Official Luthiers Forum!

Hello All,

I’ve made a small blunder. I’ve cut the purfling channel to deep. TWICE!!!
The first cut was done with a router jig I made to work with my router table.
It worked fine for a dread, but, not so good for a cutaway. So, for my second
attempt, I decided to buy the dremel attachment from StewMac.
It seemed like a good decision at the time. It’s small, and compact.
It looked to be the perfect tool for tight places. And, I’m sure it is. I guess,
I just need more practice with it. I’ve been using all types of power tools for
35 years, and thought I had a pretty steady hand. However, in this case, I have
been humbled by the dremel.

I ordered a set of b/w/b (.060") purfling to make up for the over cut on the first cut.
I was going to use b/w (.040") and maple b/w binding (.080"). But, now I needed
.140". Unfortunately, when tried to cut it again with the new dremel attachment,
I over cut in some areas as much as .040". This totals a .180" which is the deepest
part of the cut from my second attempt. Most of the cut is at .140". So, now I figure
I’ve got to go with b/w/b/w/b purfling and maple binding totaling .180", and cut it
a third time. Which, by the way, is the scary part, if I have to use that dremel again.

There is one more little problem. Even if I cut it for the third time with success, I’ve
got a slight over cut in a small area, on the top at the lower bout, about .020" to deep.
I posted a few pics below that may help.

All suggestions and criticisms are welcome!

Robert

Here's the router jig I used for the fisrt cut.













I don't know if you can see it or not. But, there is a spot
about right at the center line, that I'm sure will show after
the purfling is installed.

_________________
Everything has beauty, But, not everyone see's it!

Time for herringbone

If it were me, id cut the entire purfling channel to the same depth, and make a purflling that fills the area, if that means herringbone, or maybe a combo of strips, that is what id do.

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Lance@LuthiersForum.com

On any system that uses an adjustable guide bearing that is seprate from the bits shaft, The plane between the guide bearing and the bit must be at all times perpendicular to the surface you are cutting. If the imaginary plane line between the guide bearing and the bit is tilted off perpendicular to the cut plane you expose more bit to the cut plane. This is easy to do on curved surfaces because it is difficult to hold the router and turn corners with out getting this plane out of wack.


This is the advantage of bits with interchangeable guide bearings vs. a indipendent adjustable guide bearing or as in the Stewmac base an adjustable guide bushing. Because if the bearing and bit share the same shaft, IE the same center point at all times so no matter how you turn the router the bit and bearing are always concentric to each other and therfore always cutting perfectly perpendicular to the point of the cruve you are at. Not the case with an indipendent adjustable guide bearing that moves closer and further away from the bit

Besides needing to cut at the tangent and with that StewMac cutter you can easily tip the top of the dremel in and out and getting out off plumb. Also I think the cutting bit diameter in larger then the bearing diameter which means that you can cut deeper then the setting at the tangent. If the bearing in a larger dia. then the bit dia. you can't cut deeper then the tagent point depth unless you tip the top in towards the body.
I think you should just throw that dremel attachment in the drawer or take it to the scrap yard. I think it's not going to work for you and you are just going to have problems with it. I'd suggest making a binding machine and be done with it. I used that StewMac on my first and it worked for me but that was enough for me. It doesn't cost that much to build a universal style binding machine. Some plywood a set of drawer slides and some plexi and maybe a piece of UHMW for a shoe. But I think this has all been said before.
You might also try and see what that gap looks like. I closed up some small gaps on a spruce top at the purf lines by by wetting the gap area a tad and placing a damp cloth over the gap and running a soldering iron over the cloth steaming the gap close. It will work for some small gaps. Might be worth a try.

I agree with Lance!
Go with a nice rope or herringbone .

Always make practice cuts on scrap(checking the fit) ;before routing bindings & purfs !

Mike

_________________
Mike Collins

A few years ago something similar happened to me a couple of times before I called John Hall in a hurry and got one of his binding jigs. Get or build a secure binding jig, what you have on the router table and the hand held router require virtuosity I personally do not have.
Good advice to rout a larger ledge for multiple lines or herringbone purfs, I ended up with what's on the pic on one mishap.
Be aware that some commercial herringbone can be tough to bend on a cutaway and tight waist.

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

[quote="Michael Dale Payne"]On any system that uses an adjustable guide bearing that is seprate from the bits shaft, The plane between the guide bearing and the bit must be at all times perpendicular to the surface you are cutting. If the imaginary plane line between the guide bearing and the bit is tilted off perpendicular to the cut plane you expose more bit to the cut plane. This is easy to do on curved surfaces because it is difficult to hold the router and turn corners with out getting this plane out of wack."


Hi Michael, what you say is very valid where the guide bushing is smaller than the cutting bit diameter.
When the bit is say 1/4" and the guide bushing is 1/2" the geometry works the other way and the cut gets shallower if you get off perpendicular.
So you can go around agan and refine it to the intended cut depth.
Mine is set up like this, but post mounted not handheld.
regards
Jeff

Yes in most cases! However if the distance from center of guide bushing to center of bit is greater than the differance between the bit radius and guide bushing radius you can still over cut with a larger guide bushing than bit. But this in not likely to be the case in what we do cutting binding channels.


I addressed the smaller guide bushing situation because that was the case he was dealing with.

My main point is that the on position that an adjustable guid bearing/bushing cuts the depth properly is when the center between the bush and bit is perpendicular to the cut. Anthing else is either over cut or undercut.

Probably most of us have at one time or another installed wider purfling lines or herringbone that was made neccessary by routing uneven or too deep purfling channels, i know i have. I tried to use a dremel to bind my first guitar, and have had an unatural hatred of them ever since, the dremel just does not have the power or precise enough bearings to do the job. I dont know why stew mac still sells that base, it seems to mostly result in horribly uneven channels. I guess the point is, you are not alone and wider purflings are the way to go. The good part is you never have to tell anyone that your fancy purflings are not on purpose.

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Jordan Aceto
Ithaca, NY

Thanks everyone!

A lot of good feedback here!

Laurent, thanks for posting the pic, the multiple lines look great!
I’m sure that’s the best way to go for this guitar.

Chris, thanks for the advise. I guess I knew down deep inside you were
right the first time you told me about the binding machine. I’m going to put
that dremel attachment in the draw, and build or buy a binding machine before
I go any further with this project.

I do have one concern, and that is how well will a router bit with bearing work
having the binding channel already cut?

Robert

_________________
Everything has beauty, But, not everyone see's it!

Robert,
A lot of us have been in your boat as you can tell.
What type of bearing are you referring to? Another larger edge bearing or the center mount bearings?
The LMI bearing and cutter has set had the bearing with a spacer between the bearing and bottom of the bit so the bearing should ride on the side below you binding channel. I'm not clear as to wether the StewMac has a spacer now or not and if the other sponsers here sets do or not either.
The edge bearing all depends how it is made.
I made my own so it works as it rides below the channel.
You can get a look at mine in the tutorial and you can go here for some ideas on an edge guide also.
http://www.doolinguitars.com/articles/bindrout/
Also don't rule out using Ablam purfling. My favorite solution.