Vacuum table / fixture plate — do you probe every job, or set a table datum once and trust homing?

[u/Little_Thought_8911]

Do you re-probe for every job on a fixture plate, or establish the table datum once and just re-home after that? I've talked myself in a circle.

Hobby-class router (Onefinity Journeyman), setting up an AirWeights vacuum table. My habit has been: home, drop stock on the spoilboard, probe the lower-left corner of the workpiece, zero there. That's fine when the stock is the reference.

A fixture plate breaks it. The AirWeights grid is drawn against a nominal 24x24 box with the real 23.75" plate centered inside, so the design origin sits ~1/8" off the physical corner with nothing under it to touch off. And the ports and gasket pockets live in table coordinates — zero off the workpiece and my CAM and my fixture are in two different coordinate systems.

Which points back at homing, except mine is stall homing: it drives into the stops and detects the motor stalling, no limit switches. I have no real feel for how repeatable that is across sessions.

1. Probe per job, or set the datum once and re-home?
2. If once — what do you touch off? Plate corner with a recorded offset? A locating pin?
3. Anyone on stall homing: how repeatable is it actually? Few thou across sessions, or does it wander enough that you re-probe anyway?

[u/Gun-Aero_CNCguy]

It takes almost no time to probe and there's no other downside to doing it.

If i'm doing vise work I'll probe every part just because I can.

[u/S0u1Fire]

Depends on your machine and weather conditions. If you have low quality limit switches, you can easily drift 0.005". And then with thermal expansion from morning to afternoon, it can also mess with it.

On my machine, I have it in a climate controlled room and have good proximity switches. I tested it a few times and trust my setup, so I just turn it on and send it. And I do two side cutting, so any small offset is doubled. 

[u/Next-Mathematician23 → u/S0u1Fire]

Op does not have limit switches)

[u/S0u1Fire → u/Next-Mathematician23]

Missed that part. He needs to probe every single time then for repeatability. Stepper motors will move when powered up and no homing switches means no accurate reference point. 

[u/LevelBit8522]

I'd keep the table datum separate from your work zero.

[u/ConnectConflict1765]

My rule of thumb: trust the fixture, verify the first part. We build fixture plates for a living, so the datum philosophy is the whole game.

If the plate stays on the machine and parts load against hard location features — dowel pins, ground stops, a sub-plate with registered corners — you set the table datum once and let homing do the work. Probe every job and you're paying for a repeatability problem you already solved with the fixture. What kills you isn't homing, it's the stuff between: swarf on a locating face, a dinged pin, thermal drift across a long warm-up. So the habit that pays is cheap: quick spindle probe of one reference corner per job (10 seconds), full re-zero only when the plate comes off or you see the first part walk.

On stall homing specifically — it's fine for most machines, but if you're holding tenths, verify it once against a known edge after a cold start. Homing repeatability is a machine property; don't assume it, measure it once and then trust it.