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VMC chose wrong tool - sensor fault? Shouldn't it have 2 sensors?

learning80

Plastic
Joined
May 10, 2018
I have a Siemens 808D VMC that uses two proximity sensors in the umbrella tool changer. One for 'home' and another to count each increment of the carousel. The machine chose the wrong tool recently. I ordered new proximity sensors but tried to get it to recreate the fault before putting them in to make sure I was actually fixing something, but it is not faulting out after running over 200 tool changes. I'm wondering now whether it was residue on the sensor or some other issue that's since resolved itself (or my damn imagination?!).

In any case it seems like a slightly dangerous system to rely on one proximity sensor to ensure the right tool is chosen. Obviously if it's about to choose a 1.5mm engraving tool and then tries to engrave with a 1" drill you are going to have a nasty crash. Has anyone ever tried adding two sensors to a machine like this and then altered the PLC to do a check that they agree with one another after running a tool change? If no agreement - halt program?
 
We have a Daewoo DVC-320 with the magazine above the spindle like a brother or robodrill. We have also had a problem with the machine picking up the wrong tool. We run a lot of aluminum and the light aluminum chips sometimes get up behind the magazine and when the carausel rotates, a chip causes a miscount. The only solution we found is to make sure to blow everything in the magazine out every once and a while.
 
You both are suffering problems that are common with low end machines. To cut costs the bare minimum of hardware is used. Sure, it works when new and no unusual conditions exist. But get a stray chip that falsely triggers a single sensor and now the simple up/down counter register is incorrect and the magazine stops at the wrong position.

Best method is to drive the magazine with a servo and essentially treat it as an axis using the encoder to determine position. Nearly as good would be to use an encoder on the magazine to send a discrete binary signal representing each magazine position.
 
It could be worse. On my Fadals there is no home switch for toolchanger, only a sensor for each increment. And to make things worse, that sensor dont know if its moving forwards or backwards. If the motor reverse relay fails then the toolchanger goes the wrong way and thinks everything is fine! lol.

Despite all this, i've never ever had a single issue with the sketchy setup. I guess its not stupid if it works? But sometimes I do think about the destruction it could cause if it got messed up. Hope it never happens.
 
Yes I agree, definitely on the cheaper side of VMCs but its still a pretty decent machine. I would like to put an encoder somewhere on the toolchanger to cause it to alarm out. The controller is a Siemens 808D I guess I just need to find some way to get it to talk to the external world about its tool position after doing a tool change (then another device checks this against the encoder and sends an alarm if things are not well). I don't have any background in changing CNC controller software or modifying them, but I do have a fair bit of experience in standard computer programming.
 
It could be worse. On my Fadals there is no home switch for toolchanger, only a sensor for each increment. And to make things worse, that sensor dont know if its moving forwards or backwards. If the motor reverse relay fails then the toolchanger goes the wrong way and thinks everything is fine! lol.

Despite all this, i've never ever had a single issue with the sketchy setup. I guess its not stupid if it works? But sometimes I do think about the destruction it could cause if it got messed up. Hope it never happens.
On Fadals, usually if the count sensor miscounts, it just smacks a tool into the side of the spindle. Not a big deal.
 
Yes I agree, definitely on the cheaper side of VMCs but its still a pretty decent machine. I would like to put an encoder somewhere on the toolchanger to cause it to alarm out. The controller is a Siemens 808D I guess I just need to find some way to get it to talk to the external world about its tool position after doing a tool change (then another device checks this against the encoder and sends an alarm if things are not well). I don't have any background in changing CNC controller software or modifying them, but I do have a fair bit of experience in standard computer programming.
I don’t know if a user can modify the logic in a Siemens. If you can and have spare input addresses it should be a simple matter to use a binary encoder and have its inputs to the logic converted to decimal. Then do a comparison check of that value and the counter. If the comparison check is true then allow the toolchange logic to continue. If it’s false then halt operation and raise an error message.
 
On Fadals, usually if the count sensor miscounts, it just smacks a tool into the side of the spindle. Not a big deal.
Thats the orientation sensor, yes no big deal, just bounces off.

But the one between magazine pockets on the geneva drive has no idea which way its going. If the relay to reverse the turret motor failed it would simple keep going forward and think everything is fine. So if your running T2 and program changes to T3, you get T1 instead which could make for a really bad day lol
 
Best method is to drive the magazine with a servo and essentially treat it as an axis using the encoder to determine position. Nearly as good would be to use an encoder on the magazine to send a discrete binary signal representing each magazine position.
I liked the way American Tool did it ... there was an endless pot on the turret with a supplied voltage. Each station therefore had a specific reference voltage. Hydraulic motor drove the turret (need hydraulics for clamping and the chuck anyhow). On the interface there was a series of pots that were adjusted to match the voltage at the turret station. When the position of the turret matched the voltage of the interface pot, you'd get an in-position signal, stop the turret, and clamp. It's analog, pretty cheap and pretty reliable and easy to troubleshoot and repair.

(Yes I know but as spam goes, Anna's at least relevant and the avatar looks good).
 








 
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