|Project by thetinman||posted 04-07-2014 07:49 PM||14487 views||16 times favorited||36 comments|
Zero Clearance Blade Insert
This is written for the Delta 36-725 Contractor saw but it is reasonable to think that the thoughts and methods described here would work on other saws.
This is a fairly new design release and, as such, options/accessories have not yet been designed by the manufacturer. I made a ZC throat plate for my new saw out of ½ oak ply. Unfortunately, I had unexpected problems due to the riving knife and anti-kickback pawls being a part of the saw rather than a part of the blade guard assembly as in my old saw. Everything operated just fine except when lowering the blade. The teeth on the pawls would bite into the oak and push down on the plate rather than simply riding backwards as normally happens on the metal plate. (Pic 3 above) I had to remember to hold the pawls up with my left hand while I cranked the blade down with my right. Repeated annoyance. I am aware that others have experienced a similar situation with other saws and that the normal fix is to file the points down on the pawls. I may elect to do that for other reasons, like not leaving marks on the thin veneer on today’s sheet stock. But I was not going to do that for this problem.
Another personal issue was that I like the plate that came with the saw. Heavy steel. Nice chromed plate where the pawls ride. It was bent slightly out of the box but a bit of tweaking fixed that easy enough. What I did was make a zero clearance plug for the existing steel plate. No flex. It can be removed/changed at will. Works great. This describes how I made it using the router and table saw.
I found some “metric” ½ ply (just shy of ½”). I know this stuff is tough. It is used for flooring in boats. In reality, I think the exotic wood we all have called “whatchewgot” works well. This ZC plug will have a ½-inch wide flange that fits underneath the plate. It clears everything inside the saw, including the blade arbor nut flanges.
The first step was to put a ½-inch straight bit in the router and set the depth to the thickness of the metal throat plate.
Then set and clamp a straightedge to use as a guide for the router. Measure the length of the saw slot and draw reference lines on the plywood. As you rout you will run the router past these lines. Rout the first ½-inch dado.
Now move the straightedge router guide to make the 2nd dado. The bit is ½-inch wide and the slot in the plate is just shy of ½-inch (metric ½-inch). Add these 2 measurements together and move the straight edge router guide that much. A note here about how I work. I want this thing to fit snugly in the plate slot so I rout very slightly over size. I know that slicing with a razor knife along a straightedge later will get me a perfect fit and is not at all difficult. I don’t make a living at woodworking and don’t have dial indicator fixtures and such but I can take a minute to trim. Set the straightedge and rout the 2nd dado.
The next step is to rout one end of the ZC plug. Set the straightedge to run the router right along the edge cutting a rabbit. Rout just past each dado.
Measure the length of the slot in the throat plate and set the straightedge router guide. Again route just past each dado. The raised part remaining in the center is the plug that will fill the center of the original plate. The 1/2-inch boarder is the flange that will be underneath. Note that, to avoid any trimming of the length, you can cut the back end of the plug just a bit short. This material is going to be cut away to make room for the riving knife later anyway.
Now let’s cut this plug away from the sheet of ply. Using the tablesaw and the fence cut the ZC plug away lengthwise. Have the blade cut just at the edge of the dado leaving the ½-inch flange.
Using the miter gauge finish cutting the plug away – once again cutting just outside the dado leaving the ½-inch flange. At this point it is easier to see how this will be a ZC plug for the plate.
Now test fit the plug. Unless you are really good (for me very, very lucky) the plug will be just a whisker too wide. Using a straightedge and a razor knife trim the whisker off one side and, if necessary one end.
The plug should now go into the slot with slight pressure. It will not sit flush at this point. Turn the plate over and see that there is the spring steel retainer, the lifting hole bulge and a slightly raised portion around the spring steel retainer. Simply notch the corners of the plug flange to clear the retainer and the finger hole. The raised portion is a stamped relief to accommodate the chrome cover at the back of the plate. You could rout, saw, sand or file this a bit to remove the excess plug thickness in this area. I used a chisel and it took only 6 or 7 minutes to trim this area down. Yes, I know I’m slow but I got it done.
Now pop the plug back into the plate. Adjust anything you need to. The top of the plug should be level with the top of the plate and the sides snug.
Now we’re going to attach the plug to the plate using 3 nuts and bolts. Any small diameter ones will do. The length is not terribly critical. I used 3/4” bolts because that’s what I had laying around and they cleared everything. One person slipped on the countersink and went too deep. The 3/4” bolt hit the blade shroud. So, 3/4” is the longest you want to use. Shorter is fine.
At the back end, measure in ¼ inch from the edge of the metal and ¼-inch down from the chrome plate. At the front end measure ¼-inch from the metal centered on the slot. Leave the plug in place and, with a piece of scrap wood as a backer, drill the 3 holes through the metal and the wooden flange underneath. Countersink the holes. Add the nuts and bolts and tighten. Here you do not need to add any washers or worry about the bolts/nuts loosening. Hold the nuts and tighten just until the nuts are pulled slightly into the wood. This is your “lock washer”. They won’t come loose.
As you can see at this point, changing plugs, say to cut with the blade at an angle, is a job that only takes seconds. It may not be as convenient as just buying inserts, but this is much simpler than all that routing underneath to clear the adjusting screw mounts, motor and blade lock. And, there is no issue with the anti-kickback pawls. This was my first swipe at this plug design. It works well. Making a rectangular jig to knock out a half-dozen at a time is certainly not a daunting task.
Now lets finish this thing up. Remove the riving knife from the saw. The blade in this saw drops below the tabletop but not far enough to clear this plug underneath. It is so close that I thought of just turning on the saw and pressing down. But, I couldn’t admit to actually ever doing that could I? So, the correct/safe way – remove the 10-inch blade from the saw and replace it with a circle saw blade. Lower the blade and put the throat plate in. Move the fence over the plate close to – but not over – the blade and lock it down. Use a push stick to hold the plate on the other side if you feel more comfortable doing so. Turn on the saw and slowly raise the blade. You’ll hear it when it starts to cut. Keep raising the blade until it cuts through the plug and is fully raised. Turn off the saw and lower the blade.
Replace the circle saw blade with your standard 10-inch blade then lower it all the way down. Put the plate back in, move/lock the fence as before and start the saw. Again raise the blade until it cuts through the plug and is fully raised.
Almost there. The only thing left is to cut the slot for the riving knife. Take the plate out and extend the saw kerf all the way back to the metal plate. I used my jigsaw. Just snap out the little sliver left in the middle. This last part just takes a minute. I didn’t even bother to draw cut lines.
Now the plate is ready for use. Put in the riving knife and attach the anti kickback pawls as in the pics above.
Have a beer.
-- Life is what happens to you while you are planning better things -Mark Twain