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CraftyGeek
07-12-2009, 09:15 AM
I'm starting to think ahead to projects that I may build in the future on my DIY MDF cnc machine. I will at some stage want to make something that is double sided - the software side I can work out on my own easily enough, but I'm having a little trouble working out the hardware side of things.

How can you ensure the best possible alignment for double sided machining?
Are there any alignment approaches or stock mounting methods that can help?

I can see a lot of room for error in this, so any info would be very helpful.

Thanks

Tom

irving2008
07-12-2009, 10:43 AM
I'm starting to think ahead to projects that I may build in the future on my DIY MDF cnc machine. I will at some stage want to make something that is double sided - the software side I can work out on my own easily enough, but I'm having a little trouble working out the hardware side of things.

How can you ensure the best possible alignment for double sided machining?
Are there any alignment approaches or stock mounting methods that can help?

I can see a lot of room for error in this, so any info would be very helpful.

Thanks

Tom

The usual approch to this is some form of 2-part jig, one part of which clamps to the table and the other to the work. The two parts of the jig are joined by dowels or pegs and the workpiece part can be flipped over.

Assuming the centre line of the jig is zero'd correctly, so flipping the work over retains the centreline accurately then machining will be coincident on both sides.

CraftyGeek
07-12-2009, 11:04 AM
Thats a great start - thanks....simple too.

I just need to work out how to set the zero reliably & accurately now....any pointers on this?

irving2008
07-12-2009, 11:19 AM
Thats a great start - thanks....simple too.

I just need to work out how to set the zero reliably & accurately now....any pointers on this?

Again, consider fixing the jig to specific fixing points on the maching table/bed. If your bed is MDF, then one option is to drill through and use tee-nuts inserted from underneath. These will then act as a fixed location for the jig base.

In all cases you need some way of bringing your cutter back to a known, fixed, starting point... or some way to calibrate it to a known point. This is where homing switches are useful but how useful depends on the accuracy you are looking for.

On a milling machine you rarely rely on the table location but zero off to the workpiece using a rotating edge finder tool in the spindle.

In this case you'd zero off to the jig... jog to the jig edge util the tool gives a zero indication, then set this as a known value in Mach3 or whatever the software you use. Repeat on the other axis.

CraftyGeek
07-12-2009, 11:46 AM
I've just done a bit of searching into edge finders - that looks like the way forward, I'd not heard of these before.

I can't seem to find a 6mm or 1/4" one though - they seem to be 10mm or larger...more googling required I think.

Peter Griffin
07-12-2009, 11:12 PM
Crafty Geek, edge finders are also known as wobblers or wigglers these come in smaller diameters.

irving2008
07-12-2009, 11:24 PM
I've just done a bit of searching into edge finders - that looks like the way forward, I'd not heard of these before.

I can't seem to find a 6mm or 1/4" one though - they seem to be 10mm or larger...more googling required I think.

Chronos (http://www.chronos.ltd.uk/cgi-local/sh000001.pl?REFPAGE=http%3a%2f%2fwww%2echronos%2el td%2euk%2facatalog%2f&WD=finder%20edge&PN=Chronos_Catalogue_Centre___Edge_Finders___Touch _Point_Sensors___264%2ehtml%23a32230s02#a32230s02) have a 4mm ones, a standard one EF4, and a Z-type EFZ4

ptjw7uk
08-12-2009, 10:04 AM
Only other thing to realise is that depending on what axis you flip the piece on, then one of the cutting axis will be a mirror image!

Peter