G0602 threading issues (not disengaging the half nut)

[MontanaLon]

How are you holding the workpiece? In the chuck? Chuck and center? Between centers?

Your workpiece moved which isn't uncommon. Chuck not tight enough, or tight enough but inadequate engagement with the part.

Slap another piece between centers and see if you can cut a pretty thread. If you can then it eliminates all other possibilities but work movement.

If you had it in a chuck and with tailstock running a center did you inadvertently not lock the quill on the tailstock? I know if I don't lock mine just the weight of the handwheel knob will rotate it down to BDC and there is a bit of mechanical advantage with the threads. Granted, I have never seen it move anything it the chuck but I have seen it move and release tension on the center and with the tool pressure lead to a tapered cut.

Just spitballing, but I would lean toward the work moving in the chuck and try it again. But the tailstock losing pressure is within the realm of possibility too.

 

[fitterman1]

If you reset the cross slide to zero after pulling out at the end of each cut, and do not mess with the compound setting until ready for subsequent cuts, there is no need to remember or write down anything, you just feed the compound in a bit for each successive cut.

Why would i reset my crosslide to zero every time? Its the only thing telling my depth.
Oh i see, you swing your compound around and feed in with that.
My bad, i dont do that, i keep it normal and feed in with the crosslide.

 

[GunsOfNavarone]

I really appreciate the info guys. I mis-spoke...I feed in with the coumpond, not the cross slide. I have tried to use between centers and also no tail suppport. I am using a 3 jaw...super tight and I can confirm my work is not shift. I did stay up last night researching...I did come across the Asian scale theory, I need to check that angle.
this play i speak of can be felt the most when turning the spindle/chuck...changing directions (which being that I am reversing direction without disengaging, I feel like there could be something to this...that whole gear train has a lot of slop! I did try to to disengage the half nut and use the dial to drop in...being that I was doing 12 tpi,.I used different numbers on indicator (thought that was ok, my works was ALL OVER the place then. Seemed like every pass was somewhere new. I guess to keep it simple, i could try to thread at 60°. I new threading was difficult, this has just thrown it through the roof.

 

[GunsOfNavarone]

Looks like something was moving during the cut- tool, toolpost, compound, leadscrew or even workpiece itself- something was causing those two scribe lines and it isn't right, there should be only one, especially since you kept the half-nuts engaged
You are pulling the tool away from the work when reversing the carriage correct?

I am not pulling the work away when I reverse....should I? I am taking such light cuts, it's removing the stock so I feel it's just a light graze.

 

[GunsOfNavarone]

It is not a good idea to pull out with the compound, it should be left at the same setting that the cut was taken on, pull out with the cross slide only. Start out with the cross feed handle at about the 10:00 or 11:00 position and the dial zeroed, and take each successive cut by feeding the compound in, however much you dare, you can take heavy cuts for the first several passes, then ease up for the finishing passes. Some folks do the 29.5 deg thing, I do not, I set on 30 deg. Hot rolled is not a nice thing to thread, it does not cut freely or finish nicely. I would try disengaging the half nuts ands sucessively take trace cuts with not feeding in and see how the tool tracks on each cut, or rather does the tool track in the same location on the successive cuts.

Sorry, i meant cross slide...Compound is for feeding only.

 

[GunsOfNavarone]

It looks like the angles of the two sides of the tread are not symmetric, which could be because your compound is set to 150.5 degrees vs 29.5,

from my late night readings, this is starting to lead the pack in possibilities. I guess I am confused how I am to set it if my compound is off. If like to use my compound, but until I know what that is, am I trying to turn it 29.5° from being 90° perpendicular?

 

[GunsOfNavarone]

That thread looks like the compound was not set at 29.5°. This happens a lot with people that have smaller asian import lathes. Commonly the compound scale on asian import lathes will read 0° & not 90°. If your scale reads 0° you have to set the compound to 60.5° to get the correct angle for flank threading. Typically the scale won't even read that far (only on lathes where the degree markings are on the cross slide) & you need to set it with a protractor, etc. My 12x36 is this way also, I made my own marks instead of having to use a protractor every time.

Edit: Well the scale should read 0° instead of 90°, what I should have said is that small import lathes usually don't have the additional scale directly on the cross slide like bigger lathes do.


View attachment 297043

I couldn't explain my frustration if this is it. Bad enough I'm trying to learn...throw this curve ball at me....why would they do this?? (Rhetorical)

 

[jcp]

I am not pulling the work away when I reverse....should I? I am taking such light cuts, it's removing the stock so I feel it's just a light graze.

Retracting the tool with the cross slide at the end of the cut is mandatory.

 

[Janderso]

Let us know what you find, that's an interesting thread you cut.

 

[GunsOfNavarone]

Hold a 60 deg "fishtail" thread gage against the front side of the compound, it should appear to be 90 deg. to the ways.

So.....using that technique.....I end up with the compound at.......60.5° Perhaps this is why some people are running 60.5 and not having issues. I will run some more today and see....how ridiculous. I think you guys got this thing figured out without even breaking a sweat. I'll keep you posted. You saved me a lot of angry disassembly!