wheels17
Stainless
- Joined
- May 10, 2012
- Location
- Pittsford, NY
The recent discussion about the size of the register and the centering of the chuck on a threaded spindle led me down quite a rabbit hole.
I am a believer in the "threads center the chuck" theory, so I went to check the data I had on clearances on my chucks. I'd done this a few years ago in the context of calculating the thread engagement on various chucks and never tripped to the difference between my measured spindle diameter and the South Bend specification. At that time, I had measured my spindle at 1.498 and went on my way without realizing that the South Bend specification was 1.509.
I went back to check this and did a number of measurements using a B&S tenths reading 1-2" micrometer that had been zero set on a 1.5000" stack of gauge blocks. The overall average of the readings was 1.4969, substantially under the specified diameter of 1.509. I then took a close look at the area near the chuck seat, and it looks like the spindle may have been ground. I'm not sure what a South Bend spindle is supposed to look like in this area.
It's a very clean 1978 10K 4 1/2' underdrive with hardened ways that seems to be in excellent condition other than this spindle diameter surprise.
I realized this situation could be a good way to check chuck centering without a close fit to the register. I turned a slug of mystery metal in the 3 jaw chuck that has a register bore diameter .034 larger than my spindle. I then measured the turned area using a +/-.002" range, .00005" resolution Hamilton Watch dial test indicator set in the tool post. Four readings were taken at 90 degree locations by rotating the chuck by hand with the drive belt disengaged. Four repeats of the cycle were made.
The surface finish was not very good, but I didn't want to do anything that might alter the concentricty of the turned area. This led to some variability in the readings. The averaged readings on the indicator were converted to runout by subtracting the average of the measurements from each measurements. The runout measured .00016, .00000", -.00007", and -.00009".
This chart shows the variability in the measurements expressed as 3 sigma limits.
I then removed the chuck without touching the slug in the chuck. I screwed the chuck back onto the spindle and seated it firmly without using any momentum to seat the chuck. The measurement sequence was repeated, and the runout was calculated at 00011, .00001, .00001, and -.00014.
The chuck was removed a second time, and was seated by snapping the chuck approximately 20 degrees against the seat. The runout was calculated at -.00001, -.00008, .00012, and -.00002.
Here's the data presented in a graph.
I was pleasantly surprised by the low runout measured on the turned slug before the chuck was removed and reinstalled. The runout did vary with the two chuck changes, but even so, the overall runout remained below a total runout of .00025.
This chuck seems to center with a runout of .00025", with a register clearance of .034", over 100 times larger. It is clearly not impacting the centering of the chuck under these conditions.
I need to get a better combination of tooling and materials to get a higher quality surface finish and repeat the test, as I believe a large portion of the variability seen in the runout is due to surface finish, not chuck positioning.
I am a believer in the "threads center the chuck" theory, so I went to check the data I had on clearances on my chucks. I'd done this a few years ago in the context of calculating the thread engagement on various chucks and never tripped to the difference between my measured spindle diameter and the South Bend specification. At that time, I had measured my spindle at 1.498 and went on my way without realizing that the South Bend specification was 1.509.
I went back to check this and did a number of measurements using a B&S tenths reading 1-2" micrometer that had been zero set on a 1.5000" stack of gauge blocks. The overall average of the readings was 1.4969, substantially under the specified diameter of 1.509. I then took a close look at the area near the chuck seat, and it looks like the spindle may have been ground. I'm not sure what a South Bend spindle is supposed to look like in this area.
It's a very clean 1978 10K 4 1/2' underdrive with hardened ways that seems to be in excellent condition other than this spindle diameter surprise.
I realized this situation could be a good way to check chuck centering without a close fit to the register. I turned a slug of mystery metal in the 3 jaw chuck that has a register bore diameter .034 larger than my spindle. I then measured the turned area using a +/-.002" range, .00005" resolution Hamilton Watch dial test indicator set in the tool post. Four readings were taken at 90 degree locations by rotating the chuck by hand with the drive belt disengaged. Four repeats of the cycle were made.
The surface finish was not very good, but I didn't want to do anything that might alter the concentricty of the turned area. This led to some variability in the readings. The averaged readings on the indicator were converted to runout by subtracting the average of the measurements from each measurements. The runout measured .00016, .00000", -.00007", and -.00009".
This chart shows the variability in the measurements expressed as 3 sigma limits.
I then removed the chuck without touching the slug in the chuck. I screwed the chuck back onto the spindle and seated it firmly without using any momentum to seat the chuck. The measurement sequence was repeated, and the runout was calculated at 00011, .00001, .00001, and -.00014.
The chuck was removed a second time, and was seated by snapping the chuck approximately 20 degrees against the seat. The runout was calculated at -.00001, -.00008, .00012, and -.00002.
Here's the data presented in a graph.
I was pleasantly surprised by the low runout measured on the turned slug before the chuck was removed and reinstalled. The runout did vary with the two chuck changes, but even so, the overall runout remained below a total runout of .00025.
This chuck seems to center with a runout of .00025", with a register clearance of .034", over 100 times larger. It is clearly not impacting the centering of the chuck under these conditions.
I need to get a better combination of tooling and materials to get a higher quality surface finish and repeat the test, as I believe a large portion of the variability seen in the runout is due to surface finish, not chuck positioning.