Facets in my 16 TPI Threads - Need some help!

[mksj]

Looks like quite a bit of movement on the side of the QCTP and on the top of the tool post rest/slide. Is there a small gap between the base of the QCTP and the foot so it clamps down solidly? Is there any rocking motion when it is sitting either on a flat surface or on the tool post slide. Also is the slide locked down and is the gib set correctly? To me that is quite a bit of movement when the cutter engages.

 

[TX COWDOC]

OK, you showed your dial, so thought you were using it. I keep my DRO in diameter mode and I usually take .020 for 1st cut, then .010" for a cut or 2, then .005" up until close to final cut and maybe .002" and then spring pass or 2.

The cross dial and the DRO correlate very closely on this machine. I am, in fact, using both. I am not taking heavy cuts. I begin with 0.001 scratch pass, then increments of 0.002 radially until I am at my last 0.010 then I proceed at 0.001 with a spring pass. Everyone has their preferences to be sure. I am getting some telling me heavier cuts and others saying I am taking too much. Some say speed up and others have clearly demonstrated success in the range I was expecting to work in ~ 65RPM.

All I want is pretty threads!!

 

[joebaby]

High speed steel 10% cobalt tool bit. Much lighter cuts, esp. in roughing passes. Set compound at 28 degrees, not 29.5 (the purists will be appalled). Spend a little time making a fixture to grind the tool bit. Good luck

 

[davidpbest]

David,
I took the measurements and posted some videos. What amount of movement on the DTI is acceptable? I did get movement at the tool holder DTI @ 200 RPM as well as the Compound @ 65 RPM. All cuts were 0.002" with the cross slide from 0.000 to 0.028 then 0.001 + spring passes until 0.0375.

I've watched all your videos, and I may be overly anal about all this, but I see evidence of some rigidity issues. Hard to know if these are the source of your treading woes, but I would first try to get all these tuned out as much as possible. Here is what I observe and what I would do to check/correct things if this were my lathe:

1. Watching the first video where you are threading away from the chuck at 200 RPM and indicating to the side of the QCTP, there is evidence of jitter indicating tool chatter in the cutting operation and that something in the setup is allowing the QCTP to flex under load. Makes me wonder if the QCTP is sitting dead flat onto the top surface of the compound. If the T-nut securing the QCTP to the compound sticks up above the top surface of the compound, the QCTP will rock under load.
2. The second video from cross slide to bed ways show minor displacement, but it's enough that I question the fit of the cross slide gib. I would remove the cross slide gib, clean the dovetails surface, and stone the gib on all surfaces to remove any burrs. I would also check to see if the surfaces that mate to the dovetails are flat with some bluing.
3. In the videos where you are indicating to the cutting tool and the back of the compound, it appears to me that the compound is flexing as the thread-cutting engages. This looks like too much movement to me. The indicator to the tool shows the tool deflecting downward about 5/10ths, and the indicator to the back of the compound shows that surface being tetter-tottered upward almost 0.001". The possible sources of that flex include:
   • The bottom surface of the compound rotation ring is not flat where it sits on the surface of the cross slide
   • there is debris between the mating surfaces of the compound and the cross slide
   • the securing bolts that lock the compound to the cross slider are not tight
   • the t-nuts in the circular race inside the cross slide are weak or broken or being pulled up for some reason. Other users have had issues with one of the t-nuts pulling up when the compound is angled just so and the t-nut ends up positioned just below the access hole. This has been discussed at this thread.
   • The gibs in the compound are not fitted and adjusted properly.
   • I would remove the compound from the cross slide, clean up all the interfaces, and search for the source of the compound rocking.

Looking at the resulting thread grooves, I still question whether the cutting tool is on CL with the spindle. I would check that with a DTI.
Also, the best estimate I've seen for SFM for annealed 416 is 120. Published SFM's I've seen range from 110 to 175. If your part is 1.25" diameter, using SFM of 120, that works out to a spindle speed of 366 RPM using HSS cutting tool. If the part is 1.5" diameter, using a 100 SFM, that works out to 250 RPM. I haven't threaded 416, but with 316 and 304 if you go too slow, the material tears rather than cuts and produces the kind of finishes you are seeing. I'd love to see a thread cut at 350 RPM for comparison.

