[Uman]

I will be building a new batch of fixture plates and typically set G54 X & Y at the left back corner and use the part for G54 Z.
The fixture plate will mount multiple aluminum part work pieces that are not surfaced and are raw cut stock with mount holes.
I am wondering if it is a better method to use the fixture plate for G54 Z as well (not the mounted part).

It seems there are advantages to both approaches.
Part G54 Z advantages:
Stock thickness variations is compensated in the Z elevation (first run).
No dedicated G54 Z touch-off location; with limited Tormach travel, maximizing fixture plate usable area is a big advantage.

Fixture plate G54 advantages:
Set G54 XYZ without parts loaded on fixture plate.
Consistent G54 Z for all runs.

Standard G54 methodology is important between all fixture plates; all G54 XYZ should be achieved the same way between fixture plates.
So if a few fixture plates require maximum real estate usage, then it seems the G54 Z on part is the best solution to standardize G54.

How do you approach this dilemma?

-Uman

[the_gentlegiant]

Hello Uman,

Without seeing your parts and understanding their needs in overall thickness, it's hard to be completely certain about what to say. I can offer a few basic ideas.

To start, I've done fixtures both ways. The biggest problem I have with Z0 being the fixture plate, is now all of your tool Z heights are going to be positive numbers except for drills going all the way through the part. This can be both confusing and a little dangerous because typically most Z heights are negative numbers. Say your parts are 1/2" nominal thickness. Now your potential clearance plain is 0.6 instead of 0.1. Mess that up once and you've got a crash. In general it just gets a little confusing being opposite of the norm. Especially if using many tools, with each new one adding to the possibility of an error being introduced. My programming is partially hands on. A lot of Z stuff I control myself. If your software can nail this stuff down without a 2nd thought, then likely you'll have less to concern yourself with then I do.

(This paragraph sort of depends on what is important on our part features height wise.) One way to have the best of both worlds is touch off on a gauge block of the true nominal height of your part that is sitting on your fixture. Or the easier thing to do is set up your control (Fanuc you can do this in the "Shift" register) so when you're touching off on the fixture plate the control is in this case, is automatically subtracting 1/2" from the machine position or current tool height. (Meaning subtracting a negative from a negative which makes your tool height more positive) Say you touch off T1, a drill, and the Machine Coordinate Position screen says Z-11.0000" Typically it is this number that is entered into your Tool Height Offset Table. You want your control setup so that Z-10.5000 is what actually gets entered into the T1 Tool Height Offset. So now your tools are all set to the nominal height of your part and the programming looks normal, because the Z heights are mostly negative. Note that these ideas allow you to program normally off the top of a theoretical or ideal part, while still referencing off the fixture in a meaningful way.

It's like when I use my 4" tall tool setter. I set it on the part or fixture and set up the control to take into account the 4" height whenever I'm entering tool offsets.

Just some basic ideas. Hope they give you some food for thought.

[jwatte]

My take on this is that they're actually the same thing in the end. It's just numbers. No matter where the "0" is, you have to know what the numbers mean, or it will go poorly!
I decide to put my Z=0 where it's easy to get a reference.
If I'm working on an already nice, level, surfaced blank, it'll be on top of the part.
If I'm working with raw stock, it's almost always at the bottom.

You could put a feature into your fixture plates where you can bond a gauge block or something (or just raw material that you then surface to a known height) to get you all three of X/Y/Z from the same point.
In fact, I've done this for soft jaws, making a pocket with the walls being X/Y reference and the bottom being Z bottom. Worked great for that setup!