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OK, This is redonculous. I'm on build number 33, and have always struggled with cutting an accurate circle. No joke. I'm using the old stewmac circle cutter with a drummer and downcut cutter bits.

The finished product alway has a few flaws..
1- the width is never perfect all the way around
2- in places it appears the bit is "tilting" so the rosette isn't being cut strait down causing the edges to be at an angle.

Overall very frustrating. For a while I just bought rosettes from LMII, but I want to be able to do it on my own.

Is this circle cutter just inferior and prone to this kind of error?
The work is being held steady as it should, and I just can't figure out what the issue is

Any insight ?
B

If you meant using a "Dremel" with the StuMac circle cutter/router base here are a couple things to consider.

Dremels in my experience have less than perfect bearings that seem to wear out quickly. Could cause the bit to wander. I quit using Dremel's for precision routing.

Use the biggest dia bit you can to reduce bit flex.

I take a lot of care in drilling the guide hole in the spruce, AND a backer board, to keep the pin perpendicular to the hole. I also toughen up the pin hole in the spruce with a little thin CA.

Sometimes crossing the winter wood grain can set up bit vibrations and some error. Go slow.

Hope that helps.

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Brian R, Wood Mechanic
N8ZED

The hole for the steel pivot pin must be drilled at a perfect right angle to your work board or you will get the lack of perfection in your circle.

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Measure Twice,

Karl Borum

You need to use a real router with a good base. My diy base has a 1/2" diameter pivot point that has minimal flex.

Yea … I’ve been considering this… I do have several colts that would be more than enough…


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"You can make mistakes with a hand tool, but real destruction requires a power tool..."

Most of the 'bought' rosettes my students get are more or less out of round to begin with, uneven in width, and thin enough to brook no error in depth. I strongly encourage them to make their own, but once in a while... At any rate, when that's what you start out with fitting is a real problem, and I know of no general way to do it. I build my rosettes in place, so they're as round and uniform as the channel they go into.

I use one of the old LMI hand circle cutters and a hand routing plane for this. The circle cutter pivots on a 1/4" pin that goes into a hole in the work board and through the top. It's a tight fit, so there's no wobble. You set the blade for depth and make the (say) inside limit cut, then turn the blade around and reset the diameter to make the outside limit cut. Then the diameter can be stepped back by 1/8" or so at a time to scribe the depth and loosen the waste. The hand routers I have are a Stanley #71 with a 1/4" cutter and a smaller Lie-Nielson #271 with a 1/8" cutter. These are used to ground out the channel, and can do an accurate and smooth job if you do it in small steps. A friend of mine made a similar one out of a block of wood and an Allen key.

I like to make the initial channel a bit narrow, say by a mm or so, and make light cuts with a sharp cutter to get to the full width after the channel has been grounded out. You have to pay attention not to cut against the grain around the edge, and go slow on the cross grain sections to get a smooth cut.

The hand cutter is a bit slower than the Dremel, but I've never blown out a top with it due to bad bearings or a dull bit in the router.

I agree a bigger trim router and base will make cutting more stable.

However, I still like the StuMac router base and circle cutter as it allows you to see the bit action much better than bigger routers.

With the StuMac base, I use a Foredom with flex shaft and a Foredom handpiece.

The Foredom foot operated speed control is very handy for rosettes and other inlay recess cutting too.

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Brian R, Wood Mechanic
N8ZED

Like Alan, I cut my channel by hand and build the rosette inside the channel. I suspect he does a much better job than I do though.

I made a circle cutter with a screw adjust. Cutting a nice channel can take some effort but it is much more relaxing than going at your top with a power tool.


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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

I use the LMI grammil to mark the inside and outside lines. Then using a Bosch Colt with, I forgot the name, base rout out the waste. For me this is the best way I've ever gotten perfect lines. The router bit only needs to kiss the grammil line.

As mentioned before the hole you drill in the top and the work board underneath it needs to be perfect 90deg. Use a drill press if you got one.

You can make a base for your Bosch with some basic hardware store parts and a piece of HDPE. Here is a tutorial - https://youtu.be/9oRqUK-CMIo?si=oB56wLZ2oPdTtzMP

Here is the one I made -


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What’s the definition of luthier insanity - using the same Dremal over and over expecting different results?

