Indexing issues on two-sided parts milled on 3-axis cnc machine

[Goemon]

How do you guys prevent indexing issues on parts that requiring turning half way through the job on 3-axis cnc machines?

I've been advised that simply turning the part over manually is a better option than adding a low cost 4th axis (especially for milling metals). I just can't see how I would be able to position the stock and touch-off for the second side accurately.

Do you guys have a trick for this?

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[awerby]

You can make a fixture with a couple of dowels sticking up on the center-line. This works if you're cutting your part out of a block of material, so that it remains suspended by tabs in a surrounding frame. The dowels slide into holes in the frame, and you release the clamps, remove the part, flip it over and re-clamp. Some CAM programs have routines for this as well.

[Goemon]

You can make a fixture with a couple of dowels sticking up on the center-line. This works if you're cutting your part out of a block of material, so that it remains suspended by tabs in a surrounding frame. The dowels slide into holes in the frame, and you release the clamps, remove the part, flip it over and re-clamp. Some CAM programs have routines for this as well.

I can sort of see how using dowels could help me flip a part over while keeping it in the same place.

It seems like I'd be better off with a cheap manual rotary table for indexing though (if I'm going to have the stock suspended between two fixtures anyway).

I'll give it a try and see if I can manage to touch off in the right place after loading a 2nd gcode file for the other side. I usually gave to reset everything and re-zero in Linuxcnc whenever I load a new file.

Have you tried the dowel method? If so, please can you post a pic of the fixtures you used to hold the part?

[mactec54]

I can sort of see how using dowels could help me flip a part over while keeping it in the same place.

It seems like I'd be better off with a cheap manual rotary table for indexing though (if I'm going to have the stock suspended between two fixtures anyway).

I'll give it a try and see if I can manage to touch off in the right place after loading a 2nd gcode file for the other side. I usually gave to reset everything and re-zero in Linuxcnc whenever I load a new file.

Have you tried the dowel method? If so, please can you post a pic of the fixtures you used to hold the part?

You normally have a machined area where you can indicate your part, to reestablish your X Y Zero, that is how machinists do it, no amount of dowels is going to precisely relocate the part once you flip it

[pippin88]

I can sort of see how using dowels could help me flip a part over while keeping it in the same place.

It seems like I'd be better off with a cheap manual rotary table for indexing though (if I'm going to have the stock suspended between two fixtures anyway).

I'll give it a try and see if I can manage to touch off in the right place after loading a 2nd gcode file for the other side. I usually gave to reset everything and re-zero in Linuxcnc whenever I load a new file.

Have you tried the dowel method? If so, please can you post a pic of the fixtures you used to hold the part?

You don't rezero X and Y, that is the point.

Plenty of videos around of the process

[rcheli]

If the part has feature that is accessible from both sides like a hole or corner you can touch off from that feature after the flip. John Saunders of NYC CNC on YouTube has some good videos on work holding. Example here:

[Goemon]

If the part has feature that is accessible from both sides like a hole or corner you can touch off from that feature after the flip. John Saunders of NYC CNC on YouTube has some good videos on work holding. Example here:

I tried this plus dowels that awerby suggested and it works fairly well.

Even if a part doesn't have a hole all the way through, you can add one to the stock before starting, so I think I can use this method for most of what I do.

Even if this is slightly less accurate than using a 3+1 set up on a 4 axis machine, it may still work out better overall.

I seem to get a far better finish when I bolt work to the table than when I use any type of work-holding that suspends stock above the table (as it would be in a chuck on a rotary axis).