Threading 4140 troubles

[Mitch Alsup]

Here the setup

I have the same lathe and same cheapo carbide tool and insert.

I cut threads at 70 RPM and in 4140 I would take DoC = 0.001-0.002 on the compound not the cross slide with the part dripping oil.

 

[RPMWorks]

4130 is pretty rigid stuff so i don’t think 2.5’ is too much. If your machine can handle threading fast you can go all the way up to 1400 rpm according to the materials SFM, though i would probably only go 4-500 rpm. The cheap inserts could very well be the problem, Assuming you are in fact on center. I’m thinking it’s your lathes lack of rigidity. what is this lathe?

 

[mksj]

I have the same lathe and same cheapo carbide tool and insert.

I cut threads at 70 RPM and in 4140 I would take DoC = 0.001-0.002 on the compound not the cross slide with the part dripping oil.

I assume you mean 0.01"-0.02" per pass? The actual DOC will be about 30% less when advancing the compound. I use the cross slide when threading, usually start out at 0.01-0.02" for the first pass then increase by 0.01" per pass until close to target and then reduce and check fitment. I do one spring pass. Carbide is very brittle if not under compression, any back pressure on the insert when reversing and not clearing the work will cause it to break. SFM for inserts depends on the type and coating, although a production CNC machine may be able to be pushed to SFM's in the 250-450 range, not smaller manual lathes. At one point when using the vertical style threaders and I tried some generic (Cobra) carbide inserts, I could barley thead one piece before the insert broke. I switched to the laydown type using Carmex (Iscar) inserts and do 100's of threads with one cutting tip. I have no issues with surface finish no matter what the material. As I mentioned 4130/4140 does much better with more aggressive material removal, shallow cuts/low feeds usually result in poor finish. Another clear indicator of poor surface finish if you dialed in properly is your cutting tool is worn/damaged.

 

[FTlatheworks]

Have to ask, you are sure that the cutter is on center line? How do you check it?

I line the tip of my tool up with the tailstock center. I have in the past taken an utility knife and set it on top of my tool and I get the razor edge of the blade in the center of my tailstock center point, then I bring my tool up the width of the blade using my dial indicator. I never leave a nub facing with this method. I usually just eyeball the tip of the tool touching my tailstock center after my tail stock has been lined up with a dead center in the head of the machine.

 

[FTlatheworks]

4130 is pretty rigid stuff so i don’t think 2.5’ is too much. If your machine can handle threading fast you can go all the way up to 1400 rpm according to the materials SFM, though i would probably only go 4-500 rpm. The cheap inserts could very well be the problem, Assuming you are in fact on center. I’m thinking it’s your lathes lack of rigidity. what is this lathe?

Pm1236

 

[FTlatheworks]

4130 is pretty rigid stuff so i don’t think 2.5’ is too much. If your machine can handle threading fast you can go all the way up to 1400 rpm according to the materials SFM, though i would probably only go 4-500 rpm. The cheap inserts could very well be the problem, Assuming you are in fact on center. I’m thinking it’s your lathes lack of rigidity. what is this lathe?

I’m about 90 percent sure it was the stock had some wobble at the end and I didn’t take the time to measure it. I didn’t chamfer the work, I didn’t support it with a center, and I was using the compound back and forth. I saw and heard the insert chip at the start of the work. I’m gonna do it again this week and I’m confident I will get it this time. I will report back what happens. I don’t think a pm-1236 is a flimsy machine, compared to some of the tiny lathes I hear other people successfully do some of the same things with.

I don’t like the quality of it, but many people often talk about 1022’s and even smaller and how they thread rigid stocks all the time.

I know it’s not rigid compared to industrial machines, or American made old machines, but it’s definitely operator error. I have thread 4140 with hss just fine before.

 

[RPMWorks]

I’m about 90 percent sure it was the stock had some wobble at the end and I didn’t take the time to measure it. I didn’t chamfer the work, I didn’t support it with a center, and I was using the compound back and forth. I saw and heard the insert chip at the start of the work. I’m gonna do it again this week and I’m confident I will get it this time. I will report back what happens. I don’t think a pm-1236 is a flimsy machine, compared to some of the tiny lathes I hear other people successfully do some of the same things with.

I don’t like the quality of it, but many people often talk about 1022’s and even smaller and how they thread rigid stocks all the time.

I know it’s not rigid compared to industrial machines, or American made old machines, but it’s definitely operator error. I have thread 4140 with hss just fine before.

I am unfamiliar with those lathes so i will be curious to see what happens. Have you ever checked, with an indicator, all the places where “looseness” could occur? Watching…

 

[Swarthy]

I started using a electronic microscope with lcd screen to examine the cutter tip.

 

[WobblyHand]

I started using a electronic microscope with lcd screen to examine the cutter tip.

I have found that 95+% time when the surface finish gets crappy it's because there's a crack or chip in the insert. Many times it's not visible to the naked eye, but upon 10x or better magnification it's clear there's a fault or defect in the insert. For me, when the finish goes south on a material I have machined before, I just replace the insert. If I'm curious, I will look at it under a glass. Almost all the time, it's chipped.

 

[Janderso]

I have found that 95+% time when the surface finish gets crappy it's because there's a crack or chip in the insert. Many times it's not visible to the naked eye, but upon 10x or better magnification it's clear there's a fault or defect in the insert. For me, when the finish goes south on a material I have machined before, I just replace the insert. If I'm curious, I will look at it under a glass. Almost all the time, it's chipped.

I have a hard time seeing defects in inserts as well. I bought a used stereo scope to check the integrity of my inserts.
Now I know if I'm about to use a compromised insert.