Base Shim

Stopped into the repair shop and found our technician, Rachel, working on an overhaul.  As usual, she had a tray full of keys and mechanism components, including some round, rubbery discs that we hadn't noticed before.  So, we had to ask -- "What are these?"  Well, Rachel explained that they are "base shims."  They are used at the bottom of the cup, underneath the shims and pad.  These base shims are used with Straubinger pads, because Straubinger pads are much more compact and take up less space in the cup.  The base shim helps take up the extra space so that the builders and repair technicians do not have to put an over-abundance of shims in the cup to raise the pad to the correct position.  Rachel added, "One thick piece of material is much more stable than many thin pieces."  Certainly makes sense to us!

Taking a closer look at the base shims, we noticed two types.  One type is a rubbery plastic that is somewhat translucent.  It is used in cups that are flat on the inside.  The other type of shim is made from a harder plastic and has "rings" that interlock with the ringed surface of the bottom of cups with a "rings."  The top of the "ringed" shim, however, is flat so that shims and pads can sit properly.  Rachel also told us that base shims have been made out of different materials over the years.

How long have flute makers been using base shims?  Well, it's a bit difficult to pinpoint exactly, but for many years, flute makers used felt pads.  The felt pads are "fluffier" and take up much more space in the cup, so they do not need a base.  Felt pads may be made from either woven felt or pressed felt.  The Straubinger pad is made from microfiber, so it is much thinner in comparison with the felt pads.  Rachel mentioned that there are many types of pads available now, and many will use a base shim because they are much more thin or "compressed" than older felt pads.  Base shims are pretty sturdy, so in terms of "repair or replace," they generally would not to be repaired or replaced -- they are just part of the pad-fitting process.

Pads left to right: Straubinger, pressed felt, woven felt.

Rubbery plastic base shims for cups that are flat on the bottom.

Cups with "rings" inside take a different base shim.

Harder plastic base shim for cups with "rings" inside.  Base shim's "rings" interlock with cup, and flat side is up.

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.