+1 for ditching the Dremel and stepping up to a real router. I’d also spend a few extra bucks and replace the stock collets on your Colts - all of your routers for that matter. If you still want a rotary tool, the Proxxons are very smooth, quiet and run concentrically. https://www.proxxon.com/us/micromot/38481.php I’ve adapted both of my Stewmac bases to the Proxxon units.

Best, M

PS I don’t really have a leg to stand on cuz I cut mine using my CNC…

+2* for Proxxon. They have a nice flex shaft too. Mine is always at the ready.

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Brian R, Wood Mechanic
N8ZED

Like others above, I'm thinking that upgrading your cutting tools could help a lot. I use a Bosch Colt with an upgraded collet mounted in a Bishop Cochran router base. If you can swing the cost, the BC router base is awesome. It is rock solid and its adjustments for radius and depth are super fine and accurate with zero lash. I also have a dedicated work board that has a bushing (Ace Hardware) set into it that fits the pivot pin exactly so there is no play at all around the pin and the bushing keeps the hole in the workboard from rounding out over time with use. With this setup, I've had no problems at all getting clean rosettes.

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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

As a machinist here are the issues I see
stew mac circle cutter has some play and you need to figure that into the equation
Dremel
a lot of run out of the shaft. Under powered.
Routers are heavy and they too have run out on the shaft and collet
My tool of choice is a souix air die grinder turning at 70,000 rpm

when doing a soild wood circle there is never a perfect round cut. The stress relieved as you cut will often allow the wood to relax across the grain

to get the best result work with the variables

so make your rosette and rout the top. Knowing this will have some issues but do not cut out the sound hole

now once that is into the top and glued. you can take the smaller bit and drop in the joint and this will be more easy to
control. I always do a few passes pulling out and then pushing in. This takes the wiggle variable out.

now you can drop in purfling and this will help add some design to the rosette.

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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

Wells/Karol type jig with a DeWalt router and a precise bits collet works very well for me with bits as small as 0.020".

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Steve Smith
"Music is what feelings sound like"

Does anyone know if Bishop Cochran is still in business?

The bushing is really smart, Jay.


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There is still a website; you can Google his name to find it. However, I tried ordering some extras for mine a few years ago and was not able to get a call back from the person entrusted to fulfill orders. So, you can try, but be aware it might not work out well. Hope it does!

Here's mine:



The important things are a solid router base and backing board (and clamp your work), a decent router with aftermarket precision collet, a large diameter pivot and anti-backlash adjustment. The linear bearings are probably overkill, but they are smooth. There's no problem cutting a full annulus (or a segment of same) and fitting it exactly into a matching pocket.



More here.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Are you doing a separate rough pass and finish pass? That's the main secret to getting precise dimensions in any kind of milling work. Do your full width slotting with a small offset from the final dimensions, and then adjust and do a quick finish pass to clean up the walls and floor of the pocket.

We use the Wells-Karol jig modified as follows:

- All 1/4" rod is .0250" drill rod, to include the pivot post
- An additional 1/4-20 nylon cap-head machine screw is used to tension the jig against the pivot post, and addresses any wear that might arise or difference in pivot post diameter (hardware store 1/4" rod stock varies by more than 0.010".... drop by a big box store with a caliper when selecting stock, or better yet, see my earlier bullet)
- The body is made up of 3/4" Delrin sheet, which is both self-lubricating and threads nicely. UHMWPE works as well, but is not as wear-resistant and does not machine as nicely as Delrin, although it is generally about 1/4 the price)
- All through holes are reamed to final size using slightly oversized reamers (e.g., .250" through holes see .2505" reaming, which results in a nice fit with the drill rod).

