why two proxies on gantry?

[chopper]

I was wondering why two proxies were mounted on the gantry,
I believe this is for squaring?
at least this is what I thought but then I read the thread about squaring the gantry, and Gerald stated that the auto squaring in mach wont work because the mech mates gantry is to stiff to square with the motors, so I was wondering what the second proxy is for? am I missing something?
thanks, Chopper

[Gerald D]

Did I say that auto squaring won't work? How did I cause that perception?

[chopper]

I guess you didnt use those exact words but :

A gantry, with motors disengaged from the racks, must sit perfectly down with the V-rollers all making equally firm contact with the rails. If the motors can twist the gantry square from this seated down position, without lifting a roller, then you either have an already-square position, or a faulty (flexible) gantry. (The original ShopBot gantries could be flexed into a square position by the motors - the MechMate gantry is too stiff for that) Mach3's facility for auto-squaring a gantry tends to assume a flexible gantry - we don't use it

so the second proxy is for auto squaring and I am not understanding this correctly? Gerald I mean no insult I am just trying to understand the wiring of the proxies and in fact the whole machine.
Thanks chopper

[Gerald D]

The motors will move the ends of the MM gantry independently - I thought from your post that I had said somewhere that the motors could not work independently because of the stiffness. However, the wheels lift away from the rails.

[chopper]

thanks Gerald,
I have another question,
how does mach auto square if all the proxies are on the same loop,
how does it tell which x sensor has triggered? or better yet how does it know
when both of them have tripped when they are on the same circuit,
thanks Chopper

[bradm]

So to continue those thoughts, as long as nothing has ever caused your gantry to torque and lift a pair of (diagonally opposed) wheels a bit, you only ever need the one proxy. However, with the two proxies, this type of problem would get automatically corrected.

I'm an EMC user, and EMC doesn't currently have an auto-squaring feature, so my second X proxy is pretty much useless. In practice though, the gantry really doesn't want to torque and lift, so it hasn't been an issue, at least yet.

[Gerald D]

Chopper, if Mach is driving one particular motor, and the sensor loop triggers, Mach makes the assumption that it was the sensor nearest to the motor it was driving.

See: Using limit/home switches to autosquare the gantry (http://www.mechmate.com/forums/showthread.php?t=605)

[chopper]

thanks for the reply's
I will read the thread thanks Gerald
chopper

[smreish]

Heck, I have torqued-n-lifted my gantry at least 1/2 a dozen times! Usually due to something left on the rail (like a rag or air hose!) The prox sensor I have (I use only 1) catches the fault every time so far. I will add that second one soon. I'm just too lazy to run the new wires!

[Gerald D]

Sean, you need some old-fashioned cowcatchers (http://images.google.com/images?sourceid=navclient&ie=UTF-8&rlz=1T4IRFA_enZA279ZA280&q=cowcatcher&um=1&sa=N&tab=wi). :D

(Guards for the V-rollers and oilers for the rails? (http://www.mechmate.com/forums/showthread.php?t=796))

[smreish]

Gerald,
Thus, the reason I have made the plastic (delrin/uhmw) rail scrapers that need to be attached soon! :D

[Gerald D]

If they are really delrin scrapers ie. they contact the rail, then why not dispense with the wheels? :D