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Magnetic Zero Clearance Plate
Does your bandsaw throat look as the one on the right side of Figure 01? (Itís even worse than it looks as itís not flat anymore either). Or perhaps you have an alleged zero clearance plate like the one on the scroll saw on the left side of Figure 01 thatís worn well past having any effectiveness? The problem is like, in a way, that of working with dull lathe tools. First you have to know how to make them sharp, and secondly sharpening them has to be easy enough to easily let you overcome the mental inertia of continuing to work with dull tools. Zero Clearance Plates quickly wear outóitís just their nature. So whatís needed is a Zero Clearance Plate that mounts securely enough to stay in place while being used, but can easily be removed when not needed and moved when worn. Magnets seemed to present a way of doing this.
Figure 01: Does your scroll saw or band saw throat look like this?
Why should you bother to make one, assuming yours arenít as bad as mine? On a scroll saw, just like on a table saw, a zero clearance throat cuts down on tear-out as the blade exits the wood. It will do the same on a bandsaw, but as bandsaws generally have a rougher cut it wonít be as noticeable. When cutting small parts, or cutting small amounts off larger ones, the offcut can trapped with a bang between the throat and blade thus ruining the part and perhaps your day.
But while magnets are great at resisting tension forces (being pulled away from a steel or iron surface), theyíre nowhere near as good at resisting shear forces (being pulled along the steel surface). This presents a problem for a Magnet Zero Clearance Plate because the plate wouldnít work well if it slides around while youíre trying to use it. Luckily for this article, this is easily fixed by inserting something with a high coefficient of friction between the magnet and the table. While moving the magnet slightly away from the table lessens the downward force, the resistance to shear is the product of the downward force times the coefficient of friction so we can easily come out ahead by picking something with a high coefficient. Lee Valley, for instance sells friction discs for use with magnets (http://www.leevalley.com/US/wood/page.aspx?p=32066&cat=1,42363,42348&ap=1 ). Gorilla Tape, as shown in Figure 02, also works and is cheaper, assuming you already have some. Generic duct tape would probably work but I havenít tried it. If you have a hole-punch you can make the tape over the magnets look neater. But the tape IS out of sight on the bottom. I prefer to use 2mm craft foam. Itís inexpensive, gives the whole bottom surface a high friction surface, and, well, I have a thing for craft foam. Sort of like I do for blue tape (only not as extreme?).
Figure 02: Using Gorilla duct tape over magnets to increase the friction between plate and table.
Figure 03: Using 2mm craft foam to increase the friction between plate and table.
Briefly, a piece of Masonite is cut to size and the bottom covered with craft foam. Holes for the magnets are drilled through the foam and the slightly recessed magnets are glued in place. The plate is flipped upside down to cut a kerf with the blade to be used. When worn the plate is along the kerf or a new kerf is made. Last a couple of modified plates for special uses are shown.
Making the Magnetic Zero Clearance Plate
Masonite has a nice smooth surface on one side which suits making a zero clearance throat, but any of the various sheet goods will probably work. 1/4Ē thickness allows room to drill to mount 1/2Ē diameter by 1/8Ē thick magnets without compromising depth of cut much. Cut a square of suitable size for the work youíll be doing as in Figure 04. Iím using 8-1/2Ē, picked partly because I had a strip of Masonite 8-1/2Ē wide left over from another project, but itís worked out wellómost things you need a zero clearance plate for arenít huge.
Figure 04: Cut Masonite to size.
You can get a pack of 2mm craft foam from most anywhere that sells craft supplies. I got mine from Wal-Mart. Ordinarily I donít prefer the self-stick version, but in this case the self-stick works well. If you already have the non-self-stick foam and spray adhesive it will work fine. Cut a square of the 2mm craft foam to the same size as the Masonite as in Figure 05. Fold back one edge of the release paper and align the still coated edge with your Masonite square. Press the exposed self-stick edge down on the Masonite as in Figure 06, and then peel off the remaining release paper and press the entire foam piece onto the Masonite.
