Router Circle Jig problem. Help sought.

[Michael MacDonald]

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"

Dell -- if your radius is exactly 10", then your diameter must be 20"... Except perhaps if one of the following is happening:

1. not a circle: perhaps the center pin moved/slipped as you went around?

2. diameter measurement not taken through center: The radius is the radius, but the diameter can be misleading... did you measure the 19 7/8 through the exact center?

3. measuring tool innaccurate: if you are using a tape, the metal hook may not be moving to accomodate inside (handheld hook for radius?) vs. outside (hook on circle circumference?) measurements. Or some measuring sticks may be innacurate at the tips... I have a try square where the first 8th of an inch is a bit more.

I would suggest checking your measuring tool, and then measuring the radius in multiple spots on the circle to see if it is really 10" everywhere. I will be interested to see how this mystery is solved.

[Stephen Edwards]

Another thought: Cut a stick that is 10" long minus the radius of your pivot pin. For example, if you're using a 1/4" pivot, cut your stick 9 7/8" long. Then, with the stick firmly against the pivot pin in your trammel, adjust the router on the trammel until the bit is firmly against the other end of the stick.

For this to work there can be no slop in the pivot pin hole in the work piece and it must be perfectly 90 degrees to the work piece.

As others have pointed out, perhaps use something thicker for the jig/template that you're making.

[Dell Moore]

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…....?

[Mike Henderson]

This won't help you now, but I often take my first cut long - that is, I intentionally cut the disk a bit bigger than I want it to be. That way, if I have mis-measured I have some extra space. And when I take the final cut, I'm only trimming a small amount off the disk, which usually gives me a better edge.

Mike

[mreza Salav]

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…....?

I'm late to this thread but:
That's the way I do: measure the distance from the center of the center point
to the center of the router bit, then add 1/2 the thickness of the router bit;

So if I want a 10" radius disk cut using a 1/4" router bit, I measure center to center to be 10"+1/8".

[Ray Newman]

Finally found what I was looking out in the shop -- it was right where I put it so I could easily find it! Couldn't remember who made the circle cutting jig or where I bought I bought it.

Anyway, some might find the below useful.

If you cut a number of circles or are looking for a precision circle cutting jig do the job right the first time, I would recommend the "Jasper Circle Cutting Jig" .

A few years ago, I needed make different dia. circular cut-outs in a project. Was getting frustrated doing it : once the pin slipped, and another one or two times, I just didn't get the measurement right.

Went to the local Woodcraft and saw the "Jasper jig". And it work right-out-of-the box, the first time around. No wasting time fiddling around with measurements, etc. It is a time-saver and saves the frustration for other projects.....

Downside??: works best with a plunge router, & only cuts circles in 1/16" increments

Manufacturer's site:
www.jaspertools.com/

Other sized Jasper jigs:
http://www.woodcraft.com/Catalog/Pro...px?prodid=2707
http://www.woodcraft.com/Product/200...7---52-34.aspx
http://www.woodcraft.com/Product/200...-1---7-12.aspx