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stevec Wrote:I always believed the reason for 29° was so that the trailing edge of the 60° threading tool would be 1° shy of parallel with the compound. That way all the cutting will be done with the leading edge of the cutting tool.
Not quite *all* the cutting on the leading edge - the odd degree means the trailing edge takes a *very* fine shaving too, which keeps the thread profile clean and gives a nice shiny finish :) It also means that the tool forces act only in one direction to close up any backlash in the leadscrew and half-nuts, and the chip is (almost) all from one edge and curls better (picture a straight-in cut, the chip will be "v-shaped" and won't clear as well).
A lot of people take the final finish cuts "straight in", which has the same effect of ensuring the right profile, and some only ever feed straight in...
Re the topslide angle, it needs to be swung 29 degrees anti-clockwise from "straight in", 61 degrees clockwise from parallel with the ways (assuming an external, right-hand thread).
Re the clearance angle on the side of the tool, I make 1/6th inch (lead) in roughly-a-foot (circumference) to be less than a degree of helix angle, so your 10-degree clearance should be plenty! Bear in mind (if you haven't already) that you need front clearance too, as otherwise the bottom of the tool will be dragging against the trailing edge of the thread (just cut by the leading edge of the tool).
Just my ha'pennorth,
Dave H. (the other one)
Rules are for the obedience of fools, and the guidance of wise men...
(Douglas Bader)
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'Tis true. I think anywhere between 25-30 degrees will work, just as long as it's the correct 25-30 degrees...<chuckle> Most references suggest 29.5 degrees. That trailing edge is actually cutting with a negative rake. Smearing, pushing the metal.
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If you use the compound method, you get less load on the tool and cross slide, and it will allow a positive top rake, which is good for softer material especially. Left flat on top, you have a neutral rake and can let the tool drag on the trailing edge with no real tearing or smearing other than just from the neutral rake. For HSS, I prefer some positive rake. So if I use the compound method, I use a positive rake, and as close to 30° as possible and get a fuzz on the trailing edge. In reality, I rarely use HSS in favor of carbide, and plunge straight in. I take many light cuts and never have a problem with chip control or finish.
In CNC programming, it is common to use the compound feed method, and additionally, one method that alternates cutting the front flank with the trailing flank, with the DoC shrinking as it nears size. Very little stress on the machine or insert. I do not remember the code for that method.
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07-31-2012, 08:11 AM
(This post was last modified: 07-31-2012, 08:30 AM by Sunset Machine.)
I don't know, Tony. Consider the helix angle. Flat on top results in a positive rake that matches the helix angle, with a likewise negative rake on the trailing edge. Easily seen from the side, especially with larger pitch threads. Same reason why we need extra clearance on the leading edge.
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I can go along with that. It's a variable, as you said, dependent on helix angle, so I suppose that's why you find most people grinding neutral. Kind of splits the difference. Any additional positive grind amounts to more negative on the trailing edge, if you plunge straight in especially not too good. If you stay close to the 30°, it's not so bad though.
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Quick update. Well I reground the tool with an increased side relief angle and it did much the same on the original piece that was problematic. I think the metal is too low a quality (was a boiler plate slug), as the tool tears more than it cuts (despite being sharp).
I had an old drill coupler that was given to me by the previous owner. This was very hard and had to be annealed but finishes much nicer. The threading tool was cutting quite well in this, although the finish wasn't as nice as I would have liked but may have gotten better as I approached final dimension, taking finer cuts. However, I never got that far!
The thread dial stopped working ( I'll address this in another post), so I left the half nuts engaged and simply backed the carriage up after each pass. After several passes the thread was forming nicely and I was backing the carriage up when it suddenly stopped. The spindle was spinning but not the leadscrew. One of the change gears has walked  Given that I was turning a 6tpi thread, my lathe only requires 1 quadrant gear and I did think at the time that I should make a spacer as the shaft is sized to accommodate two gears.
I did try to pick up thread again but failed. Oh well, you live and learn!
Hunting American dentists since 2015.
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Self taught so this may not be the correct method to pick up a thread but it seams to work for my level of precision.
I just back the cross feed out, engage the half nuts and let the carriage travel till the tool is over the tread. Stop the lathe, now use a combination of compound and cross feed till your into the cut thread. Back out the cross feed and start the lathe, now slowly bring the cross feed back in till your cutting. That depth on the cross feed is your new setting as you back out and return for subsequent passes.
Free advice is worth exactly what you payed for it.
Greg
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(09-02-2012, 12:20 AM)f350ca Wrote: ...That depth on the cross feed is your new setting as you back out and return for subsequent passes.
I think that is where I messed up.
Hunting American dentists since 2015.
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And then there are a lot of us old timers that leave the compound right there at 29
for everything except taper turning. Then get or make a crossfeed thread stop,
that eases the burden. Then waiting for ever for the number I have that problem
and just live with it because I have an Atlas dial on a S.Bend cause I cant afford a
real one and the cause is, because the driven gear is like 24T and SBend needs 32T
however it does work for a Flea bay $10.00
big job, proud to be a member of MetalworkingFun since Jun 2012.
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I thought I was doing well with leaving the halfnuts engaged, even though to shoulder was approaching fast and the lathe takes a few revolutions to slow down. Had to throw her into reverse a little soon on a few occasions. Would have been good if I had made the spacer rather than making a mental note to make it
Now I find out that the dial is suppose to stop turning when the half nuts are engaged, I'm even more annoyed with myself for not noticing (or figuring it out) 
Hunting American dentists since 2015.
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