Setting compound angle when thread cutting

[Pete F]

I've seen various statements over the years as to how the compound MUST be set at precisely 29.5 degrees or whatnot (for a 60 degree thread) in order to cut threads accurately lest an outbreak of 2 headed babies suddenly be born as a result of one's heathen activities in non-complying. However does it really matter? I just make sure the compound is less than half the included angle, and thus far no 2 headed babies have resulted. It seems to me that as one gets closer to 90 degrees the tool simply increasingly cuts on the trailing edge, to the point where if straight infeeding it's cutting both flanks equally. I've cut various thread forms with no particular attention to precisely setting the compound, definitely not "half degrees" that's for sure! The only one I can think of that's critical is a square thread, where the angle is by definition 90 degrees only.

I wonder if I'm missing something here, as I see this "29.5 degree" figure often coming up.

 

[67Cuda]

Feeding in at 29.5 degrees will help in not snapping inserts when threading coarse threads.
A 13 TPI is the max I'll do and not use the compound.

Tom

 

[Metal Head]

Pete, I understand what you are saying and agree, I was just explaining this to some guys in my shop last week. As long as the compound is set at less than 1/2 the included angle your not going to blow out the angle of the thread form when using a properly ground tool. It is also impossible to cut undersized when the included angle of your tool is more than the compound setting. Chasing that last 1/2 degree is kinda pointless IMO.

 

[Pete F]

Tom I'm not suggesting not using the compound, I'm just saying that I see figures insisting that the compound be set to a very precise figure, typically 0.5 degrees off half the included angle. Presumably that 0.5 degrees is so the tool doesn't just rub on that flank, but I can't see the point in setting to that precision. I even recall reading an article in a magazine a while back saying in as many words that the compound shall be set at 29.5 and precisely 29.5 degrees. Am I missing something here, or is this a case of an urban myth that has just become popularised without people really understanding what's going on?

 

[Jorgo]

I never set my compound at an angle. If I'm worried about clearance I'll move my compound forward a few thou for every cut. Mostly I just plunge my tool and don't worry about it at all.
Jordy

 

[67Cuda]

In the world of threading, maybe 29.5 degrees is the Holy Grail.
In reality, if your setting up a threading tool using a fish tail, a degree here or there isn't going to matter.

Tom

 

[sticks]

I agree for the most part on the angular presicion. Guys thread 60^ threaded barrels into 55^-cut rifle actions with confidence (qualifying/seating to a double shoulder might add reassurance ...)

Let's say for a sec that whether to feed straight in or using the compound can be a matter of mat'l (at say 50-90rpm?), TP rigidity, root depth .. the usual stuff. IMO the simple logic behind the 29.5^ says at least that thread angle is within some tolerance and that 'scuffing' of a non-cutting edge might heat up a cutter .. as in the days of spinning handwheels and grinding one's own HSS cutters, maybe working without coolant on a toolroom 'wallflower' .. and where it must'a came from. Your call on whether 'shall' is ever just a writer's own preference, .. or yours.

 

[Pete F]

Tom using a fishtale will set the toolpost/tool and wont affect the compound one iota, I fail to see what that has to do with anything. If your tool is correctly ground, but not set square, the thread will also finish up with asymmetric flanks, but how it was cut is a different matter. I find using the compound and angular feeding provides a better finish on most of my threads, especially the coarser threads. My lathes are pretty light-weight and I feel it's also much kinder to them than just jamming the tool in.

Sorry Sticks I didn't understand a word of what you just wrote, can you confirm it's English?

 

[timopen]

I have only turned few threads so I'm not expert to comment practical situation... I understand that setting compound to angle smaller than 1/2 the thread angle enables back of the tool (assumed to be accurate thread profile) to take a cut also. When the compound is exactly in 1/2 of the angle of the thread back of the tool wouldn't take a cut in theory. If the angle is larger than 1/2 of the angle thread back face of the thread will be at the angle of compound and thread will be non-symmetric.

Well I thought about the half degree... It might be because of 60 degree threads... When having thread angle of 60 and if the compound COULD be set to 30 degrees and when starting to turn the thread and taking touch-contact with workpiece you could zero compound dial and when compund is feed the amount of thread pitch the thread is ready in theory (no thread clearances or anything considered). But this was only for 60 degree threads... so but because not good to put compound at angle of 30, but slightly under so it doesn't affect so much to compound dialing but lets back of the tool to cut backface of thread. I don't know.. maybe this was idea to choose 1/2 degree instead of 1 degree...

