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[machinist] Setting compound angle when thread cutting
- From: Lawrence London <lfljvenaura@gmail.com>
- To: machinist@lists.ibiblio.org
- Subject: [machinist] Setting compound angle when thread cutting
- Date: Sun, 12 Jan 2014 15:32:39 -0500
http://www.practicalmachinist.com/vb/general/setting-compound-angle-when-thread-cutting-278289/
Thread: Setting compound angle when thread cutting
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Setting compound angle when thread cutting
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. -
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 -
01-11-2014, 01:53 AM #3
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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.
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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?
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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 -
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 -
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.
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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? -
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
jackary2003 likes this. -
01-11-2014, 03:40 AM #10
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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).
Originally Posted by RC99
I just plunge cut with inserts...
What does a CNC machine do? -
01-11-2014, 04:06 AM #12
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Pretty much the case. Left-out was that in the TRAINING process, the typical 'school' mount was a modest SB. Not a great deal of power, nor stiffness nor strength. Anywhere. And 'cooant' was a periodic swipe with an oily chipbrush. Originally Posted by sticks
.. 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.
Try straight-in plunging with the typical 'apprentice grind' (read NOT fishtail, nor even close), expect vibration, tearing, raggedy-arsed threads to become easier than nice ones.
On a proper 'engine' lathe, 20, 30, 50 inch & up, Lodge & Shipley, even a hoary old WWI-era Niles, even though the work and threads scaled-up, there was still enough 'beef' to do it another way. Except ... it had already been drummed into the bone, so....
First time I cut threads on a lathe that had no compound, just a couple of hundred pounds of McCroskey 4-way standing tall right off the cross, I was .. well... 'alert'...
3/4" square HSS blank. Lots of grinding. Sort of 'spoon' shaped on top. Worked just fine.
But I still prefer to set up the 'Old Way' when I can do... Can't afford the double whammy of the grocery budget for no two-headed chilluns.
Bill -
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. Originally Posted by RC99
I just plunge cut with inserts...
What does a CNC machine do?
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. -
01-11-2014, 05:50 AM #14
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Of course they all 'work' .. if.... but Originally Posted by Pete F
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.
Not going to generate - for example - a 60-degree thread-form by plunging if .... the cutting tool itself is not a 'form' tool of a 60-degree included angle.
If one close-matches the compound angle to ONE flank, OTOH, AND USES the compound to advance ALONG that flank, then the compound's angle has generated one-half of the required form, and the tool-tip need only be ground to generate the OTHER half.
That ain't much of a factor when the cutters are store-bought AS 60-degree threading tools.
It 'sometimes' WAS handy in situations where the tool, material, tool holder, and machine-tool combination presented challenges. Simple chip-control, on an inside bore, worked dry, for example.
Try it. Take a 60 degree form, split the shape down the middle by grinding the right flank to the axis of the split angle eg; straight back on one side @ 90 degrees to rotational axis of the work.
With the compound set correctly, and very fine advance each go, you can will get a 'passable' net 60-degree included angle.
Same tool on a straight-in plunge gives you a bastardized one-sided buttress.
Setting the compound - and using it - stacks the deck in your favour even when the tool IS 'believed to be' proper, 'coz with HSS for-sure, carbides, maybe, the shape can change even IF the tool has a 'perfect' angle
....BUT .. the tool is not EXACTLY at the correct height.
Which was less a certainly anyway with once-common lantern TP's, lousy machines, poor environmental conditions, and long hours for crap pay.
Bill -
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? -
01-11-2014, 06:56 AM #16
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Not so much the 'intention', just an illustration. Don't forget that when this stuff came out, threads in our environment weren't all 60-degrees anyway, nor even was the inch yet 'unified' between US and UK/Commonwealth. Originally Posted by 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?
What WAS common, regardless of included angle, was to make the tool a skosh 'sharper' than 'n' degrees.
IF/AS/WHEN a thread just HAD to be more precise, the kiss of a mostly-spring-pass could be made on each flank with just a layout-hammer tap or three to angle the holder.
An exactly-on 'n'-degree tool shape didn't allow for that, hand-ground relief at the sides could change included angle as the top was ground (and we could SELDOM just leave it flat..) so if by some chance it had BEEN at a slight angle, the whole thread was tilted.
Remember - the intent is to stack the deck in favour of the machinist. Minimize the risk of scrap, partly by giving him a means of 'recovering'.
Store-bought tooling, better machines, workholding, toolholding, metrology, Unified National Fine/Course..thread shape, standardization of the inch.... then the same exercise again where metrifuckation came in .. yadda yadda .... have changed the playing field.
Don't worry it to death. Either adapt and apply it where it serves you, ELSE consider it an historical artifact, move-on, let some other pilgrim make his own choice.
BillJames H Clark and Marty Comstock like this. -
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.)Trboatworks and Pete F like this. -
01-11-2014, 07:45 AM #18
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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 -
01-11-2014, 07:46 AM #19
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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.
James H Clark and ratbldr427 like this. -
01-11-2014, 07:47 AM #20
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Note how LATE the abandonment of other angles took place. Take the history of Whitworth, US, and metric included angles just for one. Originally Posted by 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.
On 'paper', 1948 was the start of the change, 1965 the 'requested' obsolescence year. But Whitworth form persisted in softer metals, also in angles other-than 60-degrees. Variants are still in use today.
All that for what one might call 'common hardware-store threads', not Acme or the exotic Dardelet.
As-of the early 1960's American Machinist published, as one of their regular tinted-age special sections, a list of IIRC 122 different thread-forms with drawings and specifications.
You won't find store-bought cutting tools for even a small fraction of those, so don't get overly hung-up on 30 degrees or a half-degree less.
Bill
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- [machinist] Setting compound angle when thread cutting, Lawrence London, 01/12/2014
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