Circle/Radius Cutting Jig - Router Forums
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post #1 of 18 (permalink) Old 09-30-2019, 06:25 PM Thread Starter
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Default Circle/Radius Cutting Jig

I've been working on making a ships ladder for my granddaugther's fort. For the hand holds, I am making templates for using a circular saw for the straight parts of the cutouts and a router for the radii at the ends and for finishing the sides and edges. The router template needs to have a large radius for the router base, to create a template radius of 3.394" which will, in theory, create a radius of 1.5" at each end of the hand holds.

I was thrashing about trying to figure out how to make a circle cutting template for my router (Bosch 1617). I was in a woodworking store recently and saw some really cool circle cutting templates and normally I wouldn't hesitate to buy one. But, money is tighter right now than usual, so I refrained. The jigs I looked at in the store rely on using a 1/4" router bit for establishing the radius that the jig will cut. The jigs had series of radiating 1/16" holes allowing radii to be set at increments of 1/16". One reason I refrained from buying one is that I remembered I'd bought the Bosch edge/circle guide jig (RA1054) about the time I bought the router. I realized I didn't need to buy anything and digging it out when I got home proved that correct.

The Bosch jig has a cool little circle pivot point tab for inserting a 1/4" dia. rod located either in the workpiece or by using a provided suction cup with a 1/4" pin protruding. The radius I need puts the pivot tab too close to the router base to use the suction cup, so I needed to come up with a different method. Either way, I needed a way to set the radius of the router/jig. I reasoned that a 1/4" pin securely mounted in the pivot tab could be used along with calipers and a similar 1/4" pin in the router chuck for setting the radius, subracting half the pin diameter of one of the pins. I set about making a somewhat fancy one, but I realized that a partially threaded 1/4" bolt could do the job satisfactorily. I tried it and it does, but made the fancy one anyway.

Bolt Pivot Pin (head would need to be cut off and the shaft shortened)
Circle/Radius Cutting Jig-bosch-pivot-jig-bolt-pivot-pin-09-30-19-640.jpg
Circle/Radius Cutting Jig-mounted-bolt-pivot-pin-09-30-19-640.jpg

Brass Pivot Pin
Circle/Radius Cutting Jig-bosch-pivot-jig-brass-pivot-pin-09-30-19-640.jpg
Circle/Radius Cutting Jig-mounted-brass-pivot-pin-09-30-19-640.jpg

Bolt pivot mounted in the jig
Circle/Radius Cutting Jig-bolt-pivot-pin-09-30-19-640.jpg

Brass pivot mounted in the jig
Circle/Radius Cutting Jig-brass-pivot-pin-09-30-19-640.jpg

Setting jig for 3.394" finished template radius.
Circle/Radius Cutting Jig-settting-circle-radius-09-30-19-640.jpg

It appears that the bolt would work every bit as well as the fancy brass pin, but I do like the brass one better. I haven't tried either one out yet and I expect to encounter some glitches, perhaps in how well the caliper does in setting the right radius. It will likely take some trial and error, sneaking up on the desired setting. Even with a bit of fiddling, if it works, I'll consider it a success. I expect to set it shy of my final desired dimension for roughing it out and then setting it for a final light pass.

Rick

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post #2 of 18 (permalink) Old 09-30-2019, 08:45 PM
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Good thinking, Rick...I assume you'll be cutting the head off the bolt and use only the shaft...?

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post #3 of 18 (permalink) Old 09-30-2019, 09:23 PM Thread Starter
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Good thinking, Rick...I assume you'll be cutting the head off the bolt and use only the shaft...?
Thanks. If I were going to use the bolt, I would cut the head off, yes.

But, I will use the fancy brass one that I made. The shaft that inserts into the jig tang was machined to be a snug slip fit and the shoulder created by the hex brass stock is 1/2" dia., giving a fairly positive - square mounting. I trust it a lot more than a mass produced bolt, which are typically undersize by up to 0.010". The pivot hole through the tang is 0.254". The shaft on the brass pivot pin is 0.2535".

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post #4 of 18 (permalink) Old 10-01-2019, 05:00 AM
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Thanks for the idea, Rick. It looks like with a little thought, the capability could be added on to edge guides which do not have the tab for the centre pin. Might need to invert the edge guide, in order to clear the workpiece. I am sure @JOAT could come up with something.

I would not be able to achieve the tolerances you quote, but then again I probably would not need to.
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post #5 of 18 (permalink) Old 10-01-2019, 09:29 AM
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Hey Rick, it seems your precision is far greater than I usually aim for. Would love to see pictures of the final piece.
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post #6 of 18 (permalink) Old 10-01-2019, 09:50 AM
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Quote:
Originally Posted by RickKr View Post
Thanks. If I were going to use the bolt, I would cut the head off, yes.

But, I will use the fancy brass one that I made. The shaft that inserts into the jig tang was machined to be a snug slip fit and the shoulder created by the hex brass stock is 1/2" dia., giving a fairly positive - square mounting. I trust it a lot more than a mass produced bolt, which are typically undersize by up to 0.010". The pivot hole through the tang is 0.254". The shaft on the brass pivot pin is 0.2535".

Rick

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post #7 of 18 (permalink) Old 10-01-2019, 10:33 AM
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A circle jig can be made in about an hour and cost zero, as shown in this pdf.
A more ambitious one is shown in the second pdf.
Attached Files
File Type: pdf circle jig (2).pdf (339.7 KB, 62 views)
File Type: pdf New router circle jig.pdf (927.6 KB, 59 views)
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post #8 of 18 (permalink) Old 10-01-2019, 11:28 AM
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All of the posted methods work great, even the Jasper jig that you found in the store. I have used all of them at one time or another, and they all do a good job. Adding the 1/4" pivot pin to your edge guide is yet another good, and cheap, way to cut arcs and circles. Kudos for coming up with a cheap (free) solution.

Bobj3 modified his Jasper jig so he could use it with a router bushing some years ago and posted pictures here. I thought it made the Jasper jig even easier to use, just in case anyone is interested. A search on this site should find it easily.

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post #9 of 18 (permalink) Old 10-01-2019, 08:03 PM
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Rick, it looks well thought out and as some of said the precision of the tool might be greater than actually required. I do think it was prudent that you did try to hold close tolerances for the tool itself, but please don't get too picky when working with wood to try to hold very close tolerances because you will lose some of the joy woodworking gives you.
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post #10 of 18 (permalink) Old 10-02-2019, 01:37 AM Thread Starter
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Quote:
Originally Posted by MEBCWD View Post
Rick, it looks well thought out and as some of said the precision of the tool might be greater than actually required. I do think it was prudent that you did try to hold close tolerances for the tool itself, but please don't get too picky when working with wood to try to hold very close tolerances because you will lose some of the joy woodworking gives you.
Thanks, to you and the others, who have commented about the precision being a bit much. I appreciate that it is, where working with wood is concerned. As you allowed, it may be good to have held that for the tool itself. I agree. We'll see how well that translates to actually working with wood.

"If you want nice, clean, fresh oats, you must pay a fair price. However, if you can be satisfied with oats that have already been through the horse, that comes a bit cheaper."
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