Accuracy Of Dial Calipers

[randyc]

Gordon has a wealth of information on most things. But he's nowhere near the "go to" person on the subject. I respect Gordon for he has done and support him 100%. He has a nice screw measuring system for measuring pitch diameter, but like many, can't write instructions for most of us to understand and use.

BTW: He was a member for a short period before being banned several years back.

Perhaps I reveal my own lack of knowledge but Gordon always seemed to have the last word on measurement techniques and instrumentation. His manner can be condescending but everything that I've read by him made sense (to me, at least). I can't think of anyone on PM that is his obvious superior in knowledge and experience ... my personal opinion of course.

 

[RJSakowski]

The 3.000 inch measurement is a typo, one of the zeros took a lunch break. The 2.000 inch measurement - ditto, the decimal point became liberal and migrated to the left. Although it would be natural to question the measurements, I am positive that those measurements are good ones WITHIN the ability of my eyes to interpolate the divisions on the dial (which, as I wrote, is sort of a crap shoot).

I strongly suggest that others make the same measurements to confirm (or not) the surprising results, given the presumed low quality of the tool I was using !

Metrology is not a strong suit for me - at least not mechanical metrology (I'm much better at electronics metrology, specifically RF and Microwave instrumentation). On reflection, although three standard deviations would seem to guarantee a much greater accuracy than either claimed or assumed, I would be uncomfortable with accepting measurements of that precision that as I'm sure most others would be !

Similarly while calibration standards almost always require an order of magnitude greater precision than the device to be measured, when making measurements down to the tenths (as I noted in my .0003 example) I personally would prefer something with that same magnitude of precision to reduce uncertainty when making bearing fits and the like.

Without spending a bundle, however, I'm stuck with tenths micrometers and a set of gauge blocks to compare them against. A sometimes useful technique in the absence of gauge blocks is to use three instruments to make the measurement, discarding the worst measurement of the three. This is simple enough for small dimensions where micrometers are cheap but gets pricey for large dimensions which are the ones that I'm most doubtful about !

A condition that I mentioned but didn't stress highly enough, perhaps, is the temperature environment. Maybe my instrumentation (micrometers) is not as stable as could be but I have to recalibrate if shop temperatures change more than 15 or 20 degrees F. Example: A micrometer calibrated to a stainless steel standard measures an aluminum workpiece at 1.7500 inches in diameter when shop temperature is 60 degrees F. When the same workpiece reaches a temperature of 80 degrees, it now measures 1.7502 inches.

Might be trivial depends on the application -

Randy, IMO your data supports your ability to accurately interpolate between divisions. When I first learned to make scientific measurements, way back in Physic 101, we wer taught how to interpolate. I personally feel comfortable with interpolating to the nearest .2 divisions and am fairly good at hitting .1 divisions. I have a B & S pseudo-digital micrometer. It reads out every five thousandths. Past that there are divisions for half and full thousandths and a vernier scale to determine where you are between the half thousandths intervals. I usually don't bother with the vernier and interpolate directly. When I do check the vernier, I find that that invariably I am within a half a tenth of what the vernier says. It sounds like you have that same skill.

As much as we tend to rag on Chinese tooling, I believe that every manufacturer tries to to make things accurately. Like your experience, every time that I have checked one of the cheap calipers or mikes against a higher standard, I have found them to be accurate to their expected reading. At work, I had a pair of Harbor Freight digital calipers that I used for my personal work. They were included in the calibration schedule when the metrology lab came in to do our annual ISO calibrations and they were certified.
I expect that there is more error due to technique than to the tool.
Regarding the dimensional changes with temperature, I would guess that the difference is due more to the thermal expansion of the part than to the mike. An easy test: keep one at a constant temperature and the change the temperature of the other. That same metrology lab would stabilize the temperature of any instrument they calibrated before they would calibrate. Surface plates had to sit for a week before they would touch them. I am fortunate in that my shop is located in my basement and varies no more than 10F year around. Most of the time it is within 5F.

 

[randyc]

..Regarding the dimensional changes with temperature, I would guess that the difference is due more to the thermal expansion of the part than to the mike...

That's definitely correct. The example that I posted was derived from the difference between T/C for stainless and aluminum multiplied by the temperature difference and the work diameter. There are so many variables that can affect precision, I will illustrate an example at the end of this post, extracted from:

http://www.hobby-machinist.com/threads/making-a-high-performance-yo-yo-what.32255/

I have an old micrometer from Sears, purchased new for slightly more than $5 in 1965. It was cheap because of various shortcuts in the manufacturing process including making the frame from some mystery metal that is non-ferrous ("Mazak" ?). When this tool is brought into the house from the shop, you can literally watch the "zero" changing half a division in five minutes ! At a constant temperature, the tool is just fine and can be easily interpolated, as in your own experience, to .0002, verifiable by gauge blocks.

You are indeed fortunate in the location/environment of your shop ! Mine is just an insulated 2-1/2 car garage ... the insulation does little because the big garage door is uninsulated

OK, here's the quote detailing an experience that I had a few years back:

Deflection of the outer rim of the hub was problematical because high spindle speeds caused the O.D. of the rim to centrifugally expand. For example: without moving the cutting tool location and changing the spindle RPM from 400 to 950 produced a measurable difference in the turned diameter - nearly .001. This wasn't due to a "spring" cut, just centrifugal expansion of the perimeter of the workpiece due to the flexible cross section !




