Refining Plates & Bracing

Refining Plates

Using a scraper, calipers, finger planes, and fine sandpaper, I refine the graduations of each top after machining. I constantly flex and tap it, listening to what it sounds like and looking for a certain amount of lateral flexibility and just a hint of axial flexibility. I also use light extensively to help me keep things even and smooth, which seems to be important.

light

Brace Fitting

Doing repetitive, highly-skilled tasks can be annoying to me. No matter how much is automated, there are still plenty of opportunities to screw things up, so I try to minimize those opportunities whenever possible.

After spending a dozen or so hours fitting tone bars one week, I thought it would be nice to have a fixture which would automagically get the tone bars to match the geometry of the top.. at least within 5 or 10 thousandths of an inch or so. From there I can scrape it to final shape, no problem.

CNC is not an option, because I adjust the top plates by hand, and that process strays from the “perfect” CAD geometry.

fittingJig1

Its skeleton is a parallel torque reaction arm, which I initially purchased for doing binding. It keeps the cutter dead plumb but free to move.

It also counterbalances the router so it only “weighs” about 8 oz. Not enough to damage the spruce plate, but enough to keep the assembly grounded.

fittingJig2

There is a Delrin probe which slides really smoothly but doesn’t dent the spruce. The probe is coaxial with the cutter.

The workholding fixture above the unit holds tone bar stock.

Setup consists of aligning the tone bar’s location with a line on the table, and clamping the top plate in its quick release rim fixture to the table with the blue hold-downs.

The torque reaction arm keeps the cutter from moving wildly. Because of the torque counter-action of the “elbow” on the unit, this router can be operated with one finger, it is very stable and does not “dance around” like a Dremel.

With some practice, tone bars can be machined, then hand-scraped to a perfect fit in 10-15 minutes per instrument. Occasionally I mess a set of bars up.. which used to be a major frustration. No sweat, I can make another very quickly.

Finally, the tone bars must be refined and shaped to work with the rest of the soundboard design. This is done with planes and scrapers, and is gauged by tap tones and flexibility.

Comments

  • Hi Marty,

    Very clever… Does anyone else do this? Collings maybe?

    STP

    Stephen PorterApril 29, 2013
  • Nobody that I know of.. though Tom Ribbecke has a cool method of his own, where he springloads the rough brace down against the top and then slides sandpaper back and forth.

    Marty JacobsonApril 29, 2013

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