geometry genius required - Router Forums
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post #1 of 12 (permalink) Old 11-05-2015, 09:56 AM Thread Starter
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Default geometry genius required

I've obviously lost more brain cells than i thought, so I need help.

I'm using the router table, with a Trend box joint cutting bit. This is the one that cuts 4 slots in one go.

If you cut four slots in one piece, and four slots in another piece, when you invert one of them, its a perfect joint. If you place the two pieces at 90 degrees, you have a perfect 90 degree box joint. O.K. so far?

Now, Having seen that mryni mini jig thing the other day on here, where you cut 45 degree angles on the table saw to get an octagon box, i thought that would work on the router table.
The cutter is horizontal rather than vertical but then the wood is also out of phase, so it will work. Yes?
NO!.

Heres where I cant work out what went wrong.

I set the wood against the fence at 45 degrees to the normal fence, and adjusted the cut so the back of the cut slot is in front of the guide bearing.
I now have 2 x slots cut at 45 degrees to the fence. Why, when I put the two pieces together, do i get an angle of TEN degrees less than 90?

I could understand 22.5 if I had them out of sync, but why 10 degrees?
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post #2 of 12 (permalink) Old 11-05-2015, 10:26 AM
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Damn Bob, you came up with another one of those "word problems" I hated so much in school and discussed in the other thread on math education

I'm far from a genius, so a picture of the setup would sure help.
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post #3 of 12 (permalink) Old 11-05-2015, 11:39 AM
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...what Rob said...picture would be helpful or maybe some clarification...

Which bit are you using...the slot cutter...? ...and the orientation of the wood piece...? ...and what are you using to set up the wood against the fence for the 45...? What would the finished product look like...?

My SWAG would point to the initial setup of 45 deg...or, since the wood is 45 deg to the bit, one of the slots is not as deep as the other...

...dunno...Picture/drawing would be helpful...

BTW...just cuz I responded doesn't mean I'm a geometry genius...

Nick

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post #4 of 12 (permalink) Old 11-05-2015, 11:46 AM
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Oliver will know.
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post #5 of 12 (permalink) Old 11-05-2015, 12:01 PM Thread Starter
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Its late evening here, everything is locked for the night so I'll get a few pics tomorrow.

On a table saw, the blade is vertical and the wood is flat.
I watched this video
and he runs the wood across the saw blade at a 45 degree angle to make finger (box) joints to make an octagonal box.
on here, he runs the wood end on and flat across the vertical blade

Now, I dont have a table saw, so my thinking was to recreate the cut on the router table, except now the cutter is horizontal and the wood is vertical. Should work.

The fence is set normally, there is a 45 degree wood spacer clamped to the fence so the wood pushes into the blade. I worked out the lower bearing has to be allowed for, so i positioned the woodto enter the cutter in front of the bearing.

The cut pieces look good, its only when they are joined that the angle is completely wrong.

I've now realised while typing this that the 45 degree spacer is not needed. I can still use the fence but just move it over the cutter to recreate the angle. But it still doesnt answer the question.
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post #6 of 12 (permalink) Old 11-05-2015, 12:13 PM
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I'm sure if you posted a video of what your doing and could see a good example that someone would see the issue right away

I donít always insulate , but when I do .
Ok ,I never insulate
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post #7 of 12 (permalink) Old 11-05-2015, 12:38 PM Thread Starter
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posting videos is above my pay grade. I've got the still pics down pat though. i can prove that tomorrow.
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post #8 of 12 (permalink) Old 11-05-2015, 02:13 PM
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Bob, it's not clear to me how you are passing the board through the cutter. I would think you'd keep the fence straight, flush with the guide bearing, and use a miter gauge at 45 deg. to push the board through (assuming that you have a miter slot on your router table).

It sounds like the 10 degrees (you mean less than 45, not 90, correct?) is related to the radius of the cutter. In other words, the cutter is cutting an arc, and you're not fully pushing the board through the cutter.
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post #9 of 12 (permalink) Old 11-05-2015, 02:36 PM
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On a lock miter bit one piece is passed through 90* to the other but on a box joint I would think they should be the same except that one has to be started offset to the other. In other words, the tongues can't all be in the same places.

Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.
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post #10 of 12 (permalink) Old 11-05-2015, 02:38 PM Thread Starter
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Furboo, you have the answer.

I have been pushing the wood into the cutter to one side of the lower bearing, but not passing it along the diagonal fence. Therefore the projecting fingers are at 45 degrees, but the inner slots are curved, so the wood pieces are meeting at the tips of the curves.

I shall retry tomorrow and work out how to get the inner slots in a straight face rather than a curved one.
i have never been good at 3D thinking.
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