Official Luthiers Forum!

I'm designing/building a jig for my PC 310 router to cut the binding and purfling channels. It combines a few features from different designs and is a twist on a jig that Rod True built and posted pictures of here on the forum a while back. On my design, the router is mounted in a table and either the back or top of the guitar will be facing down so it's similar to a hand-held design in how it works........just upside down.

I wanted some feedback from the experienced here regarding one variable.......how much of a lip should I leave outside the router bit to control the depth of cut? To clarify the question, there is an area that will taper up meeting a flat area on which the top/back will ride. How far the bit protrudes beyond this flat area determines the depth of cut. It seems the ideal would be that the flat area ends even with the side of the router bit; however, the "lip" that the top would ride on would be roughly equal to the thickness of the bindings so it would be very easy for the body to slip off this lip. If I extend the lip so it's easier to keep the top/back riding up on the lip, then I increase the error in the depth of cut (assuming the top or back is domed and not flat). So I guess there is a trade-off here........what size lip do you use?

Another question, the lip can extend to the sides of the bit which would help stabilize the body rocking side to side. So here is another trade-off.......the further you extend this lip the more stabil the body while routing but you increase the error slightly as the body is tapered from the tail to the neck block. Conversely, you could leave little to no lip either side of the bit and have essentially no error......but the body won't be held stable and if you tip the body sideways, you could mess up the channel. So do you extend the lip either side of the router bit? Why or why not? If so, how far?

I looked at a one doughnut made to fit the bottom of a trim router for cutting binding/purfling channels and the flat lip on it was circular with a radius of 11/16" from the center of the bit. This seemed ok for the side-to-side lip but seems excessive for lip in front of the bit (would cause some error on a back with a short radius). I should mention that my design doesn't use a bit with bearings so I was planning on using a 1/4" down spiral bit. If you used a 3/4" bit with bearings it would reduce the distance from the edge of the bit to the outside of the lip which would reduce the error introduced.

I hope that rambling is understandable!

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

There just needs to be enough of a lip that it can not slip into the cut area and allow the cutter to go too deep. If you cut the purfling first then that has to be taken into consideration. I think the problem with the cutter under the guitar is seeing what you are doing as you cut.

Fred

_________________
Fred Tellier
http://www.fetellierguitars.com
Facebook page http://www.facebook.com/pages/FE-Tellier-Guitars/163451547003866

Darryl-
I understand your question- but don't have the answer, unfortunately.
I've had a similar problem with the binding depth varying a bit around the body of the guitar.

I'm not competent in the computer drawing/graphics thing at all, so I hesitate to suggest a labelled picture or drawing, but I know it would help others who may not have made a similar jig to understand the question.

I'll be watching here for the answers!

Cheers
John

Mine works from the top but I allowed an approximately 1/4" flat rim on the doughnut so that if anything slipped the depth of cut would be the same until the bit was no longer engaged, i.e. the bit would not be able to cut too deep. Probably the same thing Fred said

_________________
Steve Smith
"Music is what feelings sound like"

That's a good point Steve and Fred.......if you make the lip in front of the bit at least as wide as the thickness of the binding it would prevent the bit digging into the wood outside the channel if the guitar slips off the lip. This is my first build so I haven't done this step yet......but I assume it wouldn't be as big a consideration for the purfling since the bit is engaged much further so less likely to slip off the lip.

So if the binding were 1/16" thick, the guitar would be engaged 1/16" into the bit and if a 1/16" lip is left in front of the bit, there would be a 1/8" lip for the guitar to ride on. Is that enough?.......or is that asking for problems? Hopefully it wouldn't be an issue if the guitar slips off the lip as I don't think the design I'm considering would allow the guitar to get into the bit. Am I missing anything?

Edit: now that I'm thinking about it more, cutting the purfling may be where you are in danger MOST.......if the guitar slips off the lip. Maybe the lip outside the bit should be at least the width of the purfling channel. Thoughts?

