Threading woes

better-lathe-than-never

better-lathe-than-never

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I'm unable to get the desired 18 tpi from my Jet Lathe (JET BD920N). I've set the gears as shown in the picture. I double checked them tonight by carefully counting the teeth on the small ones (a=30 and b=30). The lever is set to Position #2 as indicated on the chart... with those settings the thread it produces looks more like 32 tpi... it's very fine.

1652456311006.png

I have the machine on the slowest speed, though I don't think the speed selection matters, since the gears make movement relative.

I don't understand what I'm doing wrong. I was able to produce 18tpi before (as discussed this thread: https://www.hobby-machinist.com/threads/trying-to-make-sense-of-the-threading-charts.99688)
 
Does your machine have a course / fine 'range' select?

GsT
 
Yes a picture of your gear set-up would help.

Two things to check:
1) check that the 40-80-40 gears are in place
2) check that gear "a" and "b" are directly running against the 127-tooth gear. Neither should be on the gear pinned to it

Stay with us, this group will help figure it out.

Brian
 
Based on the chart, the 80t and 127t gears are simply acting as idlers. The ratio of the a and b gears, along with the QC gear box gears determine the thread pitch. In the configuration shown, the input shaft to the gear box should be turning at the same RPM as the spindle.

I see two possible explanations for your result. First, the half nuts may be engaged at the wrong position on subsequent passes, giving the effect of a multi-start thread. Second, lathes that I have seen using your setup have an alternative method of using power feed; namely, there is a key slot in the lead screw which drive the power feed to produce a finer lead than would be obtained with the typical method using the half nuts. Assuming that you are cutting decent threads but the wrong pitch, it would suggest the latter.
 
RJSakowski said:
Second, lathes that I have seen using your setup have an alternative method of using power feed; namely, there is a key slot in the lead screw which drive the power feed to produce a finer lead than would be obtained with the typical method using the half nuts.
Click to expand...
@RJS - the problem must lie somewhere in what you've stated above, just not exactly sure yet how. My lathe indeed has that slot in the lead screw.
Also, I should have clarified the method which I use for cutting threads: since I'm a novice, I never disengage the half nut, once engaged I keep it there until I'm done. That is, with each pass, I simply shut off the machine, let it coast to a complete stop back off a bit away from the work piece, power feed it back, then run it forward again... and so on.

There must be some kind of setting that I flipped to force it to cut finer threads - that would be the only logical explanation because I was able to produce good threads with the right pitch before and now, without changing any of the gears I'm unable to produce that thread pitch with the identical settings.

The thread it's cutting now is so fine it takes a magnifying glass to match the threads... to the best of my ability I believe it's producing 36.
 
Do you have a selector for shifting between threads and feeds? Are you engaging half nuts, or are you engaging longitudinal power feed?
 
pontiac428 said:
Do you have a selector for shifting between threads and feeds? Are you engaging half nuts, or are you engaging longitudinal power feed?
Click to expand...
Yes, I do - I'll try to attach a photo - I'm engaging the longitudinal power feed lever (lower lever) only.
 

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