Single Point Threading Tool Angle Accuracy

[devils4ever]

So, I've been doing a fair amount of threading on my lathe and I'm starting to get pretty good at it. A question came into mind and I'm not sure what the answer is. Maybe someone here can answer it.

Like most, I use a center gauge like the one pictured below to get my lathe tool at exactly perpendicular to the work. I spend a fair amount of time making sure the lathe threading tool is correctly set so that the 60 degree included angle is perfect.

But, how accurate does this need to be? Can it be off 1/2 degree? 1 degree? More? Am wasting my time trying to get it as perfect as I can get it when it can be off a few degrees?

 

[682bear]

How accurate do you want your threads?

I always get it as close as my uncalibrated eye will let me. If it is off a half degree, the threads will have to be cut deeper to allow the opposite part to fit, meaning that the minor diameter and major diameter of the thread will be affected. The end result will be a sloppy fit and a weaker thread.

Your final result is usually directly related to the accuracy of your setup... especially when turning threads.

-Bear

 

[GK1918]

We were taught to use 29 1/2 degrees many use 30 degrees. All is well and good IF your tic marks on the compound is right on and many are not. If you have the time measure a store bought bolt you may be in
for a shock. unless you have a NASA contract don't worry. Yes all wants to be proficient the first thing they do is fry their brain with calculations. And nothing beats hands on doing. And after you thread 1,000s
theres no need for wires, thread mics, although good to know if a beginner. All of this takes time developing skills problem solving that's mainly this job. been there done that now my bones are gone
keep with it sam.

 

[RandyWilson]

The 29.5 degree compound angle is about cutting loads on the tool, not the finished thread form. There are two schools of thought here. One is you want to be as accurate and you can, always. the other is that since cut threads are weaker, if the ultimate strength was critical, you would be roll forming the threads. I set my tool post square to the chuck and rarely use the fishtail gauge.

 

[projectnut]

All good points. Personally I do use the gauge and try to get as close as possible to a perfect fit. The gauge has a dual purpose. First it is used to make sure the profile of the tool is exactly 60* when grinding for single pointing. Secondly since the tool will fit perfectly in the fishtail after grinding it should fit perfectly in it when placed against the work.

Cut threads may be marginally weaker than rolled threads, however when purchasing bolts 90+% are cut threads. The strength marking on the head reflects the strength of that bolt. A bolt with cut threads should at least meet the standard of the marking. A rolled thread may be a bit stronger, but as long as both meet the identified standard they should be able to be used interchangeably.

Just to complicate things there over the years there have been some off shore bolts found with fake markings. Whenever I need anything more than a hardware grade bolt (grade 2) I purchase them from McMaster. At least then I can have some relative assurance the bolt is the strength I need.

 

[RJSakowski]

A 1/2 degree off on a 10 tpi thread is less than .001". The left side of my HSS bit is about .1" to allow me to thread in close to a shoulder. It would be difficult to visually detect that small a setup error.

The 29.5º angle is design to make a light shearing cut on the right flank of the thread while the bulk of the cutting action is on the left flank. Any angle which is less than 30º will work. Many used the practice of advancing with cross feed which would be 0º.

 

[RJSakowski]

In fifty years of working with nuts and bolts I can count on one hand the number of times that I have had one fail in service. Most cases of snapped fasteners have been due to rusting in place. In fact, it is a common practice for me to remove rusted on nuts on bolts to tighten rather than loosen to deliberately shear the bolt. It takes a surprising amount of torque to do so.

Aside from fasteners like head bolts and the like, the vast majority of fastener situations we encounter will never come close to testing the design limits of a fastener class.

 

[graham-xrf]

A 1/2 degree off on a 10 tpi thread is less than .001". The left side of my HSS bit is about .1" to allow me to thread in close to a shoulder. It would be difficult to visually detect that small a setup error.

The 29.5º angle is design to make a light shearing cut on the right flank of the thread while the bulk of the cutting action is on the left flank. Any angle which is less than 30º will work. Many used the practice of advancing with cross feed which would be 0º.

I also wondered about 29° when looking at Machinery's Handbook about cutting ACME threads. Agreed, ACME is not a fastener class. I guess there were design reasons why ACME is 29° included, and that this has nothing to do with setting the compound angle.

What I get fro @RandyWilson is that 29.5°is about cutting loads. I thought that might mean "aim for 29.5°, and the bending under load might serve you up 30° instead".

Then we have the very clear explanation from Joe Pieczynski about 29°and 30°.

 

[John281]

"How accurate does it need to be?"
I think that is up to you to decide depending on what you're making. If you're in a production environment where time is money, then it only has to be good enough to get the job done and make the customer happy. But as a hobbyist, we can often afford to spend extra time to make things as good as we can make it even though it's not necessary. Personally, that's what I enjoy about this hobby.
I've found that having the bit angle off 1/2 degree or so is not as important as getting a nice, smooth surface finish. I rarely use my fishtail gauge. Instead, I grind the HSS bit with the 60* angle perfectly centered then align the bit to be perpendicular to the spindle by squaring the bit on the face of the chuck.

 

[RandyWilson]

This really isn't relevant to the OPs question, But to clear some things. It's a matter of relative reference. The world I come from the use of lowly SAE graded bolts is not done for critical loads. The SAE grades are meant to hold a static load in tension only. high cyclic stress and stress reversals is where things like micro-stress risers from cutting threads will really bite you.