PM25 DRO and Hand wheel off by 1 part in 1000?

I just did the test on my PM25. I cranked the X handle 40 times, the DRO reported 3.9992. I would expect ±.0002 reading the dial, so the error is .0006 ± .0002.

Is this the same a s 6 mil, I'm not familiar with that unit of measure, sounds metric.
 
I guess I'm missing something, you turn the handle 210 times (each turn .100") and the DRO says 21.025". You moved the table 21" (by the dial) and the DRO says 21.025", so your off .025" in 21", not 25 millimeters, I would think over 21" of travel, being off by only .025" would be great.

If it was a millimeter per inch, that would make the error reading in the area of 19-21 millimeters (full travel) or about 13/16"

Again maybe I'm reading this wrong
 
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Buffalo20 said:
I guess I'm missing something, you turn the handle 210 times (each turn .100") and the DRO says 21.025". You moved the table 21" (by the dial) and the DRO says 21.025", so your off .025" in 21", not 25 millimeters, I would think over 21" of travel, being off by only .025" would be great.

If it was a millimeter per inch, that would make the error reading in the area of 19-21 millimeters (full travel) or about 13/16"

Again maybe I'm reading this wrong
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My tired 40 year old Taiwanese mill/drill reads 18.396 over 18.4oo by the dial for an error of .004". The error increases to .008" mid travel where the wear is the greatest. I would expect better than .025" error from a new mill.
 
Thus "mil" tends to be used more than "thou" for the thickness of plastic sheet, while "thou" or "thousandths" tends to be used when discussing machined dimensions. [Wikipedia]
I hadn't thought of that. OK. I got it now.
 
RJ,

As I don't do this for a living, I thought an error of about .001% over a span of 21 inches would be considered excellent. That being said, after reading your post, I called one of my friends who runs a small shop that specialized in stainless steel and other more exotic metal repair and parts for the local drug companies. He said the calibration company, that comes twice a year to his shop, runs a somewhat similar sounding tests, 5 times on each machine and get an average travel error. He said the 70s era Wells-Index (completely rebuilt in 2012) is off about .045" over 36" or about .001% and the 3 year old Bridgeport, has an error of about .031" over 36", or about .0009%.

As I didn't witness the OPs test or witness the calibration procedure, that the friend said they seemed very similar, I can't say it is, but according to him, he didn't think it was excessive error. I'm not trying to be argumentative, I'm looking for information. Obviously I need to do some more reading on the subject.
 
Buffalo20 said:
RJ,

As I don't do this for a living, I thought an error of about .001% over a span of 21 inches would be considered excellent. That being said, after reading your post, I called one of my friends who runs a small shop that specialized in stainless steel and other more exotic metal repair and parts for the local drug companies. He said the calibration company, that comes twice a year to his shop, runs a somewhat similar sounding tests, 5 times on each machine and get an average travel error. He said the 70s era Wells-Index (completely rebuilt in 2012) is off about .045" over 36" or about .001% and the 3 year old Bridgeport, has an error of about .031" over 36", or about .0009%.

As I didn't witness the OPs test or witness the calibration procedure, that the friend said they seemed very similar, I can't say it is, but according to him, he didn't think it was excessive error. I'm not trying to be argumentative, I'm looking for information. Obviously I need to do some more reading on the subject.
Click to expand...

I am sorry for using a non-standard, for machinists, term of "mil". I am still learning the lingo. But folks did figure out that I meant .001" or, I gather from the guidance (most politely and indirectly phrased) a "thou" or a thousandth of an inch. It seems that other PM25MV's do not have this problem as Tom B only saw an error (with respect to his DRO) of .0008 in 4" or .0002" per inch (what I would call .02%). And his was in the reverse direction of my .001" per inch (.1%).

Hearing a Bridgeport was off by .031" in 36" gives me comfort. I guess that is how accurate mills are. Though I don't understand the source--the number of threads per inch seems invariant for a given machine.

So, before DROs, how did folks make accurate parts? Off by .001" per inch, or like .01" in 10" seems like would be material for many sorts of work. Did machinists use correction multiplications and purposely target 10.01" if they actually wanted 10"?

My Y-axis has a similar property. In 5.000" of hand wheel the DRO said 5.0045". So, just a hair less than .001" per inch instead of the X-axis which is a bit over .001" per inch. Interesting they are not the identical error. (The difference is well beyond the noise of the measurement). They simply must have used the same screw thread set up.

I do not see how sine error of the DRO could be it--the sine of 5.7 degrees is .001. That is a very large angle--10x or even 100x what seems possible based on visual inspection.

Machining is certainly filled with puzzles. Yes, I will use the DRO. But goodness, my curiosity has be spiked.

-Bill
 
The sine of 5.7 degrees is .1
My point is that you are trusting one unknown accuracy scale to test another unknown accuracy scale. "Man with one watch knows what time it is..."
 
Sorry Bob but if the DRO scale was correctly installed ( scanned with a TDI) the angle error from the tilt of the scale is way below the error of the machine, or the DRO. perhaps Dan should run his DTI along the scale to check. Also the fact that the Y Axis shows the same error is very suspicious.

Bill this raises three Q in my mind
What has PM got to say about this error?
Is this error acceptable even in a cheap Mill? Would other forum members be happy with such a machine?
And finally if you had not purchased a DRO how would produce accurate work on such a machine?
Cheers
Ron
 
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