How to square a miter slot to the blade

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  • How to square a miter slot to the blade

    I'm planning to build a pair of miter slots for my BT since the OEM ones are going for over $100 each. The construction seems fairly straightforward, but as I have been considering this project, it occurred to me that you all might have better insight on how one is supposed to square the dado with the blade.

    It strikes me that using the rip fence could work, as long as I make sure that my fence is dead parallel with the blade, but any error there will be compounded. I figure this would be the case if I cut the slot with my dado stack or with my router.

    Do I make the slot slightly wider than the miter bar and square it as I'm attaching it to the slot? Do I cut the slot and then square the entire slot holder to the blade? It seems like whichever method I choose, the likelihood for my slot to not be parallel to the blade is fairly high

  • #2
    First my disclaimer is that I don't have a miter slot on my BT3... I have found the SMT good enough for most operations and will attach normal miter slot accessories to the SMT fence or SMT.

    That said I have considered it from a construction standpoint...

    First issue is the slot.

    Either take a block of wood and make a precision dado slot or buy a piece of miter slot extrusion and put that in a less critical slot of its own. I would tend towards a miter slot extrusion because it can wear and also because you can get a T-slot extrusion that has a semi-captive miter bar as an option.
    Click image for larger version  Name:	miter track with captive bar.JPG Views:	0 Size:	16.2 KB ID:	848805
    Or you can just get a plain miter track 3/4" x 3/8" deep:
    Click image for larger version  Name:	miter track plain.JPG Views:	0 Size:	13.2 KB ID:	848807

    But it is a more expensive option. Either will work. but the store bought is more durable, is very accurate and has the captive miter bar.

    Second issue is alignment. As I suggested the slot is cut or mounted into a piece of wood.
    You have two options in my opinion.
    1. cut the wood and slots long enough to span the front and back rails and make a rail locking attachment similar to the SMT to secure it to the rails. It is easily adjustable and requires no modification to the table saw. It can also be moved from one side to the other of the blade. Downside is moving it or moving the rails requires recalibration. And you may have to move it to clear the rip fence for narrow or left side bevel rips. OTOH, you can easily move this to either side of the blade.
    2. Cut the wood and slots as long as the table top is wide. Put some threaded mounting holes into the two extensions of the saw table top casting extending to the left. Put an oversized countersunk holes in the wood base and use them to adjust the right left angle adjustment to calibrate the slot parallel to the blade. This arrangement is more permanent than (1).
    Either way you will want a mounted caliper or dial gauge to check the different points on the slot against the side of the blade for parallelism adjustment.
    Either method you can make dual slots if you want. You can even put a DIY slot and a metal extrusion slot in the same board, perfectly parallel (if you use the rip fence to make them) to each other.
    Last edited by LCHIEN; 03-01-2022, 12:36 PM.
    Loring in Katy, TX USA
    If your only tool is a hammer, you tend to treat all problems as if they were nails.
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    • #3
      I use aluminum miter track. A cut wood slot would wear out too quickly. Rockler, Powertech, and myriad other companies make and sell aluminum T track extrusion. Cut a dado into a work piece you are going to use as a small table as it were, to hold the miter track, and secure with angle brackets, T bolts and star knobs. Adjust square to the blade and lock it down tight...
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      • #4
        Ah yes, sorry I think I was too vague originally. I'm definitely planning on using an aluminum extrusion, not just a wood slot. I was looking for strategies of squaring the resulting assembly to the blade.

        I was reading up about Rod Kirby's extension table and I think that style, where I'd be using T nuts to mount the assembly to the rails, I'd have enough play that I could accurately (well, as close as my crappo dial indicator will get) square the slot to the blade.