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I used a variation of this jig to cut my soundhole rosette:



However one problem I have is that the slot is always wider at one side than another, often 180 degrees apart. So if I start the route at the 12 o clock position, the slot will be a tiny bit wider at the 6 o clock position, and narrower at the 3 and 9 o clock position. The difference is about 0.03". Not a lot but enough to cause fitment issues. What could be causing this and why such a consistent width difference?

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Cat-gut strings are made from kitten guts, stretched out to near breaking point and then hardened with grue saliva. As a result these give a feeling of Pain and anguish whenever played, and often end up playing themselves backwards as part of satanic rituals.

Typhoon Guitars
http://www.typhoon-guitars.com

Most likely a tolerance issue. There is probably play (wiggle room) in your setup. The first places I would look for issues would be with:


1). Your circle cutting jig
2). Locating pin and/or hole


I drill through my soundhole with a 1/4" bit and use a 1/4" locating pin. I found that I need to use a backing board beneath the soundboard with a hardened bushing. I then clamp the soundboard to this backing board while I route. This ensures there is no movement of the soundboard. Make sure everything is tight with no movement. 0.030" is a LOT of play. Just looking at your jig, I would make something a bit more robust. I know you do metal work -- so you should be able to make something a bit more accurate.

If that was the problem, why is the width difference so consistent? No matter what I do, how I push it, etc. the width is always narrower at 3 and 9 o clock position than at 6 o clock. Because if the jig was flexing at the same rate, then the width difference should not be so consistent.

Plus for my jigs the bar is steel, not that white plastic thing.

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Cat-gut strings are made from kitten guts, stretched out to near breaking point and then hardened with grue saliva. As a result these give a feeling of Pain and anguish whenever played, and often end up playing themselves backwards as part of satanic rituals.

Typhoon Guitars
http://www.typhoon-guitars.com

With my flimsy black&decker dremel type tool and stewmac router base, I get effects like that due to the bit pulling into the wood or pushing away from the wood as the grain direction changes throughout the circle. I doubt that would be the problem in this case with a proper router, but something to check for.

I'm using a dremel tool with very small end mills... but when I used trim routers in the past I still had similar problem. One problem is my dremel is really not that good and the runout is terrible, so I was breaking bits. Unfortunately I don't know how to draw circles in an etch and sketch or else I'd have done it on my mill (which has basically no discernible runout even doing heavy cuts). So on the last pass I basically have to pull on the jig to somehow compensate for things...

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Cat-gut strings are made from kitten guts, stretched out to near breaking point and then hardened with grue saliva. As a result these give a feeling of Pain and anguish whenever played, and often end up playing themselves backwards as part of satanic rituals.

Typhoon Guitars
http://www.typhoon-guitars.com

I found there were just so many potential areas where inaccuracies can creep in when using dremels for this type of work, and eventually I lost patience futzing around trying to find ways around their inherent limitations.

You can buy precision collets with almost ludicrously small amounts of runout for some popular models of router and lam trimmer.

Combine this with a rigid jig that features a method of eliminating runout in its adjuster and you can cut and accurately adjust the width of rosette channels by a couple thousandths at a pass with relative ease. Sneaking up on a perfect fit becomes straightforward.

A Wells-Karol-ish style jig but with provision for eliminating backlash is described by Trevor Gore in this thread - http://www.anzlf.com/viewtopic.php?t=8195

I made one and it works flawlessly.

Of course since you’ve got a mill, maybe what you really need is a rotary table to bolt your soundboard to

I do have a rotary table but it is a huge pain to setup and make it work. I put a lathe chuck on it but even so it was finicky to say the least.

Sure I could probably go spend 100,000 dollars on some German made rotary table... that have very tiny runout in addition to being smooth to operate, but then I could just use a boring head to achieve similar results (kinda like that fly cutter type jig that LMI used to sell)

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Cat-gut strings are made from kitten guts, stretched out to near breaking point and then hardened with grue saliva. As a result these give a feeling of Pain and anguish whenever played, and often end up playing themselves backwards as part of satanic rituals.

Typhoon Guitars
http://www.typhoon-guitars.com

Take a scrap and turn the grain so that it runs from 9 to 3. Cut the circle and see if the narrow spots stay at the 9 and 3 o'clock position.

Is it the same in spruce rather than hardwood?

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Did you drill the center hole perfectly vertical, through both the workpiece and the board underneath? Just looking for other explanations for consistent inaccuracies.

That could explain the consistency.

So it seems like the narrow spot follows the grain?

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Cat-gut strings are made from kitten guts, stretched out to near breaking point and then hardened with grue saliva. As a result these give a feeling of Pain and anguish whenever played, and often end up playing themselves backwards as part of satanic rituals.

