Facets in my 16 TPI Threads - Need some help!

[macardoso]

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.

Would you be concerned with added wear to the ways?

 

[davidpbest]

Would you be concerned with added wear to the ways?

I wasn't suggesting this as a permanent fix, just one method to isolate if the leadscrew is stuttering. Clearly you would not want to continue to run longer term with the carriage dragging - that would create undesirable wear. But it could help diagnose if the leadscrew is the culprit.

 

[Batmanacw]

If this was my lathe I'd be checking to make sure my change gears are too tight or running out but I don't think this lathe has change gears.

 

[mattthemuppet2]

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.

I'm not an expert but any end to end play in your leadscrew will show up in cross threading, not chatter. The left hand leadscrew support/ bushing came loose on my Atlas once and trashed a thread. Easy to diagnose, simply close your halfnuts and try and move your carriage back and forth.

The OPs problem looks like bad chatter to me. Something is loose somewhere - spindle bearings, carriage/cross slide/ compound gibs, tool post or tool holder. Vibration from motor or belts is a possibility, especially as plenty of people have described issues with their 12x36/ 13x40 class import lathes on here, but I can't see it producing faceting that bad. I've had mild vibration on my lathe before and it didn't produce this pattern. I did thread a chuck backplate with a whippy home made boring bar and HSS bit that produced exactly that pattern though.

I'd start with the list above, beginning with spindle bearing preload, and go from there.

 

[macardoso]

I'm not an expert but any end to end play in your leadscrew will show up in cross threading, not chatter. The left hand leadscrew support/ bushing came loose on my Atlas once and trashed a thread. Easy to diagnose, simply close your halfnuts and try and move your carriage back and forth.

What do you do to resolve this if you do identify it as an issue? I see probably 1/8" of movement doing the test you just described. My lathe does not have thrust bearings on the leadscrew.

 

[mattthemuppet2]

I snugged up the end nut to take up the play, but you could make a simple bronze or steel washer of the right thickness to go between the end of the leadscrew and the right hand support bearing. Hard to say for definite as I don't have any experience of your type of lathe.

 

[davidpbest]

Any update here?

 

[macardoso]

I don't want to hijack this thread if my screw issues are not the solution to the faceting issue from the OP, but I bought some hardened steel washers (thrust bearing races) and bearing bronze stock (expensive!). Once my current setup on the lathe gets torn down, I'm going to make a thrust bearing and see if that resolves some of the faceting I see on my threads just like OP.

 

[TX COWDOC]

Any update here?

Hi Dave - I'll circle back with an update this weekend.

 

[9t8z28]

Wow, better read the reviews on those Accusize tool bits ... maybe better to grind your own?

I agree about these being used only for brass and the like. They do not hold an edge very well but work great for tools that need to hang out of the tool holder quite a bit. The only reason I use them is when I need to grind a lot of material off of them for deep internal/external threading where the tool gets long and narrow but again, only for brass. You can grind a finished tool with these in a few minutes but with HSS from a well known brand you'll be there quite a while with a bench grinder.