Inconsistent Threading Depth of Cut on Leblond Regal Lathe

[tjansson]

I've been cutting a number of 1/2-20 threads in alloy steel shaft 33 HRC for some prototype parts at work. I am using a Leblond Regal lathe with DRO. I am using the DRO to bring the cross slide back to "0" after each pass and then advancing the compound slide for the next cut. I keep getting randomly heavy passes, twice now it's blown right through from a 80% cut thread to overcut (too much material removed) and I have to scrap the part. I started engaging the half nut on the same number on the chasing dial each time, but it actually seemed to make the problem worse. I might try cleaning the half nuts and lead screw next.

Any advice? Thanks!

 

[dutchgray]

Does your DRO reading on the cross slide change from 0 when this happens during the cut?
That will tell you if it's your cross slide that's moving in, make sure all the gibs are well set up.

Are you engaging the half nuts far enough away from the cut that the engagement settles in, you want several rotations at least before any cutting happens, but the more the better.

Definitely clean the leadscrew and half nuts if they are dirty.

 

[Firebrick43]

I've been cutting a number of 1/2-20 threads in alloy steel shaft 33 HRC for some prototype parts at work. I am using a Leblond Regal lathe with DRO. I am using the DRO to bring the cross slide back to "0" after each pass and then advancing the compound slide for the next cut. I keep getting randomly heavy passes, twice now it's blown right through from a 80% cut thread to overcut (too much material removed) and I have to scrap the part. I started engaging the half nut on the same number on the chasing dial each time, but it actually seemed to make the problem worse. I might try cleaning the half nuts and lead screw next.

Any advice? Thanks!

What kind of tooling are you using (carbide inserts,HSS)?

When cutting harder steels and using carbide there is no sneaking of on a cut. You have to get it under the skin or it just rubs and when it gets under the skin is going to take .004” or more in cut. You have to know your tooling and adjust/measure through the process with a thread mic so the last cut ends up correct.

If you want to sneak up on it you really need to use sharp HSS tools, either hand ground bits or inserts which are available but uncommon.

 

[tjansson]

@dutchgray
The DRO doesn't indicate any shift.

I have been engaging the half nut with the tool about 1/4" - 3/8" away from the threads, 5-8 threads away. I'll try more like an inch.

I have a hunch it happens when I'm a little late on moving the half nut lever... like right on the dash rather than right before. Not sure why that would make a difference.

 

[tjansson]

@Firebrick43
I'm using carbide tipped tools (not insert).

I've been doing about .003" DOC on the compound dial as I'm getting close to finished thread. It seem like when it takes a heavy cut it's like .015 DOC or more!

 

[slowtwitch73]

Make sure your tool is on center and that your setup is rigid.

Sometimes the part will move up (roll onto) the cutter.

 

[Firebrick43]

@Firebrick43
I'm using carbide tipped tools (not insert).

I've been doing about .003" DOC on the compound dial as I'm getting close to finished thread. It seem like when it takes a heavy cut it's like .015 DOC or more!

That is way to light of cut on carbide.

You are going to get inconsistent results.

Switch to HSS and grind the tool with positive rake.

 

[tjansson]

Make sure our tool is on center and that your setup is rigid.

Sometimes the part will move up (roll onto) the cutter.

I've been sticking the 5/8" stock out of the chuck about 1.75". Best I can do is 1.25" stick out. I might be able to sneak the live center center in there,.

 

[Mgdoug3]

I like to use a dial indicator on my crossfeed so I can tell exactly how much I'm taking off. It helps me get really close to size before needing to test fit or measure.

Edit: dial indicator is touching my tool post. This way I don't have to do math to figure out how much my compound is moving at a 29.5 degree angle.

 

[alfadan]

How much stick-out are you using? No center drilled for tailstock support, deflection may be an issue.

 

[slowtwitch73]

Besides the obvious deflection culprits, it can be the chuck, the bearings, etc etc.

I'd grab the end of that rod and yard on back and forth up and down and see if you can discern any movement/play.

Your saddle gibs/locks also need to be in order.. I usually *just* snug them a bit when threading.

Make sure you are on center with no play and increase doc as others have said (or use hss).