Ready Steady Eddy

[EddyCurrent]

Thanks Eddy, for the link I'll give them a call. Sorry to hear about your PC + IS problems, is there anything we can do to help ?
Mike

Thank's for the offer but I seem to be okay now, just letting things 'soak' for a few days to make sure.

Lighting strikes destroys phones and router, waiting for repairs, computer playing up, waiting for new parts, new parts fail after a week but only after reinstalling eveything and activating Windows, failed motherboard renders hard drive as 'Raw', manage to use PartedMagic to retrive stuff off drive, 10 day wait for replacement board so after 5 days decide to get another make/model next day delivery, eventually get everything running and installed, decided 32bit OS not good anymore so order 64bit and more memory, get it all installed and activated when hard drive comes up with bad sectors, find out new system is UEFI and not so simple to clone so order Paragon software which works great, order SSD and clone system, wating to see if all okay before reinstalling everthing this time.

[JAZZCNC]

Tony you are correct the stall detect on the AM882 only works above 300rpm. In real use that's fine because thats when your post likely to stall motors as torque is lowest. It's also when you need it most as it's the speed and inertia that does the damage.

Regards the one home switch with twin screws then like Eddy it's a preferance thing but don't be fooled into thinking your Gantry is strong enough it's not going to rack because it most certainly will and more so over time with eddy's setup of using one switch.? . . . Problem comes from the fact you will be accumulating lost or missed steps that never get reset this will eventually cause racking or binding that will cause stalled motors.
Also don't think just because you have the motors tuned well below there threshold your safe from lost steps because your not.? . . . . Every time you E-stop the machine at any resonable speed you have the potential to lose steps due to inertia pushing the gantry/motors. Now unless you always bring the machine to a controlled stop then you have to presume you have lost position and reset your home position.! . . . With eddy's setup only the one screw/motor is effectively reset and any error in the other remains! Do this often enough and It's definate fact you WILL get racking and stalled motors along with premature wear.

[EddyCurrent]

Okay, let's revisit the scenario, this is how I imagine it with 2 home switches on X

The gantry is moving back to the home position, one side hits it's home switch first so it stops and backs off slightly, now the gantry moves again until the other home switch is hit, the gantry stops and backs off slightly.
First off, is that correct ?, if not then let's have an explanation of what really happens.

Questions;

1. Do both X home switches have to set so they both activate together exactly when the gantry is at the physical 'square' position ?
2. If so, we are now relying on the repeatability of the switches for machine accuracy.
3. Imagine a cutting job has just been completed so that the gantry has been up and down loads of times and it's now out of square. When it comes back to the home position it will hit one home switch first, so then what happens ?

[Neale]

I've spent odd moments wondering how this two-motor setup could be made to home in LinuxCNC (which doesn't support it out of the box). What I think should happen is:
1. Both motors head for home at "find home" speed.
2. One home switch trips. Stop both motors; drive both motors back at "slow home" speed until same switch resets. Now one side of gantry is correct.
3. Second motor is sent towards home position at slow speed until its switch trips. Now drive it back at slow speed until its switch resets. Now both sides of gantry are correct.
In principle, this all sounds OK, except for two points. First is what happens if the gantry is really, really, stiff - this homing process is trying to deliberately rack it. Second is what happens if both switches trip closely enough together that the software can't distinguish it. You could choose one motor arbitrarily and then move the other one away from home a very small amount and then it's back to step 3. Or do you say, "Close enough - let's call this homed". Don't know what the Mach3 homing routines do.
I think that the answer to your question 2 is yes, you do have to rely on switch repeatability. However, Jazz did some measurements a little while ago (in response to some comment of mine) and they do appear to be surprisingly repeatable. I seem to remember 10 microns or so, which should be good enough. Answer to question 1 is that however you home it, assuming that it wasn't spot on first time, and then rehome, you are going to hit this situation - so my hypothetical "both switches trip together" situation is almost certain to occur fairly often. And that's the answer to question 3 as well.
I'm not even sure that this two-motor homing is possible in LinuxCNC due to the way that the internal motion control logic works with two motors - don't know if you can execute the homing logic outlined above - so it's all a bit theoretical for me at present. Guess who's going to go with single motor/belt-drive to two ballscrews, at least in the first instance...

[JAZZCNC]

Ok Both motors drive upto first switch then both back off then Slaved motor drives upto it's own switch while Master stays still then backs off.
So yes if your switches are far away from each other then you can affectively rack the gantry square but that would be silly wouldn't it and no one with half a brain would do that to any great degree. In practice then your switches are so close together they do affectively trip at the same time but that doesn't matter because it means your square anyway. Think of it like Homing the The Z axis then without moving it pushing home again, the axis still moves onto then off switch then goes back to same place if switches are accurate, same principle with slaved switch being at exactly same place as master.
This is why EACH SCREW on the shared AXIS needs it's own switch on it's OWN INPUT so that when they trip together only the Master switch is being watched and when it does trip both motors back off, then ONLY the slave motor drives back onto it's OWN switch then backs off. So if this switch is located at the same place as Master switch (which it should be if everything is square and correct) it drives on and off the switch then ends at exactly the same place.
In practice the amount of movement of slaved motor is so minimal you don't actually see it happen because the trip point is fractions of millimeters away.!!

Yes again to some degree your relying switch accuracy but like as been shown even low quality switches give micron level repeatabilty.

Now with your single switch setup then ever time you home only the one screw is located at the position it started. Lets look at it like this.?