This wouldn't normally be a problem but CAN be for precise work. A lesson for me was that we frequently change spindle speeds when transitioning from roughing to finish cuts - usually increasing speed for the finish cut. This could detrimentally affect accuracy in irregularly-shaped workpieces with thin cross-sections.

The finished diameter of the aluminum hub was fairly critical for a shrink fit with the rim material. Keeping this in mind, had I spun up the spindle for the final cut (after roughing at a low RPM), the expansion of the O.D. would have resulted in the finish diameter being SMALLER than the design value - the part would have been out of tolerance and the rims could not have been shrunk to fit the hubs. (If the rims had already been produced, they would have been scrapped - new ones would have had to be made to accommodate the hub O.D. error.)

 

[RJSakowski]

My woodworking shop is in a 100+ year old granary where the temperature swing runs between -30F and +110F. I have seen a 60 degree swing in a single day. My welding/blacksmithing shop is in an old carriage house which can go between -30 and +100. At least there, I can fire up the coal forge to take the chill out. Needless to say, woodworking is a three season event and sometimes, summer days are better off spent fishing. Fortunately, the fallback is the machine shop. I also have my old Miller buzz box and Miller MIG welder there for small winter tasks. I open the back door to the outside world and weld quickly.

The one problem with the basement shop is the summer humidity. at temperatures in the high 60's and low 70's, it is the heat sink and it is a fight to control moisture. We run the central air all summer and I run a dehumidifier all summer but the old stone foundation is porous so moisture accumulates. I pull about 20 pints of water out each day.

 

[randyc]

Holy Cow !!! I must be fortunate for experiencing only a fifteen or twenty degree swing throughout the day

 

[VFM3]

Howdy, the fellow was me and as I said, I made two measurements on each gauge block. Granted, that's not a lot of measurements to confirm repeatability but what the heck, these were $15 calipers and the two measurements agreed in every case.

I don't think pressure on the jaws is at all critical. Using the same $15 calipers and a one inch gauge block, I just made the following experiment. From barely snugging the caliper jaws (on the gauge block) to exerting a considerable amount of pressure, I note a variation of about .0005 on the dial.

Not trying to argue with you but you might try the same experiment yourself - it only takes a few seconds you might be surprised at how good your calipers are

Greetings Randyc,

I have confidence in my hands and of my calipers; however, sometimes I need to demonstrate to the other guys how measuring pressure will change readings. So I bought this specially designed (spring assisted) caliper for that purpose from Mitutoyo; Gordon C. also manufactures a better design.

Some blokes were surprised on how they were either 1.) measuring too hard or 2.) measuring too soft on various materials from plastics to steels.

I am not surprised that your cheap $15 calipers can pass calibration. Too many people believe that China is not capable of releasing decent, bargain priced stuff.

 

[RJSakowski]

Greetings Randyc,

I have confidence in my hands and of my calipers; however, sometimes I need to demonstrate to the other guys how measuring pressure will change readings. So I bought this specially designed (spring assisted) caliper for that purpose from Mitutoyo; Gordon C. also manufactures a better design.

Some blokes were surprised on how they were either 1.) measuring too hard or 2.) measuring too soft on various materials from plastics to steels.

I am not surprised that your cheap $15 calipers can pass calibration. Too many people believe that China is not capable of releasing decent, bargain priced stuff.

A tenth with calipers is impressive! Have you made a comparison of readings with a mike on real world measurements (rounds, narrow flats, at the tips, etc.)?

 

[randyc]

...I have confidence in my hands and of my calipers; however, sometimes I need to demonstrate to the other guys how measuring pressure will change readings.... Some blokes were surprised on how they were either 1.) measuring too hard or 2.) measuring too soft on various materials from plastics to steels...

If I interpret your post correctly, you are of the school that calipers are sensitive to measuring pressure to which I earlier said that they are relatively insensitive. I just made the experiment that follows:

The normally movable jaw was clamped securely with the calipers oriented vertically while the normally fixed jaw was allowed to move freely. A one inch micrometer standard was inserted between the jaws and the dial was "zeroed" with light finger pressure against the top jaw. Two different weights were then carefully balanced on top of the calipers, forcing the two jaws together. Both weights were steel, the first 1.25 diameter x 1.375 long (0.48 pounds) and the second 3.75 diameter x 2.75 long (8.6 pounds).

The photo shows the variation of the needle for the two weights (remember that the needle was zeroed with jaws closed and no weight other than finger pressure). I stand by my statement that calipers - at least THESE calipers - are relatively insensitive to pressure. The variation from almost zero pressure to almost 9 pounds of pressure was about .001.



When it comes to plastics, obviously a soft touch is best

 

[VFM3]

I am not sure what I am shocked at: the fact that the caliper withstood all that weight without damage or that you managed to balance that weight precariously on the step end .

And I would like to restate my position on this, Calipers are not sensitive to measuring pressure, but the parts you measure will be affectedif you do not repeat the pressure exerted.

 

[RJSakowski]

I find that variability in readings with applied pressure is greatly affected by the sliding fit of the movable jaw. If the adjustments are loose, it is almost impossible to get consistent measurements. My B & S dial calipers are similar in response to Randy's. Measurements are most reliable when made close to the beam.