Problems threading on a 3 in 1 machine

Pmedic828

Pmedic828

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Has anyone had any threading problems on their 3 in 1 machine? I am having problems threading as the existing threads that I have cut are being cut away on proceeding passes. I chucked a piece of HRS in the lathe chuck which extends about 1 1/2 inches from the face. I took a skim cut to make sure the stock was round. Set up the compound at 29 1/2 degrees, made sure that the cutter was square to the work and chuck. Marked the stock with an indicator mark to see if stock was slipping in in the chuck. Checked the machine for a 20 TPI thread. Zeroed the cross slide, advanced the compound about 3 thou and proceeded to cut a thread thread dial on #1 - everything looks good, thread matches up to thread gauge and rod mounted in lathe has a relief cut to prevent the cutter from crashing into the shoulder. After cutting the 1st thread, cross slide backed out and returned up to beginning of piece, cross slide returned to zero, compound advanced 2 thou, thread dial on #1, engaged half nut, cut same thread deeper - everything looks good, continued this another time with threads tracking -
now for the fun part, after re zeroing the cross slide, advancing the compound another 2 thou, thread dial on #1, engaged half nut and the threads that were already cut was eaten away due to cutter riding on crest of the already cut thread. Checked to see if rod slipped in or out or rotated in chuck - nope. This is the 2nd time I had this happen to me.
Mounted a piece of 1-1/4 already threaded rod left over from a bolt that I cut the hex head off of. Checked to see what pitch the thread was - it was 7 TPI, set threading knobs to 7 TPI, and left cross slide out a bit so cutter would track over the already cut threads. After 3 times, the cutter advanced over the crest of the threads which seems to indicate that the timing or advancement of the cutter is being influenced. it seems to be about 7 1/2 TPI as the cutter advances more than the existing thread - I have no 7 1/2 TPI setting on my lathe?
Does anyone have a comment - will make pics tomorrow if needed, but I don't think it is in the setup - the thread just seems to be increasing and decreasing in the linear distance for a specific thread setting. Comments please....:dunno: :thinking:
 
You aren't, by any chance, backing the carriage up under power, are you?

Robert D.
 
What would happen if the compound were swung 29 1/2° the wrong way?? I would think it would only cut on the opposite side of the tool as intended, but I am just bringing it up as a 'what if'?

On second thought, if the compound were swung the wrong way, after a few cuts the existing thread would tend to push the tool and the carriage into the 'backlash zone' and it would cut funny if at all. Does this sound plausible?? I guess I don't know for sure since I've never made that mistake so that I could learn from it.
 
wa5cab said:
You aren't, by any chance, backing the carriage up under power, are you?

Robert D.
Click to expand...

Are you meaning by flipping the tumblers the other way?? The headstock is running forward but the leadscrew is running in reverse?? That would definitely cause problems!! Once you start cutting a thread, you cannot disengage the leadscrew from the spindle in any way, not even to reverse.
 
Any chance the thread dial can slip, or the gear that runs on the leadscrew could be loose? Is it a metric or inch lathe?
Are you sure you have the correct orientation of 29.5*? Some lathes have the protractor markings start 0* at 90* position
The compound needs to be at 29.5* from parallel with the topslide and the handle swung to the right. The compound when viewed from above should have the handle orientated at about 5 o"clock.

cheers Phil
 
Is there a possibility that you have the gears set up for metric threads?
 
I might suggest tighten compound gibs to a snug fit. One option would be to add a 0-1 inch indicator to the cross feed so you know it is zeroed out. Last but not least before starting your lathe engage the half-nuts and see how much play your machine was but turning the handle on the apron.

I'm starting to think your thread dial is not fully engaged or the set-screw or pin is loose. It can happen to almost any lathe.

Good luck.
 
I don't even have a thread dial on my 3 in 1....
So, when cutting threads, once I start, I never disengage the half nut and I never disengage the lead screw!
Otherwise, I would have (have had) the problem you are describing... So perhaps there is a problem with your thread dial or your lead screw. You might try my method...

Again, when cutting threads, once I start, I never disengage the half nut and I never disengage the lead screw! I have to turn off the motor off when approaching the end of the cut, then cut the last little bit by turning the chuck by hand, if necessary.
Then, I reverse the motor to run the carriage back to the start position.
Frankly, if it is not a long cut, or threading up to a shoulder, I will do all the spindle turning by hand.
I can cut pretty good threads now. An "Expert" is just someone who has made all the mistakes.
I know my machine has lots of limitations, but it is what I have, so I learn to deal with it. For me, part of the fun is figuring out how to get around the limitations.

Dave
 
Found the problem = under the cover of the timing feed gears, the pin was not lining up on the selected gear but was moved over and would sometimes engage the next larger gear for about 1/2 rev then drop back on the selected gear making the travel more than it should be - the drive gear had slipped - moved the drive gear over to where it should be and pinned it - now everything back as new - now, when set on 7 TPI, cutter moves only 7 TPI for 1 inch and the same for 20 TPI - Repeats every time now - just like a timex watch! Thanks so much for all the help you have provided - I know what everybody was thinking - what has he been drinking - now I found out why! Thanks again!
 
That's a new one. Good job troubleshooting.
 
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