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Lathe spindle nose external taper out of spec

jothoc

Plastic
Joined
Nov 16, 2023
Long time lurker, finally have an issue that I can't find in the archives and looking for a little bit of help.

I am chasing concentricity and out of square issues on my new-to-me 14x40 Cadillac lathe. I have found that the external taper OD on my D1-5 spindle nose (Measurement "B" in the image attached) is coming in at 3.232". This is way outside the allowed tolerance from anything I have seen (3.2505" + .0005). It looks like the previous owner bunged up the surface in a couple spots at the front of the taper, which I believe are contributing to the out of square condition (Axial slip W/O chuck mounted is about 0.0002", Axial slip W chuck mounted is about 0.002"). I plan to stone these spots and see if that helps, unless there is a better suggestion.

For the OD being undersized by 0.018", I am unsure the contribution of this but hope to know more after fixing the dings on the taper and blue the mating surfaces. I mounted the chuck by pressing the tail stock against it to support while indicating on the chuck and prying up on the chuck to see how much motion I got and it was 0.0002", which seems acceptable, but I will need to repeat this once the dings are fixed as well.

Does anyone have experience with what minimum external taper OD is acceptable for D1-5 spindle noses? Any suggestions to fix the dings on the precision taper besides stoning?

1700149540364.png
 
Good call on the over pins, I will remeasure. I originally took it with calipers. I think this is a large enough delta that I can confidently say that it is undersized based on the caliper measurement, but to what extent, I agree, I would need to use over pins or another more precise method.

If it is undersized and not making complete contact with the chuck reference surface, I think my only options are spindle replacement or turn down the rear surface on the chuck.
 
Try taking the camlock pins out of the backplate and see how it fits. That will make .018 u/s easily apparent. There is also the chance the backplate has been worked over.
 
As stated above, take out the pins, and hold the chuck in place with the tailstock - if the taper is that much undersize you'll be able to move the chuck.
This mounting is designed so the chuck fits snug on the taper and fits up against the flat face.
One simple test might be to get some thin skim stock, maybe 5 thou or less. Cut 3 small pieces and "stick" them on the taper, oil or something. Mount the chuck and hold in place with the tailstock. See if there is a gap in the mating face, measure it with feeler gauge. With a bit of trig you can probably figure out if you've got an issue or not.
Bob
 
Do the math, but if you took .002" off the flat surface behind the taper, it would increase the taper O.D. .016" and correct the problem. The tangent of the taper angle is ⅛, so there is an 8:1 ratio. If you want more info look at this thread:
what-is-the-d1-camlock-spindle-nose-taper-ratio.331378

You would get the best results if you removed the spindle and took it to someone with a cylindrical grinder. However, if you are intrepid (read foolhardy), you could use a TP grinder to bring it close. Maybe close enough.

However, there is another possibility. Since the tangent is so high, the need for very careful measurement is paramount. You need to measure the taper diameter at exactly the right distance from the flat part of the spindle nose. Easy to get a false reading. Much better to find a spindle nose test piece and check it accurately by bluing it in. If you use wires, they need to be held tight against the spindle nose flat, and they need to be the right diameter.
 
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Do the math, but if you took .002" off the flat surface behind the taper, it would increase the taper O.D. .016" and correct the problem. The tangent of the taper angle is ⅛, so there is an 8:1 ratio.
8:1 is correct but I think you got stuff upside down. 8 off the face gets 1 on the radius. So .002 off the face gets .00025 on the radius and .0005 on diameter.
 
However, there is another possibility. Since the tangent is so high, the need for very careful measurement is paramount. You need to measure the taper diameter at exactly the right distance from the flat part of the spindle nose. Easy to get a false reading. Much better to find a spindle nose test piece and check it accurately by bluing it in. If you use wires, they need to be held tight against the spindle nose flat, and they need to be the right diameter.
^^^ What he said. Isn't there a relief cut at the intersection of the taper and the mounting face, making the B dimension measurement impossible with hand tools? What matters is that it fits. I agree on removing the camlocks and checking the free-play again.
 
Measure over pins will get you a very close size. You need more than two hands to do this. The pins have to be tight against the spindle face and are forced away by the measuring pressure. Maybe magnets?
 
The best way I've found to measure this taper is to use some of the wax threads used to check journal bearing clearance - Plastigage is the stuff I've used. A tiny dab of oil or grease will hold it, place 3-4 bits, clamp on the backplate and when you pull the backplate you'll have squished threads whose widths give you the clearance.
 








 
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