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PostPosted: Thu May 26, 2011 6:59 pm 
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Walnut
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Joined: Tue Apr 19, 2011 12:47 pm
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First name: Ben
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Country: Canada
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It is a tense time when you are at the point in your first guitar where you have to cut the binding channels. I am at that point, and could use some advice.
I am building a classical guitar, and I am wondering about the safe depth that I can cut to form binding and purfling channels. My design scheme has a total binding/purfling width of 4.5 mm, with sides that are 2.5mm thick, and kerfed linings that are 6mm thick and 16mm tall. Is it really safe for me to cut a single square channel 4.5mm wide, removing all the side wood and leave only 5mm of lining for the top/back to cling to? Will this make my top/back less securely glued?
It seems to me (in my research) that most steel string builders use a stepped method wherein the purfling channel is cut shallower than the top thickness, and a second channel that is slightly less than the side thickness is cut as a step-down for the binding (see attached diagram). Is this even true of most steel string guitars, and is it a method often used on classical guitars? Or am I just paranoid that my work up to this point may be ruined by one fell swoop of a router?


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PostPosted: Thu May 26, 2011 11:01 pm 
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First name: Tony
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Personally, I would not want to cut completely through the side wood into the kerfing if I could avoid it. I would do it like a steel string. Some might say the binding/ purfling combo will replace the wood you removed, but there is no guarantee that your glue joint it perfect all the way around, so why risk it?

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PostPosted: Thu May 26, 2011 11:11 pm 
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I'd cut a stepped channel .05-06 deep for the purfling line.
It also helps to scribe the cut line with a marking gage to prevent tear out.
If you don't have a markinking gage some shellac on the area can also help also help a little with tear out.

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PostPosted: Wed Jun 01, 2011 4:17 pm 
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I concur with the guys above. I would use a stepped binding/purfling channel. I'd not be too worried about routing into your linings for the bindings depth (excluding the purfling), though with 2.5mm sides this probably will not be an issue for you. With thinner sides or thinned cutaway areas, it's pretty common to cut into the linings. As long as you do a good job binding, you're golden.

I second scoring the top for the perfling cut if you have a gramil or marking gauge up to the task. Regardless, I would use a spit coat of shellac around the rim to protect and help reduce tearout. Also, if you are using a rabbiting router bit and bearing to route the ledge, put some green or blue painters tape around the sides where the bearing will ride, make your initial cut. Then remove the tape make your final cut. The few thousandths of an inch the tape adds means your final cut is just a few thousandths and it will be very clean with less chance of frayed wood.

Good luck!

Keep in mind I've never built a classical guitar, only steel string. But when I do, I will no doubt use a familiar stepped binding/purfling ledge.

Aaron

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PostPosted: Wed Jun 01, 2011 4:54 pm 
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Koa
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Most wood bindings are about 0.080 thick, about the same thickness as sides. It is not uncommon to route up to or even slightly into the kerfing for these. It is also not uncommon to add a second layer of something (herringbone, pearl, fiber, wood, etc) - this is purfling and is usually not as deep (into the top). That can be done with the stepped channels shown in the SM picture. I assume that yours will be multiple pieces, it will be very hard to bend wood binding that thick.

A trick that works really well with pearl is to laminate a small square piece of teflon in the purfling channel. After the binding glue has cured you pull the teflon and have nice little channel to lay the pearl into. Don't know if that would work with your scheme or not, I have used it on several guitars with style 42 purfling.

When I did my classical I used simple rosewood binding against rose, but offset it with a thin maple strip.

http://i51.photobucket.com/albums/f387/ ... detail.jpg


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PostPosted: Wed Jun 08, 2011 4:14 pm 
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Walnut
Walnut

Joined: Tue Apr 19, 2011 12:47 pm
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First name: Ben
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Country: Canada
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Thanks for the tips... shellac, marking tool, tape... I'll use these techniques to cut a stepped channel; hopefully with no tear-out. Now all I need is for my router bit set to come in.


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