Micrometer / Caliper

[Keppy]

I would like to ask a question on the proper technique of measuring with either one? I was cutting some patches today and measured with my caliper. The patches measured .022 but when I squeezed it I got a reading .018. Now that's the measurement I had before washing it. Now I measured with my micrometer and got a .018 reading. I take the micrometer with when buying patch material. Now just how much pressure do you use? I know the surface area is smaller with micrometer and when measuring a hard surface I just snug it up. Maybe I'm doing it right or maybe just lucky. Tape measure is my specialty. Thanks

 

[Zonie]

IMO (speaking as an ex Quality Assurance Engineer), there is no way to accurately measure the thickness of a soft compressible material with a Micrometer or Caliper.

The best that anyone can do is to use a light amount of pressure and when measuring several different cloths try to be consistent.

A Micrometer is much more difficult to use because it operates via a screw. It takes very little rotational force to create a tremendous amount of force between the anvils.
I think a much better "feel" can be made by using a dial/electronic caliper.

 

[paulvallandigham]

Use only enough pressure so that sliding the fabric out is slow, but possible. If you can't pull it out of the micrometer, you have used too much pressure, or turns, on the micrometer, and need to back it off.

Using any fine measuring gauge involves a lot of fine motor skills. You literally have to feel your way to a correct measurement. You can practice on different objects at home to educate your "touch". I recommend using newspaper for practice. Its strong enough to give you a good reading, but will easily tear if you crush it, and try to remove it from the micrometer.

I have seen some machinists, who have arms that make my thighs jealous, but who can turn the knob on a micrometer so delicately that for years they resisted getting a modern micrometer with the 10,000Ths click adjustment. The older men still "compete" with the new digital micrometers to see if they can set it just as accurate as the " computer can".

Washing fabric removes a starch-like additive, called " sizing" from the fabric. The Sizing is put in the fabric to help it remain flat for efficient storage and transportation. Do not be surprised that the measurement of the cloth at the store, and your measurment after washing the sizing out will differ. Measure the washed cloth at several different location to get an idea of what the average thickness is, and plan your loading accordingly.

 

[Keppy]

Zonie & Paul thanks to both of you on your reply. I guess then I was thinking I used a .018 patch and by doing as you say I would actually be useing a .021 patch. I guess it make little differnce as long as it shoots good. I'm been using the blue and white pillow ticking for .54 and .58. I use another blue color that I use in the .45. It measures .014. Washing the cloth does fluff it up. I guess I have been using a little more pressure than I should. Like I said tape measure is my specialty. Thanks its something to practice on.

 

[Dave K]

Practice getting the correct feel by measuring feeler guages. You will soon get that feel of how the object you are trying to measure, will slide through the mic and of course you can check your progress by seeing what the true thickness is by reading what it says on the feeler guage. Do make sure the mic. is "zero'd" when it is closed. You may what to get the mic. checked for acuraccy.

 

[roundball]

Practice getting the correct feel by measuring feeler guages.

Outstanding suggestion DaveK...agree the goal is to "just feel" the object...not compress it.
I stood at the bench with a handful of patches and my dial caliper, and practiced a couple dozen times...measured one, dropped it, picked up another one, measured it, etc, until I developed a "feel" for when I felt the material but was not compressing it.

 

[ToddB]

They make a paper mic that has larger diameter anvils to advoid smashing softer materials but just getting a feel for it would probably be a cheaper solution.

 

[musketman]

IMO (speaking as an ex Quality Assurance Engineer), there is no way to accurately measure the thickness of a soft compressible material with a Micrometer or Caliper.

The best that anyone can do is to use a light amount of pressure and when measuring several different cloths try to be consistent.

Having spent the last 20 years in the rubber industry I can vouch for this, we have to measure open cell sponge rubber with micrometers. We hold a small section of the item to be measured by the top and run the micrometers down its length using a light touch, closing the micrometers just enough to start to feel any resistance.

If we close too much, the sponge rubber compresses and gives a false reading.

 

[mykeal]

This reply to Musketman is really a reply to the forum:

When I load a patched round I force the ball, surrounded by the patch material, into the bore with significant force. The patch material is highly compressed at that point, and remains in that state until it's expelled from the barrel with the ball.

Why do you (the forum) suggest we measure the UNcompressed thickness of the patch material, when the compressed thickness is what's effective when 'in use'?

 

[roundball]

Why do you (the forum) suggest we measure the UNcompressed thickness of the patch material, when the compressed thickness is what's effective when 'in use'?

Because it's the only "constant" which can be reasonably trusted in making measurements.

Different materials (ie: cotton vs. teflon vs. denim), different weaves (ie: cotton vs pillow ticking), etc would result in different degrees of compressability, not to mention one person's sense of something "being compressed" would probably be different from another individual's ides of being compressed.

As far as a patch being compressed in a bore, that varies a lot as well...a PRB that can easily be thumbstarted would have much less compression than one which required a firm rap on a short starter...to say nothing of those who actually use a mallet to start a PRB, etc.

 

[Stumpkiller]

IMHO the "true" thickness doesn't matter if you have a good seal and accuracy. The method of measurement, therefore, doesn't matter AS LONG AS YOU ARE CONSISTANT. If you found something that works and measured it one way, measure the same way the nest time you're shopping and it should work out for you.

As a test, but some "professional" patching from T/C or Ox-Yoke of a known thickness and see if your measurement agrees with the packaging.

I have a clutch on my old Starrett micrometer and I double the material and tighten it until I get three clicks, then divide by two. Most cotton I've measured has at least a +/- 0.002" tolerance depending where you measure it.

Those folks that tighten a micrometer with force should do the same with an empty micrometer and observe that they get a negative reading as the jaws will deform. Always measure with the same force used to calibrate the zero setting: hence the clutch on mine.

 

[Ken Rummer]

I have a cheap $10 outside 0-1 mic bought at Sears. It has a clutch that allows the barrel to stop turning when a given compression is encountered. The clutch is very helpful for the consistant pressure that permits repeatable thickness measurements. It is not necessary to know what the actual thickness is but you can now compare your mic today with your mic tomorrow when you buy replacement patch material.

 

[renovato]

I've always measured with a caliper, lightly closed against the cloth. This conversation got me thinkin' though. It doesn't matter so much what the cloth measures uncompressed. What's important is how thick it is when it's compressed in our bores. 2 different types of cloth could measure out the same when lightly compressed, but behave entirely differently when compressed in our guns. Maybe it would work better to measure using more pressure after all? I'll give it a go next time I hit the cloth store. Alas, Walmart doesn't sell material here anymore.

 

[Stumpkiller]

Well that's easy enough. If the lands are 0.540" and the ball is 0.530" then the compressed patch is 0.005" (1/2 of the 0.010" difference). Now if your groves are cut 0.008" deep the less-compressed areas of the patch have to fill 0.016" around the ball; though the lead does compress a bit and form itself to the grooves with a tight load.

Use a patch at least 120% as thick as your rifling is deep. :wink:

If you're getting blow-by burned patches use a thicker patch. If they are tearing when loaded (pull a ball to check this) try a thinner patch or shoot 100 more and try again if it's a new barrel.

The rifle is older than the micrometer, guys. :rotf: Don't over-think this.

 

[musketman]

I'll give it a go next time I hit the cloth store. Alas, Walmart doesn't sell material here anymore.

Head for the hills, they're on to us! They know we are using their pillow ticking for patched roundballs... :haha:

 

[tecum-tha]

The blackpowderaccuracysystem suggests compressed patch thickness. And I agree, because it worked for me (and probably for a whole lot of other guys).
Here is what Doug suggests:
"Most micrometers have a little knob at the end of the handle that slips with a clicking sound when the two parts come together and hold the measured material snugly but not tightly. That´s how you normaly use a micrometer. For patching, we then turn the thicker part of the handle with thumb and forefinger only, until we can´t turn it anymore. What we measure is compressed thickness."

