Tuesday, December 8, 2015
Hi all!
I've been working on a new banjo project and came up with a different approach to getting the rim in-to-round with a router, rather than a lathe, than I had been using. Relative to my previous method of routing down into the rim layers with the circle jig ( http://www.banjohangout.org/topic/309690 ), I am now using plexiglass templates I made with my router and a circle jig that are designed to run along a bearing at the top or bottom of a router flush bit, which I have found produces a much cleaner result.
I bought the plexiglass at Home Depot, one large sheet ($20) was enough for one set of templates for a 12" rim.
The templates consist of two parts made of three pieces.
The first piece is a 13" square with the desired inner diameter of the rim plus 1/8" for play (11 1/4") cut out of it. I used a router circle cutting jig to cut out the middle.
The second piece is a 12 1/8" diameter circle I cut with the circle jig.
The third piece is a 11 2/8" diameter circle I cut with the circle jig and glued to the second piece using the small hole from the circle jig as a centering anchor.
Here is a pic of the templates apart:
Here is a pic of the templates fit snuggly together:
It is important to rout the inner cut first as you can glue shoulders to your block rim octagon or hexadecagon that can be screwed up into the corners of the 13" square of plexiglass. Take care to cut depressions in the plexiglass so the flat head screws that go through the template and into the shoulders are flush. Then place the template on the router table and guide along a flush bit with a bearing at the bottom.
NOTE: It is very important that you remove most of the material from the rim up to the line of the template with another tool, such as a band saw or jig saw, before you place this template assembly on a router table. If you attempt to remove too much material from the inside of the rim through this approach, the router can grab into the wood and hurl your project across the shop. I know this because I meditated on the potential issues that could arise for a prolonged period of time, mentally working through every possible problem, and surmised that this was possible. Not because I threw a rim across my shop. And destroyed the first template.
A good way to remove that material before attaching to the template is to glue up the block rim as two halves you can use the band saw to trim down, or to leave one joint unglued that the band saw can enter through. If you are gluing up thinner layers of a multi-layered block rim, you can also use a jig saw on the complete glued up rim to trim close to the template line.
Here is a link to some videos that cover a bit of the pre-trimming done by Dan Pennington: http://www.banjohangout.org/myhangout/videos.asp?memberID=5299
Here is a pic of the inner diameter template with the rim, note where the shoulders would be screwed to the template (shoulders not in pic):
Once the inner diameter is cut with the flush router bit, the second and third piece template assembly should slide right onto the rim with the third piece inside the rim and the second piece resting on top of the rim.
Again, when making the outside flush cut, it is important to make sure you are removing as little material as you can from the outside of the rim, trim the outside close to the template line with a band saw or a jig saw beforehand.
Here is a pic of the second and third piece assembly with the rim:
And here is a pic of the routed rim:
DPG
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