View Full Version : Off-zero caliper

02-01-2008, 09:07 PM
I use my digital caliper (cheapy, about $20.00, IIRC) for mostly quicky measurements, usually use a micrometer for anything I care about. The other day, I noticed that the caliper disagreed with the mic by about .002". Held it up to the light and could see just a bit of daylight between the jaws, but could see nor feel no burrs, dirt, or anything else. Finally decided that one of the tips had to be damaged, so stroked both a couple of times with a diamond knife sharpening hone. Bingo!

Dunno how long they had been like that, but I had checked the caliper when I first got it and it was OK.

Moral: it pays to cross-check measuring tools now and then.

Another observation: I have a friend who is a pretty fair gunsmith, even builds his own design falling-block actions. He almost never uses mikes, mostly a caliper.

02-01-2008, 09:52 PM
Despite your friend, if it has to be more accurate than +/- 0.002", that'd be the time I'd reach for a mic.

Glad you got your clipers tuned up.



Al Messer
02-01-2008, 10:02 PM
That degree of accuracy is well and good, but consider that 100 years ago, you would most likely not find a micrometer in every tool kit, just the "master" one in the office to which the machinists "set" their calipers, and they were not "dial" calipers either.

02-02-2008, 12:43 AM
Mass produced firearms are seldom made to those tolerences, and the one place that tolerance is on the large side, is headspace... which is measured with a gage anyway.

A basic before use quick zero check should be done with any measuring device that contacts the object being measured....

Light punch/press fit pins and screws, and matching holes of course, require tenths accuracy. They are fairly small part of the action though...

I use external mics when called for, and depth mikes the same.
But calipers (vernier here) get a workout due to ability to get into nooks and crannys... I would need 6 or 7 special unipurpose mics for...

Firearms are fairly loose, unlike Turbineblades/aircraft/nuke/etc
Nice toolroom tight actions, jam with the slightest bit of brass shaving, or powder buildup, or.....

When doing a blueprint or hotrod accuracy job on a benchrest gun, all stops are pulled, granite surface plate, tenths mics, etc.. but not wanted or needed for the other 90%

A jammed or locked up toolroom treasure can cost you a match, a deer, an elephant, or in places like up here where I live... Your Life...

02-02-2008, 01:39 AM
Mass produced firearms are seldom made to those tolerences, and the one place that tolerance is on the large side, is headspace... which is measured with a gage anyway.

And military issue firearms are built even looser.

02-02-2008, 09:14 AM
I am with Bguns on this. A slide caliper will repeat a dimension. Most gun work is fitting, and calipers are used to transfer dimensions, not for absolute measurement.

I also know Karl and he is pretty fussy with his work. His actions are simple and very well made. They make up into a very trim and attractive gun.

02-02-2008, 10:06 AM
I'm also with bguns, I always check zero on my digital calipers. That's what threw me for a loop; it always repeated zero perfectly. Not sure why I happened to check actual measurements, was probably measuring something "close" with the caliper, then switched to the mic and found the discrepancy. Although I'm as careful as the next guy with his measuring tools, it's obvious that I dinged the tip of the jaw somewhere along the line. I just wanted to point out the potential problem to others.

Karl builds very nice guns, the ones he's working on lately are Black Powder Cartridge Rifles. BPCR is a specialized game that's shot at ranges up to 500 yards, using metal targets. His guns are quite accurate.

02-02-2008, 05:59 PM
No auto or quick change zero to get out of whack..

A burr will be quite apparent when sliding to ..... close to zero :)

Now where did I put those Bifocals.....

02-02-2008, 07:18 PM
The reason I posted this originally was that it was NOT apparent. When picking up my caliper, I'm in the habit of sliding it to Zero to check, and there was no apparent problem, I only noticed that it didn't agree with the mic when measuring something. After measuring a mic standard and a couple of guage blocks, I realized that the thing was linear, but seemed to be off-zero by .002 or maybe .003. Even holding it up to the light, it wasn't READILY apparent, but I could see just a bit of daylight between the "outside" jaws. A .002" gap is pretty small. One can usually feel a burr with one's fingers, but I couldn't on this, so grabbed that diamond hone and took a couple of swipes on the tips. Voila, it came back .002, so I re-zero'd and made a couple of measurements to find that all was well.

My point is that something like this can happen to anyone and isn't necessarily apparent. It's a good idea to cross-check now and again. I've also noticed that many here seem to depend on a caliper and less on a mic than I do, so it's doubly important for them to check things out.

02-02-2008, 08:44 PM
Agree that a before and after check should be made, if doing more than picking metal stock off the rack. I use my mic standards and am very careful with the tips of the calipers. A quick jaw closed, look for light/gap makes sense.

Just that with a vernier, (no dial to get slightly twisted or zero button) it can't come all the way closed (force is always an option :) ) and go to zero, unless you are springing it and it should not stay at zero if a burr/dirt exists.

It is possible though if movable jaw is set too tight I suppose, but I like to feel what is going on :) and the silky smooth feel of a vernier or digital...

If the vernier zero line does not line up when jaws are closed with no pressure applied, somethings wrong.

My neighbor had (Mechanic grade) Starett Dial calipers, until I showed him the .002 gap in the jaws and the poor feel. I showed him my 23 year old Helios Digital calipers (With Swiss guts). He then bought a used older Starett digital caliper and found out that they would not repeat to zero consistantly and had a lag time in the display...

He asked me what to get for the next try :) and now he owns a decent Mitutoyo digital caliper.

A rack style caliper is a bit too easily damaged for me,(lots of small filings to damage rack teeth when gunsmithing). I have a couple for nice clean work though.

02-02-2008, 09:25 PM
My eyes & bifocals aren't good enough for me to use verniers much anymore. Yeah, there's always a magnifier (which I keep VERY handy!), but it's a strain that I don't need and there's more chance for making a mistake. Like you, I've found that dial calipers can have problems, my ancient Starrett skips every now and then, so I don't use it much.

Of course, like most here, I prefer quality tools. So, when I lost my Mitutoyo digital caliper, I bought a Brown & Sharp. POS!! It is simply unreliable, the display goes ape with no warning, sometimes requiring battery removal to get it going again. That gets tiring pretty fast. Because of the cost, it still occupies space in my toolbox, but should go in the trash.

Once, at the NAMES show, I bought a ChiCom 4" digital caliper. It shortly turned out to be much like the B&S and I pitched it. About the next year at the show, I looked around and found the one I'm now using for about $20.00. That was several years ago, and it's been working fine since. So, even having to buy 2 to get one good caliper, I'm way ahead of the B&S, which I think cost about $120.00 at the time.

I have a Mitutoyo digital micrometer, but it goes through batteries faster than I'd like, so I mostly use the older Lufkin. It has the friction thimble and a silky feel. Plus, unlike a couple of old Starretts that I've picked up over the years, it's very easy to read.