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I have the LMI circle cutter. On scrap I've tried to cut the channels for the purfling and shell rosette many times. In every instance, no matter how precisely I measure it isn't correct. I thought this part would be easy: measure, set the cutter, and cut. I've tried different measuring techniques to no avail.

Any tips?

I don't have that particular circle cutter but I've seen pictures of it. It seems to have a threaded rod that sets the diameter. You should be able to work out what 10 full turns of that rod does to advance the cutter and adjust things accordingly. One full turn or a fraction of a full turn should then be easy to work out. It's just a pitched screw. Perhaps put some sort of mark on the head of that screw to give you an indication of how much you are advancing it. Having the pitched screw seems to me to be a real advantage in setting fine increments for diameters.
Keep trying it on scrap, also use a digital caliper to help, you'll eventually get the hang of it. It's likely that you will want a tiny bit of clearance for any wood inlays as the glue will swell the grain a touch. Kind of a nice and easy fit, certainly not an engineers interference type fit.
I use a couple of types of circle cutter. The popsicle stick is good for repeated 'set' diameters, simply because those diameters never change, neither does the popsicle stick. My other adjustable circle cutter is self made, largely from a tank hole cutter. I can use a digital caliper, between the blade and the centre pivot, to set very fine increments.

What do you mean by incorrect?? Wrong diameter or offset?

What exactly are you measuring? Is the problem with getting the blade positioned accurately or does the blade appear to be correctly positioned to start the cut, but the cut doesn't come out right?

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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

Mostly the spacing in between the channels is never even. The last time I tried I used a method where I laid a paper template with a hole in it over the wood and started with the first inside diameter. With the square side of the blade facing inward I transferred the measurement to the tool using the "inside" jaws of the dial caliper referencing against the pin of the tool and the blade so that I can set the screw exactly to the measurement. You'd think this would work perfectly but it doesn't. I cut all of the inside diameters in succession.

Then I flip the blade over to cut the outside diameters and this time use the outer jaws of the caliper, referencing against the outside of the pin and the outside of the blade.

This strategy seems like a perfect method and the only potential problem would be if the pin hole is drilled perfectly square, if the pin is leaning it would cause an error depending on how high or low the measurement was taken. I drilled the hole with one of those jigs that attaches to the drill but it may not be the truest, squarest method. It's the only possible explanation I can see.

Now I'm wondering if I can just cut a single channel and put in a purfling/shell/purfling type thing in?

I don't know enough about the ramifications of the weight, effect on top vibration, or other unknowns to just do it this way?

Blade appears to be perfectly aligned. I flip it over to measure it and use calipers as a stop.

Could the blade be wandering as it crosses grain lines?

Alex

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"Indecision is the key to flexibility" .... Bumper sticker

I use a router type of cutter which is adjustable like the LMI cutter. I never measure the tool to position the router bit for the cut because I think it introduces a source of error. I put the cutter on the pin and set the position of the edge of the bit by eye to match the rosette layout lines that I've drawn very accurately on the top. The only measuring I do is when drawing the rosette layout. Have you watched Robbie O'Brien's video on the LMI site on how to use the cutter? He uses a similar approach. He marks the position of the rosette on the top and then makes trial cuts to position the blade where he wants it. No measuring of the tool needed.

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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

+1 Yeah, I would do it on the actual piece, setting the cutter to match lines scribed in the the area under the fretboard.

IF the width of the channels is varying, it sounds to me like your centering pin may be either: not square, or has some play in it, or it isn't securely pushed into a hole in a workboard below your test piece.

You should be able to test if the hole in your workboard is square by sticking a longish (a foot or two) 1/4" rod into it.

There's nothing wrong with doing a simple rosette with one channel (purfling/shell/purfling or whatever).

Is your pin in a bushing in the work board below your piece? If not, that could introduce some play.

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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

Years of darkroom work have taken a toll on my eyesight, that's why I did not use O'Brien's method.

How do you see the exact position of the blade under the tool?

I can see the router bit through the side of the router base. I haven't used the LMI cutter and I was wondering if it's possible to even see the blade when the cutter is in position to cut. However, if you do it the way Robbie does it, that's not a problem. He positions the blade close to the target radius, makes a short trial cut in the fret board extension area, and then adjusts the blade position as needed to sneak up on the correct position. Of course, you have to be able to see those trial cuts clearly. Maybe it's time for a pair of magnifying reading glasses?

