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Compound Angle for General Turning

reidry

Aluminum
Joined
Dec 27, 2012
Location
Cocoa, FL
All of my mid 20th century machining resources depict the compound at 29 1/2 or a few degrees off perpendicular or parallel to achieve small(tenths) X or Z movements.

There also seems to be a modern approach that places the compound completely parallel with the bed ways (Z axis). Some proponents of this configuration advocate use of the crossfeed instead of the compound for threading. Dorian toolposts have drilled holes in the bottom for pinning to the T slot block in exactly this configuration.

What approach do you use and why?
 
29ish degrees is for thread cutting. I just leave mine there most of the time. There's also some angle that gives you sensible fine increments- never used that one. If you put it parallel to the ways, it makes it easy to pick up a thread when needed. Sometimes I put mine at 120 degrees for cutting 60 degree points on things. Sometimes I tweak it for cutting small tapers for chucks and such. It's whatever you need it to be.
 
What approach do you use and why?

Store the compound somewhere safe in the odd case one needs to cut a taper too steep for the TA.

Makes room for a stouter and stiffer 4-Way that holds larger HSS blanks that cool better and last longer between sharpenings.

Lazy, Iyam. Also habituated to what we all did at one time. Straight-in threading included.

Downside? As-in what do I LOSE?

The "preferred" angle for a compound is commonly selected either to match your usual thread flank angle - there's more than one - ELSE provide one-tenth the travel of the dial due to simple trigonometry.

That can make it easier to sneak-up on a dimension, spring pass especially, than to try to split a thou on the cross dial directly. Most especially.. if the cross and screw/nut be the LEAST BIT.. .worn. Or so badly worn and loose it must be clamped for critical work, and the compound relied on exclusively.

WHEN I use a compound, that 10:1 ratio is the one I go for. It is the most universally handy, any given day or tasking.

Can't always do that. Sometimes the compound is simply "in the way" and has to be rotated elsewhere.
 
Store the compound somewhere safe in the odd case one needs to cut a taper too steep for the TA.

Makes room for a stouter and stiffer 4-Way that holds larger HSS blanks that cool better and last longer between sharpenings.

Lazy, Iyam. Also habituated to what we all did at one time. Straight-in threading included.

Downside? As-in what do I LOSE?

The "preferred" angle for a compound is commonly selected either to match your usual thread flank angle - there's more than one - ELSE provide one-tenth the travel of the dial due to simple trigonometry.

That can make it easier to sneak-up on a dimension, spring pass especially, than to try to split a thou on the cross dial directly. Most especially.. if the cross and screw/nut be the LEAST BIT.. .worn. Or so badly worn and loose it must be clamped for critical work, and the compound relied on exclusively.

WHEN I use a compound, that 10:1 ratio is the one I go for. It is the most universally handy, any given day or tasking.

Can't always do that. Sometimes the compound is simply "in the way" and has to be rotated elsewhere.

I guess you don't use a taper attachment much, Use the compound to adjust the cut depth when using the taper att. Set the angle parallel to the cross feed. Use a dial indicator or as on one of my lathes a DRO on the compound. You may need to use that 29 1/2 degree BS on some wimpy SB as it will allow the thread tool to cut on one side only, reducing chatter on flexy flier lathes. I feed straight in on my LeBlond Servo Shift regal and the Holbrook C13. Their is no compound on my Miltronics ML 20, It is CNC so as with all CNCs I know of it feeds straight in on the thread cycle.
 
I guess you don't use a taper attachment much,

[
As luck would have it no. Almost all the tapers I ever had to do were sore shallow, so it was TS set-over, else too sharp for a TA.

Use of the compound for adjustment, fine or otherwise, is a good point in general, though.

As to threading - with exception of things so fine and hard to support I'd probably use a die anyway, straight-in, no compound's extra stiffness seems to more than make up for having to pay more attention to tool shaping back when it was HSS.

