Official Luthiers Forum!

There is probably a discussion of this somewhere, but I'm feeling lazy! How much shoulder is recommended on the doughnut used to ride on the back or top, before it curves up?

_________________
Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

Not very much. An 1/8th of an inch should be more than enough. Don't have my jig handy, but I'm guessing it pretty close to an 1/8th.

Aaron

_________________
Aaron Craig

Here's my problem, and I've never used this method before, but my top purfling channels are wide - generally 4 - 5 mm + the binding channel. So I have a total of something in the range of 6 - 7 mm total width of both channels. Backs are less, usually. The doughnut that is on the machine has a shoulder in the 1/4" range. That would seem OK, but I know I'll get some variation in depth with that width. Am I thinking about this the wrong way?

_________________
Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

It's best to minimize the footprint but I don't think 1/4" will be a problem.

Waddy,

No, you are thinking the right way. My initial doughnut had a shoulder that was about 1/4" and I found that produced a height variation that I found unacceptable. My current one is a little less than an eigth and that is much better.

Pat

_________________
There are three kinds of people:

Those that make things happen,
those that watch things happen,
and those that wondered what happened.

Then, my question becomes, how do you have wide purfling channels with a doughnut that narrow? Maybe you don't.

_________________
Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

Do away with the bearing and cutter set and adapt the jig to accept a trimmer sitting on a second 6" lazy sue with a 1/4" downward spiral as per Dave White's conversion where a fixed bearing riding on the instrument's side held to the tangent determines the width of cut.

Cheers

Kim

Kim, then what would be setting the depth of the cut? Isn't that where the variation is at in Waddy's setup?

_________________
Formerly known as Adaboy.......

Exactly!. Kim the problem isn't the width of the cut as it is the fact that the doughnut still has to ride on the flat or not so flat surface of the uncut portion of the top or back, regardless of what bearing system you use. To me, that means the shoulder of the doughnut has to be, at least, a bit more than the width of your widest dimension. Is that not correct?

_________________
Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

I don't think so. This is off the top of my head - the hole you cut in the doughnut is slightly larger than the diameter of your rabbit bit, right? At least I think that's general practice. So no matter how wide you cut you purfling channel, you're doughnut ring will always rest on the top just slightly beyond the cutting edge of the bit.

This was a quick post, and maybe I'm missing something.

Aaron

Edit: To more directly answer your question Waddy - no, I don't think the shoulder of the doughnut has to be at least a bit more than the width of your widest dimension. The only purpose of the ledge is to provide a stable contact point with the top of the guitar. Ideally, you would make that point of contact as small as possible to decrease the affect the back and top arches have on the cut. But as Todd mentioned you run the risk of damaging the top if you go to much below 1/8, plus you loose stability.

_________________
Aaron Craig

If I'm understanding your question, Aaron has it right if you're using a bearing cutter set. Thus the cutter is always in the same relationship to the "doughnut foot" regardless of the with of cut. I doubt I told you anything new, so maybe I'm missing some in the question.

_________________
Jim Watts
http://jameswattsguitars.com

I want to that I changed to a slight angle from a flat donut. This way I locate on points as close the the cut as possible. On the top the radius is really not much of an issue , on the back the radius and angle will magnify the problem and that is where the angle helps to eliminate variation.

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Someone came up with the idea of using a wooden cabinet knob from HD as the donut. I remember that Don Williams saw the mod and gave it a thumbs up. I don't have access to mine at the moment, but I remember it being something like 1-1.25" in diameter with a slight radius that keeps most of the knob off the guitar body. I've only used it once, but got good results on a small tight waist body with a 16' radius back.

Bob

That all makes sense now. I'm used to cutting the purfling channel first, which would be backwards using this method. That would mean that the doughnut would have to reach across the channel when cutting the binding channel. I probably need to modify my doughnut now. Ugh. Thanks all, for your input.

_________________
Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

OK, I sanded a 7 1/4" radius on the doughnut. I'm hoping that will be enough to clear most of my taper and back doming. It has to be better than a 1/4" flat, anyway. It still leaves me almost an eighth between the doughnut and the cutting edge of the bit.

_________________
Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

No hijack intended, but what are you guys using for the donut - UHMW, wood?

Kevin Looker

_________________
I'm not a luthier.
I'm just a guy who builds guitars in his basement.
It's better than playing golf.

The depth of cut is set in the same way as when the trimmer is used as originally intended, you slide the body of the trimmer up or down in its housing. The doughnut can be made smaller with this system because you are only using a 1/4" bit to make the cut. Because there is no bearing involved, there is nothing but the cutting bit itself to dictate the required width of the doughnut.

http://www.defaoiteguitars.com/page36.htm

Cheers

Kim

A little update. Here is my binding machine - made by Garry Hallam.

Here is the original doughnut. Sorry for the bad picture, but the shoulder is flat.


Here it is after my modification. 7 1/4" domed shoulder now. No flat at all.

I cut the binding and purfling channels on the back of #8 with it this afternoon, and the binding channels, as near as I could tell, only had a variation in depth of 0.05 mm, which I can live with. I measured at the waist, the tail area, and near the neck joint. My backs have a side to side radius of 25' and a tail to neck radius of 15' and a downward taper of about 15 mm toward the neck. Maybe I'll find variances I didn't notice when I start gluing up. We'll see, I guess!

_________________
Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

Waddy that's a nice looking binding machine. Sounds like it works well too.

_________________
Jim Watts
http://jameswattsguitars.com

Thanks Jim. I rigged it so I can attach it to my solera. That way the back can be done without jigging. I'll have to be more creative for doing the top, but I think I can figure it out - neck rest, clamp, some wedges under the upper bout, etc.

_________________
Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

This is why I got away from this style of cutter and use the Luthiertool binding jig. My guitars have a 15' radius, but they have a 1" end to end taper, a very tight waist, and a very deep cutaway. Combine that with a wedge and it's impossible with the Ribbecke/LMI/Fleishman style of cutter because 1) It's very difficult to get all of your sides square to the table in the carriage (made easier with laminated sides); 2) when looking at the back, with the neck to your left, as you move the cutter from the end to the head the body is running away from the cutter. The contact point may only be 1/8, but the donut runs into the back as you go toward the headblock; 3) with the wedge the cutter cuts full depth on the apex of the lower bout on the wide side and runs away terribly on the bass side of the upper bout because you have both the end to end taper and side to side taper at their max in this location.

I don't have these problems with the hand held Luthiertool rig. As long as your sides are flat, you can even tilt it some to keep it resting on the back and it averages everything out. I find I do much less cleanup post-rout using this tool than I ever did with the Ribbecke (which incidentally, he intend that the router tip with the back axially on the face-mounting plate so it could follow the contours of the back).

-- db

I was using a Luthier Tool type binding cutter base, and was having terrible problems with unevenness. This is an attempt to correct some of that.

_________________
Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars