Digital Calipers

[Zonie]

As I have mentioned in other posts, one of the more useful tools I have is a Dial Caliper gage.
I use this countless times when building a Muzzleloader for everything from measuring the size of the barrel channel in the stock, determining the location of the barrel pins, the height of the sights and the depth of the dovetails, the list goes on and on.

For those who are not familiar with precision calipers they look rather like a 7 inch long monkey wrench with a dial on the side. The latest versions have a digital readout on them.

They measure outside sizes, inside sizes and depths to an accuracy of one thousandth of an inch or less and give you the answer in an easy to read mannor.

I have been using an old RCBS gage I bought at a gun show 9 years ago. Who knows how many things they measured before I got them, but they have had more than their share of use over the years. They sometimes read a thousandth or two off of what I know the answer is supposed to be (like measuring drill bits which should be within 1/2 thousandth of the size that's marked on them).
That's why I was interested in an ad by Checker Auto.
They were selling Digital readout precision calipers for the whopping price of $19.95. What the hay says I. For 20 bucks they can't ba much worse than the ones I have now.

I must admit I was totally suprised. These things measure to an accuracy of 1/2 thousandth and Everything I measured was exactly what I knew it should be.

If you have an old worn out set of calipers, or you don't have any at all, you might consider buying some like I got.
They are made by Performance Tool.

 

[blckhrn]

I bought this pair, HF might be more accessible for some. They are building stores in the bigger cities.

http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=47257

$20 plus sales tax if you can find a store.

Some of the digital calipers I've seen don't have the roller wheel for fine adjustment (I lay out hole centers with mine using them as a scribe with one jaw on the straight edge of a workpiece. Without the wheel the adjustment can be elusive sometimes). These have the wheel.

 

[Zonie]

Blckhrn: Yes, I agree the thumb operated drive wheel is a nice feature. The calipers I bought also have this.

By the way, every time someone mentions precision measurement, someone else always asks the question "how much force should I apply to get a accurate measurement?"
The answer for hard objects like metal or wood, is "just enough to lightly contact the object without any clearance between the object and the measuring jaws".

If you are new to this art of measuring and you buy a micrometer or precision dial or digital calipers (they used to be called Vernier Calipers because of the scribed lines you used to have to read to get an answer)the best way of getting a feel for the technique of using them is to measure things with them.

Boy, that's a dumb statement! Well, not really. If you apply too much pressure, the calipers or micrometer will bend slightly and give a false reading. Too little pressure will not seat the jaws snugly against the work.

If you have some good drill bits, you can measure them. Of course you have to know what the decimal equivalent of the drill is, so to make it easy for you I will give you some of them: 1/16=.0625 3/32=.0938 1/8=.1250 3/16=.1875 1/4=.2500 5/16=.3125 3/8=.3750 1/2=.5000.

If you buy the digital calipers I mentioned above, they read to an accuracy of .0005. Because this is their limits they will round off their reading to the nearest .0005. That is close enough for most gunwork.

To give you an idea of how small .0005 is, imagine taking a hair from your head. Now cut its thickness into 3 equally thick pieces. Each piece will be about .001 thick. Now slice one of the pieces in half. That piece is about .0005 thick!

If your measuring drill bits, it is best to measure the shank right where the flutes (2 grooves) stop as it won't be buggered up from a slipping chuck.

Once you get the "feel" you will remember it.
Have fun!! ::

 

Zonie

What you are needing is a micrometer in the .0000 variety (10 thousandth).

 

[Zonie]

"What you are needing is a micrometer in the .0000 variety (10 thousandth)."

Whafor? ::

 

Half of a thousands would no longer be your vocabulary :shocking:

 

[Zonie]

Well, for most blackpowder gun smithing work, IMO 5 tenths is close enough. The most precision area I can think of would be the fit of the tumbler axle to the lock plate where the total clearance of .0005 to .0015 would work very well.

In the work I do, speaking of one tenth of one thousandth of an inch is common. In fact, +/-.00015 is the total tolerance given to many of the part features I design.

When measuring features with tolerances like this, a controlled temperature room is required as a 10 degree change of temperature will cause the part to expand or contract more than .0001 .