Hope this helps.

 

[Liljoebrshooter]

The 416r barrel steel threads much easier than 304 stainless.
There are still some issues causing the problems.
Joe

 

[Liljoebrshooter]

Here is a tennon I threaded on my 1340 jet. Same material. 416R from Krieger.
I had many of these same issues when I began threading. David listed many of the problems I had with my machine.

Joe

 

[TX COWDOC]

Here is a tennon I threaded on my 1340 jet. Same material. 416R from Krieger.
I had many of these same issues when I began threading. David listed many of the problems I had with my machine.

Joe

Nice work. What is your approach? Speed? Tool type?

 

[TX COWDOC]

I've watched all your videos, and I may be overly anal about all this, but I see evidence of some rigidity issues. Hard to know if these are the source of your treading woes, but I would first try to get all these tuned out as much as possible. Here is what I observe and what I would do to check/correct things if this were my lathe:

1. Watching the first video where you are threading away from the chuck at 200 RPM and indicating to the side of the QCTP, there is evidence of jitter indicating tool chatter in the cutting operation and that something in the setup is allowing the QCTP to flex under load. Makes me wonder if the QCTP is sitting dead flat onto the top surface of the compound. If the T-nut securing the QCTP to the compound sticks up above the top surface of the compound, the QCTP will rock under load.
2. The second video from cross slide to bed ways show minor displacement, but it's enough that I question the fit of the cross slide gib. I would remove the cross slide gib, clean the dovetails surface, and stone the gib on all surfaces to remove any burrs. I would also check to see if the surfaces that mate to the dovetails are flat with some bluing.
3. In the videos where you are indicating to the cutting tool and the back of the compound, it appears to me that the compound is flexing as the thread-cutting engages. This looks like too much movement to me. The indicator to the tool shows the tool deflecting downward about 5/10ths, and the indicator to the back of the compound shows that surface being tetter-tottered upward almost 0.001". The possible sources of that flex include:
   • The bottom surface of the compound rotation ring is not flat where it sits on the surface of the cross slide
   • there is debris between the mating surfaces of the compound and the cross slide
   • the securing bolts that lock the compound to the cross slider are not tight
   • the t-nuts in the circular race inside the cross slide are weak or broken or being pulled up for some reason. Other users have had issues with one of the t-nuts pulling up when the compound is angled just so and the t-nut ends up positioned just below the access hole. This has been discussed at this thread.
   • The gibs in the compound are not fitted and adjusted properly.
   • I would remove the compound from the cross slide, clean up all the interfaces, and search for the source of the compound rocking.

Looking at the resulting thread grooves, I still question whether the cutting tool is on CL with the spindle. I would check that with a DTI.
Also, the best estimate I've seen for SFM for annealed 416 is 120. Published SFM's I've seen range from 110 to 175. If your part is 1.25" diameter, using SFM of 120, that works out to a spindle speed of 366 RPM using HSS cutting tool. If the part is 1.5" diameter, using a 100 SFM, that works out to 250 RPM. I haven't threaded 416, but with 316 and 304 if you go too slow, the material tears rather than cuts and produces the kind of finishes you are seeing. I'd love to see a thread cut at 350 RPM for comparison.

Hope this helps.

Very informative post. I will look dig into the suggestions tomorrow. I will confirm tool height this weekend per the Joe P. video shared by Jim F.

Depending on the tool, the cutting edge may not be the point touching the scale.

 

[Jim F]

If you are dead set against the reverse threading method Joe Pie does, this maybe an option to consider. Both lessen the forces on the tool.

 

[Jim F]

This a tool holder that came with my lathe. I is too big to fit a SB9 lantern, but I am thinking of making a dedicated holder for it.
It is 1 1/8 tall and 9/16 wide.