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

Here's some photos of my binding jig, maybe they will help. The doughnut is basically zero clearance. I made the final hole by lowering the rabbiting bit through it. Looks like the ledge is at least 3/16" to 1/4".

_________________
Steve Smith
"Music is what feelings sound like"

Darryl,

From your question it seems to me that you have a good understanding of the considerations. As has already been said, the lip should be just big enough to make it impossible to slip and gouge the bottom of a channel you are cutting. Any amount beyond that and you are introducing unnecessary error when you are taking the router up off flat areas of the guitar.

Pat

_________________
There are three kinds of people:

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

my experience tells me you will lose a little control by having the router below the work.

_________________
http://goatrockukulele.com

My experience is that a router table type set up works just fine for arch top mandolins.












It would probably work on guitars as well, but for those I use this jig which registers off the sides and top/back. As long as you keep the sides firmly against the bearings, the cut will be very smooth. My ”doughnut” is a washer that is glued to the bottom of the jig (not visible in the picture), and the bit protrudes through its middle. Its height is just enough to keep the rest of the jig’s sole from interfering with the cut. The exact size of this part is less critical than you seem to think, just make it reasonably small and yuou'll be all right!

_________________
Rian Gitar og Mandolin

Can someone explain what the "doughnut" is for and it's general dimensions? My understanding is that the doughnut has a lip on the inner hole's circumference which rides on the top of the guitar (when cutting binding channels) this width of the flat part (lip) can be about an 1/8". Then the rest of the doughnut falls away from the lip - at what angle ?
Do they sell these premade already ?

_________________
It's this new idea from recent decades that everyone gets a participation award. - MUX

How did you make the bearing assembly you show here?

So the only part that touches the top would be this washer, not the bottom of the jig correct? could you not angle the bottom of the jig slightly to accomplish the same affect?

It`s really uncanny how certain issues I come accross have a way of showing up in posts made by others while I`m mulling them over in my head.
I tried my Williams binding jig out for the first time a couple days ago and found that the lip on my doughnut was too small, and when it slipped off the top into the purfling channel, sure enough, it took a deeper cut on the binding ledge. Lesson learned in a hurry!
I`m still counting my blessings in the fact that I was testing it out on scrap.
From what I`m gathering, the ring should be at least as wide as the widest purfling you will use plus 1/16". This way, the binding bit will be clear of the cut should something slip.
Coe Franklin

_________________
Give me 50 cents worth of regular.
Check my oil too, if you don`t mind,,,

The doughnut provides a smaller radius surface for indexing the router bit height off of the edge of the top or bottom. Since the top and bottoms of most modern steel string guitars are radiused to some degree then the large surface of a standard laminate trimmer base would result in an incorrect and varying cut depth.

_________________
Steve Smith
"Music is what feelings sound like"

Thanks everyone! The discussion was helpful.

One last question. If I want to make the lip the thickness of the binding plus the width of the purfling to protect the guitar from the bit per chance the guitar slips off the lip, then I need to know the average sizes of the binding and purfling. It seems 1/16" is a comon thickness for binding. What is an average width for purfling? On this build I'm using herringbone for purfling that has a width of 0.135" plus I need to add the thickness of any BWB strips. What are common purfling widths? Is the width of pearl purfling similar to the herringbone I have?

I'm about to say I should just make the lip 3/16" and go for it......<smile> That would give me a minimal lip of 1/4" if the binding is 1/16" thick. That seems like plenty of lip if I'm not careless.

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

I think 3/16" to 1/4" is what mine is. Also, you will want to let the doughnut fall off from the lip at an angle a bit steeper then the smallest radius back you will use. I'm thinking it was about 5 degrees but it would be easy to just measure the angle between the side and back on your guitar with the back and sides glued together. There was a discussion a while ago that addressed this. If I can find it I'll post the link.

Edit: here's the link. Some good info from Todd Stock about 6th post down http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=20790&hilit=+binding+jig

_________________
Steve Smith
"Music is what feelings sound like"

Thanks for that link Steve!

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