Both screws start at Zero(home) and travel in sync until you hit the E-stop at which point one or both axis lose position. So you then return to home(zero) to re-register Zero position which is defined by the single switch. But this switch is only moving the Master axis back to Zero the other remains at some arbitory position depending on how many steps it lost. This can then accumulate over time so between E-stops and general positional loss thru dropped steps(which will be low if correctly tuned) then you slowly rack the machine and can never truely be sure or certain the gantry is square.!

I've said this many times but if I was using Slaved motors without out Dual home switches then your better off with Hard Stops which locates the gantry square and slowly drive upto to them. Again this way you know you are square and can Zero with confidence knowing both motors are back where they started from.
Other ways is to use cheap dial indicator on each axis and manually turn each axis until reads zero but then you may has well use 2 switches as it just the same.?

[EddyCurrent]

Ok Both motors drive upto first switch then both back off then Slaved motor drives upto it's own switch while Master stays still then backs off.

This is the bit that bothers me. After one side has hit the home switch and the gantry has backed off, the other side now has to travel the 'backed off' distance until it reaches it's home switch. What I'm saying is, I don't think my machine is flexible enough to allow that to happen, and even if it was, do I want it to go through this stress each time it homes ?

[JAZZCNC]

This is the bit that bothers me. After one side has hit the home switch and the gantry has backed off, the other side now has to travel the 'backed off' distance until it reaches it's home switch. What I'm saying is, I don't think my machine is flexible enough to allow that to happen, and even if it was, do I want it to go through this stress each time it homes ?

If your gantry is square and your switches located level which they should be if square then the amount it moves is minimal.!. . Not even 1mm.

Now No disrespect meant here Eddy but your gantry won't be that stiff that it won't flex under the mechanical advantage of a ballscrew and stepper, ive seen much stronger than yours flex. If you want to test this then Just disconnect one motor and set the other going and see what happens. .

The machine nor the gantry is stressed when things are correct, it's only when the gantry is out of square and your forcing it back square are stress's put on the machine. Like I say when machine is setup square and correctly you wouldn't even see it happen it's that small a movement.!

[Ger21]

Ok Both motors drive upto first switch then both back off then Slaved motor drives upto it's own switch while Master stays still then backs off.

Sorry my friend, but that is not correct. I didn't think so when I read it, so I just went out to the garage to check. My machine has a lot of flex, so I can easily rack the gantry one way or the other. I have dampers on my steppers, so with the power off, I can move one side by turning the dampers.

First I moved the master side about 3mm away from home, then powered up the machine, and homed the X axis. The slave axis hits the switch, and backs off, while the master keeps moving to it's switch, then it backs off.

Next, I moved the slave side 3mm farther from home. This time, the master hits the switch, and backs off, while the slave keeps moving.

Each side of the gantry (master and slave) move independently to their own switches. For all practical purposes, they are not slaved during homing.

1. Do both X home switches have to set so they both activate together exactly when the gantry is at the physical 'square' position ?

Yes, but technically, no. The Home position is not set when the switch activates, but when it deactivates as it's backing off. So technically, the switches need to be set so that they both deactivate when the gantry is square

2. If so, we are now relying on the repeatability of the switches for machine accuracy.

Yes, but as Jazz said, even cheap switches are quite accurate. The DIY Hall switches I use are supposed to ba accurate to about .01mm or better, and only cost about $3 each to make them. The most important factor is accurately positioning the switches.

3. Imagine a cutting job has just been completed so that the gantry has been up and down loads of times and it's now out of square. When it comes back to the home position it will hit one home switch first, so then what happens ?

The side that hits the switch will back off and stop, while the other side continues to it's switch, squaring the gantry.

Depending on the type of switch, you may or may not actually see the machine back off the switch. On my machine, it backs off such a small amount that it appears to just stop.

[JAZZCNC]

Sorry my friend, but that is not correct. I didn't think so when I read it, so I just went out to the garage to check. My machine has a lot of flex, so I can easily rack the gantry one way or the other. I have dampers on my steppers, so with the power off, I can move one side by turning the dampers.

First I moved the master side about 3mm away from home, then powered up the machine, and homed the X axis. The slave axis hits the switch, and backs off, while the master keeps moving to it's switch, then it backs off.

Next, I moved the slave side 3mm farther from home. This time, the master hits the switch, and backs off, while the slave keeps moving.

Each side of the gantry (master and slave) move independently to their own switches. For all practical purposes, they are not slaved during homing.

To be honest Gerry I wasn't 100% sure my self it worked like I said and couldn't can't check as I don't have slaved motors on my machine. Last machine I setup that used slaved motors was several weeks ago and was using the ESS but I'm sure that stopped both motors and backed off then moved slave.? Thou I'd have to check again to be 100%. I don't ever use PP so can't check if it works differant to Motion control cards but I will on next slaved machine I'm building just out of curiosity. Previous machine also used Motion control card but that was Russian PLCM and thought that did the same but again can't be 100%.? Esp as it happens so fast and with gantry being square so hard to see.!

But more the point was they work together to square the gantry and like as been said if the gantry's setup square to start with and switches located at same point it's a seamless operation and very little twisting forces applied if correctly setup.

Edit: One other thing for clarity for others (Gerry knows this ).? The speed at which you home makes a big difference to accuracy and how far it over shoots then as to backoff. Thick of it as driving wheels upto a line in your car.! At fast speed chances are you'll over shoot further then have to back up more to put wheels on line. Same principle go slower and chances are you'll hit the line ever time with little to no over shoot.

[Ger21]

Homing in motion controllers is done differently than with the parallel port, so you may be correct with the SS. I know it's taken several years for Greg to get slaved homing to work correctly with the SS.