Matching concave/convex spherical router bit - Router Forums
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post #1 of 15 (permalink) Old 05-27-2017, 10:34 PM Thread Starter
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Default Matching concave/convex spherical router bit

Hi Folks... I do violin restoration and periodically need to reinforce cracks in a violin top or back plate. The usual is to glue the crack and hand carve a shallow dish over the crack on the inside and then shape and chalk fit a perfectly matching patch to be glued in for reinforcement. This is a time consuming job especially if the crack is long and needs several patches.
I was thinking if I could simply use a router with an approx 1" spherical bit and create a dimple about 1/16" in deep. Trouble is I can't find a bit or method to create an exact perfectly matching counter bit to create the patch to be glued in.

Thoughts?

Thanks... Mat
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post #2 of 15 (permalink) Old 05-27-2017, 10:54 PM
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Router bits arent made to just cut a convex shape.
A CNC machine could make one though.
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post #3 of 15 (permalink) Old 05-27-2017, 11:53 PM
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I wouldn't let a router near a violin. This is a job for a small hand plane and maybe a scraper. Plane a sharp edge recess down into the back, make a patch the width of the recess. Glue, then hand plane and then scrape the patch in place. I take it these are not top of the line instruments. Finishing to match the rest of the instrument must be a chore, particularly if it has some age on it. I know the varnish is part of what makes the violin sing its sweet song, and that the wood is actually pretty thin. Every violin I've ever held was far lighter than I would have thought. The patch is probably made of the same wood as the original instrument, and I think I recall that is often maple. Sounds pretty exacting. The old instruments are glued with hide glue, aren't they?
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post #4 of 15 (permalink) Old 05-28-2017, 06:15 AM Thread Starter
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Thanks DRT... You are right... the glue is a hide glue and yes, it is exacting work, necessitating the patch to be a perfect fit achieved with chalk fitting.... hence time consuming.
I have no issue with using a router bit provided the depth of the plunge is controlled... might even use a drill press as I need to go in only 2 mm max. All the work is done from the underside (inside of the top or bottom plate), so the varnish is not affected at all. Top is usually spruce and the back maple, and yes, the wood needs to be matched and the grain offset by about 20- 30 degrees.

So my challenge is to find the tools to mechanically make the inlay patch exactly match the shape of the divot. The divot part is easy with a spherical bit... but a matching convex inlay bit?????
Cheers, Mat
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post #5 of 15 (permalink) Old 05-28-2017, 08:07 AM
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I THINK YOUR SUGGESTION WOULD BE PRETTY DANGEROUS

Since the router bit is spinning while it cuts I can't see any way you could cut a matching convex piece in one pass. You might be able to do it by using a 1/4 round concave cutter and making the first pass to make half the shape but you would then need to cut a thin piece off the original so you could rout the other half - pretty tricky and dangerous in itself. I don't know how you could hold this thin length safely while you made the second pass. I assume that commercially made half round mouldings are made using a special machine with a pair of cutters through which the wood is fed in one pass (mechanically with no fingers anywhere near it).
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post #6 of 15 (permalink) Old 05-28-2017, 08:08 AM
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A 1/16" box core bit will get you the groove you seek, a 1/16" corner round bit for the filler.

https://www.amanatool.com/45900-carb...-8-x-1-4.html/

https://www.amanatool.com/54161-carb...ch-shank.html/

cutting the groove will prove to be exceptionally difficult when working on convex/concaved surfaces! A Bosch colt with a modified base might work. 2 very narrow points of contact with the wood. the whole thing could prove to be problemmatic given that one OPS and you could have just created some very serious repairs. A Dremmel might be ideal if it can handle the load, which I think it could easily.

The corner round: create the bead on the stock you're going to use for the repairs. Use the tablesaw/handsaw/razor knife to cut out the beaded portion you just made..

You can also do a google search for "Custom Router Bits"...plenty of companies to choose from if these won't work..
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Last edited by TwoSkies57; 05-28-2017 at 08:27 AM.
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post #7 of 15 (permalink) Old 05-28-2017, 11:22 AM
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You say you want to use a 1" diameter spherical bit to create a dimple about 1/16" in deep and need a bit to cut the patch.

These are what I would recommend:

https://www.amazon.com/Whiteside-Rou...whiteside+1413

https://www.amazon.com/Whiteside-Rou...whiteside+1434

For a depth of 0.0625" for the dimple the width is about 0.48" for the patch so the thickness of the blank for the patch will depend on how much you want to trim after the patch is glued into the dimple. I would use the half round bit to route the profile for the patch with a blank that is at least 0.50" thick centered in the diameter of the half round bit then cut the patch off the blank with a tablesaw or bandsaw. You might also want to make the patch a cross grain patch to help strengthen the repair. Straight grain patch might just split if the crack tries to open again.

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Last edited by MEBCWD; 05-28-2017 at 11:26 AM.
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post #8 of 15 (permalink) Old 05-28-2017, 01:16 PM
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A crossgrain patch on the outside will stand out like a sore thumb. If this is a really expensive instrument that alone might reduce the value of the instrument. If it is on the inside, then you either have to dismantle the violin or find a way to go through the F holes. If you dismantle and repair, then the curve will be slightly inward (concave) so a convex bit would be a little more forgiving. But with a power tool, the slightest wobble will tear the instrument up. So I'm still suggesting you use a small hand plane, the kind that is used for guitars. The picture is a set of Ibex Violin Makers Planes.

I have also seen simple brass bodied planes about half an inch wide like the other picture. Not expensive. You could grind a convex curve into one of a set, concave in another and keep a third flat to smooth the patch. A plane wil let you go diagonally on the patch to smooth it out. But even with the miniature planes, I'd still create a clean edged shallow groove then cut the patch to that width laid in cross grain, then plane and then finish with a scraper. You can use a straight edge to guide the first plane so the edges are exact width. Since the patch grain is cross grain, it will bend into the groove so it will be pretty easy to plane flat to the surface of the area surrounding the patch. Finish leveling with a scraper, which will also give you a perfect surface you can't get with sanding.
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post #9 of 15 (permalink) Old 05-28-2017, 05:24 PM
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Quite possible I'm missing something here, but I just woke up from a nap. Sounds to me that making a dimple with a router would work, but rather than routing a patch, seems like having it the right size and shape, just sanding the sides to fit it in would work. Then scrape it down as Tom suggests. Way back, when it was all done by hand, I'm sure there was a way of doing just what you want and by hand, I'd research that if it was me.
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post #10 of 15 (permalink) Old 05-28-2017, 08:04 PM Thread Starter
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Quote:
Originally Posted by JOAT View Post
Quite possible I'm missing something here, but I just woke up from a nap. Sounds to me that making a dimple with a router would work, but rather than routing a patch, seems like having it the right size and shape, just sanding the sides to fit it in would work. Then scrape it down as Tom suggests. Way back, when it was all done by hand, I'm sure there was a way of doing just what you want and by hand, I'd research that if it was me.
Yup... traditionally, and now, it was and is still done with gouges, chisels, scrapers. files and chalk. Time consuming !
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