Stew Mac's Rolling Jig
© Frank Ford, 7/12/00; Photos by FF
Here's another fixture from Stewart MacDonald's toolmeister, Don MacRostie.
Most mandolin and archtop guitar builders fit their bridges to the curvature of the instrument top by sanding the bottom of the bridge with sandpaper held right to the top of the instrument. The problem is, how do you keep from rocking the thing sideways as you shape. In order to fit the feet accurately by sanding, you have to rub the bridge back and forth quite a while.
Here's Don's tool:
It's an aluminum bracket that screws right onto the bridge base, using the bridge's own adjusting nuts:
My bridge fits poorly, but it sits at the correct angle, so I'd like to preserve that angle. I hold the bridge firmly against the top, supporting the jig off the face of the mandolin:
As you can see, the little plastic wheel is at its highest adjustment, about 1/4" off the face.
Then I loosen the adjusting screw, allowing the wheel to drop to the surface of the top:
It couldn't be easier to get a perfect adjustment. If I wanted to change the angle at which the bridge sits, I'd simply view it from the side as I raised or lowered the wheel.
Now, holding some fine sandpaper in place I can rub fore and aft, fitting the bridge to the top quickly and accurately:
Notice that I have a piece of low tack masking tape protecting the finish under the wheel. It took me a few seconds to get the hang of using this device, and I found it absolutely necessary to maintain good downward pressure over the wheel to avoid the bridge rocking forward and messing up my sanding job.
Sanding the bridge in this direction rather than sideways is much faster because I'm sanding across the grain.
It is important to note that this technique depends on the top being reasonably uniform in its arching. That is to say, an old dimpled top is problematic. If I were a builder, I'm sure I'd use this method for fitting. As a repairman, I'll use it as well, but more as an adjunct to my regular bridge fitting by scraping.
The model Don sent me obviously fits the Gibson style mandolin bridge. I'll be checking in with him about these:
My narrowest adjustable mandolin bridge and widest archtop guitar bridge won't fit this prototype, but Don MacRostie tells me he's got that in hand. The real tool will be wider, with slots to accommodate a full range of adjustable bridges.
Look for the production version of this little fixture in the Fall 2000 edition of the Stewart MacDonald catalog.
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