Here is a dial caliper being used as a precision depth gauge. Read this column to learn ev
Q. The picture of a dial caliper being abused by using its points as a scriber caught my eye in the Nov. ’12 issue. Measuring tools will lose their accuracy if used for anything other than measuring (don’t use a micrometer for a C-clamp, for example). Use the caliper to set the blade of a combination square and use that to scribe lines. This will also be more accurate.
You are right that any reader getting a dial caliper will use it constantly. My $20, 6-inch caliper is as accurate as my $125 Brown and Sharpe, and it is not nearly so painful to drop the $20 one on the concrete floor. Digital calipers give an illusion if increased accuracy to four decimal places by rounding off to the nearest 0.0005 inch. The dial caliper will show out-of-roundness on a shaft better than a digital.
Facing the dial, the rod at the right is a depth gauge, and when the dial is at zero a surface on the back of the sliding part is flush with the left end of the caliper. This end is also a depth gauge and used to measure a shoulder or set your combination square. Using this end to guide a scribe to mark the center of a hole is OK, just don’t slide the caliper.
The short arms at the top are used to measure the hole inside diameters, keyway widths. The long arms measure outside diameters, length, and thickness on the broad part of the arms, and cylinder wall thickness with the narrow points. Measuring hole to hole with both pairs of arms and averaging the two dimensions gives the exact center-to-center dimension.
I personally don’t want to refer to my 50 years as a machinist, but Federal law requires that I do so, or lose my Social Security and apprentice geezer license.
A. Well, Bruce, you are absolutely right that precision calipers are not designed to be used as scribers, and that doing so will diminish the precision they were designed to achieve. Nevertheless, most of my work is with sheetmetal, and if I can achieve tolerances within plus or minus 5 thousandths of an inch, in most cases I have done an exceptional job. As you well know, this is a pretty “sloppy” tolerance in the world of precision machine work, but in my work, it serves me quite well indeed. In fact, even though this may not be recommended, my “standard procedure” for marking a piece of sheetmetal for trimming in a shear is to scribe a line with my trusty caliper.
I have had my 6-inch dial caliper for at least 25 years, and I have probably scribed several hundred yards of material with it. It’s likely that I have dulled the points one or two thousandths of an inch in doing so, but they still provide me with more than sufficient accuracy for the work that I do. I’m willing to accept the slight wear to the tool for the speed and efficiency that it provides me. I, like you, have inadvertently dropped my caliper, and although this has caused some damage too, it still meets my needs very well.
I know you and your work, and I completely understand why you take exception to this abuse of a precision instrument. I’m very thankful that you have taken the time to inform our readers that this is not how the tool is designed to be used, plus providing a good alternative, and explaining many other features of this versatile tool.
As a note to our readers, I met Bruce about 35 years ago on my first trip to the East Coast, and saw his work firsthand. In addition to being a fine machinist, he has made some of the most stunning brass radiators, gas tanks, headlights, and other accessories for street rods that I have ever seen. I’m honored to have a craftsman of his caliber contribute to this column.
You can email your questions to Professor Hammer at firstname.lastname@example.org, or mail to: Professor Hammer c/o STREET RODDER, 1733 Alton Pkwy., #100, Irvine, CA 92606; you’ll receive a personal reply! Ron Covell has made many DVDs on metalworking, and he offers an ongoing series of workshops across the nation. Check them out online at www.covell.biz, or call for a current schedule of workshops and a free catalog of DVDs. Phone: (800) 747-4631 or (831) 768-0705. You can send a request by mail to: Covell Creative Metalworking, 106 Airport Blvd. Ste.105, Freedom, CA 95019.