I made this gauge almost 30 years ago from scraps and fasteners on hand, and little did I realize just how indispensable it would become for marking out projects. It closes down to a minimum of 3/16" (4.76mm) and opens to 8" (203.2mm) and allows for very swift layout of tenons, panel fields, rabbets (rebates), boundaries, etc. This gauge is very useful around the shop, and is also a good utilization of pencil stubs. A measured drawing is included, but feel free to improvise as needed.
Step 1: The Drawing:
My apologies to my metrified friends, but you will need to convert the drawing's measures to your own.
Step 2: The Beam:
I used a #10-32 (M5 x .08) knurled brass thumb screw for the jam screw, which simply presses against the face of the pencil. It is run into a threaded insert, so be sure to match thread type among components. Also, be sure to clamp the beams’ end when drilling for and installing the threaded insert. This will minimize the stock splitting out.
Most pencils I have used seem to work best in a 19/64” (7.54mm) diameter hole, but a 5/16” (7.94mm) size should suffice as well.
A left to right reading adhesive back rule is attached to the side of the beam. A right to left version installed on the opposite side would work well for a “leftie” model. As an alternative, a section of broken tape rule can likewise be used instead, to good economical advantage. Or, no rule at all may be used, but the utility of the gauge will be diminished.
Align the 1” (25.4mm) mark of the scale to the vertical center line of the pencil hole, clamp the tape in position, peel back a few inches of the protective backing strip and lay the rule back down. Remove the clamp and finish the job.
Step 3: The Fence:
In the through section view of the fence, observe that the ¼-20 (M6 x 1) thumbscrew is bearing against a plastic slug, which in turn presses against the beam. This prevents galling and destruction of the beams’ surface. The slug is just a #6 plastic drywall anchor, trimmed to fit, and installed up through the beams’ mortise. Most anything plastic may serve as a soft nose device.
The groove ploughed into the face allows the pencil to nest in a recess, giving a minimum 3/16” (4.76 mm) dimension mark. The maximum dimension is 8” (203.2 mm). The scale is read at the point where it enters into the fence mortise.
The rabbet (rebate) rides along the work surface and provides a positive contact point. If your scale does not agree with the resultant marked distance, add or remove stock here to calibrate. A brass wear strip would be a handsome appointment, but is not a necessity.
The mortise for the beam should reflect the beam width plus the rules’, and was bored and chiseled square after boring the thumbscrew insert hole. Strive to hold horizontal movement of the beam to a minimum, as too wide a swing will affect scribed accuracy.
An alternate construction method is to dado two halves of fence stock on the table saw or router table to form the mortise and then glue them together at the centerline of the thumbscrew.
A 1/4” (6.35mm) stop dowel is glued into the beam end, all handhold surfaces are smoothed, and a finish of preference is applied.
Step 4: Parting Thoughts:
It really is a joy to use a simple, handcrafted device and get such precision results, especially when determining center of stock. Just make an eyeball guess where, mark it, position tool, set the beam to it and scribe from both sides. Adjust beam as necessary for a "null" mark and done, plus the dimension can be read off the beam's rule.