Thread stainless on lathe

[just Dave]

Yes! Set the indicator so you get nearly a full swing and land at 12:00. With some practice you'll be able to land within .010" or so.

.001 is the goal. Old men can still hit that.

 

[Cole2534]

.001 is the goal. Old men can still hit that.

I never got that good, maybe within 0.005". This is one of those times where an extra large dial face is a big help.

 

[Bsalbrig]

Yes! Set the indicator so you get nearly a full swing and land at 12:00. With some practice you'll be able to land within .010" or so.

I did try threading with a normal holder up to a large relived area and it does look better than the boring bar. I did that one at 90deg in just to test how it went. I’m going to try it again at 30deg. Im including pictures of both holders side by side. They are both 16tpi full profile Stainless geometry inserts. I’m using a monarch/weiler 517 lathe. I tried it on a emco maximat v13 that I much prefer but I believe the thread half nut is too loose or something is broken in the head because it won’t pick up on the same thread location.

 

[Kingbob]

I did try threading with a normal holder up to a large relived area and it does look better than the boring bar. I did that one at 90deg in just to test how it went. I’m going to try it again at 30deg. Im including pictures of both holders side by side. They are both 16tpi full profile Stainless geometry inserts. I’m using a monarch/weiler 517 lathe. I tried it on a emco maximat v13 that I much prefer but I believe the thread half nut is too loose or something is broken in the head because it won’t pick up on the same thread location.
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If you have a sloppy half nut then don't disengage it. Stop the spindle then reverse to pick back up at the stary of the thread. Low rpm in these situations ir friend, slows things down.

 

[Cole2534]

I did try threading with a normal holder up to a large relived area and it does look better than the boring bar. I did that one at 90deg in just to test how it went. I’m going to try it again at 30deg. Im including pictures of both holders side by side. They are both 16tpi full profile Stainless geometry inserts. I’m using a monarch/weiler 517 lathe. I tried it on a emco maximat v13 that I much prefer but I believe the thread half nut is too loose or something is broken in the head because it won’t pick up on the same thread location.
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I'm not a fan of the 30deg approach. I find it easier to plunge directly and it doesn't change your Z position.

 

[guythatbrews]

I did try threading with a normal holder up to a large relived area and it does look better than the boring bar. I did that one at 90deg in just to test how it went. I’m going to try it again at 30deg. Im including pictures of both holders side by side. They are both 16tpi full profile Stainless geometry inserts. I’m using a monarch/weiler 517 lathe. I tried it on a emco maximat v13 that I much prefer but I believe the thread half nut is too loose or something is broken in the head because it won’t pick up on the same thread location.
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Now we can see the boring bar does have a shim seat. What is the angle marked on the insert?

I don't think you mention the major diameter so I'm guessing 3/4. The helix angle for 3/4-16 is about 1.5* so you can use a 1* or 2* shim. Some bars already have an angle built in to the pocket so that must be taken into account. Understand that there is a range of helix angles each insert/shim/bar setup can effectively cut. What good for one may or may not work on another. You've got to do some figuring.

If your'e not using a -1* or -2* shim the insert is tilted the wrong way and will likely rub on the flanks, resulting in poor threads and broken inserts. Softer materials will be more forgiving but still not good.

Get two pieces of high speed xteel and a white wheel on the bench grinder and a fishtail gage. Grind a radius relief into the first one, plunge that baby into the workpiece to a little under thread depth at the headstock end of the thread. That makes a stronger thread than what you are doing.

About the radius thread relief this is 100% correct. Maybe you are just practicing threading to a shoulder but you must understand you are cutting a very poor defacto thread relief anyway. The worst you can make as far as a stress riser. A wider thread relief, even flat bottom, is a better solution and easier in every way.

Even with a CNC lathe running a canned cycle you'll have this crappy relief. A ramp in can be programmed longhand and is much preferred.

Please tell use why you are so opposed to a thread relief! Maybe you have a very good reason. We'd like to hear it!

 

[Mtndew]

If you need to cut an OD thread away from a shoulder, use a regular holder, flip it upside down and run the spindle in reverse.
Works like a charm. I've done it many times back in my engine lathe days.

 

[EMTech]

Now we can see the boring bar does have a shim seat. What is the angle marked on the insert?

I don't think you mention the major diameter so I'm guessing 3/4. The helix angle for 3/4-16 is about 1.5* so you can use a 1* or 2* shim. Some bars already have an angle built in to the pocket so that must be taken into account. Understand that there is a range of helix angles each insert/shim/bar setup can effectively cut. What good for one may or may not work on another. You've got to do some figuring.

If your'e not using a -1* or -2* shim the insert is tilted the wrong way and will likely rub on the flanks, resulting in poor threads and broken inserts. Softer materials will be more forgiving but still not good.

This is exactly correct. This is what I was also trying to communicate.

The reason the OD holder worked better cutting from front to back is because the insert relief was correct.

I suspect that if the OP used the threading bar and cut a left handed thread from front to back, he would see similar surface finish results.

The reason for the bad surface finish while using the bar from back to front is because the insert does not have the proper clearance for that cut with the shim that is currently in the holder.

 

[Bsalbrig]

I'm not a fan of the 30deg approach. I find it easier to plunge directly and it doesn't change your Z position.

Now we can see the boring bar does have a shim seat. What is the angle marked on the insert?

I don't think you mention the major diameter so I'm guessing 3/4. The helix angle for 3/4-16 is about 1.5* so you can use a 1* or 2* shim. Some bars already have an angle built in to the pocket so that must be taken into account. Understand that there is a range of helix angles each insert/shim/bar setup can effectively cut. What good for one may or may not work on another. You've got to do some figuring.

