2 sided machining

[dickdelmi]

I am having trouble setting up my work for 2-sided machining.
I use the lower left corner of the workpiece for my origin and then machine the first side.
When I flip the piece to machine the second side I again use the lower left corner.
After machining the two sides are not in sync by a small amount.
How can I set the origins so the two sides are in sync?

[johnb]

Registration pins?... or a better corner jig arrangement to make sure the flipped side is aligned exactly like the first side? Before you do anything, check your miter saw or what ever you're cutting the blank with to make sure that you are actually creating square, parallel ends (or run a toolpath to through cut the blank from one side on your cnc if you can). A miniscule amount of error in "square" will translate to a "small amount" of sync error.

If you do all of that, then you shouldn't need to reset your origin when flipping the piece for the second side, but it seems that a center-of-work origin might be better. You can put "home" anywhere you want it.

[loren]

If you're making more than a few, it might help to make a fixture to hold the parts consistently.

One of the things I've done are little bicycle themed hot plate trivets, in three steps:
- cut outside shape and center hole
- mount in fixture, carve side one
- flip over, carve side two

[309cnc]

The only way I have found to work reliably for me:

I use 3/8 (steel) registration dowel pins in holes which I make part of the design. Usually two on the vertical axis if I flip the work piece right/left or on a horizontal center axis if I flip up/down. I then use the same tool path, at different Z-height off course, to 'drill' the holes with a 1/4in upcut endmill into the work piece and my spoil board.

Not directly related but important: I screw down most all my work pieces. So I also make the location of those screw holes part of the design. I run a separate v-bit tool path just deep enough to mark these holes on the work piece and the spoil board as well. After using a drill press on the workpiece for through holes I bring it back to the machine and have my 'dimples' in the spoil board for where the screws go...

By making registration pin holes and screw locations part of the design, I avoid running bits into (hardened) steel dowels or metal fasteners. Quite the fireworks if you have never tried...;-)

I assume you do create a two sided design file to start with, correct? If so, you set the reference (anywhere you want really) once and forget it. The software will line the two sides up perfectly and as long as the work piece is flipped correctly, the tight fitting dowels will allow me to get a good match between sides. As always: If your work piece is not perfectly flat, you may also be fighting tension / warping, etc in the wood as you cut into it. I machine 2in think solid wood quite a bit and I learned about 'close enough' flat the hard way....

[traindriver]

I had some two sided machining to do for the local trophy shop (challenge coin holders), and it kicked my butt for a while.  I machined 3 dowel holes in a spoil board on the outside of my part, so I could locate the part against them.  My origin is usually bottom left.  I kept having issues with the step on each side line up with the center hole until I realized that my origin on the bottom left becomes the bottom right when I flip it over.  I made two fixtures.  The "A" side had the dowels to the left and bottom, and the origin was to the left and bottom.  The "B" side had the dowels to the right and bottom, and I made a new file with the part flipped and the origin on the bottom right.  Everything lined up after that.

[Mezalick]

Traindriver,
I think you got lucky with your origin location technique.
From my understanding and from the doumentation in the Help Manual, a two sided project uses a common registeration location.
From the manual.."Typically you will cut the top surface of your material, then manually turn the material block over on your machine, whilst maintaining a common registration position, and cut the bottom surface using a second set of toolpaths."

Michael

[dickdelmi]

Thank you all for your input.
My primary problem was that I was resetting the origin on the second side.  A big mistake as this put the two sides out of sync.
I now screw a piece of 1/2" baltic birch to the spoil board and then machine a recess that matches my workpiece usually along the X axis.  The recess only has to be about 1-2" deep along the Y axis and of course 1/2" on the Z axis.  This creates a nice pocket for the workpiece which I hold down with DF tape.  I then set the origin on the first side and forget it.
Works great so far, thanks again for your help.