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Non-Slip Gripper Options

  • What are people doing to help reduce parts being dropped from grippers?


    The robot loads and unloads a CNC machine, it blows off water-soluble coolant that acts as a solvent on the grip-tape (sticky sandpaper) I'm using.


    My applications are usually with raw, extruded alum that is often not very flat. 1 out of every 10 or 20 parts slips from the gripper.


    Thanks for any suggestions.

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  • I have not. If it has an adhesive backing, I have tried a couple of types of sticky sandpaper but it only lasts about 10 parts before the coolant starts making it not stick.


    I think some type of plastic will be better than sandpaper, especially if we are going to use it on a finished surface.


    We are considering a plastic "dip" that goes on things like pliers handles. I could mill an undercut on the gripper then dip them and the undercut would secure it so it won't slide off.


    Thanks for the suggestion!

  • I found some ~40 grit grip tape that has an acrylic-based adhesive that holds up much better to the coolant than the silicone stuff I was using. I cleaned the gripper with acetone, cut rectangles about .03" smaller than the face of the grippers and clamped the tape in place for 10 minutes to cure.


    The grip tape was < $10.00 USD for a 2 foot long x 3/4" wide strip.


    I've replaced one so far, the others have lasted dozens of hours so far and still hold up well.

  • If 1 of 10 parts is slipping, then that is a 10% failure rate.

    If 1 of 20 parts is slipping, then that is a 5% failure rate.

    Both cases are utterly unacceptable because of damaged work pieces, equipment, and disrupted operations.


    The design issue is to provide enough grip force and enough coefficient of friction to hold the work piece. It appears there is an opportunity for improvement that will provide a robust solution.


    Any of the soft stuff applied to gripper fingernails will become a maintenance issue, disrupting continuous production and uptime. As you have discovered.


    The toothed inserts will certainly provide more friction if your work pieces. But it will involve surface disruption.

    You might consider going back to the beginning with (1) properly designed fingernails (not trivial) and (2) properly sized gripper mechanisms (again, not trivial).


    Every gripper manufacturer I have ever encountered provides rigorous engineering guide for proper application of their gripper. Defining the fingernail geometry to grip multiple parts is usually a fun challenge. An expensive alternative for multiple incompatible work piece geometries is multiple grippers and tool changers. But that requires extra programming to be effective.

  • Personally and as a company we are very new to robotics to aid automation. We've been at it for about 6 months now. Our pallet size, depending on the part, is between 24 and 36 parts per "load" and they run between 1 and 2 hours.


    We typically drop one or two parts per day but since we're over there loading pallets every couple of hours it has not been a serious issue. Yet.


    I designed a modular table to attach to the robot's pedestal and it will be here next week. It is about 3 times the size we're using now so it will be important to have a solution before we move to that volume.


    We made our own grippers for the very first run we did and still use that design. It's a flat surface with no knurls/fingernails/perforations. I have been sketching various designs based on the part shape and weight trying to come up with a good design that we can make ourselves.


    So how is your gripper actuated? Pneumatic, servo, etc.,? What does it look like? Can you post a picture or two? (Please be kind to the server, mind the file size).

    The grippers are (I believe) servos.


  • Looks like you have a UR of some type. (Not especially relevant, I suppose.) Robotiq gripper, second one mounted also? What I might lean towards is leaving some extra space in the fingers to bolt in maybe Urethane blocks which can have a texture of some sort, maybe simple as milled grooves lengthwise. Quick and easy replacement for when they wear out. Also, accommodate for shims to adjust tighter or looser as required.

  • Looks like you have a UR of some type. (Not especially relevant, I suppose.) Robotiq gripper, second one mounted also? What I might lean towards is leaving some extra space in the fingers to bolt in maybe Urethane blocks which can have a texture of some sort, maybe simple as milled grooves lengthwise. Quick and easy replacement for when they wear out. Also, accommodate for shims to adjust tighter or looser as required.

    UR10.


    Yes, 2nd Robotiq gripper is attached. There's also an alum angle plate mounted between them that has an air nozzle attached so we can blow out the coolant and chips from the vise before we pick up the finished part and before we place the raw part. This is what generally causes the coolant to splash up to the grippers.


    Does urethane keep it's stickyness when it gets coolant on it?


    What do you mean when you mention shims to adjust tension? The UR gives us a value between 0 and 100% of gripper "strength".

  • I mentioned urethane because it should be pretty durable and have a little give to it. I am not sure of its properties in your environment. These are the kind of shims I am used to seeing in most machine tool applications.


    They allow for adjustment by inserting or removing from between the piece being bolted on and that to which it is bolted. You could add to compensate for wear, rather than just replace.

  • The Robotiq gripper closes to a commanded force level, correct (with internal closed-loop control, IIRC)? So theoretically, it should compensate for wear on the fingers over time, unless/until the wear becomes really excessive.


    So the real issue is making a finger surface that can grip a wet bar of soap, metaphorically speaking.


    The closest I ever saw to a sandpaper-like surface was metal that had been subjected to "plasma blasting" or "plasma coating" -- I don't recall exactly, but it wasn't hard to find a local machine shop that could handle it. It basically resurfaced the contact area of the finger to something similar to sandpapaer (the shop doing the coating could produce a requested "grit" level). It gripped quite well even when coated with some very slick lubricants, and I never saw it wear over the course of 2-3 years.

  • SkyeFire We do thermal spray here. I should be ashamed for not suggesting it. :( There would be a variety of long wearing, rough coatings which could be applied. It won't be as cheap as buying some sandpaper tape though.

    Can you give me an approximation of what it might cost to coat 4 or 8 parts with a suitable spray? I've tried internet searches and can't find much local to San Diego. Most of the results are for tanning booths. (I know, I know... who needs a tanning booth in San Diego?)

    A lot less likely to have a sudden "glue failure," though. 8o

    Haven't had any sudden failures. They always slip off slowly so I can see it happening.


    Once we get into larger volumes with the new table I won't be hanging around there as much.

  • Those look like the material is Aluminum, important to know what the base metal is. What are the dimensions? When you get back to me with that I could see if sales could give a ball park number. There are places out your way. Look for metallizing, plasma spray, flame spray and ITSA (International Thermal Spray Association).

  • They are alum, and the "box size" is about 2 inches square. The face that will get the coating is about 1/2" x 3/4".


    Thanks for the search term tips. Will try it when I get back.

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