Threading 4140 troubles

[Jason812]

Standard of Depth of Cut (External Threading)
mitsubishicarbide.com
https://www.mitsubishicarbide.com › files › tec_th...

Numerous manufacturers have infeed tables for threading. I calculate the depth based on 30 degree compound angle. You have to lighten up the cuts the deeper you go to keep the insert tip intact.. Recently I cut 1.5"-8 threads in 4140 at 160rpm using Anchor Lube with zero issues.

 

[Mitch Alsup]

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.

No, I mean 0,001" (1 thou) DoC on 4140, maybe 0.003 on aluminum and I also mean at the slowest RMP the machine has.

 

[mksj]

Surprising a doc of 0.001", it would take one almost 100 passes to do my last threading job which was a 1"-14 thread and required a 0.098" thread depth in 4140 which took me 9 passes. A PM-1236 is rigid enough too do at least 0.01" starting pass (and this would be ~0.007" DOC with the compound) and the finish improves with higher SFM. I use DOC and SFM's in the range of the table below. The 1" threading I was running at 300 RPM in 4140 which works out to around 80 SFM, when doing aluminum I would be running at around 600 RPM. Smaller threads I could run higher RPM, but never had an issue with the cutting the thread, and materials that might work harden need deeper continuous cutting. You can also try to thread using the cross slide if your lathe is rigid enough, I used that on my 1340GT and also my current lathe which is an ERL-1340. Bottom line, is he blew out the insert and then tried to thread with it. I would do some more practice runs and see what method works well on your lathe.

 

[Mitch Alsup]

Those speeds and feeds are fine for 5 HP 3000 pound lathes in proper working order.

You don't have on of them........It appears you have a 1200 pound lath with 2 HP. Different animal.....

On the other hand, I have not worn out (or chipped or damaged) a threading tip in 4 years......

 

[taiwanluthiers]

I bought some cheap carbide insert threading tools from Amazon with 5/8 shank. I was trying to thread 5/8 4140 stock with 18 tpi. I had 2.5 inches of stick out. I was going to use my live center, but decided not to because I couldn’t fit the tool with it. I took a scratch pass then proceeded with .005 doc. Everything was good until I hit .040 deep out of the .067 I needed to cut to hit minor diameter. The tip of the tool broke off, but I continued because the thread looked ok and it was the very bottom of the tool.When I hit .050 it snapped even more, but very jagged, but I’m pretty sure the thread was ruined at .045 before the insert broke the second time. I was running 400 rpm by the way. My compound was set to 29deg with my qctp turned so it was at zero degrees. I advanced my depth with compound.

My question is, why didn’t this work? Was it too much stick out without support? Was it speed? Was it my process? Or I did everything wrong?

Consider mounting your tool upside down and turn your lathe the wrong way. Somehow this improves rigidity a LOT, don't know why. I was able to deep part 4140 this way with the parting tool sticking out almost 2 inches. I notice CNC lathes run this way too. If you do this you will have to make a relief cut, but at the same time you don't have to stop the lathe on a dime either.

 

[FTlatheworks]

Surprising a doc of 0.001", it would take one almost 100 passes to do my last threading job which was a 1"-14 thread and required a 0.098" thread depth in 4140 which took me 9 passes. A PM-1236 is rigid enough too do at least 0.01" starting pass (and this would be ~0.007" DOC with the compound) and the finish improves with higher SFM. I use DOC and SFM's in the range of the table below. The 1" threading I was running at 300 RPM in 4140 which works out to around 80 SFM, when doing aluminum I would be running at around 600 RPM. Smaller threads I could run higher RPM, but never had an issue with the cutting the thread, and materials that might work harden need deeper continuous cutting. You can also try to thread using the cross slide if your lathe is rigid enough, I used that on my 1340GT and also my current lathe which is an ERL-1340. Bottom line, is he blew out the insert and then tried to thread with it. I would do some more practice runs and see what method works well on your lathe.

