Help with plug thread gauge

[mega arc 5040dd]

I have a lathe with a threaded spindle that I am making a drive dog plate for and need some advice. Because I don't want to have to remove the chuck with a larger piece in it to check the threads as I go I decided to make a thread gauge to check the threads on the drive plate I am making. I am a hobbyist so could use some guidance on this. From what I understand from my research when measuring the pitch diameter of the threads on the plug I should me cutting them to the B pitch diameter and not to the A. I am making a 1.5" 8tpi gauge. Machinery's handbook says pitch diameter for 3A max 1.4188 min 1.4133 and 3B min 1.4188 max 1.4259. I cut my threads to a pitch diameter of 1.4188 and my other chuck that threads onto the spindle would not thread on to the new gauge. When I brought the pitch diameter down to 1.4152 the chuck threads on perfectly with absolutely no play. They are the nicest threads and fit I have ever cut other than for a gauge my understanding is they should be cut to the internal thread pitch diameter. But if that is true the why would the chuck not thread on? I can't imagine the spindle or chuck are an out of spec size since they were mass produced and interchangeable. Where did i screw up or am I just not understanding things correctly?

I know for what the drive plate does as long as it fits the spindle it will work so I will go ahead and make it using the plug gauge I made. I just wanted to practice making something to specs properly and would like to figure out where I am going wrong. All the numbers are from machinery's handbook 28 oddly i could not find anything on 1.5" 8tpi in the 6th edition from 1924 not really sure why but thought that was interesting.

 

[TeachMePlease]

If it's a male thread, you cut it to 3A specs.

Think about what you just wrote. You made your MALE gage a line to line fit with the female threaded part. Of course they won't go together.

The male gage has to be SMALLER than the female thread, or it's not going to go in...


Ignoring the thread, what you just posted is equivalent to saying "I can't push my .5000" pin into my .5000" hole"

 

[mega arc 5040dd]

I completely agree with you however in this video Pitch Diameter, Wires and Numbers Galore. Threads in detail ! - YouTube at about 14 minutes in give or take 30 seconds he says for a plug gauge to use the measurements for an internal thread on the external threads of the gauge. Or at least that how I understood it. I have also seen that mentioned multiple times on this forum when I googled how to make a threaded plug gauge. Am I just miss understanding the proper way to do it? I do realize that what I originally posted is the equivalent to putting a .5000" pin into a .5000" inch hole but that's how I interpreted what I have watched and read.

 

[TeachMePlease]

I completely agree with you however in this video Pitch Diameter, Wires and Numbers Galore. Threads in detail ! - YouTube at about 14 minutes in give or take 30 seconds he says for a plug gauge to use the measurements for an internal thread on the external threads of the gauge. Or at least that how I understood it. I have also seen that mentioned multiple times on this forum when I googled how to make a threaded plug gauge. Am I just miss understanding the proper way to do it? I do realize that what I originally posted is the equivalent to putting a .5000" pin into a .5000" inch hole but that's how I interpreted what I have watched and read.

I mean... He knows more about it than I do, I guess. I'd never heard that. I also don't manufacture plug gages so...

I guess follow his advice...

 

[Bobw]

A plug gage has the PD's of the INTERNAL threads, so they are bigger
than the actual thread that has to go in.

Where the problem lies with turning a pitch gage to exactly the minimum.. Is exactly
what street speed said. .5000 pin in a .5000 hole. Its not going to go.

And of course with threads. There are a million things that can effect it besides the
PD. mainly the flank angle. So if you make a go gage .0005" smaller on the PD, should
work fine.

But you had to go .003 under.. Could be what you are checking it against isn't very good
to begin with. Possible that you messed up on your measurements, or your tool is out of
angle a bit, or your major on your gage is too big (they are usually pretty darn small on a
plug gage). There is a million reasons. Some may be your fault, some might not be.

But.. All you NEED to do is cut the thread to work, and now you have a gage that will do that
for you.. And you probably learned something along the way.. So.. I would call it a successful
venture.

 

[4GSR]

You should make your gage pitch diameter the same as you measure on your spindle thread. IT's no telling what the lathe manufacture made the spindle thread to. What ever they wound up with in the beginning, thread gage wise, is what they used for all the lathes they built. Rather they followed the national standard or not. Remember this was way before the Unified thread standard most of us are familiar with today.

 

[JST]

Spindle threads were probably ground ..... very smooth.

Cut threads have a good chance of being rough. The actual PD may be just what you want, but the "fuzz" on them is taking up the clearance and more.

Hit it with a brass bristle brush, then see what you have. Maybe even a steel bristle brush. If you don't bear down forever, it won't remove anything but fuzz.

 

[rogertoolmaker]

The screw is Always made to the basic dimension. The mating thread or nut is where the fit is made. If you just want the practice. With thread rolls measure the thread on your lathe. Then go for a .0015 to .003 over the thread dimension measured on the spindle. However, I think your over engineering the project. Why not just fit the thread in your dog plate to the spindle thread by a little trial and error.

Roger

 

[basalt]

Whenever I have had to make an internal thread to spec. I always made my male gage to maximum material condition then I stayed away from maximum
material condition on the internal thread regarding the minor diameter and the depth of thread. That way when my gage fits I know there isn't going to be any interference on any of the 3 diameters - major, minor and pitch.