This article was published in the June 2011 (#31) edition of Woodturning Design.
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When I first saw an ad for the Galbert Caliper (see WTD #28 for a review), a caliper that could substituted for a gaggle of traditional calipers and gives a reading of the diameter as you go, I went straight to my computer and ordered one. But it didn’t really work out for me. It was my own fault, really. First I didn’t read closely enough to notice that you weren’t supposed to measure under ¾” with the lathe running—and I tend to do small work. Secondly it tended to foul on my non-standard home-made tool rest. The Galbert Caliper seemed way too complicated to duplicate on a smaller scale at home but too good of an idea to give up on.
Ordinary calipers measure a circle with two points—but have to be on a diametric line to do so. Adjustable springy calipers tend to give you different diameters depending on how hard you push (at least for those of us with limited patience) and “hard” calipers, whether adjustable or not, tend to really compress the wood where you measure.
After a bit I remembered a bit of High School Geometry to get over the diameter problem. While one has to be on a diametric line to define a circle with two points, ANY three points that are not on a straight line define a circle. Thus I should be able to make a Direct Reading Caliper with a simple pivoting mechanism (like a pair of scissors) with a pointer and scale on one end, and a rounded tip and a rounded V-notch at the other end. This article describes how to make the Direct Reading Caliper I came up with—in the end. I’m not going to tell you how many tries it took me. First of all I didn’t count, and secondly I’d be embarrassed.
The Direct Reading Caliper basically consists of two pivoting parts cut out of brass sheet. The Pointer has a pointer on one end and a V-notch at the other. The ends are 90 degrees apart (rather than 180 degrees as in scissors) to put the pointer where you can see it. The Feeler has a rounded feeler and a curved scale. A fine threaded machine screw and two nuts provide the pivot. A bent piece of music wire serves as a spring. A plywood arc is glued onto the second part to give you a place to make the scale. The rounded feeler hits progressively further from a diametric line as the circle measured gets larger (see Drawing1) so the scale is not completely proportional. Thus the scale is calibrated after assembly by measuring known diameters (drill bits).
I’m pretty happy with the design. It’s at least as accurate as even hard calipers (I used it to measure the starting diameter of a run of tapered tenons which is fairly demanding) and I don’t tend to bruise the wood underneath. As the pointer and scale are twice as far from the pivot as the V-notch and feeler, the scale is expanded and easy to read.
It is possible to extend the range of diameters the caliper will measure. There are, however, unavoidable compromises in doing so. There’s a limit to how far you can open your hand so if the caliper is designed to measure larger diameters the scale can’t be expanded without getting awkward. Furthermore, larger diameters require a larger V-Notch to be accurate, which will get in the way of cutting small diameters. None the less I’ll try to get a design posted to my web site for larger diameters (didn't happen--sorry).
I cut out the first few iterations on a scroll saw using fine toothed metal cutting blades. They have to be fine toothed because control depends on at least two teeth being engaged at all times. It tended to take a long time because of the fine blades, be uncomfortable because of high feed pressure required pushing on the thin metal edge, and lead to breaking three or four blades each time. Blue tape to the rescue (hope you’re not getting tired of this trick). By temporarily fastening the brass to ¼” plywood I could use a much larger #12 blade which cut faster, required less feed pressure, and gave a more comfortable edge to push on.
Begin by printing out or copying the pattern, Drawing2. Cut out a 4-1/2” x 5-1/4” rectangle of ¼” plywood. Trace the plywood rectangle on a corner of the brass sheet as in Fig01. Apply blue (or other extended release) masking tape to the marked area of the brass sheet and one side of the plywood as in Fig02. If you lightly sand the blue tape the glue will bond to it more securely.
Drawing2 as a pdf.
This will print more accurately--be sure "none" is selected for page scaling.
Drawing2 as a pdf. This will print more accurately--be sure "none" is selected for page scaling.
Spread a thin coat of ordinary wood glue (use wood glue because you need no lumps and complete coverage) over both taped surfaces as in Fig03. Clamp together with several clamps or a heavy weight as in Fig04 and leave overnight for the glue to cure.
After the glue cures remove the clamps. Apply some spray adhesive to the back of the pattern as in Fig05. Give the spray adhesive a few minutes to dry (follow the directions on the can) and then fasten the pattern to the brass sheet as in Fig06. Be sure to pick the same corner and orientation as your plywood.
You’ll probably find the pattern easier to cut out on a scroll saw if you first cut out on the rectangular lines on the pattern. I used a hand held scroll saw with a 17-24 tpi metal cutting blade as in Fig07. Turn the orbital motion on if your saw has this feature. It’s easier if you clamp the brass sheet to something substantial. I used a saw horse with a ripping slot but a workbench will do.
Take the cut out rectangle to your drill press. Be sure to clamp the rectangle down for drilling operations as if the drill grabs spinning sheet metal would be awfully hard on your hands. First drill access holes for the spring tabs. Measure the width of the scroll saw blade you’ll use (about a #51 drill for a #12 blade) and pick that diameter drill bit. Clamp the rectangle down the drill at one end of a spring tab as in Fig08. Repeat for the other tab.
Measure the fine thread machine screw you will use for the pivot and select the smallest drill bit the screw will fit through because if the fit is sloppy the caliper will be less accurate. Make a test hole outside the pattern and test the fit as in Fig09 to be sure. Then drill at both pivot locations as in Fig10. It’s only necessary to drill through the brass.
