Cutting zero clearance throat plate (ZCTP)

[LCHIEN]

I was machining zero clearance plates for my BT3000 and wanted to cut the slot by installing the ZCTP and raising the spinning blade into it. The warning being that if the plate is screwed down and the blade trapped you can break your belts when starting the saw.

So there has to be clearance between the top of the blade at its lowest position and the bottom of the plate, and/or a relief cut in the plate if the pate is thick.

The plastic ZCTP blanks form Ryobi are 1/2 inch thick and have an arched relief in the blade area. I was using a 1/2" thick piece of flooring.
I measured the blade clearance top tip blade fully lowered, to the top of the table

and I could get a 3/16" setup bar with clearance but a 1/4" setup bar touched both sides so at least 3/16".
I put in a relief of 0.25 deep, but I think I'm going to have to increase it to .375 leaving only 1/8th inch at the apex.



Also, the three hole pattern for mounting the ZCTP puts the hole in the thinnest part, very close to the edge, you can see on my plastic ZCTP where it has broken out. I decided to use the four hole pattern which is thicker area farther from the edge.

[SpinDoctor53]

I have a bunch of blanks just sitting for making new ZCTP's. The blank are 19mm Jatoba. AKA Brazilian Cherry (1). What held them up getting finished was I lost access to a milling machine at work. One thing I think I would do is cut a slot and pocket on the top surface so that the sacrificial part is made from 1/4 Masonite. Have master made up and profile the new blanks with a router.

[LCHIEN]

Here's some post experience notes

I cut the blanks, then routed the edge and end rabbets with the router and a straight bit or using the table saw with the regular blade to cut rabbets

I made micro adjustments to the thickness of the rabbets using micro adjust on the router table raizer or using a small rabbet plane to take off a hair.

I find the thinnest edges around the ends of the zctp are too thin on both the supplied plastic plates and the use of flat head screws eventually splits this thin edge and the 3-screw pattern. Using the four screw pattern the edge is much thicker. But I had a very hard time getting the holes for the four screw pattern in the right place because I didn't have more than 6" calipers to measure the screw distances on the saw table. and no original plate with four holes to use as a template.

Someone eventually gave me one of the later factory accessory plastic plates uncut that has the four hole pattern I could use as a template.

[capncarl]

I remember making my zero clearance thick and replacing the blade with a 7 1/2” skill saw blade to cut the starting slot, then changing to the proper blade and cutting till blade was aprox 3/4” through. No sense cutting any deeper than you normally saw with, that can be deepened as necessary.

[SpinDoctor53]

I did some checking of the distance from the table to the surfaces the throat plate sits on. The surface the steel throat plates sit on varies about .0010" (.109 to .119) on my saw. The surfaces that the our ZCTP screws thread into range from .374" to .384" *. I checked all of this with a depth mic. The factory ZCTP's measure .360+/- a couple of thou over the countersunk holes. So tightening the hold down screws enough to hold it in place are going to tweak the plate somewhat.
Ways to fix this involve three different options.
1) Disassemble the saw and set the table up on a milling machine to machine the surfaces relative to the top of the table. Unless you have access to a milling machine probably out of the question.
2) Use a router to take very light skin cuts until you clean up the surfaces. This is is going to require either a very careful free hand or setting up stops to run the router body against.
3) Making up shims to even out the surfaces the that mounting screws thread into. And then super glue the shims in place.
Now comes the question. Just how close do we really want to get to Zero. And how flat is the die casting the table is made from in the first place. Chasing thousands of an inch could very well be a frustrating experience.
* The tapped holes the leveling screws are in.

[LCHIEN]

My requirement is that the throat plate must be at or just below the table surface at the front and just at or above the table in the back. That is so a square edged workpiece won't catch as you feed it into the blade. By just above or below I would consider 0-.005" OK.
I check by taking a sharp edged steel ruler and running it against the edge and make sure there is no catch. I machine or shim .002 to .004" as I adjust it. I use a rabbet plane to shave a couple of thou and some metalized address labels with sticky on one side that are .004" to shim,

The throat plate opening has two sets of holes, one set is three and the metal OEM plates use these. The other set is what I call the 4-hole pattern.

The three hole pattern is used by the steel OEM plates where the steel is 0.115" or so thick. The plastic plates or DIY plates that use the three hole pattern are handicapped because the material is so thin; you have to use a flat head screw (because of the limited thickness) and the thin-ness of the material and the angle tends to wedge and spit the thin wood or plastic with use. Because of the flat head screws, the locations of the holes has to be very accurate.


The later plastic throat plates had both hole patterns. The distance from the top of the table to the cutout where the 4-hole pattern is, as you say 3/8" or 0.375". That's enough to allow counterbored screw holes for a recessed pan head screw And the thickness will not fracture or break from being too thin. Its easy to make the thickness match the 3/8"

Which brings up the next point, with the four hole pattern the plate can be made much simpler, You don't need the 0.115 lip at all, Eliminating a lot of precision milling. You end up with a 0.115" deep groove running around the edge of your plate. On the sides it's a thin recess only .1" wide, on the front and back, it's like 1/2" wide recess. These recesses are much smaller than the wood being cut, nothing can fall in, and the depth is not much different than the depth of the groove between. I think the small moat is not consequential.

I'm working on one but I have these pictures vs a conventionally cut one showing how much more simple it is.

[capncarl]

Another not mentioned method of cutting zero clearance slots without spending a long weekend setting up.
1. Mark a line where you want the slot, not necessarily the entire length front to back. 2. Using your oscillating saw cut a slot along the line.
Now that you have a relief cut 3. Put the piece on the table saw and cut it through .

[gjat]

I have 4 new zero clearance plat, Craftsman 922212. I'm never going to use them. No point you going to all that trouble and risk. I'd be happy to send one or two to you, gratis. Send me a PM.

[doktrred]

I made a ZCTP out of lexan. I clamped a scrap piece of wood over the TP as I raised the blade up to make the initial cut. Wasn't sure how it would go, but was fine. I have used the ZCTP for a long time. I wasn't sure how it would work being so thin, but it has been very stable so far. If I get a chance, Ill take a pic and measure the thickness. I can't remember how I made the opening long enough for the riving knife to come through, it's been a long time ago
Mike

[d_meister]

I made a ZCTP out of lexan. I clamped a scrap piece of wood over the TP as I raised the blade up to make the initial cut. Wasn't sure how it would go, but was fine. I have used the ZCTP for a long time. I wasn't sure how it would work being so thin, but it has been very stable so far. If I get a chance, Ill take a pic and measure the thickness. I can't remember how I made the opening long enough for the riving knife to come through, it's been a long time ago
Mike

I like this approach. If it needs to be stronger, one could add stiffeners to the underside, as Lexan (polycarbonate) can be solvent-welded. Simply cut strips of the same material and glue them on edge, box frame style. Much easier than subtracting material.
The same can be done with acrylic, which is easier to source than polycarbonate, but is not as strong.
https://www.homedepot.com/p/LEXAN-8-...810A/100244149