BP CNC Upgrade

[contractdesign]

Hood; I have a question for you concerning the limit switches. I read somewhere that the safety limits can be used as home switches as well as safety switches. The thread also said something about Mach switching from home mode to safety switch mode once it is started. I'm unclear as to what a home switch actually does. I am considering wiring the limit switches in series to save inputs. (I'm using a PMDX 122 breakout board). I understand what the safety switch does, but confused about a home switch. Any advice?

[NC Cams]

HOME switches are mechanical, unmoveable reference points for that axis. HOME on a a 3D number line is 0, 0, 0. IF you send the machine to HOME, you can then reset the internal counters to 0,0,0 and the thing "knows" where it is starting from with respect to preprogrammed "soft" limits that are in say a parameter file.

However, something somewhere somehow has to tell the computer where HOME is with respect to the + & - limits that will ultimately be hardware contstrained.

Bridgeport used a single home switch on all 3 mill axes for their limit controls. ON bootup, you'd find home. This would then be used to reset the internal register to 0,0,0. No matter what you did afterwards, the computer added or subtracted to keep you from over runnning software controlled + & - limits on the axis. The did NOT have actual + & - limit switches wired on machine _ save cost and complexity.

Even if you reset the DRO to 0,0,0 someplace else, the computer kept track of the new and 'old' HOME and software limits. If you asked for a move that would blow you past the soft limit. The PC wouldn't let it happen.

Wiring in HOME logic is NOT something to leave out to simplify and or cut costs....

[Hood]

NC_Cams has answered you regarding what the home is for but I would like to add a wee bit concerning the way Mach works. I have an 1 optical switch on each axis, this is triggered at the extent of travel in either direction, ie the switch is in the middle of the axis and the stops that trigger are at opposite ends of the axis. When you ask Mach to Reference (Home) it will move the axis until the limit switch is triggered and then back off slowly until it is released, this then as said above is registered as machine coordinate 0 for that axis, it does this for all three axis. Any other time (other than a Home move) that a limit is triggered it is treated as a limit and the machine will stop.
Mach can also be set up with "soft limits" which will stop the movement at either extreme of an axis before it trips the physical limit but it should be used in conjuction with the physical limits and not instead of for safety reasons.
As can be seen from the above you will only need to use three pins of the parallel port for all axis (limit and Home). The only disadvantage is if a limit is triggered Mach will show a LED for both the + and - of that axis but as you can look at the table and see which way it is you will know which way to jog to move off that axis. (hope this is clear)
Hood

[contractdesign]

Thanks guys, I understand how the BP control kept track of where the safety limits are in relationship to the home position. Seems redundant though because I would imagine an operator would zere out everthing anyway and not rely on a switch for 0,0. I also understand the explanation on the Mach, but does the Mach still remember where the actual safety limits are when you zero out all three axis. ie, if I set a mill vise in the middle of the work table with a stop and zero X and Y to that stop, will Mach still know where the safety limits are? If so, one would not need a home switch. Today is the day I get into the Mach IO connections. The electrical panel is all built and passed the smoke test. I need to install the breakout board today. I could actually start to bench test the system using the Mach sometime today.

[NC Cams]

Home is a fixed point the machine needs/uses for reference. It does not need nor should it necessarily coincide with the 0,0 point you use for the origin of your world.

Although they CAN have the same function, you'll find that your needs ultimately WON'T coindide with what the machine tool maker or you decided on for HOME.

By "homing" the machine at each and every start up, yes Mach will know where home is. It may recall what home was the next time you boot it up but don't count on it. PC's count real fast. They have memories that will amaze you. But they can easily forget when you power them down.

Besides, if the controller fails to HOME on power on restart, you know something is wrong BEFORE it surprise you by trying to bury a cutter into the table later on in the exercise.

Don't try to scrimp on home and limit stop switches by even thinking about eliminating either. Have a machine runaway just once and you'll be thankful they're there. See a machine runaway w/o limits is an unforgettably expensive and humbling experience.

[Hood]

All that NC Cams said regarding not scrimping on limits is seconded by me, they are your safety net, things could go wrong with soft limits so make sure you have a physical limit.
Mach will keep a track of the machine coordinates no matter how many offsets you make, also if you check the persistant offset box it will remember them after a shut down. When you start Mach the next time you will have to do the reference and then once done you can go to X0 Y0 Z0 and it will go to the position you last saved.

Hood

[NC Cams]

Your other naive comment deserves a reply:

"..... Seems redundant though because I would imagine an operator would zere out everthing anyway and not rely on a switch for 0,0......"

Yes, and he'll also read and follow ALL the instructions before he walks up to the machine and simply TURNS IT ON TO USE IT. Are you dreaming???

Did you ever have to reboot a computer???

Did you ever lose data on a PC when Windoze went blue screen on you and ran sacked your system???

Did you ever use a machine, walk away and find that someone else had used it and reset zero to some alternately logically illogical "zero" point becasue yours sucked???

If you didn't, I did and the machine did NOT return to 0,0 and I had to start all over.

I think the safety aspect of "home" and limit switches has already been addressed.

[contractdesign]

After doing my homework on home switches and safety limits, I now understand the function and purpose of each switch. I cannot however seem to locate the "Z" axis upper limit switch. Does anybody know where it is located on a rigid ram bp. At the risk of revving up some people, Mach 3 says you can use the z up limit as a home switch. I am curious if anyone has done this, or should a second switch be added? Except for this switch, my new upgrade up and running. There are a few loose mechanical ends to tie up. When it is fully complete next week I'll post some pics.

[Hood]

I have made up a single optical switch for the Z Axis, it does all three function Z+ Z- and Z Home. So dont think you should have a problem.
Hers a pic of mine although I now have a spiral type wire on it.

Hood