More recently, if it works for CNC in factory-shaped insertables, probably works the same manually. 10EE now the 'small" lathe here, but I don't count the Cazeneuve any stiffer.

"South Bend"? Pleasant enough city. University town IRRC. Dunno if it was little red wagons. Thot they used to have a major noodle factory out that way didn't they? Or was it Vermicelli?

:)
 
5.712° from parallel to Z should give 0.0001" infeed per 0.001" of compound movement.

ISTR they were even teaching such things in class. Around 9th grade "Industrial Arts", late 1950's suburban Pittsburgh, PA.

Then one gets out in the "real world" of tool wear, machine wear, the inevitable flex, and whether a "spring pass" even cuts reliably at all with a given tool and material.

End up settling for 5:1 or 2:1 as far the more practical settings.

Utility of any of these depends a great deal on your particular lathe and its tasking circumstances, IOW.
 
5.712° from parallel to Z should give 0.0001" infeed per 0.001" of compound movement.
I keep mine at 29-1/2 degrees for two reasons. Firstly, it is already set for cutting 60* external threads and secondly that is close enough to 30* to give you a 2:1 infeed for fine adjustment. This is based on the fact that the sine of 30* is 0.5 so one thousandth on the compound dial is a half thousandth actual infeed. If the compound's dial is large enough, a half thousandth or even a quarter thousandth of compound movement is easily attainable giving a quarter or eighth of a thousandth of actual infeed.

I make a practice of setting the compound dial to zero before commencing a turning operation that I need to be done precisely. That way, it's all set for fine infeed when it is needed.
 
that is close enough to 30* to give you a 2:1 infeed for fine adjustment. This is based on the fact that the sine of 30* is 0.5 so one thousandth on the compound dial is a half thousandth actual infeed.

And that.. is about where we were when this door was opened.

Sanity check, folks.

Most of us are playin' wit' ourselves if we think "our machine" can do better than split ONE thou. Working it to "tenths" at the mere rotation of a dial?

Well.. the nice way to put it is "highly optimistic"!

:D
 
And that.. is about where we were when this door was opened.

Sanity check, folks.

Most of us are playin' wit' ourselves if we think "our machine" can do better than split ONE thou. Working it to "tenths" at the mere rotation of a dial?

Well.. the nice way to put it is "highly optimistic"!

:D
I've been meaning to see if my lathe will split 0.001" repeatably. Use the normal 3 pass technique to hit a x.xxx5" dim using an indicator on the x-slide. I suspect that it will not, and I won't be able to throw-out my scotchbrite and files. haha
 
I've been meaning to see if my lathe will split 0.001" repeatably. Use the normal 3 pass technique to hit a x.xxx5" dim using an indicator on the x-slide. I suspect that it will not, and I won't be able to throw-out my scotchbrite and files. haha

The only file I ever thought worth a pre-nup and formal marriage to, as reliably as they put food on my table - were 1960 +/- a few years Nicholson Black Diamond "long angle lathe bastard" in the 22" length. Hard to find even in 12" these days, if even Nicholson hadn't hoored the quality out.

Any brand - files lacking that "long angle" cut MEANT for lathe use are not fun to use atall.
 
I set my compound for whatever cut I need to take. Chamfers, steep tapers, short tapers, whatever. And I have never fussed around trying to set 29.5 degrees for threading.

2-axis DRO, even on a pure manual machine, means you don't need to pay trigonometry games with the compound to get a reproducable small cross-feed motion.
 
I set my compound for whatever cut I need to take. Chamfers, steep tapers, short tapers, whatever. And I have never fussed around trying to set 29.5 degrees for threading.

2-axis DRO, even on a pure manual machine, means you don't need to pay trigonometry games with the compound to get a reproducable small cross-feed motion.

The DRO on my mill was the best accessory I've ever purchased and I may end up adding one to my 10EE. I see that some DRO units have multiple tool presets that store the specific location of each tool's cutting edge. To take advantage of tool presets, doesn't the compound need to stay in a fixed position as the center of rotation is not the cutting edge? Seems a DRO would be pretty compatible with a fixed tool post.