Typhoon Guitars
http://www.typhoon-guitars.com

TBH, if you're having problems with that set-up, which looks a bit rickety, why not just make a Wells Carol (Karol?) jig - simple to make and can give a very high degree of accuracy.
If you can make a a copy of the Elevate binding jig, it should be easy for you.
Mine was made with a saw and a drill + taps
Some were made in UHMP type stuff, but laminate faced ply is perfectly usable too.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Grain will affect how a bit tracks... on a rosette, the pass through end grain at the 6 and 12 o'clock position loads the system (bit>Foredom/Dremel/router>mount>base>pivot pin) differently than at 3 and 6 o'clock. If the various connections allow any flex or slop, that will be translated into cuts which are not of uniform radius.

There are various methods which might work to produce a usable channel (e.g., narrower channel with light cuts to widen, and final cut loaded towards the milled edge of the channel), but they all attempt to correct for that slop and flex in the channel milling rig....you might consider starting there.

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A constellation only takes shape when one maps the whole.
- Beth Brower

Enthusiastic second for the Wells/Karol jig. Built one out of UHMW. I've used it on all the rosettes I've installed (6). Inexpensive and precise. Installed a dedicated router.

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Peter Havriluk

I built a Wells Karol jig out of acrylic, I can't really get UHMW here, I heard chopping board is UHMW but they are not supplied at sufficient thickness to work. I will probably use aluminum next time though.

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Cat-gut strings are made from kitten guts, stretched out to near breaking point and then hardened with grue saliva. As a result these give a feeling of Pain and anguish whenever played, and often end up playing themselves backwards as part of satanic rituals.

Typhoon Guitars
http://www.typhoon-guitars.com

I gave up on Dremels and routers years ago, for just that reason. When going across the grain they tend to vibrate just enough to make over size and fuzzy channels. These days I use something akin to the Wells Carol jig, but with a very sharp and rigid knife for the cutter. It is beveled on one side with a double bevel that comes to a point, so that it can make a vertical sided channel cutting either direction by proper swapping of the cutter. I usually cut the channel under size in width, and then make very light trimming cuts after clearing the waste with a hand router plane. Prior to getting the metal cutter I used the 'popsicle stick method' with a stick on a pivot pin and an X-Acto blade for the cutter. It works well, and you could use it to true up the channels you've cut with the Dremel, so long as they're not too over size to begin with.

I have sometimes used the "popsicle stick" method to delineate the edges of the rosette and then come back with the router to excavate most of the field, finishing up with a chisel to remove the last little bit along the edges. I like that the router gives a flat bottom to the rout and requires less effort than pushing a chisel. The blade in the popsicle stick does help avoid fuzzy edges.

As others have said, do not use a Dremel. I started out with a Foredom which is supposed to be a bit better than a Dremel -- and it was awful. Use the Dewalt 311 with high quality collet/nut from precisebits.com

Like others above, it seems likely that a lack of rigidity somewhere in your rig is allowing the cut to not be exactly circular. The cut may be going out of round because of the grain, but the lack of rigidity is what allows the grain to deflect the bit.

Does the board that's supporting the top you're routing the channel in have a bushing as Toonces mentioned above? A metal bushing set into your workboard for the pin that the router rotates around contributes a lot to the overall rigidity.



The best rig out there, IMO, for rigidity and precision cutting is the Bishop Cochran router base. It is rock solid and dead accurate. Pricey, but it's a worthwhile investment. The one I have gets used mainly for rosettes, saddle slots, and backstrip channels.

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Exactly. Making a slot with a bit the total width of the slot stresses the bit and the Dremel, especially when cutting cross-grain. In addition, a straight fluted bit will vibrate more than a spiral flute. I use a smaller bit and make several passes. The final cut on each side of the slot should be less than half the diameter of the router bit.
If you insist on using the maximum size bit, make several passes by gradually increasing the depth of cut.
I like carbide dental bits for the smaller cuts, and I use the older ball bearing Dremels, i. e. model 380. It is much more solid (shorter nose) than some of the later models. I found that some Dremel models are totally unsuitable for precision routing.

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John

It should be called a Karol jig. He created it. He posted a picture in this forum and then people started copying it. Give credit where it’s due and stop using Wells’ name.

If he posted it on the OLF it was before my time, but Wells makes and sells them, so for me it's a Wells Karol jig and that would cover BOTH gentlemen.
http://wellsguitars.com/Available_Instruments/Luthier_Tools.html

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

I don't believe either one of those gentlemen invented it. It's a pretty obvious design, I made one back in 1995 from aluminum for my porter cable 7310 trimmer that I still use. I probably saw something like it before that.

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Jim Watts
http://jameswattsguitars.com

If you want t to REALLY have fun, use a Dremel with the circle cutter that comes with it! That's what I do. I just did the rosette on a tiny baroque in,A, that will be like an 8 string uke. I'm gluing purfling in, and I need to cut out the wooden rosette for the center. No screw adjustments at all!

I don't do many guitars, and I keep those real simple.

I have The circle stuff done, so I should get through this part of it. This one has a lot of veneer. I've never done that.

You can feel the cutter going free with the grain, and cutting across it. Where I can, I go around and around both ways.

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Why be normal?