This article was published in the March 2018 edition of More Woodturning

STO-Gauge as a 3 page pdf

STO-Gauge

Introduction

This is admittedly a specialized project.  But if you often turn face grain mounted work from square stock (such as salad bowls, or in my case, drop spindle whorls) it can save you time.  For face grain work less than 8” in diameter it’s sufficient to bandsaw off the corners rather than saw the stock to a disc.  For small discs it’s often easier to saw an octagon than to cut a circle,  particularly if you routinely keep a monster blade on your bandsaw.  Guessing at how much corner to remove can result in extra turning work or undersize discs.  The STO-Gauge (Square To Octagon) lets you quickly mark the corners for removal and also mark the center of the square.  You only have to set the slide on the gauge to the size of the square on a scale.

The STO-Gauge is simple to make.  Just print out the pattern and glue it to thin sheet goods stock such as hardboard.  Then cut out the pieces following the pattern, glue a few pieces together and drill and tap one hole for a thumbscrew.

Make the Pieces

Download the pdf Pattern or the Metric Pattern.  The metric pattern has a metric scale, but the pieces are still measured in inches.  Print out the pattern, making sure “actual size” is selected in the printer dialog box.  Cut out the pattern pieces leaving a 1/4” or so margin outside the lines.  Place the pieces upside down on a piece of newspaper or other disposable surface.  Spray the pattern pieces with a spray adhesive such as 3M #77 as in Figure #1.

Figure #1:  Spray the pattern with spray adhesive.

Follow the directions on the can if adhesive for curing time and attach the pattern pieces to thin sheet goods such as 3/16” hardboard (Masonite®) as in Figure #2.  I’ve also used 1/4” hardboard for some of the earlier versions, and 1/8” hardboard would probably be fine.  To prevent the gauge from soiling during use cover the pattern with wide clear packing tape as in Figure #3.

Figure #2:  Attach the pattern to hardboard.

Figure #3:  Cover the pattern with packing tape.

Next cut out the pieces as in Figure #4.  There are several options here.  The photos follow cutting out the pieces with a bandsaw with a 1/4” blade, except for the miter cuts which are done on the table saw.  As the gauge is used to layout cuts for removing the corners prior to roughing to round rather than planning space shuttle parts, you could use a bandsaw for the miter cuts, perhaps followed up with a flat file.  If you have a scroll saw you could use it instead of the bandsaw particularly on the interior cuts and dispense with the slide bridge.  If you cut the interior cuts with a bandsaw make an entrance cut on the line at the bottom.  You can follow the curves with a 1/4” blade with a little backtracking.  Cut on the inside of the lines.  If you don’t have a narrow blade, you can just cut an arc—the S curve is only there so that you know to trace the straight edge.

Figure #4:  Cut out the pieces.

Set the blade on your table saw to 45°.  Then set the height of the upper edge of the blade to 2” above the table as in Figure #5.

Figure #5:  Raise the blade to 2”.

Attach a sturdy miter gauge extension to your miter gauge.  The one I’m using is 4” high and about 1” thick made from two pieces of 1/2” plywood glued together.  Shift and remount the miter gauge extension if necessary so that you’ll be able to measure from a fresh kerf.  Saw a kerf through the miter gauge extension.  Remove the miter gauge from the saw and draw a perpendicular line 1-1/2” from the obtuse side of the kerf at the bottom.  Then replace the miter gauge in your saw slot as in Figure #6.

Figure #6:  Saw a kerf though a miter gauge extension and measure 1-1/2” from the kerf.

Clamp a right angled scrap piece to serve as a stop to the miter gauge extension at the mark as in Figure #7.

Figure #7:  Clamp a stop to the extension 1-1/2” from the obtuse side of the kerf.

Hold the base piece against the miter gauge extension against the stop with the pattern side facing the blade and make a miter cut as in Figure #8.  Then flip the base around so the pattern faces the miter gauge extension and make a second miter cut as in Figure #9.  The cuts must at least be high enough to make a clean V point.  If a nub is left, which will prevent the base from registering properly on a corner, raise the blade slightly and redo both cuts.  If your cuts are slightly too high it is of no consequence for using the gauge, but you’ll have to determine the zero position (done later) with the gauge on a square corner.

Figure #8:  Make the first miter cut.

Figure #9:  Make the second miter cut.

