Cutting a perfect triangle on the TS

[Brian Behrens]

I'm building a sled for my table saw, and adding a piece of plywood cut as a right triangle for miter cuts. It will attach when needed via star knobs to the kreg track I put on the sled.


My goal is to cut a perfect right triangle for this attachment. Running through it in my head, I can easily square the 90 corner on my sled, but I can't figure how to accurately cut the hypotenuse. I've done the 5 cut method to test the sled and it is square to .002 over a 20" cut, so I'd like to get the miter attachment highly accurate also .


Yes, I can cut it easily with a miter gauge, but I'm looking for a tip to make the 45s perfect. I want to be able to cut any miter from either side of the jig, not have to keep track of which piece was cut on which side in order to cancel out any error in the jig. I can't see a way to cut the hypotenuse to where the right angle legs are the same length within a few thousandths.

This is the exact sled I am working on. I was considering using William Ng's method of cutting the miter base, but that would change the design, and I want to learn a new trick if anyone has one. Thank you for any help

[James Tibbetts]

Half of a square piece will do it. Whatever method works for you.. miter gauge on a table saw.. hand saw.. straight edge with a circular saw

[johnny means]

You could attach it to the sled in place and rip it off using the sled sideways.

[Pat Barry]

Once you can cut a perfectly square or rectangular piece you have your saw set up. Only two cuts required then to make a 90 degree corner.

[Brian Behrens]

So once I have my perfect 90 angle, how do I cut the long side of the triangle and have the two legs be exactly the same length. To a few .000 tolerance. That is the part I can't figure out. Any of the methods mentioned will likely result in one side being longer than the other, and defeat my goal of true 45s on the corners .I don't want a 44.9 and a 45.1

[Art Moore]

How about a Drafting triangle to align your miter gauge to the blade?

[Bradley Gray]

If you can cut a perfect square then draw the diagonal cut line. Temporarily screw or tape another piece of ply with one edge on the line and run the opposite edge of piece 2 against the table saw fence.

[Steve Demuth]

If you nail the right angle at the apex, then:

1. Getting the hypotenuse perfect is a matter of getting the two short legs identical in length and perfectly seated against the main fence. I would focus on that. In fact, for your application, I'd focus on getting that right with an inch or so of the hypotenuse on each end straight and 45 degrees true and undercut the rest of the hypotenuse - makes it less likely that a woodchip will make it difficult to seat the miter block against the fence.

2. Even if you don't get the hypotenuse perfect - say it's a few tenths of a degree off - you can still cut perfect miters as long as you always cut your pieces with the show side up (or down - just needs to be the same for every piece). This will cause you to cut the left miter of every piece on the right side of the jig and right miter on the left side, so they will always add to 90 degrees. A couple tenths of a degree variance in "rotation" won't impact the length of the miter cut enough to be noticable.

[Jamie Buxton]

Cut it as best you can using the saw. Then use a handplane to adjust the triangle. Set the plane to take off only a fine shaving. Take an extra stroke or two or three at one end of the hypotenuse, and one or two long strokes to clean up.

Or if you don't like handplaning plywood, cut the plywood roughly to size, edgeband the hypotenuse with a little solid lumber, then trim that. Or if you're averse to hand planing, but have a power jointer, use it to adjust the triangle.

[Brian Behrens]

Thanks for all the tips. A member sent me a video that shows a great technique for doing this to a high level of precision. It is a very interesting method. I'm not sure if it's proper to post the name of the video here, but I can pm it to anyone interested.

[Pat Barry]

Thanks for all the tips. A member sent me a video that shows a great technique for doing this to a high level of precision. It is a very interesting method. I'm not sure if it's proper to post the name of the video here, but I can pm it to anyone interested.

Not sure why both legs need to be exactly thesame length. You could put a series of equidistant markings along each rail ... ala ruler for example. That way you could make any lengths necessary

[Steve Demuth]

Not sure why both legs need to be exactly thesame length. You could put a series of equidistant markings along each rail ... ala ruler for example. That way you could make any lengths necessary

I was referring to the legs of the triangular insert. For the sled to cut perfect miters, that triangle needs to isoceles.

[Simon MacGowen]

I think I know the video you are talking about. All the suggestions given here are no match to that video. By the way, all those suggestions will not give the results you are looking for, even though many are good in theory.

Any method that relies on pencil marking, handplane (even Paul Sellers won't get the kind of tolerance you want) and what not is not fool proof at all. That video will get what you want.

Simon

[Steve Demuth]

Any method that relies on pencil marking, handplane (even Paul Sellers won't get the kind of tolerance you want) and what not is not fool proof at all. That video will get what you want.
Simon

I don't know the video, since it wasn't posted in this thread, but if as he says Brian already has the right angle on his jig a highly accurate 90 degrees, then measuring the short legs to be equal and adjusting the hypotenuse with a handplane will get him quickly and easily as close to perfection as the material in his triangular insert (wood, after all) permits. You can easily plane to within less than .01" variance at the ends of the hypotenuse, and a hundredth there will translate into less than a thousandth gap in an actual miter - or it would, if he were working with a material in which measurements less than a thousandth of an inch were meaningful. Pencil marks are beside the point - you can use an actual rule to measure the legs rather than plane to a mark, or just cut a stick to length and use that. And again, you don't need the actual hypotenuse perfect - you only need the ends to seat the jig so the short legs of the triangle are truly isosceles. You can plan a swale into the entire rest of the hypotenuse's length as long as you get those seating points where the jig insert sits against the sled fence right.

[Simon MacGowen]

0.01" variance is a lot when compared to the video the OP refers to. I watched that video a while back and my memory might be wrong and I think that video woodworker was talking about 0.001" tolerance or something like that.

I am also a hand tool user and all I can say is, other than hit or miss/by luck, there is no way a handplaned approach could match that youtube method (verified by a digital caliper).

If we are talking good enough mitres, the suggestions we see in this thread are good enough. But, if I understand the OP's requirements correctly, none of them will meet his needs.

Simon