suited for my needs. What would you machining gurus recommend?
Many thanks in advance!
Click to expand...
What size CCMT inserts? The nose radius is a critical factor and very well could be the source of your issue. The DOC and feed rate both need to take the nose radius into account to get the best results.
 
pontiac428 said:
If I were peeling a rusty, mill-scaled skin off of a part like that, I'd be using M2 or even a tougher but softer steel in the Mushet or Rex-aaa range, or maybe T7, T-8, or T-15 tungsten steel with increased wear resistance. I would not choose a carbide cutter for rough surface cuts, but that's my opinion and the opinions of the old men who wrote the books on tool steel.

Other than being able to change a carbide insert like a cartridge, I don't really see the appeal. I use them less than 10% of the time. I will never understand how HSS fell out of "popular" culture. I guess grinding facets is difficult?

I can buy 20 lbs at least of good, mixed species tool steel bits for what one box of Iscar or Kennametal inserts costs. Maybe I missed the boat on Chinese carbide too, but I don't own, use, or want any.

Chip formation is something that happens after the cut, as far as lathes go. Chip breakers break chips. Some engineers in the past have done a great disservice to machinists everywhere by making a science out of chip breaking. It takes a lot of math to design a breaker to clear a certain chip during a given process envelope, but it's not critical in any way on a manual lathe. The only point to having one at all is to get the dangerous mess clear of the mechanicals. Any type of chip breaker will do that just fine without calculations. All I do is grind a step on the tip of the tool behind the cutting edge without changing geometry at all, and no more wicked stringies.
Click to expand...
I purchased tangential tool holders. Not purchasing inserts has paid for them at least once. With crobalt tooling, regrinding is rarely needed, just a quick hone. I can grind, just did a cast iron finishing tool for my shaper, but find I rarely need to.
 
pontiac428 said:
If I were peeling a rusty, mill-scaled skin off of a part like that, I'd be using M2 or even a tougher but softer steel in the Mushet or Rex-aaa range, or maybe T7, T-8, or T-15 tungsten steel with increased wear resistance. I would not choose a carbide cutter for rough surface cuts, but that's my opinion and the opinions of the old men who wrote the books on tool steel.
Click to expand...
I agree with just about everything your wrote, except this. The "old men who wrote the books on tool steel" are, through no fault of their own, not up on the latest in inserts. I'm as big a proponent of HSS in the lathe as anyone, but it's simply not (or no longer) true that inserts can't handle mill scale. Modern (quality) inserts can be remarkably tough. Not so tough as steel, but easily tough enough for the turning in question. Even the old saw that carbide shouldn't be used for interrupted cuts is questionable anymore (though it should at least be given consideration).

I use HSS for probably 80% of my lathe turning, but I'd probably choose an insert for peeling mill scale - I have inserts that will cut that with no discernible deleterious effect whereas it would probably at least warrant a re-honing of any HSS cutter that I used. Merely convenience, both can do a good job here.

GsT
 
You must log in or register to reply here.
Back
Top