Purfling tool setup (?)

[Erik Hokanson]

I've been away from building for a while, but now I'm going to build a violin (I hope).
My new purfling tool. The knives, as they are in this new tool, together with the faceted faces facing each other (refer to photo) measure .083.
My purfling measures about .051.
Do I:

A. Grind down the thickness of the knives so they finish in the neighborhood of the purfling using stones or wet sandpaper on table saw.

B. Turn them around so they cut with the facet side out and shim them apart until I'm in the neighborhood of the purfling. (This would push the grain away during the slice. Would the glue swell everything up nice and neat?)

C. Find wider purfling.

I hope everyone on this forum well and it's nice to have some time to whittle again.

-Hoke

[Erik Hokanson]

Turns out it's B. I looked at Stewmac website.
If B is in fact not correct, then what is? I'm just dyin to know the WAY.

[Barry Daniels]

Crushing the outside of the cut and then expecting it to swell back does not sound like a good idea to me. Sometimes you should ignore conventional wisdom and do what you think is correct. I would go with A. Also move the blades up into the handle more so that they don't tend to flex.

[Michael Lewis]

You can also retract one blade at a time so you cut with only one blade. After you make your cut lower the raised blade and raise the one you just made the cut with. This way you can cut both edges of the channel to the size you need.

[Barry Guest]

I would only consider option C. That tool is designed to cut a groove to suit standard width purfling. If you grind down the cutting blades you might be sorry later on. Turning the blades around will give you a tapered cut that presents a whole raft of problems that will show in the finished project. The one cut at a time option would be a second choice for me. The second cut requires a steady hand and a keen eye.

[Nick Middleton]

Replacement blades are $12 and easily changed. I don't see the harm in grinding-down the existing ones to make it work. Just buy the backup set now in case they ever discontinued the tool. Then you'd be sorry for modifying the blades!

[Barry Guest]

Fair call Nick. But as you said, buy the replacement blades before modifying. Although you can always make the two-blade channel wider by shimming, I am part of a school that would rather trust a professionally engineered tool than my unprofessional attempt at modification. Just sayin'

[Erik Hokanson]

I think the purfling tool is shipped with the blades like that so the delicate points are not touching each other. I tried option B, and excepting my lack of chops, which in this case are improving, the result is correct.
People have been doing this for about five hundred years. I'm sure there is a proper method.

[Barry Guest]

No........they ship the tool with the blades in the ready to use position. That's the way it's meant to be used. Option B will not stand any test that seeks to demonstrate the term "fine work". The "proper method" is to face the ground surface of the cutter to the waste wood of the groove to maintain a clean cut that is perpendicular to the plate.

[Tim Put]

It's not a purfling groove cutter. It's a purfling groove marker.

B) is the correct answer. You stick a piece of the purfling you are going to use between the blades. This sets the points precisely the right distance apart (there is some room for tweaking in how much you squish the purfling shim). You then run over your instrument marking the purfling groove. You then finish the knifed lines to depth with a hand held knife.

[Charlie Schultz]

Hi Tim and welcome to the forum!

[Barry Guest]

Hi Tim,
We will have to agree to disagree. I was taught to set the blades with the calipers according to the width of the purfling and face the ground edges toward the groove. I can't challenge my teacher....he passed many years ago. By the way, the tool in question is known by all the luthier supply houses as a cutter, not a "marker".

[Tim Put]

Thanks Charlie. I've been around (quietly) for years.

Barry: I hope I didn't come off as combative. I merely intended to straightforwardly give my thoughts on the use of this particular tool.
Could you clarify how you set the width between the edges of the blades? Do you set both blades to the same 'depth of cut" and mark/cut both lines at the same time?

[Barry Guest]

Hi Tim,

You were not combative at all. We all have different ways of doing things and were taught by people who have learned the craft in varying ways.

However, the blades on my tool are 1.5 mm width and I always used 1.5 purfling, whether I made it myself or bought it ready made. I think the Ibex tool may be different, but I don't know.
Yes, I used to mark with both blades set at the same depth. I would mark the whole instrument, then come back many times and scribe the lines deeper, before i would attempt to clean out the groove. Mind you, a steady hand and an eye in front of the cut is a must. I am speaking in the past tense because I don't use that tool anymore. I have an overhead router which has an XYZ axis and cuts the groove perfectly.

[James Ansara]

Erik, I agree with Tim. I shim the blades with metal. 1.2mm is the thickness of purfling most violins have, 1.5mm seems out of proportion to me. An overly tight fit of the purfling in the groove will make gluing the purfling in a nightmare. Use the tool as a "marker" and proceed as Tim wrote. A stiff curved blade sharpened to a double bevel works well for this. I use a 1mm. chisel as a picker to clear and flatten the bottom of the groove. Hot hide glue will swell the compressed wood very tightly to the purfling. The distance of the outer blade to the edge of the violin looks nice if it is the same thickness as the edge. This always seems easier to do on maple than spruce. Take your time and enjoy the process.

James