View Full Version : Thread forms
06-15-2001, 08:01 PM
First try cutting threads on the lathe.
I've got (and have read) various handbooks and books, I ground and set the cutting tool with a center guage. There seems to be enough fore-and-aft relief on the leading lower portion of the cutting tool.
I'm getting a thread form that looks like a reverse buttress thread (flat on the wrong side).
This is probably not the first time this has happened. Is this a tool form problem or a set up problem?
Edit: Singular on 'tool'
[This message has been edited by Ron LaDow (edited 06-15-2001).]
06-15-2001, 08:42 PM
Are you sure there isn't slop in the carrage?
06-16-2001, 09:25 AM
I keep the gibs adjusted to a light 'drag'. There's some backlash in the screw, but I always 'losd' the travel into the cut.
Is there another source of movement I'm missing?
06-16-2001, 12:52 PM
IT SOUNDS LIKE YOU MAY HAVE A SET UP PROBLEM, IS YOUR COMPOUND SET AT 29 1/2 DEGREES PARALLEL TO THE CHUCK?
06-16-2001, 03:43 PM
OK, turns out that I can solve the problem with a different set up.
I called a guy who originally taught me my way around a lathe XX years ago, back when I had hair and didn't even think about it. In spite of every book and article mentioning the 29-1/2* set on the compound, he cuts them square which means the tool is cutting on both flanks.
I tried it. It works. This was the only change from the first try (it was set at 29-1/2*).
06-17-2001, 03:32 AM
Dumb question, but I have seen more than one person do it.
Was compound set at 29 deg from cross slide. Or set at 29 deg on protractor base of compound, some compounds are graduated one way and the next is graduated different.
Saw this happen at work this week, not the first time either.
Yes I go 29 not 29.5 , close enough, the whole idea is to cut on one side and just shave the other.
Then there are the English and the Germans, they leave the compound set at 90 deg from cross slide and advance both compound and cross slide with a fomula that I am sure they have to memorize as a apprentice.
I am an American and I leave my compound set at 29.
06-17-2001, 12:22 PM
The compound rest was set such that the cutting tool advanced along one flank of the thread, cutting only (or mostly so) on the leading edge of the cutting tool.
All the geometry (and close looks at the cutting tool with a hand lens and bright light) tells me it should have cut a good form with the angled compound rest. I still have no idea why I got such wierd results.
06-17-2001, 07:45 PM
Kinda hard to type while I'm wiping the egg off my face...
You and Jbelanger had it. The compound rest was set at 29-1/2* from right angle to the work centerline.
This is to say that looking down on it, the compound screw was at say 10/4 o'clock, not 11/5 o'clock. The result was a thread form 60* on the cut flank and 30* on the 'shave' flank.
Takes a while, but thanks, guys.
06-17-2001, 10:59 PM
Don't feel bad, as I said, saw it happen at work last week.
I had a problem similar with my old bradford long time ago, wouldn't get in groove. I had readjusted my halfnut when I had carriage apart, later when I went to cut a thread it wouldn't repeat. I had to readjust back to the way it was and worked fine.
I think this is one of the reasons a Atlas or Craftsman threads so nicely, the halfnuts are worn in really well.
I hate the feel of the halfnut on a Cholchester. Unsupported lead screw.
Good luck in future.