Figure 05: Cut a piece of 2mm Craft Foam to the same size as the Masonite.
Figure 06: Align and stick down the Craft Foam to the Masonite.
Now move over to the drill press and chuck up a 1/2Ē Forstner bit (or whatever matches your magnet diameter). What to do if you donít have a drill press? As an unrepentant tool junky my first suggestion is to buy them. But if that doesnít work for you, then drill through holes of the proper diameter near each corner. Turn a dowel to the same diameter, then glue the magnet on the end of the dowel with CA glue and then glue the dowel into the hole. Repeat for the other corners.
Set the depth stop of the drill press so that the Forstner bit is just barely below the surface of the foam as in Figure 07. If you use a wooden auxiliary table as I do, tape the magnets to the top surface of the Masonite at each corner as in Figure 08. If you drill with the piece directly on the metal drill press table, place the magnets on the table to support the entire Masonite square.
Figure 07: Set the drilling depth to just below the surface of the foam.
Figure 08: Tape the magnets to the top surface of the Masonite.
Now drill a hole at one of the corners as in Figure 09. Theoretically, setting the drill stop to the surface and then raising the surface the distance of the desired hole depth will give perfect results. However, when you are trying to actually do something, as opposed to harmonize with the spheres of the universe, empirical evidence trumps theory, so check the depth with a magnet and adjust if necessary. Then drill a hole at each remaining corner. Glue the 1/2Ē x 1/8Ē disc rare earth magnets into each hole with CA glue as in Figure 10.
Figure 09: Drill a hole at one corner and check the depth.
Figure 10: Glue in the magnets.
Using the Magnetic Zero Clearance Plate
After giving the glue a chance to set, take the plate over to the saw youíll use it on (thereís no reason you canít use it on more than oneójust cut a separate kerf for each) and turn it up side down. Then cut a kerf from one edge to about a third of the way across as in Figure 11. Turn off the saw (moving the plate with the saw off will postpone kerf wear), flip the plate back to right side up slide the blade into the kerf. It seems easiest to do this with the plate held up off the table. To maintain the kerf, besides moving the blade in the kerf while the saw is off you should also remove the plate when doing heavy work where itís not needed. As itís magnetic you can just slap it up on the top wheel cover or the like to store it out of the way.
Figure 11: Cut a kerf with the plate upside down.
Inevitably the kerf will wear as shown in Figure 12. When it does, just slide the plate to a fresh spot in the kerf as in Figure 13. To further minimize wear, you can go back to a worn spot for non-critical work.
Figure 12: A worn kerf.
Figure 13: After moving to a fresh spot in the kerf.
You can also use the magnetic method for other modifications of the table. Both of these ideas probably have more utility for scroll saws. Iíve illustrated them with a bandsaw as more turners have a bandsaw than a scroll saw.
For instance if you are cutting irregular objects you can make a convex pad as in Figure 14. Warning: cutting irregular objects on the bandsaw is inherently dangerous. A convex pad makes it less so, but proceed with caution. To make this pad I drew a dome on two adjacent sides of a square cut from a 2x4 then cut it on the bandsaw using the old trick of not quite finishing the first cut. I put blue masking tape (original, not sharp edge) on both mating surfaces, and as I was in a hurry, put dots of CA glue on the taped surface of the convex pad, then pressed it into place on the magnetic pad. Figure 15 shows the convex pad in use.
Figure 14: A convex pad for cutting irregular objects.
Figure 15: Using the convex pad.
Another extension would be to make an angled pad for angled cuts you use a lot, to save setting and resetting the table. This will only work for small objects that will fit on the angled pad. To make the angled pad I cut a chunk of 2x4 at 15 degrees (angle selected arbitrarily). I put the bandsawn surface down to use the smoother surface as the platform. I attached it the same way as the convex pad only I used blue masking tape for rough surfaces as regular blue tape doesnít stick well to a bandsawn surface.
Figure 16: An angled platform.