Coaster Addendum

Error Magnification

Since I’m asking you to go to the trouble of making recursive jigs to size the feet of the coaster, I thought I should try to explain why.  The edge of the coaster square is fairly close to being tangential to the arc that defines the inner surface of the feet.  Thus a small change in the diameter of the arc leads to a much bigger error in the separation of the feet.  Have a look at ErrorMag.pdf.  This drawing shows that a change of 1/50” (about the width of a pencil mark) in the radius changes the separation of the feet about 1/10”, which is five times as much.

Roughing Corian

The trouble I have with turning Corian is patience, particularly when I have a lot to remove, such as hollowing the cork recess.  I get disappointed with the slow progress of the cut and next thing you know the tool grabs and rips out chunks.  I tried different tools.  What worked best for me was my round tip shear scraper held at a really steep angle such as in Figure #1.  I made the shear scraper myself from HSS drill blanks (see http://davidreedsmith.com/Articles/ShearScraper/ShearScraper.htm).  Most any round nose scraper would probably work fine, although I do suggest that it be sharpened using the Veritas Scraper Burnisher.  A rounded tip limits the amount of tool engaged in the cut and allows you to smoothly traverse the tool.  The steep downward angle ensures that if the tool does grab that the cutting edge will be thrust away from the Corian, not into it.  With a freshly sharpened and burnished edge it removes Corian fairly quickly but in a controlled manner, as shown in Figure #2.

Fig01

Figure #1:  Use a steep downward angle.

Fig02

Figure #2:  It removes material fairly quickly.

Depth Gauge

If you want a glass to sit securely on a coaster, then it helps to have the surface be flat.  Although sighting over a straight edge placed across the recess is likely accurate enough, you can make a simple jig to check for high spots.  The jig is just a 1/2” square piece of wood that is more than twice the width of the recess long.  Drill a hole with a #7 drill bit in the middle, and tap it for 1/4-20 threads.  Then insert a 1/4” bolt and adjust the protrusion of the bolt to the depth of the recess.  You can add a 1/4” nut to lock the bolt in position.  I used nylon all-thread so I could use the gauge with the lathe on, but I usually don’t.  I added another 1/4” nut and wingnut at the end so I could easily turn it by hand.  You can get nylon thumbscrews, but mostly in minimum quantities of 50 or 100.  Figure #3 shows the completed depth gauge.  To use, just slide it across the recess holding one arm of the gauge against the rim of the recess.  If the other arm moves away from the rim then the position of the nut indicates a high spot.

Fig03

Figure #3:  A simple depth gauge.

Permanent Clamping Substrate

If you get tired of putting fresh waxed paper over sheet goods for every waste wood glue up you can make a more permanent substrate.  I had some polypropylene flexible cutting mats (<$5 for three 12x15 mats) that I forget what I got for.  The polypropylene releases cured wood glue easily.  However, you can use 3M #77 spray adhesive for a durable bond.  Spray both the polypropylene and the surface of a suitable piece of flat sheet goods with spray adhesive.  Let the adhesive set up, following the instructions on the can, and then place them together.  You can use a folded over piece of waxed paper to allow you to position the plastic accurately in position.  Then trim the edges on a table saw if you want a neater edge.  Figure #4 shows the completed substrate.  Also shown in Figure #4 are two hold-downs to clamp multiple pieces to the substrate with one clamp.  One has a layer of 2mm craft foam to allow for minor differences in thickness.  The other uses cork for the same reason.  Cork resists permanent dents longer.  3M #77 was used to bond the cushion and the polyethylene.  It is possible, but perhaps not an excellent idea, to hold down the coaster square and all four waste pieces at once in pinwheel fashion with one clamp and an X-shaped hold-down.

Fig04

Figure #4:  A permanent gluing substrate.

Fig05

Figure #5:  Using the substrate and a hold-down.