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About a year ago I bought a cnc router built by a company I won't name. I bought it slightly used from another guitar builder (not on this forum) who thought it was alright but a little large for his small work space.
Well it just wouldn't cut accurately, I'd get all kind of strange steps and stuff like that in my test pieces. I decided to check repeatabilty and it was of as much as .1 inches! over a 20 inch run. Well as I started my trouble shooting phase I discovered that the screws and nuts were just standard acme nuts, and the machine was also over constrained and mis-alinged. In short, it had about .030 of backlash on one of the X axis screws and bearing runing on non flat surfaces.
So I rebuilt the X axis with precision acme screw and anti-backlash nuts, much better.
I still had problems however as the machine was not alinged right and it would drop steps on one side of the x axis.
I thought I had everything working well and tried a a new rosette design for me with it. You can check it out in the discussion "yet another rosette" The pockets came out OK but the inlay pieces were all over the place. I was able to recover the top however by finishing it off by hand.
Well it was still droping some steps. I've since taken the machine apart again found an over constraint on the X axis which created a lot of friction, I have since fixed that problem and the machine seems to be cutting within about .002 over 5 inches and good repeatability, Yea!

After going through all of this however I am convinced that if someone wants to build a machine that you could do a very nice job for about 4K. This would be much better than buying a cheap machine in my opinion.

Anyway, I'm finally up and going, well the tables a 1/4 inch out of flat , but that can be fixed pretty easily. I used door shims to shim my fixture simi flat on the rosette piece but now that it's working I'll replace the table.

Just venting I suppose.

Jim

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Jim Watts
http://jameswattsguitars.com

Jim--It sure can be a frustration chasing down gremlins like you've been doing. Sounds like you've made a lot of progress.   Some of the control software "out there" has backlash compensation but much better to fix the source of the problem as you've done.
I'm guessing you will probably machine your new table top to make it true to the X and Y axis travel.
Best of luck.
Nelson

That's the suggestion I was about to make, Nelson. I know Fadal, after they build a machine, uses the spindle in the machine itself to grind the table square. Of course, if your axes aren't perfectly straight then your table won't be flat, but it sounds like you've got that covered now.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Thanks for the tip there, I was planning on that.

A quick question aboout cutting inlay. After it's inlayed into a fingerboard for example, has any tried surfacing the top of the inlay along with the finger board. That is cutting the fingerboard radius after the inlay is in place.
I would think that this might chip the shell but thought I'd check anyway. Also if it does chip the shell has anyone tried a diamond grit cutter to do this?

Thanks,
Jim

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Jim Watts
http://jameswattsguitars.com

It can be done with a regular end mill without chipping. You'll need to figure out the variables for your own setup, but I've done it on the Fadal. It does leave tooling marks on the shell which are more visible than the marks on the wood, so you'll need to hit it with some sandpaper, but it's still a lot faster and cleaner than sanding inlay flush after the fact (especially if, like me, you use compound radius fretboards).

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Thanks Bob.
I'll play with it.

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Jim Watts
http://jameswattsguitars.com