If you retain the compound with a 2-axis DRO it seems compound parallel to the ways would be the quickest setting to restore in the event you needed to swivel the compound to cut a short taper - complete the taper then swivel back to close to parallel, install a tool on the face of the tool post parallel with the ways and use an indicator to verify parallel.

Does anyone have a different procedure when using a machine equipped with a compound and DRO?

Thanks,
Ryan
 
Does anyone have a different procedure when using a machine equipped with a compound and DRO?
As I mentioned above, I don't have any particular "set" angle for the compound. When I change the angle, I almost always(*) loosen the QC toolpost, push its left face up against the chuck or faceplate to square it off and retighten. With the exception of cutoff operations, there is nothing I do that is going to be adversely affected by +-0.5 degree in lead angle or toolholder non-squareness. Indicating the toolpost square would be one of those time-burners that thermite was just yakking about.

(*)Sometimes when I am cutting both inside and outside chamfers, I'll use a tool that can cut either side and not bother to reset the QC toolpost, even if I'm swinging it 90 degrees.

I think my DRO might support a couple tool presets, but as you've noted, that whole idea is not really compatible with the use of the compound slide. What I do use constantly, however, are the local zero and the preset measure features. Do some eyeball rough cuts to get into the ballpark, take a micrometer reading, put the tool back on the surface (light touch), plug the mike reading into the DRO, then direct read the finish cut diameters from the DRO. Or, if I've got a relative feature, put the tool on the surface, zero the appropriate DRO axis, and direct read the cut depth/length from the DRO.

Tip: always know whether you are in radius or diameter mode on the DRO X-axis. :-)
 
I usually set to 29.5 degrees. Just seems to put everything in about the right spot and is ready for 60 degree threads. Compounding in on slightly less than the half angle when threading has more to do with chip flow than lathe rigidity. If you go straight in a chip is flowing off both tool flanks at once and is crowded at the tool tip. This leads to shorter tool life and poor finish. Off topic but to address a previous post any cnc lathe I've used in the past 40 years can infeed at the half angle or feed straight in. Or fancy cycles where one pass on one side of tool, next pass on the other. The programmer has control. FWIW My experience straight in leads to shorter tool life, showing up as damaged point of threading tool.
 
I usually set to 29.5 degrees. Just seems to put everything in about the right spot and is ready for 60 degree threads. Compounding in on slightly less than the half angle when threading has more to do with chip flow than lathe rigidity. If you go straight in a chip is flowing off both tool flanks at once and is crowded at the tool tip. This leads to shorter tool life and poor finish. Off topic but to address a previous post any cnc lathe I've used in the past 40 years can infeed at the half angle or feed straight in. Or fancy cycles where one pass on one side of tool, next pass on the other. The programmer has control. FWIW My experience straight in leads to shorter tool life, showing up as damaged point of threading tool.

"Fancy cyc;e" - one side, the other, clear the center waste, repeat 'til close, fresh tool, finish straight-in was the way for manual as well.

As to tool life, it didn't matter. One cannot water-cup chill a Rex 95. You set it on a steel plate and grind its siblings whilst the others are cooling. Typically, three tools have been prepared in the same clock time as one, plus what would have been wasted as cool-down waiting time. HSS it is, but grind too hot, edges microcrack. You experience that in reduced edge life, whether you can see it with the eye or not.

The shape, BTW is chosen to manage chip from both sides, and one does get as nice a finish as would be had by side-riding. Neither would perfect. No need.

We weren't selling threads. We were paid for the whole continuous miner back together and functioning at end of rebuild.
 
The OP asked for personsl preference on compound angle. I responded with what i thought might be helpful. It is a fact tool tip will suffer jamming a threading tool straight in. I do not recall the OP trying to get a continuous miner back on line, but I bet he wants to cut a pretty thread, even tho he may not be selling threads.
 








 
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