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View Full Version : Dumb question: LH nut on RH screw possible?



sansbury
03-31-2014, 09:36 PM
Is it possible to put a left-handed nut on a right-handed screw? The reason I ask is I have a motion application where I want to have two fingers (think a gripper for a robot) move towards or away from each other simultaneously. My thinking was that if I used a screw to drive them, and then had one finger with a RH thread and the other LH, I could get them to move spinning just one screw in one direction which is nice and simple. Not sure however if this would require violating the laws of physics. For the record the strongest substance I have consumed today is cheddar cheese :confused:

brian Rupnow
03-31-2014, 09:46 PM
No, you can't do that. You've got the right idea though. The part that spins must have half its length a right hand thread and the other half a left hand thread. Then you use a right hand nut and a left hand nut. By spinning the threaded part one way or the other, the nuts will move farther apart or closer together.

Rich Carlstedt
03-31-2014, 09:49 PM
I think what you want is a turn buckle, except you use a solid screw with RH and LH threads and two nuts

Rich

darryl
03-31-2014, 10:20 PM
Doesn't work very well with traditional threads, but with a low tpi it can work. A rod can be threaded both left and right, then both left hand and right hand nuts will work on it. When you thread the rod, the first thread looks normal. When you add the other thread, it will be cutting through the first one, leaving trapezoidal islands with both helices on them. Because the nuts would each have an uninterrupted thread, they will roll on smoothly in their perspective directions, ignoring the opposite thread.

While this does work, neither nut would ever go past center of the rod, so you'd be better off anyway just to thread half the rod left, half right. Or- re-think the method of opening and closing the fingers.

Robin R
03-31-2014, 10:25 PM
Handscrew kits might offer something that would work for you. http://www.leevalley.com/en/Wood/page.aspx?p=69826&cat=1,43838,47843&ap=1

loose nut
03-31-2014, 11:11 PM
Is it possible to put a left-handed nut on a right-handed screw?

Yes but only with a really big hammer.

lynnl
04-01-2014, 12:53 AM
Can't you just turn the LH nut around backwards to make it fit a RH screw? :D

....seems like that oughta work

darryl
04-01-2014, 01:01 AM
Related question- what would I have to do to make a metric nut fit a unc thread- stretch it or shrink it?

rode2rouen
04-01-2014, 01:25 AM
Yes but only with a really big hammer.


Actually, I've found that a stout air impact wrench works much better.
Also works well with metric nuts on SAE bolts.


Rex

wtrueman
04-01-2014, 02:17 AM
You could try fine thread on one nut and coarse thread on the other nut ? Do not really have a answer, Wayne.

darryl
04-01-2014, 02:30 AM
Looked at a lot of gripper designs and haven't seen any that I really like-

Ian B
04-01-2014, 03:05 AM
Yes, it's possible to have left and right hand nuts running on the same shaft. Have a look at these things: http://www.rccoh.com/rc.tools/yankee.screwdriver.html

No reason not to have a left hand nut for one finger and a right hand nut for the other.

Happy April 1st,

Ian

darryl
04-01-2014, 03:43 AM
Here's two nuts running on the same shaft-

http://www.d.umn.edu/cla/faculty/troufs/Buffalo/images/af001549.jpg

Paul Alciatore
04-01-2014, 04:19 AM
like!


yes but only with a really big hammer.

boslab
04-01-2014, 04:20 AM
A preemptive April the first strike, how cunning
Mark

Euph0ny
04-01-2014, 06:20 AM
The part that spins must have half its length a right hand thread and the other half a left hand thread. Then you use a right hand nut and a left hand nut. By spinning the threaded part one way or the other, the nuts will move farther apart or closer together.


Happy April 1st!

Tom Lipton briefly explains such an opposing-motion device (it's on a grinding stone dresser) here:

http://youtu.be/DOCfQV5cU2A?t=12m12s

Might give you some ideas. There is a still shot of it at 52:37, but you don't see the inner workings.

bob ward
04-01-2014, 08:18 AM
Is it possible to put a left-handed nut on a right-handed screw?

This used to happen all the time when car makers were putting LH wheel nuts on one side of the car and RH on the other. Chrysler I think was the main culprit, this was during the 30s up to the early 60s.

Lew Hartswick
04-01-2014, 08:51 AM
The level winder on a fishing reel .
...lew...

DR
04-01-2014, 11:41 AM
FWIW, the 6" faceplates on old Delta wood lathes were threaded both right hand and left hand. This would be equivalent to tapping with a right hand tap then tapping again with a left hand tap. Being a 1"-8 thread there was enough "meat" to withstand the forces in woodturning.

It was done so the same faceplate could be used on the inboard end of the spindle and the reverse threaded outboard end.

winchman
04-01-2014, 01:36 PM
You could put two right-hand threaded rods side by side with a gear on each so that one turns clockwise while the other turns counter-clockwise. That would make the nuts move in opposite directions and they could even pass by each other.

wierdscience
04-01-2014, 02:22 PM
Ditch the threaded rod idea and use two worm wheels one on each side of a single worm.Attach the gripper arms to the worm wheels and drive the worm with a stepper or servo.