Hopefully my english is somehow understandable

 

[RC99]

I just plunge cut with inserts...

What does a CNC machine do?

 

[timopen]

I just plunge cut with inserts...

What does a CNC machine do?

I think CNC could do it in any way. In picture below there are ways that have been presented in lecture slides at universitys basic machining course (not sure from where the picture is borrowed to these slides, maybe from Sandvik).

 

[Pete F]

I just plunge cut with inserts...

What does a CNC machine do?

Well there's a lot of guys with loads of CNC time here, but my understanding is most machine lead by a small number of degrees, so they're primarily cutting on the leading edge of the insert. Unless it's set the other way, hence cut on the trailing insert edge. But I'd be grateful if someone from CNCland could confirm that.

Once again, my question wasn't necessarily related to plunging v using the compound, I think that's been done to death in the past. Instead my question is why some people insist on very specific angles (and that 29.5 invariably pops up). I mean to say, if 29.5 degrees works, and zero degrees works, then surely everything between the two should also work, other than as I said at the top, the fact the tool will be cutting increasingly on the trailing edge as the angle approaches plnging straight in.

 

[Pete F]

Bill many of my thread tools I've ground myself, I honestly didn't have any trouble getting the required angles ... and I pay crap I think the last bit I ground for threading was a 45/5 buttress, and I'd rather set up to grind a tool accurately once than to have to set a compound accurately every time.

I do however understand the point you're making, but if the intention was to have a "one sided" thread tool, instead of a form tool, wouldn't the compound then be set at 30 degrees (for a 60 degree thread) and not this 29.5 degrees?

 

[maxh]

From what I read, it seems the OP asked why does everyone always say exactly 29.5 and not 29.6, 29.9, 29, or 25 for example.

My guess is that since the compound is typically marked in whole degrees, 29.5 is an easy way to say "as close to 30 as reasonably easy to read by eye without going over 30.

I'd guess a reason to shoot for as close as possible to 30, rather than 25 or 20, is that it's easy to do the math for 30 to calculate how far to expect to advance the compound to reach the end point (so you can get close before starting to measure.) And the more you deviate from 30, the more the calculation will be off.

I'm assuming most already know the reasons for advancing at an angle rather than just straight in (cutting mostly on one flank rather than two so half the cutter engagement means less likely to chatter/better surface finish, cutting force pushing against direction of drivetrain so backlash kept out and cutter stays on right path, etc.)

 

[Nick Mueller]

On the manual lathe, my compound is almost always set parallel to the Z-axis. And always while thread-cutting.
For a pitch of above 1.5 mm (or so), when I feed in with X, I also feed a 1/4 of that feed axially on the compound.*) That is the same as having the compound at 26.6°.
After having feed in X by he theoretical value (root diameter), I adjust fit with just feeding the compound along Z.

A compound set at 29.5° for thread cutting is quite uncommon here in Germany (maybe even whole Europe).

*) Feed in X by 0.1 mm -> feed in Z by 0.025 mm (not that precise).

Nick

 

[Red James]

The 29.5* rule was made when lead screws / half nuts had a great deal of 'backlash', i.e. when the half nut is engaged, it is easy to advance the cutting tool ahead of where it should be cutting with minimal effort on the hand wheel. If larger than 30 degree compound setting is used the thread tool trailing edge tends to over run the lead screw and cause your part's thread pitch to slowly lengthen. When threading with a ball screw controlling the precise movement of the threading tool this non precise cutting tool position is eliminated.

 

[Forestgnome]

I think 29.5 is just a way of saying "a little less than 30".

 

[Sunset Machine]

If you hunker down to eye level and look at a coarse thread being cut straight in, that trailing edge is a negative rake, and the leading edge a positive. The inconsistency leads to bad things, worsening with increasing pitch. We overcome that by stepping over or feeding at an angle, or both.

 

[fallriverbryan]

So. How do most CNC machines approach the problem

 

[jim rozen]

"...I can't see the point in setting to that precision. "

Look closely at that compound dial.

Then look CLOSE at it.

No, closer!

Make a wild stab in the dark about how accurate the stamped divisions really are.
Compare them in your mind to a dividing head, a good rotary table.

Yeah, I though you'd figure it out. Not real accurate at all.

probably readable to a half division, right? You can basically split a
division with the index without too much trouble.

One div = one degree.
Split the division = 1/2 degree.

That's where it came from.