Pick ‘n Place Head

This weekend I got some parts in and put together a preliminary placement head for my open-source pick ‘n place project. My requirements are that it be buildable with off-the-shelf parts (ideally same-source, to save on shipping) and no special equipment, allow +/-180 degree rotation while maintaining an undisturbed vacuum, and support interchanging of the “tools” (vacuum needles). All that’s really needed to build this are the discrete parts shown, a bit of drillable plastic (e.g. Delrin) for the base material, and a drill. A drill press would be handy (a CNC mill *really* handy, and not such an out-there thing to have considering you are probably retrofitting this onto one).

This head consists of a hollow rotary shaft with a Luer lock fitting on one end, right-angle flexible tubing barb on the other end, and a large toothbelt (notched belt, timing belt) gear in between. The shaft is held in place but allowed to rotate by a pair of bearings, and the rotation is provided by a small stepper motor at the other end of the toothbelt. The gear ratio is approximately 5.2:1, providing a rotational resolution of about 0.35 degrees/step with a common 200 step/rev stepper motor (if no microstepping is used). Finally, just to the left of it is a 1024×768 Webcam with manually adjustable focus and a ring of built-in LEDs for lighting. The webcam mounting is definitely not ideal, given the camera’s weird eyeball-like shape. Tentative plan is to lash it down with some string, align it nicely with respect to the CNC table, then backfill the opening the camera’s butt sits in with epoxy.

The hardest part was finding a combination of parts that would all fit together nicely. Currently, the fits are mostly exact to “pretty damn good”, but a bit of adhesive is needed to join them permanently.

Parts List
Unless otherwise noted, all of these parts were sourced from Small Parts Inc. in the US due to the large selection and an actually competent parametric search engine, which was a great help in finding combinations of mutually-fittable parts. Accordingly, measures are in Imperial unless noted otherwise (that’s just how they come here).

Partnumber Desc.
3002DSTNTG18 Nice Ball Bearing 3002DS, .250″ bore x .6875″ OD x .250″ width
BFM-250-P Mounted sleeve bearing, .250″ ID, 1 17/32 center-to-center bolt spacing
B00137SITY Steel tubing, 1/4″ OD .152 ID, 12″ long
40DP-14/S-01 Timing Belt Pulley Delrin, 0.0816 Pitch, 40DP, .350″ Diameter, 1/8″ Bore, For up to 1/4″ Wide Belt, 14 teeth
40DP-70/S-01 Timing Belt Pulley Delrin, 0.0816 Pitch, 40DP, 1.806″ Diameter, 1/4″ Bore, For up to 1/4″ Wide Belt, 70 teeth
TB188-090-01 Timing Belt Urethane/Polyester, Single-Sided, 0.0816″ Pitch, 0.1875″ Wide x 7.3440″ Long, 90 Teeth
LCX-LC005 Male Luer Lock to tubing adapter, .145″ OD hose barb (mates well enough with .152″ ID steel tubing)
F1-EL001 Elbow Connector , Classic Barbs for 3/32″ ID Tubing, .145″ OD
HSTA-08-24-10 1.5″ aluminum standoffs, 8-32 thread
B00137UP68 (Optional) 11ga Steel Tubing, .120″OD, .094″ID (for mating 2mm shaft stepper motors, if used, to the 1/8″ pulley

Misc. Parts
2x 2″ x 4.5″, 1/4″ thick pieces of Delrin or similar
4x 1/2″, #4-40 bolts and nuts (assuming 2 bolts for stepper motor)
4x 1/2″, #8-32 bolts for standoffs
1x Webcam, manual focus, hi-res and builtin lighting strongly recommended
1x 3mm shaft stepper motor*
1x Method of attaching to your mill – a bit of aluminum angle bracket, or a dovetail, etc., depending on your mill.
1x 3-way air solenoid valve
1x Vacuum source (see here for a cheap one)
Air tubing and appropriate couplings to your solenoid valve. The head’s air path terminates in a 3/32″ hose barb, so you’ll want a 3/32″ to (whatever) barb adapter, or a 3/32 that screws directly into your solenoid. Those using metric are on your own :-) but will probably have an easier time of it anyway.

