Newbie- threading

[crustdog7]

Hello there, thanks for taking the time to read this. I have a Central 9x20 lathe that i just got for a steal. (only reason I got that brand). I'm a beginner to machining and had a question about threading on this machine. I was taught to set my cross slide to 29.5 degrees and square everything up to thread but when I start treading, I'm cutting the top of my threads off or I'm not getting them to take the normal V shape. I know I'm missing something but not sure what. any help would be appreciated. Also, any advice to improve my machining skills? what should I do to get better?

Thanks for the help and hopefully this post is in the right forum.

[packrat]

Does your lathe have a thread dial on the carriage? If so on even number threads you can engage the half-nut on any line. On odd number threads engage on numbered lines.

On a lathe with no thread dial, you have to leave the half-nut engaged and reverse the spindle direction after backing out the cross slide.Then run spindle forward for next cut. You never disengage the half-nut till thread is done/to size. Then run spindle forward for next cut. Hardinge tool room lathes are like this.

Another mistake made by beginners is feeding in with the cross-slide instead of the compound. On some threads this will cause problems.

Hope this helps, cary

[Cruiser]

The only reason for setting the compound at 29.5 deg is for cutting the threads on the slant depth using the compound. That way it cuts one side and scrapes the other side of the thread as you cut into depth. But, the depth is trickier to calculate due to the angle.
Otherwise what packrat is saying holds true !

[Cmailco]

Good guidelines from these gentlemen. Only advice I might offer to those starting out with single-point is, do a lot of air cutting to get a feel for the speed. I've seen people walk up their first time and run right into the chuck because they didn't know quite what to expect. The pucker-factor can be high if you're not prepared.

Cut plastic, whatever it takes to get comfortable first.

Regards,
Chuck

[packrat]

Originally Posted by Cruiser The only reason for setting the compound at 29.5 deg is for cutting the threads on the slant depth using the compound. That way it cuts one side and scrapes the other side of the thread as you cut into depth. But, the depth is trickier to calculate due to the angle. Otherwise what packrat is saying holds true !

If you look on most thread tool gages, there is a chart telling the depth of cut for most thread pitches. This is the number of thousandths the compound moves in for full depth of thread.

[PixMan]

Now don't take this the wrong way, but....

Are you sure you've selected the correct gearbox settings for the thread you wish to cut, and are familiar with using the half-nut rather than the feed clutch?

[basalt]

Good morning Crustdog7,
Lots of good advice here, for what it is worth I will add my 2 cents.
I wrote a paper on UN threads, it has all the information required to calculate the 3 diameters including the depth of cut. I also put in a section called "A Practical Method to cut a UN Thread". I hope this doesn't offend anyone, as there is not much that is new except my interpretation, however, I wanted to include the novice.
One more thing, I have a small bench lathe, Craftex 7" X 8", and as the carriage is moving along cutting the thread I rest one finger on the carriage handle to apply a drag, the reason is that the carriage doesn't have enough mass to consistently repeat, as the depth of cut varies, this way I eliminate the backlash and the carriage always stays up against one flank of the leadscrew.
Hope this helps, and good luck,
The link to the paper is below.
un-unr-unj-threads