Threading with compound

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1949DC
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Threading with compound

Post by 1949DC »

I am having trouble with results using compound vs feeding straight in with cross feed. Hopefully my photo will appear so you will see. My lathe will easily handle a straight feed with much better finish on threads. I’ve threaded from 5/16” to 1” with very good results this way. Straight feed is done with .005” cut compound .003” or less. I’ve tried setting compound from 25-35 degrees and about every degree in between. The left side of tool cuts the appropriate angle but the right side angle appears to leave a shallower angle. The tool is ground near perfect on a 60 degree angle. Yes photo appeared but is upside down.
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SteveM
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Re: Threading with compound

Post by SteveM »

I've seen the same thing sporadically.

The resulting thread looks more like a buttress thread.

I've never been able to figure out why, and I'll cut another thread with the same settings and it works fine.

Steve
JimF
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Re: Threading with compound

Post by JimF »

If you are over 30 deg, you are not cutting a good thread.
John Hasler
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Re: Threading with compound

Post by John Hasler »

Looks like the tool wasn't square with the part.
1949DC
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Re: Threading with compound

Post by 1949DC »

I googled buttress and yep exactly what I get. If nothing else I learned what that is. I only stated 25-35 degree thinking my degree marks are Incorrect. Once again this old lathe amazes me how counter conventional it wants to work. Frustrating because I believe a much larger diameter shaft and deeper cut may be more difficult with a plunge cut. I’m going to try cutting an 1-1/2”.
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rmac
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Re: Threading with compound

Post by rmac »

I can't find it now, but I recently saw a video where somebody was having the same problem.

As it turns out, the scale for the compound angle on some lathes reads zero when the compound is perpendicular to the spindle centerline, but on other lathes it reads zero when the compound is parallel to the spindle centerline. So depending on your lathe, you might be off by 30 degrees if you just go by the scale.

Could that be what you did? The picture shows how it should look.

-- Russ

compound_angle.png

Edit. I found the video:

https://www.youtube.com/watch?v=c0xcyI0hZOI

The part related to this discussion starts at about the 22:00 minute mark.
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Steggy
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Re: Threading with compound

Post by Steggy »

I do my single-pointing with the compound set at 29.5° relative to being perpendicular to the lathe's center-line. I use the compound to feed the tool into the work and only use the cross-slide to clear the tool from the work before moving back to start. You must verify your tool is exactly perpendicular to the work, which may be determined with a "fish tail."

With the compound set to 29.5° from perpendicular, multiply the theoretical thread depth by 1.1489 to get the amount of compound feed required to produce the full thread. A go/no-go gauge or a nut with a class 3B fit is your friend when single-pointing, as working only from tool depth rarely produces a properly-sized thread due to variables such as tool, work or machine deflection under load.
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Harold_V
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Re: Threading with compound

Post by Harold_V »

What rmac said.
What setting the compound, the best possible reference is to talk about the hands of a clock, disregarding the markings of the compound. Because there is no standard, the 0 point may or may not be properly located so the marks read correctly (when threading). Armed with that thought, set the compound at 5:00 and see what the marks read. If you're close to 30°, they read correctly for threading. You then should correct the setting so it reads 29½°, although I prefer to set mine @ 29°, which assures that should there be an error in the marks, you won't be set beyond 30°, which leaves a staircase configuration on the trailing thread face. The tool should NEVER be fed beyond the half angle of the thread.

Assuming your marks are not in keeping with the above, you'll find that when the compound is set @ 5:00 that the mark reading will be near 60°. No problem. Just correct so it reads 61° (or 60½°).

One more thing. The compound setting should be determined by the thread being generated. If you're cutting towards the headstock, the compound should be set on the right hand side (5:00). If you're cutting towards the tailstock, it should be set on the left side (7:00). Failing to set the compound accordingly offers the risk of developing a drunken thread. It's important that the pressure of the cut causes the carriage to load against the screw.

H
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liveaboard
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Re: Threading with compound

Post by liveaboard »

Harold_V wrote: Sat Dec 19, 2020 2:50 am drunken thread.
H
Never heard that term; so descriptive of my early work.
And a recent error too.
1949DC
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Re: Threading with compound

Post by 1949DC »

So I got a big smile when I looked at compound for 5 o clock. Sure thing my lathe reads 60 deg perpendicular to spindle center line just like Blondi. I’ve watched her work before but forgot plus dismissed it because of lathe origin and age. I just threaded a .750” - 10 tpi with compound with great results eyeballing degree mark near 60. I’ve watched Joe Pi video on setting compound for fractions of a degree. I’m no trig wizard so still taking that in. If I can use his method I can know exactly my degree mark. Keep in mind I believe this old lathe was reworked over the years. The degree marks don’t appear to be factory but later lathe repaired. I’ve owned this lathe since 2000. It came with change gears but I had little interest in threading until recently. I figured out the brass plate describing TH SP SC. A year ago I tore it down for a cleaning and paint. I discovered bolts that had very odd thread pitch. Knowing the former owner had made some repairs I thought he had replaced bolts with metric or 9/16” bolts but the thread pitch was odd. So I gave up understanding that oddity. It wasn’t until I read how to run a lathe by southbend. The chapter on whitworth and American national threads. Commissioned in 1918 the American national was created but not until 1948 thread degree and pitch was universally adopted. So I had a light bulb moment. Those odd bolts are 1/2”-12 tpi . My very first thread made was a 1-5/8” - 7 tpi. Because my spindle is that size and I wanted to have another back plate made. But.... I quickly gave up on the compound method and corrected the thread by using only my crossfeed. Over the last year I have made parts for the old girl. The tailstock quill i made to within .0005” for sliding fit. The spindle bearings were replaced with bronze. I replaced both cross and compound lead screws and nuts. I made cross and compound bearing supports and replaced the dials. I used Bridgeport 200 division dial on cross and little machine shop 100 division for compound. Backlash on cross is .014” compound is .004” I mounted a 10” 4-jaw Kitagawa Chuck on head stock. Perhaps I should start a new thread but if anyone reading this can anyone tell me the age of this lathe? It is a Sheppard lathe of Cincinnati lathe Co. Given the 1/2”-12tpi can I assume it is -1918?
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Steggy
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Re: Threading with compound

Post by Steggy »

liveaboard wrote: Sat Dec 19, 2020 3:39 am
Harold_V wrote: Sat Dec 19, 2020 2:50 amdrunken thread.
Never heard that term; so descriptive of my early work.
And I admit a recent error too.
My high school shop teacher referred to it as a "wandering thread." Guess it wasn't PC in those days to use the word "drunken" around a bunch of teenage boys. :D
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John Evans
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Re: Threading with compound

Post by John Evans »

I don't bother with the scale on the lathe. Rather I use a 30-60-90 triangle and a 1-2-3 block off the chuck face.
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