Official Luthiers Forum!

The success of this type of jig would rely heavily upon the quality of the draw slides, you want a pair with no slop, the quality of the lazy sues used, once again no slop and if there is any a little peening of the race to tighten things up, and the size and design of the small doughnut that fits to the base around the cutter as this is the only actual contact point with the top or back when your cutting the ledge.

Have a look at these things Filippo, especially the doughnut as the problem you describe make me think maybe your base is making contact with the back or top where it should not be and this is lifting your base. The doughnut is meant to give clearance to the inner edge of you base all the way around the surface you are working on whilst the indexing bearing is running upon the side.

Cheers

Kim

One important thing to check is that the guitar top is level all round, and in the same plane as the trimmer base. Also that only a small part of the doughnut is riding on the top. I use a small doughnut about 6mm thick to give me the clearance on the top. I use the same trimmer and set up as you and I have no problem.

This is the underside of my trimmer base.



Colin

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I don't believe in anything, I simply make use of a set of reasonable working hypotheses.

As Colin indicated leveling the guitar top via the holder jig is important. I level it at the four locations that the holder clamps onto the guitar. I also do a dry run where I check all around the guitar to be sure that the donut sits on the top.

I find that I have to apply some pressure, not much, on the router and in the direction of the guitar to not only engage the router bit but to get a consistent cut. Making the 4 climb cuts first is cheap insurance too against blow-out.

But having to apply some pressure on the router in toward the guitar and downward is normal - at least it is in my shop.

This operation is messy eh?

Good job Filippo!

What Colin and Hesh said. I use a mechanics square at the neck block and tail block to make sure they are level with the bench. I also so do same along the sides, but find most time if level at front and back they are level along sides, but why take chances for a few seconds. I lke the carriage to have supports at the front, back, lower and upper bouts and waist. I also do a dry run, but again for an extra probably not need test cut to check and make sure biniding and purf channels will be right depth and width. I mostly do same sizes so usuallly not an issue, but just one bad cut and can have problems. Shouldn't need to press down or lot of pressure, the wieght of the router rig and hands holding it as use it should be enough.

The thickness of the doughnut ensures that the router base is kept clear of the dome on the guitar top or back so that the base only touches on the doughnut.


Colin

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I don't believe in anything, I simply make use of a set of reasonable working hypotheses.

Fillipo,

I assume you are talking about the depth of the channel cut rather than the width.

One thought - are the joints of the parallelogram a little too tight? The area from the waist around the upper bout is where the rim will drop (depending on the doming you are using) and if these joints are too tight then the donut may not be riding on the edge (marginally above). The jig is so stable that you could easily feel as though you were on the rim with the donut. One thing to do is to crouch down so that you are looking up at the router bit and do a dry run around this part watching what the donut is doing in relation to the top. The upper and lower bout are the areas where there is most curvature gradient from the edge of the rim and if the donut is too wide the outer edge of the donut will ride above the rim edge. The ideal shape of the donut is to have as small as possible a rim around hole where the router bit goes through and then to slope up away from there to give a minimal contact area. This means you have to take care that the donut does not slip down off the edge, but with the stability of the jig and if you are pushing into the body as you go around this is doable.

Having the rims perpendicular to the top edge the donut rides on is more critical to the width of cut. If the sides that the bearing rides on are "inwards" from the rim then the channel cut will be too wide (and probably a little too deep), whereas if they are "outwards" then the channel will be too narrow (and probably a little too shallow).

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Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010

you refer to an "up spiral" bit. spiral cut bits all have up spirals. the orientation of the spiral determines whether it is an up cut or a down cut. if you are using a down cut, which will leave a cleaner edge, it may have a tendency to clinb a bit, which could explain you depth of cut problems. an up cutwill tend to dig in more an hold itself down more and maintain a even depth of cut with less down pressure.

the other problem can be with the width of your dough nut's flat area around the bit opening. if it is at all wide it may raise the bit . i curve mine up immediately with no flat to prevent that.

you refer to an "up spiral" bit. spiral cut bits all have up spirals. the orientation of the spiral determines whether it is an up cut or a down cut. if you are using a down cut, which will leave a cleaner edge, it may have a tendency to clinb a bit, which could explain you depth of cut problems. an up cutwill tend to dig in more an hold itself down more and maintain a even depth of cut with less down pressure.

the other problem can be with the width of your dough nut's flat area around the bit opening. if it is at all wide it may raise the bit . i curve mine up immediately with no flat to prevent that.

Ya know, I studied that design and really considered all the possibilities. But being a newbie myself, I decided to go with the standard Williams and the LMI bearings (crap, they looked a LOT bigger in the LMI catalogue!, kind of feel like I did after I bought my wife's engagement ring).

That Dave White setup looks cool, but a bit advanced for me at this point in time (not knocking it, just admitting my level of competance). Dave, it really is quite cool! Good news is, I can always switch to it later.

Good luck!

Mike

Filippo, I think Michael answered it. I use a williams jig on archtops, with the stewmac binding bit. (So my doughnut is 3/4" thick to get enough clearance for an archtop....the "flat" however is only a 1/4" ring around the cutter - yours looks big but maybe no problem). As has been said, level the body neck to tail & side to side. Then level the Jig the same way (forget the bench). Power off, move rig around the perimeter to see if it rises - if it does, the doughnut isn't thick enough (or the flat is too big). If it doesn't, the downcut bit may be causing it to climb. Narrower cuts ("till bearing contacts) or just use a regular rabbeting bit.

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Dave
Milton, ON