Zero clearance inserts or throat plates are a really useful addition to any tablesaw. They provide both extra support near the blade, making it safer as well as reducing tear-out on the underside of cuts made.
However many tablesaws in the UK and indeed most contractor style tablesaws out there have a common problem. They come with a very thin throat plate which therefore does not allow for standard zero clearance inserts, out of 12- 18mm plywood or MDF, to be made. If you have a saw with a thin throat plate not is all lost, the method I used to compensate for this problem is actually very simple .
The first step is to rip a strip of hardboard or any material for that matter, perhaps plywood? This needs to be the correct thickness to sit in the throat plates “ledge”. It can be fractionally thinner (0.5mm max) however as this can be dealt with later.
Next the board is cut to the correct length. To get the best results you should cut it roughly to length and then remove a small amount of material at a time to get it to fit in the slot perfectly.
Here I am sanding off the corners to match the shape of the current insert. Depending on the shape of your insert it may be necessary to cut the ends roughly to shape at the bandsaw first.
The hardboard alone will not provide adequate support near the blade which is neither safe nor will give the best results when making cuts.
To address this I attach a 18mm piece of MDF to the underside which was cut roughly to the inside shape of the insert slot. Making the support out of solid wood would have be even stronger, as the grain would stretch along the length of the insert.
The inserts support is then glued to the hardboard with PVA, be sure to posistion is correctly when clamping both together.
The final stage is to bring the tablesaw blade through the insert. Because the MDF support came into contact with the blade I flipped it over to make this cut. I clamped the the insert down whilst I did this, using a piece of scrap wood.
After the first cut was made I could flip it back round so it is now in the correct orientation. With a clearance made for the blade it could now sit down in position and I could raise the blade once more.
If you own a smaller tablesaw blade you could swap out the blade and make the first cut that way.
Some final touches to the insert: To drill a hole in the front so the insert can be lifted out when needed, trust me this makes life a lot easier! Also a small cutout was made with a forstner bit to leave room for the blade bushing when the blade is raised to the full height. You may also need to remove some material at the back of the blade slot for the riving knife to fit in, I simply use a handsaw to do this.
As I mentioned at the start a slightly thinner insert can be contended with quite easily, I used a few strips of electrical tape to bring the insert flush with the surface of the tablesaw. This method is a bit crude but does allow you to level the insert quite effectively.
An alternative to glueing a support to a piece of hardboard is to take a piece of thicker plywood and using a router cut a rebate around the edge to the correct depth so the insert sits flush with the surface of the saw.
Be sure to watch the video bellow to clarify the method explained, hope this helps.