OK... what did I do wrong.

[DarbinCo]

Been a while since I was on here, but I just got my lathe set back up after moving and having to build a new shop for it to sit in. Terrible huh?
I got it new last july and it was set up for two months before moving. Prior to that the last time I ran a lathe or threads on a lathe was about 20+ years ago in school. I was pretty good at it then and I thought I had remembered how to do it. Well I think I missed something somewhere.

I turned a practice piece to play with, 1/4 20 thread. Got about 2/3rd the way thru and ruined it. I dont exactly know what happened but figured I would try again. This time with a 3/4 10 pitch.

I set the compound at 29.5
Found zero on the cross feed, reset dial on cross feed and DRO on lathe (that thing is handy, didnt have those in school!)
Started passes removing .002 to .003 a pass using the compound to advance the tool. I also used the same number on the dial each pass. I didnt care about time. Everything was looking good but I could see near the end it was starting to look a bit funny but dont know what I did wrong. Without a doubt someone here knows whats up. I was using a Micro 100 brazed carbide threading tool. I think the finish turned out good, the speeds were right but they have that ugly long side???

Heres a pic.......... Help!

[BadDog]

Looks like you have your compound set closer to the lathe bed ways rather than 29.5* off the cross slide ways.

And unless your lathe is very limber (for instance, a 9x20), or on a long unsupported piece, on thread that fine I would just feed straight in anyway...

[GlennW]

http://www.chaski.org/homemachinist/vie ... 44&t=88862

There is occasionally some confusion over the position of the compound slide setting for single point threading on a lathe so I thought I would post these to help clarify

The compound is set at 29° to 29.5° from the cross slide or roughly the 5:00 position as you need to infeed the tool bit in the direction of the cut.

Compound slides come indexed in two manners. My compound slide is indexed with the spindle axis being "0", so it shows roughly 61.5°. Some compound slides are indexed with "0" being parallel to the cross slide, in which it would be set to 29°-29.5°.

[DarbinCo]

BadDog
Yup your right on the money. I read your post, then went to the link Glen posted. That pretty much sewed it up.

Oh I had it set at 29.5 but my chinglish machine zeros off the bed ways like you mentioned not the crossfeed.

Heres another pic. Now my scale goes to 55. Would a person count back from my mark and scribe a new line in the compound base so I could use it to align the 29.5 on the existing scale even though technically its not the right 29.5 but would still be an accurate 29.5 from the crossfeed? I hope you get that! cause now Iam confused!!

[BadDog]

If your compound zero is parallel to the bed ways, then parallel to the cross slide ways will be at 90*. So you need the compound set for 60.5* (90 - 29.5). If your compound protractor does not provide that option, then you will need to use another method of setting the angle with reasonable accuracy.

But honestly, on that little thread, you can probably feed straight in, though that does not help next time you need 13 TPI or something.

[GlennW]

but figured I would try again. This time with a 3/4 10 pitch.

But honestly, on that little thread, you can probably feed straight in,

?

[BadDog]

Oops, I missed that. I thought he was still dealing with 1/4-20...

[DarbinCo]

Its all good Dog,

I put an indicator on my bed and trued up my compound by moving my crossfeed (X) in and out.

After getting the compound parallel with my X feed. I peened a small mark in the top of my x feed at the existing compound arrow mark (90 out) Then I placed tape on the compound base and put a pencil mark on the 29.5 mark on the old scale and rotated it back to zero. I then scribed a new line on the compound base where I placed the peen mark on the x slide. Thats about as close to 29.5 as I can get.

I turned another 3/4-10 and it came out perfect.

Thanks for the help guys!

[GlennW]

Oops, I missed that..

I thought that may be the case!

[stevec]

First reverse butress thread I've seen
It's like riding a bike, you may still fall off at restart but not as often or hard.

[MikeA]

Hello all,

I can feel your pain! Only within the last week or so have I discovered why I've been struggling with threading on my Emco Maier Maximat - a high quality Austrian machine. Same situation with the compound protractor having a zero set 90° different from my previous lathes (all American manufacturer.)

Instead of setting to 29.5° I use 60.5° and suddenly the threads are good! BTW - I understand some other lathes are setup similarly, supposedly one fellow who is using a Hardinge has the same thing.

Best,
Mike

[BadDog]

It actually makes some sense to orient it that way. If you are making a lathe part and need to cut a non-critical angle suitable for (tolerance for) cutting using the compound slide protractor, then how would it be specified? It is probably as "included angle" or absolute angle from axis of rotation. It's usually not specified relative to the perpendicular plane, except for maybe on a shallow taper on some facing turned seating surface or something.

But regardless of orientation of the protractor zero, there are always going to be cases where you need to subtract from 90 based on how the print was specified. You have to focus on what you are trying to achieve, and one of those points when using the compound slide is accurate angle calculation and setting the for desired outcome based on how the angle was specified. Also note that the compound protractor is intended only for relatively loose tolerance angles. Anything else requires setting approximate location using the protractor, then using a suitable method of tweaking for accuracy required (sine bar, bevel vernier protractor, angle blocks, whatever).

And particularly on cheaper machines, if you don't verify the compound protractor and general construction, you don't know how accurate the protractor is anyway. When you set it to zero (or 90) using the protractor and then make a test cut, how close is it? I wouldn't be surprised at all to find that it may be over a degree out on HF class lathes. What about that ground flat compound side, is it parallel to the ways? If you plan to use that flat side with a sine bar, or with a bevel protractor, you also need to dial it to see if it is actually parallel to the compound ways. On a Hardinge or Monarch you can likely trust it to be well within the range of good enough for anything where you would be justified in using it as a reference surface (i.e. wouldn't expect to be dialing the travel anyway), but still not a bad idea to verify, particularly on an old machine of uncertain history.