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Getting circles straight
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=50309		 Page 1 of 2
Author:  mqbernardo [ Thu Feb 15, 2018 6:38 pm ]
Post subject:  Getting circles straight
see what i´ve done with the title? :)

anyway, a little help please. i made my first rosette groves with an home made circle cutter. they were circular and reliable.

then i upgraded to a stewmac thing with a dremel. i never got the circles to be perfectly circular - which puzzled me. i mean, the theory is pretty simple and fool proof and the result seemed concentric, the grove looked circular and ends met where they should but if i went at it with my manual cutter, refining the edges, there were always places where it cut more than others, normally from 5 to 7 o´clock it was a bit short. it was cumbersome to rout wood circles for inlay, as they never fit 100% - which meant quite some time fiddling with my manual cutter. the same happened with inlaying veneers strips. in some places they´d be a perfect fit, in others it´d be too tight or too loose. i blamed it on some poor manufacturing of my dremel tool.
so, eventually, i got a router and a circle cutting jig (home made one, like the Fleishman (?) one, but should work good) - and the same happens. i can get them 99% circular but not quite 100%. i can work around it but it bothers me, specially because there must be something simple im missing. i don´t suspect that a piece of TCT blade rotating at 30000 rpm would care if its cutting end grain or side grain of 1,2 mm thick soft spruce but who knows?

any ideas?

TIA,
Miguel.
Author:  bluescreek [ Thu Feb 15, 2018 7:12 pm ]
Post subject:  Re: Getting circles straight
what bit you using? I have these and modified them for a die grinder . There are 2 problems with this design.
A only two points of attachment for the dremel this allows for a lot of slop and movement.
B you have to to take 2 passes one in and another out to allow for the slot in the center bushing

on top of this you need a good sharp bit. I do not use down cut bits in this as the chips end up in the bottom and this builds up heat

I use a fresh bit and none larger than 3/32
Author:  Ken Lewis [ Thu Feb 15, 2018 7:34 pm ]
Post subject:  Re: Getting circles straight
What John said and make sure there's no play anywhere, especially in the center pin. You may be doing
this already but clamping the top to the work surface is a good practice also.
Author:  Alex Kleon [ Thu Feb 15, 2018 8:41 pm ]
Post subject:  Re: Getting circles straight
Maybe some runout in the Dremel shaft plus the transition from side to end grain can be part of the problem.

Alex
Author:  Terence Kennedy [ Thu Feb 15, 2018 10:30 pm ]
Post subject:  Re: Getting circles straight
As mentioned earlier the two big issues for me have been less than a tight fit for the centering pin and forgetting to clamp the top to the work surface.
Author:  BobHowell [ Fri Feb 16, 2018 1:06 am ]
Post subject:  Re: Getting circles straight
I had this problem with Proxxon tool and cut down on error by drilling new pivot hole, carefully set at perpendicular. I still had slop so must keep tension even as I went around circle. Multiple rotations cancels out error. Don't notice it now.
Author:  ernie [ Fri Feb 16, 2018 8:05 am ]
Post subject:  Re: Getting circles straight
FWIW if your using the stewmac jig with a die grinder or whatever tool you have to make sure that you align your bit with the center of the circle in the base of the stewmac circle cutter jig . My machinest friend centered it for me before adding the die grinder purchased from john hall. He is a very accurate machinest and said that the base needed squaring up which he did on his60+ k cnc milling machine including finding dead center on the stew mac base and making sure that the base itself was square which it was not. I am switching over to lathe tool bits for cutting grooves on the drill press . See peter howlet/ts video on turning a drill press fly cutter into a rosette cutter for uke or guitar rosettes on youtube
Author:  arie [ Fri Feb 16, 2018 11:42 am ]
Post subject:  Re: Getting circles straight
i use a cnc mill and a down cut bit. results are more round then a wooden construct needs to be and the depth control is far better.

you can always use your homemade circle cutter to part the grain to size then use your fixture and dremel to cut out the waste. i did this for a while and it worked out fine.
Author:  jac68984 [ Fri Feb 16, 2018 1:17 pm ]
Post subject:  Re: Getting circles straight
As others have mentioned, a very tight and perpendicular hole for the index pin is a must. Sharp bits are essential too. And don’t try and cut the full depth in one pass. I am more aggressive with removal of the central portions of the channel, but I still take it down in multiple passes to the final depth. Once I’m down to the final depth, then I sneak up to the final inner and outer dimensions of the channel (i.e., for all prior passes, I am leaving a few thousandths on both inner and outer dimensions). Doing this negates the significant risk that your bit wanders off line while trying to hog out too much wood. If you are trying to take single passes at final dimensions, my guess is that’s where your issues rest. A small diameter end mill spinning at 30,000 rpm is actually surprisingly prone to wandering off track regardless of the jig used if you are attempting full depth cuts (not by much, mind you, but enough to cause gaps and tick you off - Ask me how I know).



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Author:  stumblin [ Fri Feb 16, 2018 2:56 pm ]
Post subject:  Re: Getting circles straight
I had the same problem and overcame it by gluing a bearing into a recessed hole in my workboard, same bore as the jig registration pin and used a brass rod in it and a square when setting verticality.

