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threading problems on Craftex lathe

#1
I inherited a Craftex CT089 lathe and am learning how to use it. Watched videos on threading, but am having difficulty cutting a thread. I have a 3/4" blank installed and both slides zeroed. The gears are set for 11 TPI which is what I want. The compound is at 30 degrees and I am only advancing that slide. The half nuts are engaged and I advance a few thousands and cut towards the chuck. This first pass looks perfect. Keeping the half nut engaged all the time, back out the cross slide and reverse to the start for another pass. Few more thousands and begin to cut. New cut is definitely not synchronized with the first one. Repeated cuts do not find an average, or a defined thread, rather, there is a really rough looking piece of round stock! Thinking the problem was the lash in the carriage attaching to the leadscrew, before making a pass, I began advancing a half inch or so before the cut would start, which should take care of that? Anyway, I am out of ideas here. Hoping for some direction here.
 

PeterT

Ultra Member
Premium Member
#2
On your subsequent passes, are you returning the cross travel dial to the exact same zero position as when you made the initial skim pass? It always has to return to zero if you are infeeding compound. And lock it each pass so it doesn't drift.

And AFTER you set your compound to 29.5-deg, did you ensure that the V of the cutting tool was perpendicular to the work like with a fish tail jig? If you first set the cutter, then swing the compound, the tool will be off & that will mess up threads.

What does your pitch measure to on the scratch pass. Sometimes these offshore lathes have incorrect gears &/or leadscrew combos, but hopefully you have checked all that
 
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Dabbler

Well-Known Member
#3
Someone with threading expereience should check your setup end-to-end. Peter has identified several of the common mistakes. If subsequent passes obliterate your ridge, I'm inclined to believe your thread start is shifting. That can be caused anywhere in the drive train, and needs to be checked as well.
 

PeterT

Ultra Member
Premium Member
#4
If you do a skim pass & the pitch advancement checks out, what happens if you reverse out & repeat without advancing the tool? Does it track that same skim pass exactly or deviate with a new displaced spiral. If it deviates & you are keeping the half nuts engaged, then I suspect there is something amiss in the drive line especially if you are giving it a bit of space to eliminate any backlash.
Is the clamshell assembly tight? Do the threads look in good shape?
 

DPittman

Active Member
#6
I don't want to insult anybody's intelligence here with this....and only because I had this problem myself do I make this simple suggestion.

Are you sure your compound is set at the angle (29-30) you think it is? On my craftex lathe the compound angle readings are not the same as many (especially old American made) lathes. In order to have it set properly at 30 degrees my compound says it's at 60 degrees! This caused me all sorts of grief when I first started out and I finally learned the hard way after many poor threads. This was very confusing for me.
 

RobinHood

Active Member
Premium Member
#7
@DPittman : excellent observation. Both my Standard Moderns are indicating 60* when set properly. The difference comes from what the manufacturer chooses as their reference 0*.
 

PeterT

Ultra Member
Premium Member
#8
He SAID he watched all the YouTube vids!!! HaHa.

This is a common thing when we start out because the protractor scales may be different between lathes. Maybe its better to show a visual of the compound itself. If you 'run out of scale' which is common on lathes including mine, just use a fine felt pen to mark some convenient reference datum and add or subtract so your compound looks like the picture. Some guys first set up the compound relative to face plate, get that dealt with, then set up the part. But that's not always convenient if you have successive machining steps.

This link shows some setup including cutting tool in feed (explaining the 29.5 deg business)
https://www.hobby-machinist.com/thr...mpound-setting.24500/#lg=thread-24500&slide=0
 

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#11
Before testing I moved the compound to zero degrees to eliminate any problems from there. Did a scratch pass towards the chuck, then reversed back to the tailstock without changing anything. Same depth of cut. half nut engaged the whole time. The alignment is way way off. Should travelling a quarter inch or more into the clearance not remove lash?

I tried to post a video of the above but it's too large. Here is a pic of the final result.
 

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PeterT

Ultra Member
Premium Member
#13
Thought the same but I think if he has the half nut engaged throughout then that eliminates re-engaging out of phase.
Does it have a thread feed traversing direction lever that hopefully you are keeping consistent each pass towards the head stock?
 
#14
Thought the same but I think if he has the half nut engaged throughout then that eliminates re-engaging out of phase.
Does it have a thread feed traversing direction lever that hopefully you are keeping consistent each pass towards the head stock?
You mean the lever to change direction -- the one I took a picture of below. Right? I traverse towards the chuck with the lever pointing to the right, on reaching the clearance, move the lever to the center to disengage. Then move lever to the left to return towards the tailstock. Am I missing something here? The little drawings under the lever are not enlightening me....
Does yoir unit have a threading dial?
Yes it does have a threading dial. I'm unclear how to use it. With the nut disengaged, I can move the carriage to line up with a number on the dial, but to engage the nut, the carriage has to be rocked back and forth by the handwheel till it engages...and then it is not lined up anymore. Now say I advance a first pass to make threads. on returning the carriage to the approximate location where i started, the dial does not line up near the chosen number. I did not see on the videos detail on how the thread dial works.
 

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DPittman

Active Member
#15
Most threading videos explain the threading dial and lever including "This Old Tony" video referenced above, but here is a shorter and pretty concise video on the subject. I can not explain it well enough through type but the videos will do a good job.

 

DPittman

Active Member
#16
Oh ya, you generally only use the half nut lever when you are cutting/threading in either direction. To return the carriage back to the start of the thread use the carriage hand wheel
 

Johnwa

Active Member
#18
A shot in the dark. When you are reversing are you using the electrical push buttons or the leadscrew reversing lever? It has to be the first method.
 

DPittman

Active Member
#19
Ok I reread your story and if I understand you correctly I believe the issue is that you are using the half nut lever to move the carriage back toward the headstock. This will change the position of the threading dial and the relation it has to the lead screw. Only engage the half nut lever at the correct dial position when you want to cut threads.
 
#20
A shot in the dark. When you are reversing are you using the electrical push buttons or the leadscrew reversing lever? It has to be the first method.
That is exactly what I was doing wrong!! Whenever I had to reverse, I would use the reversing lever, and after a few passes there were no discernible threads. Just came inside after using the electric motor reverse switch, and beautiful threads hey hey. Thank you so much for the advice.