 

[Kelhammer]

I'm with you MYKEAL. At least it's exactly what I was thinking. To me you would want to measure the material in a simmilar state to it's application. I'm not sure of the price but there are also spring loaded thickness gauges. I think they are mostly used to check the thickness of sheet metal. With these you push a button to open the anvils, and release it to close on the object to be measured. You get your reading off a dial or digital readout. Pressure would always be the same. This style of tool would be my first choice. Dial calipers would be my second option, as mics are easily over tightened. :nono:

Ok I looked these up at MSC's website. The ones they list are expensive ($100), but I'm sure SPI or somebody would make them cheaper. If you want to see what I'm talking about go to MSC's website and search for thickness gaugeing, then to dial gauges. MSC is not know for best priceing, but they have everything.

 

[roundball]

To me you would want to measure the material in a simmilar state to it's application.

But...what does this statement mean?
If we're talking about precise measurements, then what is meant by "similar"...that's certainly doesn't mean precise.

For example, how would you know what the compressed thickness would be to measure for, if the application was for thumb starting a loosely fitting PRB?

What would the measurement be for thumb starting a medium fitting PRB?

Or for thumb starting a firmly fitting PRB?

What would the measurement be for a firmly short started PRB?

Or a PRB requiring a mallet to hammer it in?

What would be the compression thickness to measure for using teflon patching for each of those applications above?

By now you should see the problem...there is no constant in that apporoach...there is no norm or standard by which all patching thicknesses can be uniformly measured the same way by all people due to the endless number of variables if its attempted to be done by 'application'.

 

[paulvallandigham]

agree! I know that comes as a shock. I also disagree with Dutch Schoultz's advice of using a crush fit. It doesn't solve the problem of variables from one person to another. I believe a " crush fit " as a standard was thought up by people who have the luxury of owning a micrometer with a " clutch", that clicks when you exceed its maximum pressure. If you use such a micrometer, the clutch will provide some consistency. Whether that is a crush fit or not depends on how the clutch is set.

I prefer to tighten my micrometer until the fabric can be pulled out of the instrument, but comes out slowly. I can repeat that tension fairly reliably, and I can practice that using feeler gauges or some fabric, or paper, like a business card, that has a known thickness.

If I try for a crush fit, the number of threads per inch of the weave affects how much I can crush the fabric, as will the kind of cotton, or, heaven help me, linen, or other fabric I might try.

I know from personal experience that the .005, and .010" patches I have bought made by the old Ox Yoke company, when damp, could be crushed down to about .002" of an inch, but that actually told me nothing about their performance in my gun. ( .45 cal.[.451 diameter bore] rifle with scratches for rifling!, and a .445" diameter ball.)

All my strength on the barrel of my old micrometer showed was that both patch fabrics were too thin for what I was expecting them to do. I eventually settled on .440" balls, and .015" thick patches, and 55 grains of FFFg powder. That combination proved very accurate out at 50 yards. Using the clutch micrometer, those thin fabrics measured pretty close to the advertised thicknesses.

 

[Stumpkiller]

That´s how you normaly use a micrometer. For patching, we then turn the thicker part of the handle with thumb and forefinger only, until we can´t turn it anymore. What we measure is compressed thickness."

Just out of curiosity, what reading do you get with no fabric in the jaws and using that much pressure? I get negative 0.021" by twisting until it can't turn on the normal zero calibration by abusing my Starrett No. 230 micrometer that way?!? What is that? One anti-patch thickness? Shouldn't you add that back in to the result to get the true measure less measurement/calibration error?

I can just image Mr. Fine, my 10th grade shop teacher, cuffing me upside the head and yelling: "It's a micrometer, Pearsall. Not a damned "C" clamp!" :rotf: