Continued from Previous Page
A Shop-made Thickness Caliper
by Peter O'Connor
from the ideal will result in the end of the gauge arm not landing where you had hoped. For this reason, take your time at this stage; check, double-check and then check again that the drill bit will be passing through the aluminum arm in the way you need it to. You will not regret spending more time than you think you need to at this point. The photograph shows that the caliper body is secured with three clamps and that the position in one plane is determined by the stop block of the fence. What cannot be seen is that following my own careful observations, I placed a 1/32" block from a set-up block between the caliper body and the lower part of the fence to get the right drilling angle. Again, it cannot be stressed enough - take your time at this point and make sure you'll be happy even after you have drilled this hole.
Once the first hole is drilled, test-fit the gauge. The instrument should fit into the hole without too much effort but should not be too loose. The next step is to drill the hole for the set screw that will secure the gauge. The fence on my shop-built drill-press table is not very high, so I made things more secure with the aid of a couple of scrap pieces of plywood, as can be seen in the image on the previous page. This hole isn't as critical as the first hole you've drilled, but again take your time and make sure you are happy with your set up. The set screw I intended to use had a 1/4" diameter and in wood I have in the past used a 7/32" bit, but here this would not work as I was unable to successfully drive the set screw into a test scrap of aluminum. The next size up in my drill bit set was 1/4", obviously too large, so following a quick trip to Lee Valley, and for less than $4, I was the proud owner of a very nice HSS 15/64" bit that was perfectly suited to the task at hand.
I decided to use an acorn nut for the gauge arm to rest on. This means that the material being measured - the soundboard or back of an acoustic guitar, or similar instrument - will be held between two round, reasonably softwood-friendly shapes. I did consider using a plastic cap that is designed to fit over the head of a bolt, but ultimately I thought an acorn bolt would be fine, and I didn't want the measurements reported by the caliper to be possibly compromised by the compression of soft plastic. Certainly there are other alternatives, and you might have a better idea, but whatever solution you end up with, be aware that this hole is also a critical one. You'll be drilling the hole from above, but the really important consideration is not where the hole begins but where it exits (as this will determine how successfully the gauge arm and bolt - or whatever - mate). And so for this reason, once again check and double-check your set up until you are sure you are happy with it.
What's left to be done? At this point the tool is functional. But most of these types of caliper have two further desirable features: a pivoting arm engaged by the thumb of the hand that holds the instrument, enabling one-handed use and freeing the other hand to hold the item being measured, and some type of stand or "feet", allowing the tool to be securely placed on edge. I expect that both of these features will be relatively easy to fashion from the thin aluminum. The final thing I intend to do is to sand and polish the tool to a mirror finish. In the next issue, I'll include a photo of the completed tool. I'm satisfied with the progress I have made so far, and if you wish to try making your own I wish you every success.
SOLG Newsletter - Issue 2 - September 2020
Page 8