How to accurately measure short taper in D1-4 lathe chuck backplate?

[jaek]

I'm trying to figure out a good way to measure the size of the short tapers in my D1-4 lathe chuck backplates. It seems like they are a little loose on the spindle nose. Spec says that the taper half-angle is 7.125 degrees and that the theoretical diameter at the spindle face is 2.5003" -0/+.0003.

The best idea I have is to put the backplate mounting side down on a surface plate, put two precision balls on the surface plate inside the taper, and then see what size gage block stack fits in between the balls vs the distance calculated with trigonometry. This seems to be giving me a number that is noticeably too small, but if the faceplates are loose the number should be too big.

Is there a better way? Or should I just take a bit off the backplates until they fit on the theory that cast iron backplates are cheap and easy to machine and precision measurements are hard?

 

[GeneT45]

The balls can't rest on a tangent to the radius (of the ball) if they're touching the "bottom" surface. If you can space the balls far enough apart that they engage the taper, while not protruding past the mid-point of the ball you should be able to make a good measurement. That said, it sounds like a very difficult thing to set up correctly.

I'll be watching in the event that someone has an ingenious way to measure this reliably.

GsT

 

[RJSakowski]

I would use the balls separately. Insert the smaller ball and measure the distance to the surface of the plate then repeat with the larger ball.
Knowing the difference in the measurements and the ball diameters, you can calculate the taper.

The math can get a bit hairy. I would use SolidWorks. Here is a sample measurement.

 

[jaek]

I would use the balls separately. Insert the smaller ball and measure the distance to the surface of the plate then repeat with the larger ball.
Knowing the difference in the measurements and the ball diameters, you can calculate the taper.

The math can get a bit hairy. I would use SolidWorks. Here is a sample measurement.
View attachment 510279

The short taper on a D1-4 is like something like 5/8". Using CAD software to measure seems like a good idea.

 

[GT-6 Racer]

Chuck it up and get it running true and then simultaneously use a 10ths DTI for radial and a 10ths dial indicator for axial to measure the taper. Then trig it out to get the angle. If you have a DRO on your lathe it’s easier for sure. In any case use blue checks to test the backing plate to the spindle. I’ve fit my “loose” backplates by setting them up on the surface grinder and take a few 10’ths off the face. Repeat and check until the taper blue matches and the face blue simultaneously. If you need to turn the taper use a 10ths dti to set the compound to track the taper on the back side of the spindle taper with no movement of the needle. Then touch up the taper. You will have to then turn the face to get the relationship back. I’ve done several chucks and backing plates both old ones and new ones with good results. Make sure every operation is dialed as precisely as you can. You should install the back plate on the spindle once you get the taper fit and check runout and if needed, skim the interface with the chuck to get it back true again. Best of luck !

 

[jaek]

i think I see what you are getting at here. With a DRO and some dial indicators the lathe can be turned into a poor man’s coordinate measuring machine. Need to noodle on it a bit and make some sketches.

 

[GT-6 Racer]

Exactly

 

[verbotenwhisky]

i think I see what you are getting at here. With a DRO and some dial indicators the lathe can be turned into a poor man’s coordinate measuring machine. Need to noodle on it a bit and make some sketches.

That is what I did, put a 0.0005 indicator on the quill set it to 0 then move the quill down using the quill DRO a specific travel distance and used the reading to calculate the angle, your dealing with a right angle....

 

[jaek]

Think you all are on to something here. Bought a cheap 3d touch probe off of Amazon and the spindle taper definitely looks a thou small in diameter. The taper angle itself looks good. Will try to measure a chuck back plate tomorrow.