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Help Sought - Router Circle Jig

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Forum topic by Milo posted 02-21-2010 01:02 AM 1771 views 0 times favorited 12 replies Add to Favorites Watch
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Milo

869 posts in 2780 days


02-21-2010 01:02 AM

Topic tags/keywords: jig question

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

-- Beer, Beer, Thank God for Beer. It's my way of keeping my mind fresh and clear...


12 replies so far

View cstrang's profile

cstrang

1829 posts in 2629 days


#1 posted 02-21-2010 01:06 AM

Did you account for the dia. of the router bit?

-- A hammer dangling from a wall will bang and sound like work when the wind blows the right way.

View grizzman's profile

grizzman

7796 posts in 2764 days


#2 posted 02-21-2010 01:12 AM

you should have measured from the center of the hole where the bit is…that would give you the 1/16 on each side..make up your 1/8th…...:)>

-- GRIZZMAN ...[''''']

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Milo

869 posts in 2780 days


#3 posted 02-21-2010 01:29 AM

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…..

-- Beer, Beer, Thank God for Beer. It's my way of keeping my mind fresh and clear...

View cstrang's profile

cstrang

1829 posts in 2629 days


#4 posted 02-21-2010 01:41 AM

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

-- A hammer dangling from a wall will bang and sound like work when the wind blows the right way.

View stefang's profile

stefang

15512 posts in 2795 days


#5 posted 02-21-2010 08:16 PM

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.

-- Mike, an American living in Norway.

View jevarn71's profile

jevarn71

83 posts in 2621 days


#6 posted 02-21-2010 08:24 PM

I use the method Stefang suggested. It works every time!

-- Jason, JEV Woodworking

View Karson's profile

Karson

35035 posts in 3861 days


#7 posted 02-21-2010 08:38 PM

This is another of these been there, done that moments.

-- I've been blessed with a father who liked to tinker in wood, and a wife who lets me tinker in wood. Southern Delaware soon moving to Virginia karsonwm@gmail.com †

View Tony_S's profile

Tony_S

605 posts in 2544 days


#8 posted 02-21-2010 09:31 PM

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.

-- It is the mark of an educated mind to be able to entertain a thought without accepting it. Aristotle

View Gotwood1962's profile

Gotwood1962

30 posts in 2480 days


#9 posted 02-21-2010 09:43 PM

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

-- Gotwood1962, If sawdust were valuable... I'd be a millionaire

View EEngineer's profile

EEngineer

1061 posts in 3074 days


#10 posted 02-22-2010 02:33 AM

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.

-- "Find out what you cannot do and then go do it!"

View a1Jim's profile

a1Jim

115202 posts in 3038 days


#11 posted 02-22-2010 02:39 AM

Just fix it glue a patch over your hole and redrill 1/8 further out or make what your making with a 19 7/8 circle.

-- http://artisticwoodstudio.com Custom furniture

View Milo's profile

Milo

869 posts in 2780 days


#12 posted 02-22-2010 06:39 PM

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…....?

-- Beer, Beer, Thank God for Beer. It's my way of keeping my mind fresh and clear...

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