Tools and Materials:
Dremel Rotary Tool
Dremel Router Base
Carbide Channel Routing Bit
Binding Router Guide
Design Your Binding & Purfling Strips:
Decide on your design and do a rough mock-up of the pieces so you can make actual measurements before making the channel cuts. Installation of the purfling and bindings is usually a multi-step operation as you are dealing with very fragile pieces of wood or other marquetry and taking it slow and easy is the best course of action.
Purfling Options to Consider:
Although the options are almost limitless, lets look at a few standard options you may want to consider for your purfling (the purfling is the marquetry that is used between the top plate and the binding strips.
Standard Black/White Strips: You can purchase standard strips already glues to save you a lot of grief if you like. They are available in BWB, WBW, and WB with wood marquetry. Generally plastic strips are available in the same patterns with several thickness options.
Abalone Strips: Abalone comes from Abalone Shell and is very colorful and iridescent. Colors range from Green Abalone to Red Abalone and installation of this trim will be the subject of a future article.
Herringbone: The Herringbone trim was make popular by the C.F. Martin Co. with the prewar Martin D-28, also referred to as a herringbone or just a “bone”. We will cover installation of Herringbone trim in an article as well. It is best to purchase pre-made Herringbone strips for installation purposes.
Route a Test Strip:
To be assured of a perfect routing job, route test channels in a scrap of wood and test fit your mock-up purfling/binding design. Cut a bit on the shallow side that will allow about .5mm of your binding/purfling strips sticking above the top or back plates. This is easy to scrape down after securing the binding/purfling.
Positioning the Guitar:
It is best to place the guitar in the inside mold so you have it solidly captured and it will not move. You will want consistent access to the entire perimeter of the guitar, so clamp the tongue (the portion of the form for the neck) on your workbench and have the form sticking out from the workbench at a 90 degree angle. This way you can walk the router around the guitar perimeter. Make sure you have a clear path to do so.
The Dremel Rotary Tool:
I have found that the best purfling/binding channel cutting tools is the Dremel Rotary Tool, that is fit with a
Binding Channel routing adapter. The setup allows micro-adjustment of the channel depth and width and when used with a proper Down-Cut Routing Bit, will make perfectly cut channels, with very consistent results.
Another popular option is the Bosch Colt Palm Router. It has a lot more power that the Dremel and is still small enough to be easily guided around in instrument top or back without worry of dropping or slippage. This router can be used best with a 2 blade rabbeting bit. Use bearing to adjust the cutting width for the purfling and binding. If you can’t find an exact match for the binding/purfling design you intend you use, you can easily finish the last cut with the Dremel. It is wise to have both these tools in your shop.
Start Routing the Channel:
Obviously you will want to start by routing the purfling channel first, so double check the depth setting. For each pass around the plate edge, do not cut more than about 1 mm of width with the router bit at one time using the Dremel. The Bosch can cut the channel in one operation. Also when you rout the hardwood for the back, listen carefully to the RPM’s of the Dremel. When you hear the router slow down, slow down your feed rate. You should not notice much change in the Bosch as it is much more powerful.
When routing, keep consistent pressure on the the router and be careful not to tip it or your channel will vary in dimension. Under no circumstances should you burn your wood while routing. This is VERY easy to do by the way. You will end up overheating your bit and getting inconsistent results on your channel.
Continue to make your passes until you get within about .5 mm of your required purfling channel width. Now it is time to make at least one and preferably two passes to take just a whisker off the channel as you close in on your goal. This is just as important for the soft top plate as it is for the hardwood back plate.
Here are some top plate cautions:
When routing against the grain, you will have to go very slow as you tend to get fuzz from the grain. I usually reserve the last trip around the guitar for the tiny last cut to clean this up.
When routing with the grain (this is mostly when you are approaching the waist and in the waist itself) it is very easy to pull out some of the softwood portion of the grain, so go especially slow through there, as the softwood may be deceiving to you as it cuts so easily.
Make a Test Fit:
You should always test fit your purfling and binding in several locations around the guitar perimeter. I usually do this with a 1″ or 2″ sample strip mock-up. The sides of the binding should be as close as possible to flush with the sides of the guitar.
If you have some variation, it is possible that your binding jig is not tracking correctly. It should be just below the channel you are routing. If it is not, any little inconsistencies that you have in the guitar sides will be telegraphed into the binding channel and you will get variations.
GLS Tip: Cutting the Back Plate of an instrument with an arched back can create problems with the binding channel quality. As you rout around the edge of the instrument, the cutter of the router will tilt inward and thus undercutting the binding channel to some degree. We have a free tool plan that is intended to be used with the Bosch Colt to counter the effects of the arched back. By placing the router in this jig you will minimize the undercutting of the channel and wind up with a much more acceptable and consistent binding channel that will require minimum touch-up. Check out the Free Tool Plans Section of Georgia Luthier Supply for this free download.