Assemble the Gauge

Use a square and a utility knife to cut through the paper pattern of the Base on the red lines 1” from each end.  Peel off the tape and pattern at each end as in Figure #10.  Scrape off any remaining paper or glue residue.  The pattern is removed on the ends so the guides can be glued on.

Figure #10:  Cut and peel the pattern ends off the Base.

Apply glue to the bottom of the Left Guide.  As the bottom of hardboard is textured you’ll need to use more glue than usual.  Clamp the Left Guide in place.  Put the Slide in place against the Left Guide and check that vertical straight cut aligns with the top of the V notch as in Figure #11 and that the horizontal straight cut aligns with the top edge of the Base.  Remove the slide and let the cure for several hours.

Figure #11:  Clamp and align the Left Guide on the base.

Similarly, cut on the pattern of the Slide on the red line near the bottom (you can omit this step if you used a scroll saw for the interior cuts).  Peel the pattern off the tip of the Slide, then glue and clamp the Slide Bridge in place as in Figure #12.

Figure #12:  Glue and Clamp the Slide Bridge over the entrance cut of the Slide.

After the glue has cured, remove the clamps from the Base and Slide.  Scrape any glue squeeze out away.  Wrap waxed paper under the base of the Slide (this will prevent excess glue from gluing it to the Slide, and shim the Right Guide out far enough to let the Slide slide).  Put the Slide against the Left Guide and apply glue to the Right Guide.  Push the Right Guide against the Slide while clamping it in place as in Figure #13.  Allow a generous amount of time for the glue to cure, then remove the clamps and scrape away any glue squeeze out.

Figure #13:  Glue and clamp the Right Guide to the Base.

Mount a #7 drill in your drill press and drill a tap hole through the Right Guide/Base at the cross mark as in Figure #14.  Then mount a 3/4” Forstner bit in the drill press and drill shallow (approximately 1/16”) hole on the same center as the tap hole as in Figure #15.  Then tap the #7 drill bit hole for 1/4x20 threads as in Figure #16.  The thumbscrew will be easier to turn if you use an oversize tap.

Figure #14:  Drill a tap hole for a thumbscrew.

Figure #15:  Drill a slight recess for a washer.

Figure #16:  Tap the hole for 1/4x20 threads.

Find a 1/4” washer and a shouldered 1/4x20 thumbscrew that’s as long as the thickness of the Guide plus Base.  I had a bunch of nylon thumbscrews that were the right thread but 1-1/2” long.  I threaded the thumbscrew through the washer and tapped hole and cut off the excess length with a small hacksaw as in Figure #17.  If you have a steel thumbscrew that’s too long, you could thread it all the way through the washer and taped hole and mark it with a marker.  Then remove the thumbscrew from the Base, screw a 1/4” nut all the way on, and use a hacksaw or cut off disc on an angle grinder to cut the excess off at the mark.  Grind a slight bevel on the end to clean up the cut, then remove the nut.

Figure #17:  Cut the thumbscrew to length if necessary.

Calibrate the STO-Gauge

Insert the Slide between the Guides on the Base.  Align the horizontal straight line of the Slide with the peak formed in the Base by the miter cuts.  If your miter cuts were a bit long, slide a wood square into the V formed by the miter cuts and align the horizontal line of the Slide to the peak of the corner of the wood square. Clamp the slide in place with the thumbscrew.  Make a nick with a knife on the edge of the Left Guide at the zero mark of the scale on the Slide as in Figure #18.

Figure #18:  Make a knife nick at the zero position.

Remove the Slide.  Select the smallest triangular file you have. Hold the file with one V-edge down and slide it along the guide until you feel the nick.  Enlarge the nick to a small visible notch as in Figure #19.

Figure #19:  File a notch at the zero mark.

Remount the Slide.  Find or create a sample wood square.  Measure the side of the wood square and set the Slide to that length using the scale.  Hold the STO-Gauge firmly on each corner of the square in turn and use a pencil to trace the vertical and horizontal lines as in Figure #20.  Then remove the Gauge and check the results as in Figure #21.  You can see that the center lines don’t meet at a point, as either the Gauge alignment is off or the square isn’t quite square or both.  It’s a good habit to make four centering lines and visually select the center of the box they form.  You can further check your results by drawing a circle around the center point with a compass.

Figure #20:  Mark a sample square.

Figure #21:  Check the results.

You can keep the gauge handy by mounting a magnet or two on the back of the gauge so it will hang on your bandsaw as in the Main photo.