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Ok, this is just plain stupid, and I am pretty sure I am missing the forest for the trees here.

Cut out a pattern for a circle jig the other day so I could cut out a 20" circle out of plywood. simple jig made of 1/8 paneling I had laying around. I measured out 10" from the edge of the bit, drilled my center hole and started my circle. I made a very shallow pass to see how it was going. Nice clean circle, BUT it was 1/8 of an inch short! So 19 7/8" rather can 20" I re-measure my radius, and is was still 10"

OK, what did I do wrong? What did I forget to compensate for? This was kind of frustrating to have happen, I though I had it cold.

Has anyone done the same as me? I still haven't cut the circle yet, so all is not lost.

Anyone have a SIMPLE cutting jig like this?

Thanks for the advice.

Milo
 

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Discussion Starter · #4 ·
But if I measured from the edge of the carbide, shouldn't that have not made a difference? Or maybe I SHOULD have measured from the center and subtracted the radius of the bit so as not to mis-measure from the edge if the bit? Is that what you are saying guys?

Oh, using a 1/2 bit…

hmmm…..
 

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try measuring from the center and then subtract the radius of the bit like you said, that should work for you.
 

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Maybe you should measure from the center point out 10" , make a mark and then put the bit edge on the mark, instead of the other way around. It could also have something to do with the way you are using your rule. If it is a tape it might have something to do with the sliding end. They are notorious for inaccuracies. Try using a ruler instead. I hope you don't think me patronizing. I have made about every measuring mistake in the book.
 

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Pretty sure the problem is this….

Your measuring your center point accurately, but your not making allowance for the thickness of your 'pivot point' ie: a nail, or screw that is more than likely an 1/8" in diameter. You mark your point, drive in the nail or screw accurately….buuuut….the outside edge of the nail or screw (the true pivot point) is a 1/16 off (half the diameter of the nail) your original center point. In 180 degree's that makes your diameter 1/8 smaller.

Put the nail, screw or pin in, and measure from the EDGE of the pin…out.
 

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I agree with stefang, thats the sure fire way. Also if you are using 1/8" paneling for the jig it could flex a little but it shouldn't in 10".
Also make sure your pivot point is tight as a 1/2 inch bit can have a slight ammount of pull. All it takes is a little off both sides to make an 1/8. Hope this helps
 

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Naw, the pivot diameter makes no difference. Whether you use 1/8", 1/4" or 1/2" if the pivot pin is the same size as the pivot hole in the jig the jig will rotate around the center of the pivot. If the pivot pin and pivot hole are NOT the same diameter then you can't guarantee the cut will be round, anyway.

Now the cutting bit; you have to add 1/2 of that. So you have to measure from the center of the pivot point to the inside edge of the cutter to get a proper diameter.
 

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Discussion Starter · #13 ·
I wanted to follow up for you guys on what finally happened.

I went back and measured my distance from the CENTER of the router bit and added the radius of the bit (1/4") to the total length. This worked perfectly and I have 2 nicely rounded plywood discs for my project.

This left me thinking though, what does this say about router bits? Or my measuring technique? Or where did I put my 12" ruler after my two year old wandered up and wanted to taken to her momma…....?
 
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