GM Kuka KRC1 - Hard Stops and Mastering questions

  • Hey folks,

    Firstly, thanks for the ridiculous amount of useful information you've all posted to this board in the past, it has proven invaluable to me. I've been tasked with setting up a pair of older GM KR150 robots running off of KRC1 cabinets in an incremental approach to automating our assembly here at my job. Up until this time there has not been a question either a search of this forum or a peruse through the programming manual hasn't been able to solve. For that again, I thank you.

    I've come up with two questions though, one of which I've already managed to half answer, and they are as follows:

    Question 1)

    Currently I've mastered what we've deemed our R&D robot using the pen trick, not having either the EMT or dial indicator adaptor. To this point it has worked well enough to get proof of concept running, however when I go to a pre-production cell and then full production cell, I will be required to more accurately master the robot.

    I've found rumor in a few threads of a pdf or drawing floating around where I could machine a dial indicator adapter to manually master the robot more accurately. Does anyone have a drawing or even a dimensional description of the adapter? I have the capability to make one in-house until I can secure an EMT.

    Question 2)

    This one is a little quicker, I've found a need to reach further on a particular operation than A2 and A3 will allow. I can extend the soft envelope in the programming to allow this but I've found I will hit the rubber and steel hard stops. Am I able to adjust these hard stops to allow for a greater range of motion, or are these set from the factory to protect internal cabling and hardware?

    Thanks once again for your help, I'll continue to peruse the search function and manuals in the meantime to attempt to help myself.

  • With regards to question 2, Software limits are there to protect the robot from hitting the hard stops, the hard stops are there to protect the robot from crashing into itself. I don't think that there are any internal cabling, or other, that the hard stops are protecting through A2 & 3; probably more mechanical protection. For this I would suggest that you do not alter or remove any hard-stops.
    For example: I think your A2 will have a counter balance, there could be a point where the rod-eye/pin or rod of the CB could become the hard-stop if you removed the real ones.

    In short, surely during the design of the cell/process you could raise/lower or move in/out the robot or the work-piece to reach it without the need to butcher a robot?! If not then you need a wrist extension or a different robot, not a cut-up version of the one you have.

  • As Mr E said, the "factory" hard stops are there to stop the robot before it seriously damages itself. A2's relationship with the counterbalanace cylinder is the best example, but even A3 could "smash" different parts of the robot together if the hard stop were tampered with. Those hard stops are designed to take the hit first, in places where the robot is designed to handle that stress -- they're intended to handle even full-speed collisions caused by axes gone fully out of control in the worst-case scenario.

    Now, you could remove the rubber bits from the hard stops and try it, but it would be a risk -- I also don't know if the operating system will allow you to change the A2 and A3 $SOFT limits past the factory maxima. Newer robots won't let you do it -- if you try, the MADA file gets re-written automatically back to default.
    But, assuming you can get the software to cooperate, the issue becomes one of risk acceptance. If you remove the rubber bits to get more range of motion, any collision with the hard stop will now be hard, and quite likely break something very expensive (the motor, the harmonic drive, the arm casting). And the higher risk is the non-obvious damage -- going back to A2, you could be stressing the counterbalance cylinder beyond tolerance but not be aware of it... until it gives out violently after X-dozen or X-hundred cycles.
    That's the risk that worries me the most -- tampering with the robot this way could be setting up a time bomb with no idea when it might go off in your face. While I'm not predicting the robot would fail explosively, it would likely fail expensively.

    Long before tampering with the hard stops, I would try extending the mount of my end effector, or buying a wrist extension for the robot. Or even mounting the robot to a 7th axis of some kind.

    For the dial indicator... I've never seen a drawing for the adapter, but I'd be surprised if one isn't floating around out there. Still, the threads for attaching the EMT are a standard metric fine thread -- I imagine a good machinist ought to be able to gin something up. All you really need is a way to mount the dial indicator where it makes good contact with the contact plate (where you push in with the pen). A normal magnetic indicator base would do it, except that the robot is mostly aluminum. So something that can screw onto the gauge cartridge and provide an attach point for a regular dial indicator arm would probably suffice.

  • Mr. E -

    Thanks, I don't know why I never even considered the counterbalance on A2 as a possible source of interference were I to mess around with the stops.

    I have been looking into different ways to mount the robot, and raising it up on a base will likely be the path I'll take to get a little more reach, but I was curious as to whether or not I could fudge it into working without doing so. I just wasn't certain if the hardstops were meant to be adjustable, and now I know :smiling_face: Also having an expensive failure versus an explosive failure around here is oddly almost worse, as upper management has historically been pretty averse to spending money, so now I have some good ammo to use in justifying a proper pedestal.

    Skyefire -

    Thanks for the note on the thread for the EMT attachment. I'll still continue to attempt to source the EMT tool, but in the meantime I'm going to take a closer look at the attachment points and fab something up per your notes.

    I really appreciate the responses, gentlemen, keep it up!

  • Just wanted to follow up on my solution to the dial indicator mounting - I managed to fab something up with a 3D Printer and some hardware store bits including a collar and a spring. Pictures forthcoming, but for those interested the thread is an M20 x 1.0 pitch.

    I ran into a problem where a normal dial indicator does not exert enough force with the return spring to overcome the spring built into the plunger in mastering port, so I had to source a spring with an inner diameter that would fit over the dial indicator probe, and an outer diameter that would fit inside the mastering port. (you can see it barely poking out at the bottom end of the adapter).

    I'll likely get a final version fabbed up in aluminum or steel by a fab shop locally but this will do for the process I'm managing at the moment.

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