Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: astr on September 16, 2010, 07:09:29 PM
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I'm in the process of adding home switches to my x and y axes. I'm trying to decide exactly where my "home" position is going to be. From a convenience standpoint, I'd like Y home to be close to the Y++ limit and X home to be about in the middle of the X motion. I have no problem with the Y as it is a safe bet that the homing motion need to be in the Y+ direction. When I "Ref All Home", and the Y axis motion begins, eventually the Y home switch is set active, Mach backs off a bit and sets the "Home off" value in the Y DR - all as expected.
The X works the same way if I begin the homing operation with the table positioned to the right of home. If I begin the homing operation with the table to the left of home, I get the message to the effect that the homing switch is active, correct the problem and try again.
According to the manual, Section 5.6.1.1, (I actually read the manual :)) I would expect that when a home switch is active, the initial homing motion would be reversed from that specified in the "Home Neg" check box.
What am I missing here?
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I am not sure, but it would seem that there is no way to tell the mill what way to go home, in your situation. To the right of home is OK as home is normally at rest on the left end of the machine. If you start to the left of the home switch position there is no way for the mill to know when the axis is home, as there is no switch to trigger the axis stop.
I could be wrong, but I don't think it would be possible to have it both ways.
George
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You need to make sure you are at the correct side of the switch before homing. There is a setting on General Config page called Home Switch Safety or something like that, all that does is throw a warning if the switch is active and stops Mach homing that axis. You could have the switch triggered constantly from it to the other extent of the axis which would mean it would never home if active thus throwing the warning and allowing you to jog off then home properly.
Hood
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According to the manual, Section 5.6.1.1, (I actually read the manual :)) I would expect that when a home switch is active, the initial homing motion would be reversed from that specified in the "Home Neg" check box.
It does say that - you're right - the manual implies that if you're already on the switch it will just do the second part of homing i.e. back off the switch and call it home - but unfortunately it would seem the manual is wrong - now that's something you don't see every day ;D
Ian
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I was hoping that the manual was right and that I was doing something wrong however, I can live with moving my X home position to the extreme left of the table.
Thanks for the lightning quick responses. Sure is a pleasure working with this software :) :) :)
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I think moving the switch to the left is the right thing to do - that way you can't (likely) end up to the left of it. But - regardless of where the switch is - home is where you want it to be. So if (just for example) you wanted the middle to be zero or whatever, just set home at the left to be minus 10 or whatever.
Ian
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I went back and reread the responses and the key was in Hood's reply. I went to the General Configuration menu and UNCHECKED the "Home Sw. Safety" box and now the homing works as described in the manual.
So now I have my X home switch set up so that it is active when the table is positioned left of center and inactive when the table is to the right. When I click "Ref all home" and the table is to the right of center, the table moves to the left until the X home switch becomes active then backs off to the right until the switch becomes inactive.
If I click "Ref all home" when the table is left of center, the table moves to the right until the switch becomes inactive.
PERFECT!
Thanks Hood and Ian!
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Yes that is exactly how I operate my X axis home switch on my X3 mill. It is mid travel.
Also I have X axis to home a second time after a short delay.
The idea being that for best accuracy the axis always finally homes in exactly the same manor whichever the starting point.
Probably not necessary but satisfies my peace of mind.
Greg
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shout at me if i'm barking up the wrong tree here, but...and this may be a big BUT. If you're homing to either side of a point, are you sure the activatiing point is exactly the same. Let's say you have a microswitch as your home switch and it's operated by a cam, the home point on the left side of the cam, may be in a different place to the home point on the right side of the cam.
As greolt has just said, re-homing on one side everytime may be the answer to this, if it is a problem.
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From the sounds of it, it should actually trigger at the same point as when normally homing it moves onto the switch and then backs off until the switch opens again. With this way it is already on the switch so only does the back off part.
Speed may be the difference though so personally the double homing would be the preferred option and would only take a second or so more to do.
That is the way my servo drives do the homing, if a switch is active they will back off first then return and activate then look for the index pulse.
Hood
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Let's say you have a microswitch as your home switch and it's operated by a cam, the home point
on the left side of the cam, may be in a different place to the home point on the right side of the cam.
For this mid axis home switch system to work, the home switch must be held active from the activation point all the way to one end of travel.
It can NOT, for example, be activated by a cam momentarily as the axis moves and then comes off the cam as movement continues in the same direction.
For this reason, to my mind, non contact switches are the best option. Proximity or optical.
Mechanical roller type switch would wear out the roller in normal use.
Greg
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"Let's say you have a microswitch as your home switch and it's operated by a cam, the home point
on the left side of the cam, may be in a different place to the home point on the right side of the cam."
The part I think you missed is if approaching from the correct side, lets say right side. It moves left untill it trips the switch then back s off to the right untill it untrips.
NOW if you are to the left of the switch. It moves right untill it trips then KEEPS going right until it untrips the switch. So the untrip points should be the same point as it occurs in the same direction AND the same point on the cam ,going right.
Just a thought, (;-) TP