Thickness Sanding Back Plates

One of the harder operations in the luthier trade is sanding the top plate and back/side sets to a consistent thickness. This is most easily completed with thickness planer and or a thickness sander, and if this is the equipment used in your shop, you need not read this article.

If you do not posses a thickness planer or a thickness sander, then this article if for you.

The goal here is to obtain a the top plate, back plate and sides that are very consistent in thickness.

I prefer to thickness sand the back and top plates after they are joined together at the center seam. This assures me there is enough wood to thickness sand the plates to proper thickness after the joint is glued.

Next: Sand one side of the plate after they are joined. Sand just enough to get all the roughness off the plate consistently and do not concentrate in any one area as you will thin that spot too much. Is is essential to produce one side that is consistent over its’ entire surface – so go slow at first.

Setting The Drill Stop Depth With A Feeler Gage

Setting The Drill Stop Depth With A Feeler Gage

Next comes the trick to obtaining the proper thickness of the plates. After one one side is completely smooth, chuck a small drill bit (1/8″ will do) in a stationary drill press or a drill press attachment for your hand drill. Set the depth stop so the bit will stop a bit thicker than the final thickness for the top plate or back plate. (for this example let’s say you want the top to finish out at 3mm. Set the drill stop to 3.5mm. To obtain accuracy for the drill stop depth, it is best to measure the thickness with a feeler gage set – the kind that is use for gapping spark plugs. Find a combination that matches the desired plate thickness.

Place the plate on the table of the drill press, finished side down and protect the top from denting with any dirt, chips, dust etc. Make sure it is pressed down firmly against the table and not bowed upward. Begin to  drill down into the plate in all different areas of the plate – roughly every 2″ o.c. is adequate. All the time make sure the plate is pressed flat on the table, by placing fingers closely on each side of the drill.

Back Plate With Drilled Dimple Pattern

Back Plate With Drilled Dimple Pattern

Once this operation is finished, you can now take out the plate and sand with a power sander, metal scrapers or a power hand planer. Gage the progress all of the time to make sure the drill dimples do not completely disappear. Continue to sand or plane until you just have tiny little dots or depressions very consistently over the entire surface plate. This would be the top side of the plate and it leaves the final thickness sanding until the plates are mounted on the instrument.

That’s it! Very simple and effective.

Common Issues:

If the plate is allowed to flex upward, even a little bit, you will ruin the top. Accuracy of the drill press and consistent downward pressure near the drilling site for optimal results.

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