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So for the past week or so I've been pondering an indexing scheme for my new machine. I'll be making several two sided and at least one four sided part so I'd like something that allows me to machine and flip relatively quickly. Here's the idea I had:

My machine has homing switches so I have a very repeatable 0,0 position. It's a typical home type machine that uses 8080 extrusions to hold the rails for the long axis. It's got an Aluminum T-slot table bolted to the extrusions. My idea is to drill a series of 1/4" holes for dowel pins.

4 or of these holes would go through the table all the way through the extrusions. One pair of through holes would be located near 1,1 and 1,Ymax-1 (on one section of the T-slot table) and the other pair would be at Xmax,1 and Xmax,Ymax-1. This should allow me to re-place my T-slot sections exactly where they are now if I have to take them off. (The actual holes won't be at exactly Xmax and Ymax as that's not the ideal spot to drill a hole in the table but I'll stick with that just to get the concept acrross)

So what I've done is drill 4 holes defining a rectangle roughly 1" in from the limits of the machine.

another two more non-through holes would be drilled at +6" and +12" along the X-axis and another one or two holes along the Y-axis would also be drilled.

With these pins, one could place a square fixture against the pins to get a repeatable position with respect to the home position. If the fixture is then milled with the machine, you should be able to get very repeatable stock holding. The other thing is that since the coordinates of the dowel pins are known, the fixture itself could be drilled with holes for quick positioning.

Here's what I see as pros and cons:

Pro - Once a fixture is made, very quick positioning of stock, basically a no brainer. G54 would be used to tell the machine what's going on.

Cons - if the home switches ever get out of whack or I have to re-mount the spindle, the locations of the holes with respect to the new home must be determined. That doesn't seem too bad though because the through holes are square with the travel of the machine so one could use the probe feature of Mach 3 to determine the new X and Y offsets.

Clear as mud? What am I missing? I'm hoping to drill on Saturday (more questions on best way to drill pending)....

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I use a series of indexed jigs/vacuum clamps for banjo parts. Each jig lines up with the table via index holes in the table like you mention, but each jig also has a registration spot for x/y zero. I re-zero home whenever I change jigs--it's rarely if ever off, but doing it like this is insurance. Each of my jigs actually has several sub jigs on it, so I us g54, g55, g56 etc. By resetting home to the jig, I never have to worry about tweeking multiple work offsets. It doesn't take long and it takes switch/sensor accuracy out of the equation. I use my switches for limits only any more.

Probably not the most efficient way, but it works for me.

Dave