* The timing pulleys and stepper motors only come in a handful of diameters (imperial for the pulleys and usually metric for the small motors), so a 3mm (.118″) motor onto a .125″ ID pulley was the closest I could come up with in a reasonable amount of effort. Any small, el-cheapo permanent magnet (“tin can”) stepper motor should work here, but sourcing it may be annoying. Off-the-shelf 3mm-shaft motors I found are Jameco’s ValuePro 42BY48H08, Anaheim Automation’s TSM42 series, and Portescap’s 42/44 series e.g. 42×048 and 42S100. This surplus stepper is also worth a look, but you’ll have to remove a pre-attached plastic(?) gearhead. Beware, many of these smaller motors are 7.5deg/step, so even with the gear reduction, you will probably want to look at microstepping them to ensure adequate rotational resolution. Also, most of the companies selling them have no online click-and-buy ordering; you’ll have to phone up a salesdouche at the least, hope that you are worth their time to buy One Lousy Motor, and possibly haggle (“Request a Quote”). How companies that don’t know what their product costs stay in business is beyond me, but that’s a rant for another day.

If you want to skip that hassle, I’ve taken an alternative approach and simply bodged a surplus 2mm-shaft motor up to a 3mm shaft by gluing a short piece of thicker steel tubing onto the existing shaft (see partlist).

Design Files
CamBam drawings with machining operations for the top and bottom plates. The machining ops assume 1/4″ thick plastic, 1/8″ endmill for most cuts, and .166″ (#19) drill for the #8-32 bolt holes into the standoffs. It’s designed for the Cubeternet webcam (or equivalent eyeball-cam) and a stepper motor with 42mm center-to-center mounting holes. You can get the free CamBam at http://www.cambam.co.uk/.

Note, I made some small improvements to these files after the above prototype was carved, so what you see in the file will not match it exactly. In particular, the standoffs were moved to more optimal places and a feature has been added allowing the motor to be slid to remove any slack in the belt.

Assembly
The assembly should be pretty self-explanatory. One bearing on each of the Delrin plates (inset the ball bearing if you can; otherwise gluing it down should be fine). The hollow shaft goes thru the bearings, Luer adapter and hose barb go on either end (use adhesive, just don’t clog the air path with it). The motor shaft center should sit just a hair (50-100 mils?) over 1.75″ from the hollow shaft. On mine, an online calculator produced the 1.75 figure, but the belt turned out to be a bit loose once built this way. If possible, make an elongated hole for one of the motor screws so the motor position can be adjusted to tension the belt. You could probably also insert a peg somewhere in the belt path to push it inward and take up the slack. With the camera focus set such that the largest part you will ever populate can fit in-frame, find the resulting camera height (distance from part) and set the depth of the shaft so that the camera is “in focus” maybe an inch or two above the placement position (i.e. with the needle touching the board). This will allow the head to focus on parts without touching the needle down.

For my build, I used instant glue to attach the Luer adapter to the needle shaft, and hot-melt glue to tack down the large pulley and hose barb. None of these parts should be seeing significant force; if they are, you’re Doing It Wrong and the hot glue should hopefully break loose before something more important does. Using a non-permanent adhesive for the hose barb and pulley also allows this assembly to be disassembled later if needed. The shaft itself is free-floating and the large pulley rests against the ball bearing due to gravity. This will prevent damage due to crashing the needle into the table or too-tall part, but if you experience problems with the shaft riding up on its own, try adding a bit of extra weight or put a dab of adhesive where the shaft passes through the lower bearing.

Suggested “v2″ improvements
–Move air valve (if/when one is specified) onto head to minimize air volume between valve and head. Needed? (may be beneficial for reversed aquarium pumps or other weak vacuum sources)
–Bump detect: rather than firmly adhere the shaft into the bearing, allow it to float up and down, normally resting by gravity with the large pulley against the ball bearing. Place a contact switch just above the pulley: if the head/part contacts the surface with more than minimal force (enough to lift the shaft), contact switch is triggered. This could be used to halt the machine if a bump was not expected. If the switch’s trigger position is reliable enough, it could be used intentionally to automatically determine component heights.
–Probe function: There is a conductive metal path from the needlepoint all the way up to and including the bearing outer race, so it would be easy to touch a contact here and use the needle to probe for any conductive objects (e.g. find the tabletop if it is metal, or some capacitive shenanigans for PCBs/etc.). Useful?

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12 Responses to “Pick ‘n Place Head”

  1. Dafydd Roche says:

    Hello Tim!