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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

At least 2 or 3 folks have told you most likely the pivot hole. If the pivot is not tight in the hole and has no support other then the top your line width will vary. I also think this is your problem. You may also be trying too deep a cut. For me the best way to cut rosette groves is with router, Dremel , or laminate trimmer on a suitable jig. Less drama and trauma.
Tom

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A person who has never made a mistake has never made anything!!!

Year 2015

Ditto Tom --- $.02

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Ken Cierp

http://www.kennethmichaelguitars.com/

i use one of thesehttp://www.rockler.com/circle-cutt ... gQodAm4NCQ
you can make a jig that mounts to the drill press that holds the top and locates the circle cutter. then you just grind the cutter to a chisel tip and to the size of the desired slot. if your table is flat and your drill press is solid, it will cut a perfect circle.

I have a number of these and they work very well. There are a few things to note.
A clamp the top DOWN . Your drill press may also be a fault if there is play in the shaft.
Also when you tighten the spacers and blades be sure they are clean without anything in there. If you need to shim use alum foil instead of paper or tape.
Also be sure you are well seated and tight.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Don't worry about some of the sarcastic type comments Jimmy. The LMI cutter is quite clearly capable of cutting very neatly and precisely.

I assume this was for me. Forgive me if it is not. Learning to build guitars is hard work, even harder for folks who have not been trained in precision work with their hands. Jimmyjames seems to be having a struggle at times. I don't know his background ,so can only go by what he posts. To him it may not be obvious that a slight bit of slack in the pivot hole could cause a problem. Several folks pointed that out but he did not seem to react. I was trying to reinforce that opinion and get him to take a look and make sure it was tight. Maybe you were upset about my advice to use a machine that uses a rotary cutter rather then a pulled blade. I stand by my remark about the rotary cutter causing less drama and trauma then a pulled blade. I try very hard to contribute constructive advise that may help someone with their struggles in building. I have been building since the 70's and still have struggles myself. Check my signature under my name. Have a good day Michael.
Tom

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A person who has never made a mistake has never made anything!!!

This is a great reply.
Check it out .
Mike

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Mike Collins

I used to use one of these when I first started, still have it kicking around the shop somewhere. Never had any problems, think I did 6 rosettes with it.

Most of this has already been mentioned but...

-You want at least a very tight, accurate hole in the work board beneath the top and, ideally, a bushing.

-You need to ensure your top isn't moving. Camp it down, or use enough (and strong enough) double sided tape.

-Make sure the cutter is very sharp and use very little pressure (pretty much just the weight of the tool for your first pass to score things then add a little pressure/go a touch deeper each time around. If you try to cut full depth in one pass it'll have a tendency to want to follow some of the grain lines.

-No measuring necessary. Have a scrap piece set up to make test cuts and sneak up on a tight fit.

Apart from that I don't know what to tell you.

Regarding the pivot hole, no bushing here.
I use an minimum 18mm MDF board, and tape the top to it at each corner.
Mark and make the pivot hole a little undersized, about 1 mm, then use a 3 degree tapered reamer to open it up.
Insert the pivot pin (through the rosette cutter to hold it vertical) and tap it reasonably firmly into place.
Zero movement since I started doing this.
No claims for originally, I just eventually read some instructions.....
Test the cutter for slack on the pivot pin before cutting.

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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.

Sarcastic type comments??? Aside from one comment that I didn't understand, I see zero sarcastic comments. I do see helpful comments from people trying to help Jimmyjames find a solution to a frustrating problem.

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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

No comment aimed at you Tom. You have a good day too.
It was more aimed at the post that said '2015', which I assumed was some sort of reference to 'get modern'. I too apologise if that wasn't the intent. In any case I don't know what that has to do with someone who has obviously gone out and bought the LMI cutter and is having trouble getting it to work correctly.

I pretty much use the measurement method that jimmyjames does, and it works fine so long as the pivot is tight in it's hole, and there's no slop in the top hole or the top is held down. It's almost always better to sneak up on a measurement, of course. I make the cuts for a slightly narrow channel, clean it out carefully with a hand router plane, and test the fit. If, as expected, it needs to be enlarged a bit it's pretty easy to do that, taking just a shaving off the edge with a very sharp cutter. It's best to make the channel a loose fit anyway: the glue makes the wood swell, and if the rosette is a tight fit you won't be able to get it in. A far larger issue IMO is the fact that most Classical rosettes you buy are out of round, and not uniform in width anyway.