View attachment 437445

I agree with you, while a 1340 is a much higher quality machine I don’t think it’s 3000lbs and 5 hp. I could be wrong about that. My machine is fine, but the inserts and shanks were like 18 dollars. The only time I have problems with rigidity is when I try to knurled with cheap single wheel knurling tools and I can physically see my qctp and compound flexing up and down.

I had a good thread until it broke. Had I changed the insert I believe it would have potentially worked. Had I used a center and chamfer. I’m positive it would have. I have threaded plenty before this without problems. The pm1236 and all of their Chinese machines are trash in my opinion. They leak oil, they have holes that aren’t tapped straight/stop too early, the saddle and cross slide have a finish that looks like someone intentionally tried to gouge with dremel. Having said that, many people do fantastic work on these little machines. Especially ones smaller than mine.

Here’s some of the examples of the quality of these machine for anyone wondering. When they tell you to buy the Taiwanese, or the 1340’s and up, do it! Had they told me your machine will leak oil and be mostly bondo, and the carriage will stop because the gears don’t mesh well underneath the ways I wouldn’t have bought it.

 

[FTlatheworks]

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…

I have not checked for looseness. How would I do that properly? Also, when I find looseness, what would I do about?

 

[FTlatheworks]

UPDATE! The website I got my thread dimensions from was wrong. It listed a minimum diameter of .557 with a major diameter of .625, so a thread depth of .067.

I setup making all the changes we discussed and at exactly .050 depth the finish went bad and the threads started to chip, but my tool did not break this time. This was the exact depth things went bad the first time too. Surprisingly a 5/8 18 nut threads on just fine, but there are threads that completely chipped on the very top of the crest. I checked a reliable source for thread dimensions and it should be .037 deep. Im guessing what was happening is I was cutting over a 60 degree thread and the insert was breaking the crest of the threads off.

That’s what I get for using google instead of my machinist handbook.

Anyways, thanks for the suggestions everyone. I made an even dumber mistake than everyone originally thought. When I get home I’m going to check my machinist handbook to double check my dimensions.

 

[RPMWorks]

UPDATE! The website I got my thread dimensions from was wrong. It listed a minimum diameter of .557 with a major diameter of .625, so a thread depth of .067.

I setup making all the changes we discussed and at exactly .050 depth the finish went bad and the threads started to chip, but my tool did not break this time. This was the exact depth things went bad the first time too. Surprisingly a 5/8 18 nut threads on just fine, but there are threads that completely chipped on the very top of the crest. I checked a reliable source for thread dimensions and it should be .037 deep. Im guessing what was happening is I was cutting over a 60 degree thread and the insert was breaking the crest of the threads off.

That’s what I get for using google instead of my machinist handbook.

Anyways, thanks for the suggestions everyone. I made an even dumber mistake than everyone originally thought. When I get home I’m going to check my machinist handbook to double check my dimensions.

.557 is correct, but that is the biggest it can be and still be in spec. Your OD is oversized if you went with .625

 

[RPMWorks]

I agree with you, while a 1340 is a much higher quality machine I don’t think it’s 3000lbs and 5 hp. I could be wrong about that. My machine is fine, but the inserts and shanks were like 18 dollars. The only time I have problems with rigidity is when I try to knurled with cheap single wheel knurling tools and I can physically see my qctp and compound flexing up and down.

I had a good thread until it broke. Had I changed the insert I believe it would have potentially worked. Had I used a center and chamfer. I’m positive it would have. I have threaded plenty before this without problems. The pm1236 and all of their Chinese machines are trash in my opinion. They leak oil, they have holes that aren’t tapped straight/stop too early, the saddle and cross slide have a finish that looks like someone intentionally tried to gouge with dremel. Having said that, many people do fantastic work on these little machines. Especially ones smaller than mine.

Here’s some of the examples of the quality of these machine for anyone wondering. When they tell you to buy the Taiwanese, or the 1340’s and up, do it! Had they told me your machine will leak oil and be mostly bondo, and the carriage will stop because the gears don’t mesh well underneath the ways I wouldn’t have bought it.

Looks like a beaver’s first day on the job.