How to thread stainless

[FTlatheworks]

I'm a little late to the party here, but I'll throw in my 2 cents anyway. I thread 100's of 304 and 303 parts every month (1/2-20), both internal and external.

Internal is all done with a spiral flute, powered metal Guhring machine tap, designed for SS, and using a 50% thread depth drill.

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External threading is done with an Accusize insert holder. https://accusizetools.com/products/...threading-inserts?_pos=3&_sid=9c18780a2&_ss=r

500 RPM, 5 passes with a finish spring pass. I use a reduced step down strategy where each pass has less depth than the previous and starts at about 0.01 DOC and finishes at about 0.002 DOC. Flood coolant.



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I use these Mitsubishi MMT16IRAG60 VP10MF inserts, designed for SS
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I get a little tearout on the 303 SS, but the 304 SS threads are nice and smooth. I could work with the cutting parameters to clean up the 303 threads a bit, but they meet our needs as they are.

The 304 is much harder on the tools than is 303. I can get a few hundred parts per edge in the 303, where the 304 I get a couple hundred at best.

Also I just received my new Shars threading insert holder today. https://www.shars.com/3-4-mtvo-external-threading-grooving-tool-holder-right-hand

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and these inserts https://www.shars.com/tnmc-32nv-tin-carbide-insert

I bought this because we are changing the design of the parts and need the extra clearance. on the face.

Very interesting technique. The reduced depth of cut is for cleanup of tearing?

 

[JimDawson]

Very interesting technique. The reduced depth of cut is for cleanup of tearing?

Not really to relieve tearout, but more to keep the load on the tool more even throughout the process. When threading, as you cut subsequent passes the chip gets wider because the tool is cutting on the side of the thread. So reducing the cut depth reduces the load on the tool. I used to actually break tools before I started doing it this way. The finish spring pass at 0 stepover is really the cleanup to try reduce the tearout. I could most likely eliminate the tearout by adding a couple more stepovers, thus reducing the depth of cut a bit more for each pass, I'm really cutting kind of aggressively.

 

[ttabbal]

Very interesting technique. The reduced depth of cut is for cleanup of tearing?

Keep in mind that threading is a form tool. So as you go deeper, you are cutting along the sides as well. You can start out with a deeper DOC, but once you get in a little more, the pressure on the tool increases fast. You control that by taking a shallower cut the further in you go.

For tearing, you can do the last couple of passes really shallow and a spring pass or two at the end will help clean up the cut. If you want them really clean I guess you could use a thread file to do the last thou or two.

 

[FTlatheworks]

I cleaned up my cutting tool on a table top disc grinder. I attempted to cut threads (1/2-20). I made 6 thou cuts and it was going great until I got to about 18 thou in. I believe it work hardened, but I’m not sure. I was so close to have a perfect set of threads which is exciting for me.

it sounds like I should have backed off after taking two cuts of 5-6 thou and only taken cut 1-2 from there on? Also, I bumped up my rpm from 65-100. That seems to be fast, but that’s the closest I have ever been to having nice stainless threads. I went in at a 28 degree angle, and I used rapid tap, or magic tap.

Any suggestions?

 

[FTlatheworks]

Keep in mind that threading is a form tool. So as you go deeper, you are cutting along the sides as well. You can start out with a deeper DOC, but once you get in a little more, the pressure on the tool increases fast. You control that by taking a shallower cut the further in you go.

For tearing, you can do the last couple of passes really shallow and a spring pass or two at the end will help clean up the cut. If you want them really clean I guess you could use a thread file to do the last thou or two.

What is a spring pass? Here’s my successful thread. I can put the nut on by hand, but it doesn’t look pretty.

 

[NC Rick]

That’s awesome! I tried a 5/16-18 thread on 304 today using a die. I canceled the project do to not needing it badly enough. While it was working, i would have been there a long time to produce 6 Of ‘em. Nice work!

 

[FTlatheworks]

That’s awesome! I tried a 5/16-18 thread on 304 today using a die. I canceled the project do to not needing it badly enough. While it was working, i would have been there a long time to produce 6 Of ‘em. Nice work!

Ya it took a long time. I used a hss 3/8 blank. Now I know if I invest in fact tooling it will only get better. I calculate the sfm and came up with about 500 rpm. Can anyone confirm that’s correct?

on your project, do you have a tailstock die holder? I hear that’s a guaranteed way to do it.

 

[mikey]

Your compound angle is off. Asian lathes don't have consistent markings. If your index says 29.5, try setting it to 61.5 and try again.

 

[JimDawson]

Great start. You are almost there.

It kind of looks like your thread is not formed correctly and maybe are cutting too much on the back side.. The top of the threads seem to be folded over away from the chuck. The bottom of the threads look to be well formed. It's hard to tell but the front and back side angle does not look symmetrical. Also looks like it is cut too deep. What might help you is to take a few passes with a file before you take the final couple of threading passes, that should eliminate the folded over sharp tops and give you a more visually pleasing thread form. It almost looks like your cutting edge is not exactly perpendicular to the work.

A 28 degree approach is maybe a little much, 29 or 30 degrees might be better. With a properly ground 60 degree tool, you can plunge straight in with the approach perpendicular to the work, but this does not always work well. If you swing your compound around to parallel to the work and step over 1 or 2 thou on each pass then feed in with the cross slide it works well also. A spring pass is a final pass without going deeper on the compound. It just cleans up the fuzz on the threads.

On a 1/2-20 thread, I turn the major diameter to 0.495 and thread to the proper minor diameter. This gives a better thread form with a bit of a flat on the thread peaks.

Here is an example of a 1/2-20 thread in 303 SS I did a few days ago. I'm not sure exactly where the ''chatter'' marks are coming from, but it may actually be chatter that I can't hear. Or it could be just the way the chip forms on 303 SS. 304 SS seems to be smoother on the thread faces, but we switched to 303 to save on tooling costs.

 

[JimDawson]

Ya it took a long time. I used a hss 3/8 blank. Now I know if I invest in fact tooling it will only get better. I calculate the sfm and came up with about 500 rpm. Can anyone confirm that’s correct?

500 RPM is what I use. Turning the tool upside down, running the spindle in reverse thus feeding away from the chuck will save your nerves when threading at higher RPMs. Also turning a relief near the shoulder will be very helpful.