Accuracy Of Dial Calipers

[randyc]

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.

Ha-ha, you're right to be shocked at that balancing act. It probably required a full minute to stabilize the larger weight on the end of the calipers and since I live in earthquake country, I hastened to take the photo before quickly removing the weight.

Regarding damage, recall that we are discussing a $15 set of calipers (that are over ten years old, BTW). I had little to lose but these devices are so simply constructed and sturdy that I would have confidently used a heavier weight if I could have balanced it, LOL.

OK, your position is now clear that you are NOT of the school that feels caliper pressure is important Thanks for the clarification.

Insofar as parts being measured being affected by pressure, well sure for very soft materials, that's why I mentioned using a light touch on plastics. I don't think repeatability of measurement pressure is an issue for the typical HSM (unless measuring urethane foam or something similar - in which case precise machining is all but impossible anyway).

In an industrial environment (like yours, I infer), you are best qualified to determine the proper measurement technique for characterizing the dimensions of the material you are processing.

For harder materials, the experiment indicates that the pressure exerted and repeatability is uncritical. For softer materials, I simply repeat the initial statement:

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

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.

Deflection of the jaws due to fit will of course multiply the error depending on the location of the part in the caliper jaws. In the experiment that I conducted, the near nine-pound load would have multiplied the error at the end of the jaws resulting in about .002 rather than .001.

In my opinion, that's a fairly trivial difference given the magnitude of the load. And the fact that this incredibly cheap set of calipers performs much like your B & S pair suggests a few things

• this particular pair of imported calipers are obviously of good quality

• as a mass-produced product one might extrapolate that most calipers like this one are of good quality

• B & S calipers are known to be of high quality

• based on performance comparison between low end and high end (my cheap calipers and your nice B & S pair) we might make a presumption that the design/construction of this class of tools in general is excellent, capable of most tasks (other than high precision) required by the HSM

Now the above comments - all of them - are not meant to be argumentative and I mostly agree with the posts made by the two forum members ! And, if one accepts my bullets, the consistent accuracy and repeatability of these common tools is outstanding - the mechanical design/construction is adequate to minimize measurement uncertainty even under extreme conditions !

Makes me want to order a few more sets from HF

 

[RJSakowski]

Ha-ha, you're right to be shocked at that balancing act. It probably required a full minute to stabilize the larger weight on the end of the calipers and since I live in earthquake country, I hastened to take the photo before quickly removing the weight.

Regarding damage, recall that we are discussing a $15 set of calipers (that are over ten years old, BTW). I had little to lose but these devices are so simply constructed and sturdy that I would have confidently used a heavier weight if I could have balanced it, LOL.

OK, your position is now clear that you are NOT of the school that feels caliper pressure is important Thanks for the clarification.

Insofar as parts being measured being affected by pressure, well sure for very soft materials, that's why I mentioned using a light touch on plastics. I don't think repeatability of measurement pressure is an issue for the typical HSM (unless measuring urethane foam or something similar - in which case precise machining is all but impossible anyway).

In an industrial environment (like yours, I infer), you are best qualified to determine the proper measurement technique for characterizing the dimensions of the material you are processing.

For harder materials, the experiment indicates that the pressure exerted and repeatability is uncritical. For softer materials, I simply repeat the initial statement:






Deflection of the jaws due to fit will of course multiply the error depending on the location of the part in the caliper jaws. In the experiment that I conducted, the near nine-pound load would have multiplied the error at the end of the jaws resulting in about .002 rather than .001.

In my opinion, that's a fairly trivial difference given the magnitude of the load. And the fact that this incredibly cheap set of calipers performs much like your B & S pair suggests a few things

• this particular pair of imported calipers are obviously of good quality

• as a mass-produced product one might extrapolate that most calipers like this one are of good quality

• B & S calipers are known to be of high quality

• based on performance comparison between low end and high end (my cheap calipers and your nice B & S pair) we might make a presumption that the design/construction of this class of tools in general is excellent, capable of most tasks (other than high precision) required by the HSM

Now the above comments - all of them - are not meant to be argumentative and I mostly agree with the posts made by the two forum members ! And, if one accepts my bullets, the consistent accuracy and repeatability of these common tools is outstanding - the mechanical design/construction is adequate to minimize measurement uncertainty even under extreme conditions !

Makes me want to order a few more sets from HF

The last two digital calipers that I got from HF were terrible. The grinding was rough and uneven on the pairs (Pittsburgh brand). I bought two pairs because they were cheap. Throwaways almost.
One pair will randomly jump in multiples of .200"; someone else mentioned the same problem. The second pair was adjusted so loosely that it was impossible to get consistent readings. When I tightened them up, they would bind terribly at about 2.5 -4.0 inches. Undoubtedly, this was the reason they were set so loose. A little judicious hand work with my stones and they are now serviceable.
In all fairness, over the course of more than ten years, I have purchased eight pairs of HF digital calipers for work and for my own use and found them to perform well with the exception of the last two pairs. But I will definitely check any I buy in the future before I leave the store parking lot.

 

[randyc]

Oh well, experiences differ I have three pair, all are excellent but all were purchased over ten years ago.

 

[18w]

Just like death and taxes, the one thing you can count on these days, what you bought a year ago will invariably be better than the same item tomorrow. It doesn't seem to matter whether we are talking Starrett, Craftsman, or Harbor Freight. At least that has been my experience lately.

Darrell

 

[RJSakowski]

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.

I would argue that calipers are sensitive to pressure. The opposing forces are not directly in line giving rise to a torque moment. If there is clearance in the beam tracking, it will twist the head slightly, giving a slightly different reading. How much is dependent upon the clearance. I adjust mine more on the tight side to minimize the effect.

If you are measuring metal, the amount of force you apply by hand will not significantly compress the metal. Plastics, wood, and rubber are obviously more susceptible to pressure, although on a piece of 1/2" Delrin, I couldn't detect any effect.

 

[David S]

As an ex quality assurance person, the best way to validate what you are doing with a set of caliper, or any other measuring device is to conduct a GR&R study. By going through the exercise you can determine if the measurement error is due to the instrument or the person. Quite simple to do, we used three people and did three readings of each feature. Google will give the method for those interested.

David

 

[Optic Eyes]

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 -

Just use the same instrument for all the measurements, measure a shaft to bore a pully, use a caliper for both measurements.