Parallel grippers...

Yes that is it. You use two smooth rods for shafts to stabilize the two fingers. The third rod is threaded 1/2 left hand and 1/2 right hand. Each finger is threaded appropriately to match of course. A gear motor spins the threaded rod and it then causes the fingers to move in or out. Next some hall effect sensors and or limit switches are used to avoid going too far one way or the other. You can mount some micro switches or a flex strip on the fingers to determine when to stop the fingers as they move inwards too. Depending on how strong you need it to be determines the size and weight on the gripper.

You could probably use two same threaded rods with a reversing gear or two motors running in opposite directions to the same effect, but it doesn't seem to look as neat though.

( note: pardon my bad english, not my native language) For low-tech solution, you dont need ball-screw type mechanism. If you have a chance, rip apart a broken CDROM drive. You can find "linear gear" ( ? ) from inside the CDROM opening mehanism, use two of them on the opposite sides of gearhead, and youre good to go. My attempt at ASCII art:

|\/\/\/\/\/\/\/\/\/\/\/\/ O * O \/\/\/\/\/\/\/\/\/\/\/|

where the * is motor shaft with gear, "\/\/\/\/" is "linear gear rod", and "O"-s are gripper fingers. This way you get parallel "synchronous gripper". It has the advantage ( or, in some cases disadvantage ) of automatically centering the parts that you pick up. If you want asynchronous, with independent fingers, you could use two different motors somewhat like this |\/\/\/\/\/\/\/\/\/\/\/\/ O * * O \/\/\/\/\/\/\/\/\/\/\/|

The suitable motors could be small steppers, like found in 3,5" floppy drives. BtTW, those come with ball-screw type mechanisms, easiest thing to try if you wanna try hand at independently moving fingers. More ASCII: H---------------|---- O O ----|--------------H

H is the stepper motor scavenged from floppy drive, --- is the screw, | is the "bearing" and O-s are the fingers

Industrial automation often uses pneumatic grippers, with small cylinders to actuate the fingers, but thats good only if you have one fixed type of part to pick up. Sometimes, grippers with "linear servos" are used, those have the serious disadvantage of having low torque, plus they were quite bulky ( last time i had anything to do with such )

-kert

Thanks Earl, great to know im not out in left field with the idea. I later found some info which further confirmed this. Your suggestion of using guide rods was exactly what i was thinking. To make things alittle clearer i made a CAD drawing of the design:

I have all the parts to build this except the dual threaded rod (?) I doubt whether i'll find one at the hardware store. Im planning on using a small gearhead DC motor to drive the gripper,the steppers i have do not have a gearhead so wouldnt be hefty enough to drive the gripper, also a stepper would add abit more complexity to the control circuit.

Kaido , i had toyed with the idea of using a rack-n-pinion drive scheme and i have some of the parts you mentioned but it proved to be abit too complex to build. But if the screwdrive concept doesnt pan out i'll have to give it another shot.Thnks for the ideas.

I got the left hand taps and dies from

was someplace that actually already had prethreaded brass and steel rods in 4-40 6-32 8-32 and 10-32 sizes but now I can't seem to locate them anymore. But show some to check out.

I took a plain brass rod and threaded both ends towards the center with LH and RH dies. But the alternate was to use a short threaded tube in the center to connect the two rods together. I used brass/bronze inserts threaded for the grippers and a couple of threaded short tubes as bushings on the ends of the rods. One end has a small gear motor coupled to the threaded rod one one end. I made my gripper arms one peice full length, I don't know why I never thought about having removeable finger ends.

Actually I had this same thought too. I took apart a couple of old CD rom drives to get at the screw drive mechanisms But I needed to get a couple of small gear motors to drive the screw shaft, such as those from

I then wound up going the simpler singe screw drive rod method, but I might still build it up using the dual rod method.

Wouldn't it be easier to use a plain threaded rod and make (or buy) a left hand threaded nut for one of the nuts?

Mitch Berkson

Mitch, thnks for the tip, again im not sure if i can get those locally but i'll have a look see.

It would help if you mentioned where locally is, but McMaster-Carr

as well as many others, has all this stuff.

Mitch Berkson

Mitch,

How would that work? A left-hand threaded not won't go on a right-hand threaded rod. -Alan

Wouldn't that have to be used in a parallel, but parity flipped universe? Last time I tried to put a left handed threaded nut on a right hand thread, it wouldn't go. Am I missing something?

Doh. Sorry.

Mitch Berkson

You've gotten some ideas for using jackscrew-type mechanisms, and those are fine (but can be slow).

Anyway, to throw out a different variation: parallelism of a gripper can be achieved using two pivoting rails for each finger. An example of this is on page 411-412 of my book Robot Builder's Bonanza, Second Ed. The illustration shows rather small pads for the grippers, but the "grip" can be made longer.

A single servo motor, with a double-arm horn, is used to open and close the gripper. One end of the servo arm goes to the left rails, and the other end of the servoarm goes to the right rails. Use ordinary aircraft servo linkage to connect the servo arm to the rails. For maximum leverage and least binding-up, connect the linkages only to the outer rails on each side. Let the inner rails be used just to keep the gripper in parallel.

With this approach there's no need to buy anything special, and there's nothing to machine. It's all at the hobby and hardware store. Select a servo with the torque you need. GWS makes servos with a 110 oz-in torque, for example. A standard Futaba S-148 or S3004 will have over 45 oz-in at six volts. Given the right size of gripper your robot can readily lift a 12 oz can of soda/beer.

-- Gordon Author: Constructing Robot Bases (Forthcoming) Robot Builder's Sourcebook, Robot Builder's Bonanza

snipped-for-privacy@aol.com wrote:

Here's a kit for $15 so you can use two hobby-servos.

It looks quite nice.

Mmm... i was questioning that myself, but would we still require a left-hand threaded nut for half of the rod ? Isnt half of the rod threaded lefthanded while the other is righthanded ? Im treading new ground here.

Thanks for the advice Gordon. I have Robot Builders Bonanza 1st edition (BTW great book), i dont have the 2nd (yet) but i think i understand what you are discribing. I built a gripper awhile ago based on what i saw and read in the 1st edition of your book, pics here:

The problem ive encountered with these types of designs was holding pressure. The arm ive built, that this gripper is to be mounted to, can lift up to 8 lbs. The gripper could not hold onto this weight while it was being lifted, i had to literally tie the weight to the gripper to lift it.

To let the cat out of the bag, the reason im exploring parallel grippers is for their holding force. Ive read that "pound for pound" , screwdrive parallel grippers can exert more holding force than any other gripper design, excluding neumatics and hydrolics of course. If this is indeed the case then thats the design i'd like to build by. Is this true ?

I just remembered where I saw prethreaded left and right hand threaded rods.

has them. I was looking for something else when I saw them there. of course you still need to make some threaded nuts or something. But they might have those too. You can make a simple joiner for the center section, heck maybe a tube threaded and a brace for the center would make it even more stable too.

Earl, thanks for the link, a definite keeper.