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Question.
I have my homemade tower which has worked great. I used it on two previous guitars with no issues. On this last guitar as I was routing the purfling channel, which is only .050" deep, through the waist area the cut was rising to around .035 to .040", on both sides. My first thought was that the base the router is attached to was riding on the edge and holding the bit high, which is probably what happened, but when I did a dry run around the waist, I could not see that this was going on.
My question is this. What thickness do you use for the donut around the bit on the bottom of the base? I think mine is around 3/16 to 1/4" thick, cut from a poly cutting board. I'm thinking that it may need to be thicker so there is NO chance that the edge of the base is riding around on the rim.....
Even when doing the top, which is basically flat and level this happened.....can't figure it out.
I use rubber bands to take some of the router weight off like a counter balance, and I removed some so there would be more weight, and that helped but the cut in this area was still shallow.
Any thoughts, ideas or suggestions? I looked at a picture of the donut on the SM tower, and it looks alot thicker then what I have.
John, how thick is yours? Perhaps I need to just get one from you.

Kevin

could the tower be tilting back just a bit? Not exactly square or in plane to the sides.

well I don't think so, it is clamped to the bench and the tower is square to the bench. It cut fine around the lower bouts and on the tail and neck end....

Does the donut taper all the way to the bit or does it have a small flat spot around the hole the bit protrudes through? basically if it does not taper all the way to the cutting edge there will always be a certain amount of shallowing at some point or another. Usually less than .010" on my rig on my tighter waists with my parabolic back.

post a pic
the donut and carriage work together. If the donut rides away from the edge it can cause this problem. The fewer points of contact the better you will be and they need to be as close to the cutting area as possible.
I use a 1/2 in dount.

I measured, mine is 1/4" thick. It is tapered away from the bit, but I do have a flat area about 3/16" in front of the bit before the taper starts. Perhaps this is too much? It also fans out about 1" from the bit, maybe this is more than I need. There is almost no clearance between the bit and the beginning ledge of the donut.

Also, if this matters, I cut the binding channel first; .070" (no problems here), then the purfling channel at .210".

I just went to take a picture of this and the battery croaked in the camera, it is charging and in a few minutes when there's enough juice I'll get the pics and post.

Kevin

here are some pictures of the donut, etc.

Kevin

Is it possible that the clearence between the donut and the router bit is acting as a flat spot, so that instead of riding on one point of the guitar top it's riding on two points in the front and back of the router bit? I hope you get what i'm trying to explain.

my money is the base us hitting the back. Be sure the only part touching the body is the donut. One reason I don't use that mounting system. It can work but you may need to trim it back some.

Thanks John, I figure so too, but when I ran it around the rim turned off I couldn't see where. I am making a wider purfling cut than I have in the past, perhaps that puts everything in a different place. I think I will replace the donut with a thicker one to be sure.

Kevin