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Aug 17 2011

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A Jig For Routing Round Objects

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I’d left off writing about my yo-yo project, when I ruined the sides trying to rebalance them. To re-true the yo-yo sides I needed to create a jig that would hold one of the sides a set distance away from a router bit and allow me to slowly rotate it into the router bit. Such a jig would not only be useful for truing circles, but would make routing profiles on small wheels and other circles much safer than how I was previously doing it — spinning it by hand holding the small piece against the router bit and the fence.

IMG_2916-1The first thing I needed a was a way to hold the circular piece securely an adjustable distance away from the router bit. The simplest way I could think of doing this was to create a sliding dovetail out of some scrap Baltic birch plywood. I set my table saw to 10° and my fence to about 1-1/2″ and made two cuts in a piece of 6×8″ plywood — each on the flat edge. This gave me the sliding part of the dovetail. I used a similar-sized piece of plywood for the bottom plate and would attach the other two pieces to make the rest of the dovetail.

I realized that I hadn’t replaced my stock blade insert with a zero clearance insert. Not that I needed a zero clearance insert, but my factory insert isn’t very flush, so when the short pieces reached the insert they tipped and the cut wasn’t perfectly straight. This made sliding dovetail slide less smoothly than I would have liked, but I didn’t want to spend any more time and materials to fix it.

 

slotsTo lock the sliding dovetail in place I milled two 1/4″ slots in the bottom plate to accept 1/4″ bolts. On the bottom of the slots I milled another hidden 1/2″ slot for the bolt heads. I simply glued and pin nailed the fixed side of the dovetail to the bottom plate.

 

IMG_2921To finish off the top assembly I drilled two holes in the moving section of the sliding dovetail and attached it with some 1/4″ bolts, washers, and wing-nuts.

For the turning mandrel I used a 3/8″ bolt. To hold the bolt securely, I glued a scrap piece of MDF to the sliding dovetail and drilled a 3/8″ hole through the whole thing.

 

IMG_3039Next I needed a way to secure the jig to the table, I used a larger piece of plywood and drilled two holes at the edge to accept T-bolts to bolt the jig to the table. Then after determining the total height I would need, I added another plywood scrap between the base and the top of the jig and nailed the jig together from the bottom side.

 

IMG_2922To use the jig to fix the yo-yo, I cut a template out of some 1/4″ MDF about the same diameter as the yo-yo sides. Since my hole cutter uses a smaller diameter pilot, I ran into the same problem that enlarging the center hole would move it slightly out of center, but this time I fixed the problem by mounting the template on the new jig and slowly moving the template towards a straight router bit and turning the template with my ratchet. I removed material until the bit touched around the whole circumference.

Once the template was made, to fix the yo-yo sides, I mounted the side and the template on the jig and chucked a pattern bit into the router (see first picture). This was how I was able to turn the oval into a circle again. Then I just mounted the other yo-yo side onto the jig and repeated the process. Now I had two round, matched yo-yo sides again.

 

I also used the jig to round over the inner and outer edges again. After resanding the sides, I put the yo-yo back together again and tried it out. The yo-yo was no longer out of center. I used two different ways to verify this: by touching the edges when the yo-yo is sleeping and feeling the vibrations of the string. When I touched the edge of the yo-yo when it was sleeping, it felt almost like one continuous surface, not like before when I could feel that it was only momentarily touching my finger in the high spots. I also noticed that the string was vibrating much less. The bad news was that the yo-yo was still wobbling a bit and listing to one side. I can make the wobble worse or better by twisting the sides to different orientations, but I can’t get it perfect.

I think one of the problems is that I don’t have a more accurate scale. Even being a 1/2 a gram off will make a huge difference to something spinning this fast. Maybe a better tool would be a balance. I don’t need to know the exact weight, just that the two sides are exactly equal.

Permanent link to this article: http://workshop.electronsmith.com/content/jig-routing-round-objects/

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