Violin Forum/Message Board

[sdantonio]

Given a tool like Michael's http://darntonviolins.com/purflingmachine.php or some similar device (dremel, etc.) and given a purfling width of 1.2mm, in selecting a router bit would you go right for a 1.2mm bit or would you get something oversized? And if you would select an over sized bit, by how much? Lets ignore spindle runout in the dremel and stuff like that for now.

(the 1.2mm is just a reference number I pulled off of a chart somewhere, I realize not all purfling is always going to be the same size. But this gives a number to at least start off a conversation.)

Thanks
Steven

[Chad48309]

I would much rather squeeze in some purfling by making the channel only slightly wider with a scraper, if necessary, than make a channel oversized and have to fill it.
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-C.S.

[rs]

If this helps, here is what I do.

I use purfling that is 1.3 mm. I use a bit that is 1.3 mm. I have found that the bit, although the same size as the purfling, still cuts a slightly larger channel than the purfling. If I set the machine (dremel) on its highest speed, the problem is less. Also, if I move the router with confidence, take command actually, and cut as rapidly as the machine will reasonably allow, the cut is "tighter". Also, cutting around the plate counterclockwise causes the bit to naturally drive the stop I am using on the dremel to the edge of the plate. Cutting clockwise causes the bit to want to drive it away and the cut is harder to make clean. Being left-handed would help, but I am strongly right-handed, and I just had to teach myself to get used to the difference.

When I cut the channel, I vary a bit on what alot of others do. These two things helped me get a tighter fit. First, I do not scoop any of the inside of the plates out, but begin my purfling as soon as my outside is to a level that I can attack the purfling. Second, I do not start a scoop in the plate before purfling the plate. What I mean is, that I take the plate edge down to about 4 mm (around 4.5-5 mm at the corners), but I do not then start shaping the plate inside of this to about 3 mm which is customary. The reason, I found, is that if I have 4 mm, I can still cut the channel to the proper depth, the machine is doing the work and not me, the purfling is actually recessed or countersunk at this point, and this is handy for the next step. I set the purfling, glue it, (by the way, Michael Darnton's explanation of glue sequence of corners helped me a lot) and then take a fairly wet cloth and wipe the entire purfling joint to about 3/4" inward and the whole edge. The water swells the wood, and it tightens the joint dramatically. By not scooping the plates, either outside or inside, more wood is actually present to absorb water and force a tighter fit. That the inside is not scooped yet eliminates possibility of distortion. After it dries, I begin my shaping and recessing and scooping. Knowing the purfling is already at the desired depth enables me to simply uncover it.

I hope this helps, and I am sure others have good ideas as well.

Best wishes

P.S. I guess I could start with a smaller bit, but I usually find that the sloppiness of the channel is more pronounced on the waist bouts. I suppose this is because the grain is changing direction more rapidly and it induces a shimmy into the bit, but I do not know for sure. So either way, the channel is not going to be absolutely uniform.
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Randall Shenefelt

[L P Reedy]

I use a Foredom and purfle after the corpus is closed. Well, presently I do the top before attaching the back. If you are careful the Foredom cuts almost exactly the width of the bit. I use a 1.3 mm bit and 1.3 mm purfling. I compress the purfling to about 1.1 to 1.2 and it is still a fairly tight fit. The glue swells it back to a tight fit.

[Michael Darnton]

I use the same bit as the size of the purfling. After I cut the groove (once around, only) I fold up a bit of 150 sandpaper and run it around the groove leaning in and out, to cut off the fur turned up by the cutter, making a fiber-free edge with the groove a bit wider at the top as a consequence, to help funnel the purfling into the groove. Once it's in, pushing it home isn't hard, but getting purfling into an undersize groove is a problem.
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[sdantonio]

Thank you for the replies. They help a lot. My plan (as I have always done) is to install the purfling before cutting the scoop. I'm just finding that, if I can move some of the steps along easier with some modern machinery, then that is a good think.

Thank you all.
Steven

And let me wish you all a very Merry Christmas and a most prosperous new year (although some in my country would scream that is is politically incorrect speech to do so).