Thursday, May 30, 2013

Close-Up on Shimming

For those of us in the flute world who are not repair technicians, the concept of "shimming pads" seems pretty straightforward.  All you have to do is take out the pad and put a circular shim in the cup, and you'll be set -- right?  Well, not exactly.  Shimming is a repair skill that takes a lot of practice and a very careful touch.

Shimming is used for pads in two scenarios: pads that are "leaking," and pads that are "light."  Repair Technician Rachel Baker tells us that there is a distinctive difference, and she can determine whether a pad is leaking or light by using the feeler gauge.  When a pad is "leaking," there is no "drag" at all on the feeler gauge.  In other words, you can close the pad down on the gauge and pull the gauge through without any resistance.  Technicians check the pad all the way around its circumference, so it is definitely leaking when there is no drag at all completely around the pad.  When a pad is "light," that means that the pad seal is not even, and you can determine this (again) by using the feeler gauge at different points around the pad's edge.  In the case of a light pad, there will be some drag on the feeler gauge when you close the pad and try to pull out the gauge.  As you check around the pad, if there is less resistance or "drag" on the gauge in certain points, the pad is considered to be "light" in some areas.

So, given the different scenarios of light and leaking pads, shimming may require the technician to use layers and partial sections of shims of varying thicknesses.  Rachel tells us that she uses adhesive to secure the shim in the cup and for holding shim sections together when layering.  For a full shim, she said that shims of any of the varying thicknesses may be used.  For partial shimming, she most often uses the thousandth or half-thousandth (of an inch) thicknesses. These shims may be seen in the photos -- orange for the thousandth, and silver for the half-thousandth.  The silver shim is the thinnest -- and it is also the same thickness as the feeler gauge.

As we can see, shimming definitely takes time and a careful hand.  How does one know where to shim, how big of a section with a partial shim, and which to use?  Well, it all takes practice and a careful hand... 

Frequently used for partial shims -- orange and silver.
Using the feeler gauge to check for "light" and "leaking" pads.
Cutting a partial shim.
Adding adhesive.
Positioning the partial shim.
Close-up on partial shim.

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