Zero Clearance Insert: What? Why? How?

What’s a Zero Clearance Insert?
Well thanks for asking! Your timely question sure makes it easier for me to introduce this rather specific topic.

What is a Zero Clearance Insert?

A Zero Clearance Insert is the first customisation woodworkers make when they get a new saw or new blade – it’s so common that it often goes without mentioning. Zero Clearance Inserts fit into the gap surrounding the blade on the bed of the saw – this is called the throat. You can, and probably should make a zero clearance insert for every saw you own and every blade you regularly use on each saw, but the tablesaw is where they are the most common, and of the most benefit.

Why make Zero Clearance Inserts?

Zero clearance inserts hug closely to the blade and perfectly match the height of the sawbed. It’s crucial that the insert matches the height/depth of the sawbed to insure your workpieces, mitre, and jigs slide over the insert without getting stuck in a valley, or catch on a bump. It’s also critical that the insert be really, really smooth. having a decent zero clearance insert will greatly reduce tear-out on the back side of the cut, holds the blade truer to reduce drift and wobble, and will slightly reduce the kerf. Don’t use the angled blade setting with a standard zero clearance insert, you’ll either widen the gap, or crack the insert.

How to make Zero Clearance Inserts – Tracing.

Making your own inserts is easier if you start with one in hand. Most older saws come with a metal or hard plastic insert (with generous clearance) that you can trace for fit. Start with a piece of solid wood – cheap pine works just fine. A lot of woodworkers use plywood, but unless you’re using a high-quality void-free plywood like Baltic Birch, the voids will cause you much more trouble than it is worth. Make sure you start with a piece that’s thicker than the insert – including the adjustment screws that you’ll often find in the corners. You’ll want a little extra depth to start with so that you can level the piece properly. Sadly you can’t level the piece until after you’ve fit it to the throat of the sawbed. Well you might be able, but I sure as hell can’t. Once you’ve traced the insert you’ll want to cut it out – probably on the bandsaw if you’ve got one.

How to make Zero Clearance Inserts – Cutting.

Following the common refrain, leave a little extra on the piece so that you can fit it incrementally – gaps are bad and slipping inside the traced line will mean a gap. I decide which side is the top (unless your throat is asymmetrical, in which case the choice has been made for you) and outline where the blade will come through on that top. Then I put a nice round hole into it. This is the fingerhole that allows me to dislodge the insert from the throat – it’s gonna be crucial to fitting and levelling the insert and without it you’re hooped. Since you want the hole to have smooth sides and minimal tear out, I recommend a fostner bit on the drillpress with a flat sacrificial piece underneath.

How to make Zero Clearance Inserts – Shaping.

Once the hole is in place you can start incrementally removing material from the sides and ends until you get a snug fit. I used my benchtop oscillating bench sander which certainly makes life easier, but before I had that I used my belt sander clamped to a table, and before that I just sanded it with a large flat sanding block to keep the edges reasonably straight. Once you’ve got a good fit, your ready to level the insert.

How to make Zero Clearance Inserts – Levelling.

Before I got a planer I sanded the top and routed places for the supports to get the depth I needed. I’m not all that confident with my router, and sanding for level is a tedious affair, but it works well enough. Now that I have my planer I just take a tiny slice off each side until I have it dialled in. More accurately I take a couple of passes at what will be the top, and then the remaining passes are all on the underside. I take the first passes on the top to ensure I’m happy with how smooth and level it is, and then the rest on the underside to avoid potential tear out on the top side.

Finishing the Insert.

Once I’ve got a snug fit and the piece is entirely level with the sawbed, I finish the insert. Since my inserts and tools aren’t as protected from moisture as I would like I overprotect them. I use a thin coat of poly all over, a second coat on the edges, and then rub in paste wax the next day. This helps prevent the absorption of moisture and swelling, which will ruin the perfect fit you’d just worked so hard for.

Friends, I know this post has gotten long winded. The sentences are uncomfortably long, and the use of passive voice is atrocious, but I’m really quite ill today, so I hope you’ll forgive me.

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