Facets in my 16 TPI Threads - Need some help!

[savarin]

Same here, I only replace the compound for small taper cuts.

 

[TX COWDOC]

I did not receive your PM for some unknown reason.

just sent another. Hope it make it this time. Thanks again -

 

[Downunder Bob]

In all my years I have never seen single phase induced chatter. Does it only happen in conjunction with carbide inserts. Or is it an excuse for poorly set up machinery.

But yes, if I had chatter I would try HSS. I would also check the mounting of tool post and holder, compound gibs, tool post overhang and compound overhang all common causes of chatter. I have seen chatter just like that on 3 ph machines.

This works much better if you use beer. Especially something dark and hoppy...

David- what is the point of the insert holder being tilted and counteracting the angle of the insert? Is this purely for clearance? This tool is labeled as an external threading tool by the manufacturer so clearance would not be an issue?
Robert

I always though misadventure was bad planning , and adventure was good planning.

 

[TX COWDOC]

Downunder Bob,
Best I can tell, everything is tight and not extended. I will try HSS this week. As you can see in the videos I posted earlier, there is vibration that I cannot localize. The lathe came with a separate Baldor motor that I swapped out when assembling the lathe. It is tight and has proper belt tension. Beyond that I'm not sure what would account for 'poorly set up machinery'. I am not a trained machinist and appreciate the insights as I sort through this.

 

[pdentrem]

Drive system
Lumpy belt?
Motor mount is not solidly bolted?
Drive pulley tight to their shaft?
Drive pulley is clean without build up?

Other possible items
Can you lift the carriage either at the front or back?
Compound is sitting on a chip?
Pierre

 

[davidpbest]

Drive system
Lumpy belt?
Motor mount is not solidly bolted?
Drive pulley tight to their shaft?
Drive pulley is clean without build up?

Other possible items
Can you lift the carriage either at the front or back?
Compound is sitting on a chip?
Pierre

These are all good things to check and could be contributing factors. Put a dial indicator from the cross slide top surface to the top of the threading tool, and observe the readings while cutting threads. That will give you some indication of the rigidity of your setup.
Based on your water-glass videos, it does not appear to me to be single-phase motor induced harmonics. When those issues have come up, the videos make it crystal clear that the motor harmonics are at issue, and I do not see that in your videos. What I do see is probably belt thrashing or out-of-balance pulleys on the motor or on the headstock end where the belt attaches. Have you observed the belt-thrashing behavior while threading? I replaced both pulleys with balanced high-quality version, and the OEM belt with a notched Gates Tri-Power belt on my 1340 and was rewarded with better surface finishes.

But the kind of surfaces you are getting while threading suggest more severe fluctuations at the cutter-to-part interface. This could be rigidity issues stemming from the relatively weak mounting T-nuts for the compound on this lathe, or it could be some other loose gib in the compound or cross slide, or a loose tool post. An indicator from the cross slide to the top of the tool doing the cutting will tell a story.

 

[TX COWDOC]

Here's a video of the pulleys at 65 RPM. I'll double check the compound / cross slide tomorrow after work. I hope to also visit with PM tech support as well.

 

[ChrisAttebery]

IMHO the SFM is way too slow for carbide. I have an internal threading tool that uses those inserts. It’s been a while since I used it but I think I used 190 or 300 rpm for a 1.935” thread.

What DOC are you using? Are you taking any spring passes?

Another thing to try is locking down the compound and use the cross slide for infeed . If that makes a difference then the compound is too loose or flexing.

You really should try a HSS tool. This is coming from someone who has a rack full of carbide tooling and a 13x40 lathe.

I have this type of threading/grooving tool holder. I use the HSS threading insert for most of my threading.

Grooving & Cut-Off Tool, 1/2" Indexable HSS Inserts, A R Warner Kit #24 4819

Cut-Off Tools 4819 For external grooving and cut-off operations; Includes two 1/16" (.062") groover T-15 high speed steel inserts; Includes two 1/32" ...

littlemachineshop.com

Insert, 60 Degree Threader 4603

Indexable Inserts 4603 Cuts 60 degree external threads; Made from T-15 high speed steel; For PN 3572 Grooving & Cutoff Tool; These inserts also fit th...

littlemachineshop.com

Here’s another style that would work for your lathe.

Threading Tool, External, 3/8" Indexable, HSS Triangle Inserts, A R Warner Kit #17 3695

Lathe Threading 3695 For external threading, 8 through 36 TPI; 3/8" shank, 3/8" IC Inserts, 5" long; 2 HSS inserts included: TNMC-32-NV and TPMC-32-NV...

littlemachineshop.com

They also sell sharpened HSS tool bits if you want to try one.

Tool Bit, 5/16" HSS Threading 2221

Lathe Threading 2221 The 5/16" High Speed Steel Tool Bit Blank is a versatile and essential tool for any metal lathe operator. Made from high speed st...

littlemachineshop.com

 

[macardoso]

I have a 12x36 lathe and have been fighting this issue for 2 years. I get the exact same pattern on most of my threads, regardless of material, size, or part shape/stickout. I have only used carbide, and I have a single phase motor. Swapped belts, strengthened the setup, etc. and nothing has made any difference.

My thought is slop in the leadscrew. My half nuts are in great condition, but there is endplay in the screw itself due to a lack of thrust bearing at either end. I wonder if cutting forces can push and pull the carriage to slightly advance/retard the position relative to the screw pitch. Once it begins the harmonic forces will tend to continue the pattern on subsequent passes. I have it on my project list to add thrust bearings to the leadscrew.

 

[davidpbest]

I have a 12x36 lathe and have been fighting this issue for 2 years. I get the exact same pattern on most of my threads, regardless of material, size, or part shape/stickout. I have only used carbide, and I have a single phase motor. Swapped belts, strengthened the setup, etc. and nothing has made any difference.

My thought is slop in the leadscrew. My half nuts are in great condition, but there is endplay in the screw itself due to a lack of thrust bearing at either end. I wonder if cutting forces can push and pull the carriage to slightly advance/retard the position relative to the screw pitch. Once it begins the harmonic forces will tend to continue the pattern on subsequent passes. I have it on my project list to add thrust bearings to the leadscrew.

Before you tear into the leadscrew to add thrust bearings, try snugging up the carriage lock (carriage, not cross slide) a bit so the leadscrew has to labor a bit to advance the carriage. Kind of a poor-mans preloading of the leadscrew. That should take out any leadscrew stuttering, and if that does solve the problem then the leadscrew has it's own backlash issues that new thrust bearings and preloading the leadscrew would correct. Just a thought.