Take the rectangle over to your scroll saw and thread the blade through one a hole for a spring tab as in Fig11. Turn the saw on (select a moderate speed) and cut on the line for the tab as in Fig12 and then unthread the blade from the cut. Repeat for the other spring tab.
Cut out both pieces as in Fig13 starting at an edge. Don’t try to make sharp inside corners, but cut them separately as in Fig14. The parts after cutting out are shown in Fig15. After cutting out the pieces, split the brass away from the plywood at the tape line. A safe way to get this started is to hold the part at the bottom in a vise and split with a putty knife as in Fig16. This way all fingers are above the blade if something slips.
If you don’t have a scroll saw you could consider bribing someone who does with a few packs of blades. If that doesn’t work you could cut all but the tight curves with a hand held jig saw and then finish up with a hand fret saw or coping saw and files. It will just take longer.
Peel any remaining tape off the backs of the parts. Then apply a solvent such as mineral spirits to the pattern with a paper towel as in Fig17. Let it soak in for a few minutes and then the pattern will pull off easily. Remove any remaining adhesive with a quick swipe of the paper towel.
Trace the scale area onto a piece of ½” plywood as in Fig18, then cut out the scale.
If you orient the pieces as in the pattern all tabs will be bent towards you. Clamp the feeler piece in a vice just below the stop tab as in Fig19. Bend the tab forward at a right angle. Reclamp the feeler piece just below the spring tab. Bend the tab about 30° away from about you as in Fig20 using a hammer and punch. Now clamp the pointer piece in the vise just below the spring tab as in Fig21 and bend the tab away from your about 30°. The tabs are bent away because the pieces are clamped reversed in the Figures.
Sand the scale area of the scale piece to promote good glue adhesion. Also sand the mating area of the cut-out plywood scale. Fig22 shows the set-up for gluing on the plywood scale. Spread some CA glue on the scale and quickly align it with the scale area of the scale piece. Give the glue a few minutes to set and then sand the scale flush with the scale area of the scale piece as in Fig23.
Temporarily assemble the pieces with the machine screw and a hand tightened nut. Use a fine line marker to mark where the pointer crosses the top of the scale as in Fig24. Rotate the pointer through its entire range making sure the mark is visible the whole time. Now check how the feeler lines up with the V-notch and mark any modifications required to get it to line up with the center of the notch as in Fig25.
Disassemble the pieces. Bend the pointer just above the mark as in Fig26. Use a file or sander to reshape the feeler if necessary.
Assemble the caliper by inserting the machine screw through the pivot holes, and then tightening a nut on the screw until the pieces still move easily but not sloppily. Add another nut and tighten it against the first nut as in Fig27, to lock the nuts in place. Bend the feeler and V-Notch towards each other so that they line up with each other.
Cut a 4-1/4” piece of spring wire (I used 0.055” stainless) and pre-bend it in the middle as in Fig28. Grind away the sharp peaks left from cutting the ends of the wire so it will be less likely to cut you if it springs out. Put the spring in place under the tabs and under the machine screw as in Fig29. Trim the wire if the ends protrude into the finger-holds. Test the resistance of the spring—you want it to stay open when unattended but close with gentle finger pressure. If the feel isn’t to your liking you can pre-bend the spring more or less, or switch to a different size wire.
To calibrate the scale you need cylinders with known diameters. I used drill bits (although you could use a traditional caliper to turn a stepped cylinder of wood instead), and held them temporarily in a vise because both hands were busy. Fig30 shows marking the scale using a 1” drill bit. Apply the caliper beyond the flutes. Center the V-notch on the cylinder by feel and then close the caliper until the feeler contacts the cylinder. Then make a mark at the pointer. Fig31 shows the same process with a ¼” bit. If you calibrate the scale with known diameters every 1/8” you can safely interpolate for sixteenth inch and finer measurements. Fig32 shows the completed scale.
I don’t ever remember having a caliper catch in more than 30 years of turning, but that doesn’t mean it couldn’t happen. As the Direct Reading Caliper is made of thin sheet metal, it would be seriously painful if it did catch. So use it prudently. Always establish a groove with the parting tool before engaging the caliper. Never try to use the caliper when the area being measured still has flat spots. And just because the caliper is thinner than a 1/16” parting tool doesn’t mean you should use one. That doesn’t leave enough clearance, especially for deep cuts.
To use the Direct Reading Caliper, first establish a groove in the work. Gently squeeze the caliper to keep the feeler in contact. Stop cutting when you reach the desired diameter. It seems to be more accurate if you part down close by eye to the desired diameter and then engage the caliper by centering the V-notch by feel. Staring at the scale during a long cut seems to lead to the parting tool tilting or some such. You can tilt the scale towards you to make it easier to read and to keep the caliper from fouling the parting tool. Fig33 shows using the caliper. Fig34 shows using the caliper to cut down to a smaller diameter (3/16”) and the result. You’ll notice, compared to Fig33, that the scale is tilted forward more.
If you need measurements that are not on the scale, or are hard to find on the scale, such as 19/64”, then you can make a special scale by applying masking tape over the scale and making a mark on the tape. Fig35 shows marking the diameter of a crochet hook on masking tape applied over the scale. Much as I like blue tape, pencil is easier to see on the cheaper stuff.
Tools and Materials
0.050” Brass sheet
Blue extended release masking tape
10-32 x ½” machine screw and two matching nuts
Drill press and various drill bits
0.055 music wire
David Reed Smith is a basement turner and tinker who lives in Hampstead, Maryland. He welcomes comments and questions via email at David@DavidReedSmith.com. This article and more than 50 others will be available on his web site: www.DavidReedSmith.com