Router Circle Jig problem. Help sought.

[Dell Moore]

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.

Dell

[David Christopher]

what size bit are you useing ??? it sounds like you measured from the center of the bit insted of the cutting edge

[Myk Rian]

Did you measure from the center point to the inside of the bit carbide?

[Stephen Edwards]

Are you doing this with a jig on a router table or with a hand held router and a trammel of some sort? Thinking out loud to myself here.......could it be the diameter of whatever you're using for a pivot point that's causing the 1/8" difference? I wouldn't think so 'cause center is center.

Try it again with another piece of scrap, being sure that your circle is a bit oversized this time. Make a small portion of the cut, measure the radius and then sneak up on your target of 20". Perhaps that'll even help you figure out where the problem is! I'm kinda stumped, too.

[Dell Moore]

No guys, I knew not to measure from the center of the bit! I measured from the edge of the carbide.

I made a trammel type rather than use table. As was said, center is center, so I am really stumped!

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?

[David Christopher]

if you measured from the edge,,,,,,then Im stumped too

[Jim O'Dell]

When I built my jig so I could cut holes to make blast gates, I found that I could measure all I wanted, and it never came out quite right. Use a compass if you have one large enough, or make one by drilling a hole, the same size as the bit you are using in the router, in a piece of wood, slip it over your bit and hold a pencil where your current pivot hole is. Mark and arc, go 1/16" past your current pivot hole, and drill another pivot hole away from the first one using the arc you drew. Then try again. At least that's the way I had to do it. Not very scientific, but it worked. Jim.

[Ray Newman]

Which "edge" did you measure from?

Think of ripping or cross cutting a board and not taking into account the kerf.

Measure a 10" radius from the edge which is closest to the pin.

If measured from the edge farthest from the pin, the bit will cut a smaller circle.

[Dino Makropoulos]

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.

Dell

19-7/8"?
You are off by 1/16" only.

Next time measure from 5" to 15".
You get better results if you don't use the end ( clip) of the tape measure.

Good luck.

[Leo Graywacz]

Are you sure you were at the apex of the bits travel? It won't be at the center of the diameter but slightly before it because of the angle of the carbide.

[Tom Hintz]

I had something similar happen once and finally realized that I was measuring to the inside edge of the bit body, but not the true outside edge of the carbide cutter tip which is offset slightly, but enough. That little offset could make up the 1/16" difference you are having.

It's worth a look anyway. If not, that's all I got. Feel free to be frustrated until the light goes on for some reason. I've been there and usually get even more frustrated when I (finally) realized what was happening.

[Dell Moore]

Leo, I thought I was measuring from the furthest edge in, I was using a straight edge (not a tape) to measure, and measured to a center point.

Today I am going to try and measure from the middle of the bit and subract a 1/4" and see if that give me a better measurement. I'm also going to use my compass to draw the 20" circle to see how that works.

Thanks gang,

Dell

[Joe Scharle]

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.

Dell

If you're absolutely sure that your measured radius is 10" statically, then perhaps your bit is experiencing severe run-out. Does your bit cut an exact width channel?
Next, a tip from neighbor and fellow creeker Bruce Wrenn.
Use a piece of rectangular scrap (I use 1/2" MDF) of suitable length, bore a hole to accept a large template guide bushing that will become the sole attachment point for the router. No screws through the base. Allows the router to pivot all around the circle without moving your hands; keeps the cord and vac hose from twisting as well. Using a flat steel rule, measured from the edge of a 1/2" centering pin, center punch a mark anywhere on the jig at the exact radius point. Drill a zero clearance hole for the pivot pin (no slop). Install a 1/2" spiral bit, and go.

[Chip Lindley]

There may not be any problem with your measurements. The thin 1/8" panelling may be the problem. There may be enough *give* within the pivot hole to push the bit back the slight amount you speak of (1/16") Try making your jig of something more substantial; MDF or ply, and see if your measurements are not *right-on*! And, use something larger than a drywall screw or nail as the pivot. Try a 1/4" or larger bolt.

[Paul Atkins]

My bet is on the 1/8" stuff too. 1/4" to 1/2" BB is my choice.