We use high speed mini die grinders for inlay work, but the stands available for these tools generally lack the rigidity and thus repeatability (but see ProLuthierTools) of the heavier, albeit far stiffer DeWalt DWP-611/modified Wells-Karol jig. Sioux also seems to be in the midst of an offshoring effort for their Force tools, so worth checking on any die grinder purchase to verify US manufacture. That said, several of our US-sourced Sioux air tools have been horrible re: the triggers, resulting in replacement and ultimate retirement. We had several Sioux pencil grinders, but the last time I was over, two new Pan American Tool pencil grinders had appear, as well as both palm drills being swapped out for Pan Am units. I suspect much of this was driven by the airplane project, but upgrading tools is apparently a team sport with the boys... a new TIG welder was still in box awaiting work on a stand and an argon bottle from the local welding gas company..

https://www.panamericantool.com/nova-system-pneumatic-tools/nova-additional-tools/novar-pencil-grinder.html

A few tips:

- Spruce and redwood are quite soft, so crushing versus cutting is an issue
- Both woods prefer to be routed with bit rotation running with the rise in grain
- Sharp, fresh down-cutting bits make for better wider cuts, with standard up-cutting mills working well on cuts of less than 1/8" width. Decimal-sized end mills with 1/8" shanks are available down to 0.002" or so, but keep in mind cutting depth is usually no more than 3x diameter, so depth of cut is limited on very small diameter bits.
- A few coats of fresh super-blonde shellac over the channels to be milled make for a crisper cut (make sure the shellac dries overnight)
- As mentioned, a two pass (0.020" inner/outer rings on 18 series rosettes) or three pass technique (wider cuts) with slight diameter change gives you a better chance at avoiding micro-tearout... and elimination of slop in the routing jig (see above) allows a 'drop-in-and-mill' approach where the wood is overly cranky and the cut is relatively shallow
- Collet run-out seems to be the main issue with most trim routers, as Mr. Hall mentioned. More accurate replacement collets which eliminate that source of runout are available and we run those on machines which require that accuracy... http://www.precisebits.com is one source for varying degrees of precision (down to 0.0002 at 1" from collet)
- Precise Bits also carries 1/8" and 3/16" collets, which avoids the out-of-round and runout error inherent to 1/4"-to-mill bit-shank adaptors offered by some vendors. For 1/8" or wider channels, 1/4" shank downcut bits are available and avoid those adaptors as well.

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For the times they are a changin'

- Bob Dylan

For most hand tool only builds I use a shop-built compass gramil and excavate the pocket using chisels. Using power tools I use a full size plunge router with downcut bits with the inexpensive Jasper M400 Pro circle cutter baseplate from Jasper Tools http://jaspertools.com/. Downcut bits eject chips from the bottom and so in use are working to lift the router. A heavy router helps a lot when using these bits. Repeatability with this tool combination is excellent. The only issue with pre-drilled circle cutting baseplates is the (gulp) math needed to figure out the circle size. I have a calculator on my website here https://liutaiomottola.com/formulae/jasperRosette.htm that does the math for you. That page also describes a better way to install the baseplate on the router.

PS +1 for Trevor's comment about using a solid backer board!

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R.M. Mottola
LiutaioMottola.com

Author of the books Building the Steel String Acoustic Guitar and Mottola's Cyclopedic Dictionary of Lutherie Terms.

https://youtu.be/dgciZQc1ARU?si=D1tyE29UmTSmutWr

I use a Dewalt trimmer in the Elevate circle cutting jig. Before that I used a Porter Cable in the Bishop Cochran. But I like she’ll rosettes and haven’t don’t a wood rosette. I wonder if this video might be helpful or suggest some techniques that might transfer regardless of the cutter and circle cutting tool?


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While I have a StewMc base and a Dremel, I've never used it for rosettes because it has far too much play IMHO. I use it to rout a slot in the rim's kerfing for running brace ends through. It is light and easy to see my pencil marks to rout up to them, and perfect accuracy is not needed.

I used the Karol circle cutter until the HDPE bolt threads stripped so that it was no longer accurate. It was also always hard to turn the adjustment threaded rod in the HDPE due to friction, even when I tried to lubricate it. So, I broke down and bought the Elevate circle cutter two weeks ago and a DeWalt plunge router kit. I'm impressed with how solid it is and easy to change by a few thousandths. I look forward to cutting four rosettes this week. The problem is/was: $358 for Elevat plus $202 for DeWalt kit = $560 for equipment for rosette cutting is a chunk of change. At my age I finally figured out that I can't take the money with me and good tools are a pleasure to use together with their results.

Added 11/3/2023:
The Elevate circle cutter with the DeWalt DWP611 trim router and plunge base are very accurate and easy to dial in. For "off the shelf" equipment without me having to design and build something, they were worth the price.

Yep, I fit in that category too

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Steve Smith
"Music is what feelings sound like"