Roy
Author:  Haans [ Fri Feb 16, 2018 3:52 pm ]
Post subject:  Re: Getting circles straight
What they said. Use as thick of as a bit as you can get away with, and getting as much of that shank inside the collet as possible, even if you have to cut some off. You don't want much of the shaft showing outside the collet.
Consider that in different areas of grain, tops cut a bit differently. Cutting with the grain is easier, and may end up a little looser than end grain cutting.
I used to go full depth most of the time, but was using hard red spruce and a couple of good coats of lacquer rubbed in too. Consider the softness or hardness of the spruce you are using. Consider the speed at which you are routing the slots too. Final cuts mean VERY slow cutting both ways.
Author:  Trevor Gore [ Fri Feb 16, 2018 9:02 pm ]
Post subject:  Re: Getting circles straight
Tight tolerances on the exactly perpendicular holes in the work board and top panel, large diameter pivot pin (10mm in my case), clamp the work to the board securely, rigid circle jig (but still don't lean on it) with zero backlash adjustment, precision aftermarket collets with minimal TIR and precise bits.

Attachment:
DSCF0391s.jpg


Gives the accuracy I want.

Attachment:
DSCF7793s.jpg
Author:  Colin North [ Sat Feb 17, 2018 4:36 am ]
Post subject:  Re: Getting circles straight
I taper the hole for the centre pin with a bridge pin reamer and tap in the pin with a small hammer while it is held vertical in the jig - no wobble or slack.
I also use a scrap of Teflon tape to take up any possible slack in the jig's centre pin hole but not create any friction.
I use a Makita laminate trimmer in a wells-carol type jig (no wobble like my old Dremel/SM jig) and fix the top my work-board with masking tape.
Author:  mqbernardo [ Wed Feb 28, 2018 6:07 pm ]
Post subject:  Re: Getting circles straight
thanks for the replies (and sorry if i´m late). there´s good advice here, some of which i already use (clamping work board, perpendicular pivot, multiple shallow passes, etc.) but some others worth investigating. as i said, it´s nothing i can´t work around but its annoying and a time waster.

cheers,
Miguel.
Author:  bcombs510 [ Wed Feb 28, 2018 8:28 pm ]
Post subject:  Re: Getting circles straight
I don’t know if it was mentioned already, but cord management was an issue for me early on. I use the SM base and a Dremel. If the cord was hanging down off to the side it could pull the Dremel just slightly. I starting looping the cord through a hook on the ceiling directly above the work area. That helped a lot.

Brad


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Author:  Terence Kennedy [ Wed Feb 28, 2018 8:38 pm ]
Post subject:  Re: Getting circles straight
Not so much about the circle but the smoothness of the cut is the direction of the bit on the outer edges of the channel.

By routing the channel with a separate pass for each edge being sure it was a climb cut towards the outer edge things cleaned up significantly where the cut passes through end grain.

Took me a few years to figure that out. :)
Author:  Pmaj7 [ Thu Mar 01, 2018 1:48 am ]
Post subject:  Re: Getting circles straight
Terence Kennedy wrote:
Not so much about the circle but the smoothness of the cut is the direction of the bit on the outer edges of the channel.

By routing the channel with a separate pass for each edge being sure it was a climb cut towards the outer edge things cleaned up significantly where the cut passes through end grain.

Took me a few years to figure that out. :)
So, are you saying, for the final pass on the inside of a ring you would do clockwise and the outside of a ring you would do counterclockwise?
Author:  Terence Kennedy [ Thu Mar 01, 2018 9:30 am ]
Post subject:  Re: Getting circles straight
pat macaluso wrote:
Terence Kennedy wrote:
Not so much about the circle but the smoothness of the cut is the direction of the bit on the outer edges of the channel.

By routing the channel with a separate pass for each edge being sure it was a climb cut towards the outer edge things cleaned up significantly where the cut passes through end grain.

Took me a few years to figure that out. :)
So, are you saying, for the final pass on the inside of a ring you would do clockwise and the outside of a ring you would do counterclockwise?


Yes that is what I do now. It seems like if you go the other way you can get some micro tear out as you enter the area of end grain that can show up as slight irregularities after the rosette is installed.
Author:  surveyor [ Thu Mar 01, 2018 9:43 am ]
Post subject:  Re: Getting circles straight
If you have a drill press you might give this a try. They sell for about $20. Just look up "circle cutter"
Author:  AndyB [ Thu Mar 01, 2018 6:57 pm ]
Post subject:  Re: Getting circles straight
Have you used that to make rosette cuts, Mike?

Andy
Author:  surveyor [ Thu Mar 01, 2018 7:11 pm ]
Post subject:  Re: Getting circles straight
Yes, I've used it and it works really well as long as you can rig up something to hold the top really flat and steady. It's definitely a "measure twice and cut once" type of tool.
Author:  Barry Daniels [ Fri Mar 02, 2018 11:35 am ]
Post subject:  Re: Getting circles straight
I used circle cutters in the old days to cut rosette rings before jigs were invented;-) You have to modify the cutter so it is cutting a flat bottom slot instead of an angled cut. And it is still a scary operation.

A router in a jig is much better and more flexible.
Author:  Terence Kennedy [ Fri Mar 02, 2018 12:36 pm ]
Post subject:  Re: Getting circles straight
I cut out the sound hole with a circle cutting jig at slow RPM for many years. Now I use the router with a climb cut and the edge is a lot smoother over the end grain.

I think John Greven used a multiple cut adjustable rosette jig that LMI marketed. Not sure if he still is.

Anyone here ever try that tool?

http://www.lmii.com/products/tools-serv ... tte-cutter
Author:  ernie [ Sat Mar 03, 2018 12:52 pm ]
Post subject:  Re: Getting circles straight
Am using lathe tool bits custom cut for the fly cutter .I get the exact width and the depth has about a .005- 015 variance in the depth of cut .Easier to cut my wood veneers oversize, and trim when done.
Author:  jonsse [ Thu May 03, 2018 7:59 pm ]
Post subject:  Re: Getting circles straight
I would advise taking the first cut at such a shallow depth that it barely scratches the surface. Another tip would be to not hold the router, rather the base itself as low as possible to the work and pull it away from the centre pin throughout each cut.
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