If your'e not using a -1* or -2* shim the insert is tilted the wrong way and will likely rub on the flanks, resulting in poor threads and broken inserts. Softer materials will be more forgiving but still not good.



About the radius thread relief this is 100% correct. Maybe you are just practicing threading to a shoulder but you must understand you are cutting a very poor defacto thread relief anyway. The worst you can make as far as a stress riser. A wider thread relief, even flat bottom, is a better solution and easier in every way.

Even with a CNC lathe running a canned cycle you'll have this crappy relief. A ramp in can be programmed longhand and is much preferred.

Please tell use why you are so opposed to a thread relief! Maybe you have a very good reason. We'd like to hear it!

The inserts I am using have a .007” radius at the point. For the external bar I’m using an Iscar 16ER16UN IC908. The insert on the boring bar is a carmex 16EL16UN BMA (external 16EL insert on a holder meant for a 16IR)

Shimming inserts for cut angle is something I’ve never done nor knew that needed to be done. I’ll look into it. I was making assumptions that the angle was set on the insert and there was a standard tilt on the tool holder.

I don’t have a good reason not to thread to a relief other than I was trying to avoid the pressure of disengaging the screw at the right spot. I wanted to try it and see how it did. I bought both sets of tools before hand should it not work out.

The goal is a 1 1/16-16tpi thread. I’ve been cutting on random diameter SS bars just to figure out how to run the cut. Im not a machinist just a hobbyist. I’ve had to learn most things on my own but so far I’ve been able to figure out and have satisfactory results.

 

[guythatbrews]

Thanks for the information. You picked a great hobby. You'll never learn all there is to know, but it's a fun journey.

We try to avoid threading with no relief groove. It's just not the best way to go. On a CNC no big deal just chamfer out of the cut and it looks great. Not so much on a manual lathe. A tracer attachment on a manual lathe with the proper template can make a really great looking ramp out on the end of the thread. Even if you want to thread a RH thread away from the chuck, use a thread relief groove. The part will look nicer and be less likely to fail down the road.

Thread helix angles are all over the place. 5/16-18 takes a 4* shim seat. 1-20 takes a 1* shim seat. That's for single start threads. Multiple starts threads are much higher and will require some fiddling to get high enough tilt on the insert. Put some coarse and fine pitched threads together and you start to see what's going on.

The formula for the helix angle is

Helix Angle = Arctan(lead/pitch diameter circumference)

Here is a link to a calculator that uses the circumference of the major diameter. Strictly speaking this is incorrect, but practically for figuring what seat to use it's ok.

Helix Angle Calculator - Calculator Academy

Enter the lead of the screw and the circumference of the screw into the calculator to determine the helix angle.

calculator.academy

A small mistake but It does nicely illustrate the possibility of incorrect information being posted. The web is rife with errors and care is needed.

We don't know your level of knowledge, but for pretty cheap you can get decent books that really help. Some of the information is outdated but is still useful. It's not passive entertainment like watching a youtube vid that is quite possibly made by someone with less knowledge than you. But a book like this has been corrected and vetted and likely passes along no errors.

How To Run A Lathe - Volume I (Edition 43) The Care And Operation Of A Scre... 9781473337572 | eBay

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[GregSY]

Another 'cheat' you can do is thread 'pretty close' to the final depth, then run a die down the threads to finish them. I don't like to do this as I believe you can get a better thread by single pointing them...but in some cases it makes a project fly. It's also a good way to thread all the way to a shoulder. We used to sometimes grind a die on the surface grinder so its threads had no lead-in at all and you would effectively run the thread on the part into the shoulder. But....I have to wonder why you need threads that close to the shoulder....

 

[Cole2534]

Another 'cheat' you can do is thread 'pretty close' to the final depth, then run a die down the threads to finish them. I don't like to do this as I believe you can get a better thread by single pointing them...but in some cases it makes a project fly. It's also a good way to thread all the way to a shoulder. We used to sometimes grind a die on the surface grinder so its threads had no lead-in at all and you would effectively run the thread on the part into the shoulder. But....I have to wonder why you need threads that close to the shoulder....

I may have used that trick to get metric threads on an inch-only machine. Just once, in a pinch.

 

[rons]

I’m turning the lathe as slow as it will go at 46rpm.

I'm using about 325 rpm with a little oil. Depth of cut something like .005 each pass until end cuts which may be .002 - .001.

 

[technocrat]

46rpm is so slow you'll never get a good finish.

Cut a proper thread relief, get the RPMs up around 200-250 and practice it. You'll be surprised how much better the threads will look.

dead right here. Carbide inserts and hss operate in different metal deformation zones of the parent material. Either of them working in the wrong zone will give you a bad finish. In order of increasing surface speed:

HSS / carbon steels operates in the conventional cutting zone where the chip is not torn from the parent material and tool wear is low. The tool has the material flowing over the cutting face.

Next is the zone where hss tool wear is high and material is torn from the parent material, or carbides rub rather than cutting. The material separation zone is not controlled.

Finally, carbides operate where the material has entered the plastic state. Higher force (hp) is needed, but no tearing and much faster machining speeds result. The material separation zone is ahead of the cutting edge and lower tool wear and higher cutting speeds result.

Tool geometries do effect where in each zone the cutting tools operate, but the don't generally overlap the zone.

 

[henrya]

Even though that stub looks real stiff, a center supporting the end will make a better set up and possibly eliminate some of the roughness. Also it will give you more room to work in away from the chuck.

 

[henrya]

A proper relief groove is not an indicator of lack of skill, its sound engineering practice. You can learn to pull out of a thread to a shoulder - just practice some.