    This is exactly what I’m looking for. I’ve been looking for ways to do the pick and place head, along with the rotation of the component. This is perfect!

    I love the fact that you’ve done most of the shopping list too!

    Whats your thoughts regarding cutting the delrin? do I need access to a CNC machine? (it’s kinda like one thing stopping you from making the next!)

    Cheers

    Dafydd

  2. Hi Tim!
    This is something many people of the RepRap community are also working on or dreaming of (or both). You should definitely upload the files to Thingiverse.com, along with the parts lists. I’m sure you’ll get lots of nice and useful feedback. Chances are that you get people to adopt your design and improve it (for your benefit and others’ too).

    I think the design files are 3D printable too. Including the timing pulleys (actually, I uploaded a parametric script for printable 3D models of those on Thigiverse). I’d love to print out a pick-n-place head which accommodates some of the shelf parts!

    Cheers, and keep up the good work!

    Erik

  3. redlored says:

    This head concept is used in some industrial pnp machines with one expception…many of the industrial pnp’s like small DC motors with encoders which adds a layer of complication. Stepper best way to go for the open pnp community. =o)

  4. Tim says:

    @Erik
    Done! It is at http://www.thingiverse.com/thing:4070
    I had some trouble getting the parts list feature on Thingiverse to work, (the partnumbers, supplier URLs, etc. are missing) so the text points to this blog entry for a proper parts list. It will be interesting to see what people do with this thing!

  5. Arcol says:

    I dont understand two things:
    1. how is it solved, that the needle is fixed vertically. Ie. if you are using bearings, it would simply slide it down. Have you purpose machined a steel rod?

    2. I dont understand exactly the vacuum. You say, you are using aquarium tubing. Are you also using a compressor motor? Or are you “flexing” the tube, to make the vacuum?

    Best regards,
    Laszlo

  6. Darcy says:

    Hi!

    That head looks nice.

    Am I understanding that it rotates the piece but does not lift the piece.

    I am looking for something to lift the piece but not rotate it.

    Any ideas?

    Darcy

  7. Tim says:

    The needle assembly can be fixed in place vertically with a small dab of adhesive on the inner race of either the top or bottom bearing. I haven’t even bothered with this on mine; the steel rod and big drive pulley add some weight; the drive pulley will rest flush against the lower bearing’s inner race due to gravity. I prefer this because in the event the component height is misjudged, crashing the needle into the part/table will just cause the needle assembly to “lift off” rather than break the part or needle. (The drive belt may slide off if the needle is pushed far enough, but it’s a lot nicer than having to do repairs!) One of my suggested “when I get around to it” improvements is to place a contact switch just above the big drive pulley, so that crashing the head can stop the machine (or, if the switch point is repeatable enough, use it to actively probe for component heights).

    The steel rod is just the B00137SITY part listed in the parts list. It comes in 12-inch lengths; just cut with a hacksaw to the desired length.

    For a vacuum source I attach the tubing to a reversed aquarium pump (for now). This is just an aquarium pump sealed inside a small Tupperware container, with its “out” port poked through to the outside and sealed, and a new “in” port poked through the Tupperware and sealed. This should work fine if there is a rubber suction head on the needle, but with a bare needle this can only pick up small parts reliably.

  8. Tim says:

    @Darcy – Yes, this head just applies vacuum and rotation – the head design assumes it is bolted to the Z axis of a CNC mill or equivalent.

    For a part lifter, probably the easiest (if not fastest) way is to build a leadscrew raise/lower axis (leadscrews are my solution to everything ;-), or attach the suction needle to a solenoid or air piston if ‘infinite’ position control is not needed.

  9. Steve says:

    Excellent work. I seem to keep coming back to your design while doing my research. I am also working on a DIY Pick and Place.

    Have you had a chance to use this head yet on a CNC? How do you think it will handle 0805 and SOT23 parts?

    Steve

  10. Martin K says:

    Tim,
    I’m working on a simple xyz stage design utilizing Makerslide. I live in Arlington and I’ve met you via Dorkbot a few years ago.
    Send me an email. We should compare notes on p&p ideas.

  11. Murilo M. Ag. says:

    Hello Tim, any hint of cam software to generate paths for the machine, do not know where to start!
    Thanks, congratulations for the work!

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