# Matching concave/convex spherical router bit



## bowfrog (May 28, 2017)

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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## sunnybob (Apr 3, 2015)

Router bits arent made to just cut a convex shape.
A CNC machine could make one though.


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## DesertRatTom (Jul 3, 2012)

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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## bowfrog (May 28, 2017)

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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## katabrontes (Nov 12, 2014)

*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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## TwoSkies57 (Feb 23, 2009)

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-carbide-tipped-core-box-1-16-radius-x-1-8-dia-x-3-8-x-1-4.html/

https://www.amanatool.com/54161-car...ius-x-45-64-dia-x-5-16-x-1-4-inch-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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## MEBCWD (Jan 14, 2012)

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-Ro...qid=1495987384&sr=8-1&keywords=whiteside+1413

https://www.amazon.com/Whiteside-Ro...qid=1495987436&sr=8-1&keywords=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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## DesertRatTom (Jul 3, 2012)

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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## JOAT (Apr 9, 2010)

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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## bowfrog (May 28, 2017)

JOAT said:


> 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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## MEBCWD (Jan 14, 2012)

DesertRatTom said:


> 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.





JOAT said:


> 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.


One of my grand fathers made violins and repaired the instruments for the local schools for a living. I do know he made several of his own tools and that included several planes and scrapers of different kinds and sizes. Most of the planes and scrapers had curved blades, some convex and some concave.
You might be able to make a set that would do exactly what you are wanting to do and they may be better that trying to set up a router.


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## JOAT (Apr 9, 2010)

bowfrog said:


> Yup... traditionally, and now, it was and is still done with gouges, chisels, scrapers. files and chalk. Time consuming !


OK, first off. What is your definition of time consuming? Hours? Days? What?


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## DesertRatTom (Jul 3, 2012)

JOAT said:


> OK, first off. What is your definition of time consuming? Hours? Days? What?


To make a new one, or to do this kind of repair. Most hand work takes time, but for things like this, it's the only way to go. Thin wood that is essentially carved to an exact shape and curve. I guess you could make a new violin that was passable with a CNC, but I don't think repairs could be done that way, at least affordably if it would even be possibly technologically. 

This particular repair done with hand tools would probably take several hours at least, depending on how big the crack was, if you had to disassemble the instrument to get to the inside, if could take days, off and on while glue dries. 
Violins are held together with hyde glue link to article here Woodworking with Hide Glue so it can be taken apart using moist heat. The article is on using hyde glue and pretty interesting. It comes in dry flakes or beads and must be applied hot. We used to use this glue to make water based paint stick to theatrical flats. It also acted as sizing so the fabric stayed taught. It also smelled pretty awful.

I've really enjoyed the discussion and research on this topic. If you get the chance, there is a very interesting movie called "The Red Violin," which concerns a violin maker who mixed his dead lover's blood with varnish, which produced a perfect violin that was much sought after centuries later. Gives an idea of what the art of violin making was about, not just the craft. Here's a synopsis of the film The Red Violin (1998) - Synopsis

Certainly above my pay grade.


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## JOAT (Apr 9, 2010)

DesertRatTom said:


> To make a new one, or to do this kind of repair. Most hand work takes time, but for things like this, it's the only way to go.


I know a repair like that will take some time. I'm just wondering how much time it takes him to do it. 

Somewhere in my mind is stuck a similar type of repair. Not on fiddles, or any other musical instrument. But I could easily be applied to make such a repair on a fiddle. And cannot for the life of me remember more than that. I do knows it doesn't take forever to do it, probably less than the repair addressed here, and simpler, but very accurate and nice fitting. Ah well, it'll probably pop up 6 months or so from now.


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## bowfrog (May 28, 2017)

In this case, I will need to inlay about 12 patches over the full length of the violin, each one will take about 30 - 45 min. The alternate will be to lay surface patches (cleats) which will be much quicker.... but not nearly as professional.... and intuitively, I think not as strong.
As to previous comments re laying the patch cross grain... a cross grain inlay at 90 deg is definitely not recommended as with the variance in expansion and contraction rates and constant vibration, the patch can release. Laying the patch parallel to the grain, as previously commented, can split again.... therefore the standard practice is to offset the grain by only about 20 to 30 degrees , which seems to give the strength needed in this application. The thickness of the plates are in the order of 2.8 to 4.2 mm.

btw, when i am referring to a router bit, I am not sure I would actually use a router but rather a drill press, as this would create an accurate simple plunge (without sideways movement) to create the divot ( about 2mmm deep) that is perfectly round and spherical.... sort of how plugs are made but with the end result of an accurate spherical shape

So... the challenge still exists to find a way to make a precision convex inlay to match the easily made perfectly spherical concave divot. ... any suggestions for a custom router bit manufacturer?

Thanks for all your kind responses... Cheers, Mat


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