Threading conundrum

[jonwinchester]

Long story very short. 29.5° has been the compound angle I've used to cut threads for the past 20 years. The lathe I run currently, when threading at 29.5° cuts the back of the thread off. I ask one of the older guys how he threads and he tells me he sets his compound at 60° . Wtf. I try it and it works perfectly. I'm hoping someone can help me out to understand what's going on. Thanks! Oh, I run a Chinese made conventional lathe.

 

[L Vanice]

Not all lathes number their compound angle scale with zero in the same orientation to the bed axis. Whether you use 29.5 or 60.5 degrees for threading depends on where the maker put the zero mark.

You have to put the compound at 90 degrees to the bed axis and then, looking from above, rotate the compound about 30 degrees counterclockwise. Now look at the angle scale and see if you are close to 30 or 60 degrees.

Larry

 

[mhajicek]

Is the 60 on this lathe where the 30 is on most lathes?

 

[William Lynn]

Once our shop started buying carbide threading inserts, I quit worrying what the compound angle was. Just square the tool to the part and go straight in with the crossfeed.

 

[APD]

Long story very short. 29.5° has been the compound angle I've used to cut threads for the past 20 years. The lathe I run currently, when threading at 29.5° cuts the back of the thread off. I ask one of the older guys how he threads and he tells me he sets his compound at 60° . Wtf. I try it and it works perfectly. I'm hoping someone can help me out to understand what's going on. Thanks! Oh, I run a Chinese made conventional lathe.

30 and 60 are complimentary angles
30 is the same as 60 from a different reference point.
As others have said, the scale on your lathe is referencing a different zero point.

Like on a miter saw: zero degrees on the saw makes a 90 degree cut on the workpiece

 

[farmersamm]

Don't trust the markings on the compound.





I never trust anything. I always check to make sure. The markings on these machines are sometimes strange. Make your index mark where it belongs, and you're golden.

This might not apply to your machine, but it's a good idea to check.

 

[EPAIII]

There really is no, "most lathes". They seem to be divided roughly equally.

I just ignore the numbers stamped on the scale and do as L Vanice suggested. That works on ALL lathes that I have ever seen.

Is the 60 on this lathe where the 30 is on most lathes?

 

[Cole2534]

I just ignore the numbers stamped on the scale and do as L Vanice suggested. That works on ALL lathes that I have ever seen.

That's how I threaded on the South Bends, bigger lathes I feed directly with the cross-slide.

 

[AlfaGTA]

Just use your center gauge.
Put the two ends of the straight end against the stock, and left side of the
60* will be parallel with the side of the compound.
Cheers Ross

 

[john.k]

All the high speed single point auto threaders feed the tool in at 90deg,and produce a perfect thread ......so any angle less than 90 is probably good.

 

[michiganbuck]

Originally I cut my threads right on 30 and then took a .002 or what cross-feed to get a nice cleanup on the following side.
I used a fish gauge to grind and set my tool bit to be square to my part.

Agree it is best to just look over a protractor the first time you use a lathe to see that the compound has the 30* line in the right place...and then check the accuracy of the line (but don't cold chisel mark it just remember it).

 

[MilGunsmith]

My Dad, a 91yo retired Toolmaker, has a drilled, reamed hole for a dowel pin in his compound to hold at precise 29.5*. He has limited vision so that lets him get around seeing the fine lines on the compound now.

 

[Forestgnome]

I hate threading conundrum. It's a very difficult material to work with.

 

[Conrad Hoffman]

Stolen from my website-
Loosen the fastening screws for the compound and rotate it so the feed handle points directly at you. The compound will be parallel to the crossfeed. Look at the degree markings and note how they chose to number it. Ideally this position will be zero, but some lathes call this 90 degrees.

Rotate the compound counterclockwise (CCW) exactly 29° and lock it down. Some people use 29.5° and that's OK too. You want just a bit less then half the thread angle, never more. If your compound started out at 90° it should now read 61°. If it started out at 0° it will now read 29°. Unless they dreamed up some other way to mark it, but the key is pointing the handle towards your belly button, then going 29° CCW.

 

[Doozer]

Just stop stop stop and bring your heads out of the weeds
for a reality check. You know what a 60° thread looks like.
You look at 100 a day. For single flank threading, you set
the compound parallel to one flank of the thread. As in 30°.
You can eyeball your compound to SEE IF YOU ARE CLOSE.
If you are 15° OFF or whatever, it should be clear as day.
And for the guys who argue over 1/2 a degree or whatever,
just stop. It does not matter. Just get the compound parallel
to the thread flank being cut. That is the objective. Be sensable.
Me personally, I cut MOST of my threads by plunging straight in.
Unless they are 4 TPI or something huge. But just to parrot
what some lathe book wrote, 99 years ago says, and not to
think about what you are actually doing is nuts. Become part
of the process and understand threading and using a lathe.
Being a casual observer of life will guarentee perminant status.

-Doozer

 

[Lanso]

This 29.5 degree discussion always reminds me of high school shop class!
I’ve been single point thread cutting of many sizes and materials for 50+ years and never waste my time with this, there are more important details to focus on.

 

[eKretz]

I'm in the straight infeed club too. 99% of the time I just lock out the compound and use the cross slide. Every now and again, angled infeed is useful for issues with finish or chip control.

 

[farmersamm]

I think it might have a bit to do with the rigidity of the machine.

Plunge cutting, aka cross slide threading, requires that the cutting tool widen the full shoulder width with each increasing depth of cut.

If you're coming in at 30*, you're only widening the root of the cut as you increase depth. A smaller area of the cutting tool actually engages the material. A smaller bite of the apple, so to speak. A bit easier on smaller machines.

 

[gbent]

Infeed with the compound is easy when you have a ball stop on the cross slide, whichever angle you choose. Angle infeed helps with chip control on flexible or gummy workpieces. Just remember to add 10 or 15% to the thread depth on angular infeed (13.4% is the correct amount, 10 or 15 is easy mental math).

 

[ratbldr427]

No matter how the lathe is marked if you are feeding from the compound you are feeding 30 deg from perpendicular to the center line of the work. If you feed more than 30 then you will get steps on the back side of the thread as the tool is fed away from the back side. If you feed less than 30 then you will start cutting on the back side, no steps. The idea with the 1/2 deg (29.5 from perpendicular ) is to just shave the back side. The closer you get to perpendicular the more you cut on the back until at straight in both sides are cutting equal. Except the front is more positive and the back is negative. So it would seem that the back thrust would tend to push the tool to the front easier cutting edge. In actual practice the cuts are small per cut and probably won't make much difference. However try cutting the whole depth at once perpendicular and see what happens.

Personally I use 29.5, no big deal, just seems less likely to cause any problems.