Bob Fisher
04-01-2014, 02:30 PM
Grippers I have seen use simple levers to open and close. Why would you want to use a screw, be much slower to actuate the way I see it.Bob.

Peter.
04-01-2014, 03:46 PM
Could you make 2-start thread but make one start LH and the other RH? You'd have to make nuts that had most of the thread machined away but if you made them trapezoidal/acme thread they would be pretty strong.

PStechPaul
04-01-2014, 04:43 PM
You could also use fine threads on one end and coarse on the other, or even thread both the inside and outside of a piece where the outside would be much coarser than the inside. The rate of feed would be the difference in the threads as expressed in inches per turn (inverse TPI), or directly using metric measure (mm pitch).

There are some reversible push drills that have CW and CCW helix shafts:

http://i.ebayimg.com/00/s/MTIwMFgxNjAw/z/DlAAAOxyTMdTKQ7M/$_58.JPG
http://www.ebay.com/itm/like/251486477752

sansbury
04-01-2014, 05:24 PM
Grippers I have seen use simple levers to open and close. Why would you want to use a screw, be much slower to actuate the way I see it.Bob.

I care more about strength for weight than actuation speed. Lever mechanisms are much faster but require more motor torque which means a heavier motor, and since it's at the end of the arm, every additional ounce of motor is an ounce less of payload. I don't care if it takes 5 seconds to actuate instead of half a second. It's not for a production line robot.

sansbury
04-01-2014, 05:28 PM
Could you make 2-start thread but make one start LH and the other RH? You'd have to make nuts that had most of the thread machined away but if you made them trapezoidal/acme thread they would be pretty strong.

Interesting idea, but I'm trying to stick to plain-vanilla components to keep cost down in case I wanted to make these by the dozen.

sansbury
04-01-2014, 06:20 PM
Ditch the threaded rod idea and use two worm wheels one on each side of a single worm.Attach the gripper arms to the worm wheels and drive the worm with a stepper or servo.

Are you referring to something like this?
http://www.robotee.com/index.php/robot-arm-4-bar-linkage-end-effector-robot-gripper-11037/

I considered that but decided I want to have a parallel gripper so that the point of contact doesn't move depending on the width of the object to be gripped.

wierdscience
04-01-2014, 10:28 PM
Are you referring to something like this?
http://www.robotee.com/index.php/robot-arm-4-bar-linkage-end-effector-robot-gripper-11037/

I considered that but decided I want to have a parallel gripper so that the point of contact doesn't move depending on the width of the object to be gripped.

Okay,if you want to do that,same mechanism,but use a cam followers running in machined slots.Depends of course on how much travel you need.Here's a primative cave drawing to better explain it.

http://img.photobucket.com/albums/0903/wierdscience/gripper_zps8b9310a5.jpg (http://smg.photobucket.com/user/wierdscience/media/gripper_zps8b9310a5.jpg.html)

brian Rupnow
04-02-2014, 09:57 AM
In your original post, you say that you want to accomplish the gripping by "turning one screw in one direction". Surely you mean to turn it in one direction to grip, then in the opposite direction to release---Didn't you?

sansbury
04-02-2014, 01:09 PM
Surely you mean to turn it in one direction to grip, then in the opposite direction to release---Didn't you?

http://x1.fjcdn.com/comments/4872230+_981637170075f217904bd7e4966a14fd.jpg

Yes, you're correct :)

Current thinking is two screws with a gear to reverse direction and motor driving one screw.

Boostinjdm
04-02-2014, 04:38 PM
how about a crank and linkage set up like metal shop cabinets use to lock the doors?

brian Rupnow
04-02-2014, 05:43 PM
Does the gripper have to center the load when it picks it up? Is the thing being picked up rigid enough that the gripper won't crush it? How do you adjust the amount of "grip force" applied by the gripper? Can the motor driving the gripper be allowed to "stall out" when the gripper closes on the load it is picking up? Since it is "end of arm" tooling, and you don't want to add weight, can the motor be reversed to "ungrip" the load and return to a fully open position and then shut off and wait in the "fully open" condition until it is positioned properly to "close and grip" again?

greatbasin
04-03-2014, 11:07 AM
I would just thread over a right hand thread with a left hand thread of the same you, same results as previously shown and simple my two bits:

sansbury
04-04-2014, 07:38 PM
Bearing in mind that this is currently more a fancy toy than a commercial application- (not that any of your questions are wrong, just to help calibrate expectations)


Does the gripper have to center the load when it picks it up?

I'd strongly prefer that as I think it makes programming easier.


Is the thing being picked up rigid enough that the gripper won't crush it?

Assume yes.


How do you adjust the amount of "grip force" applied by the gripper? Can the motor driving the gripper be allowed to "stall out" when the gripper closes on the load it is picking up?

I've been assuming I'd use a stepper or DC motor with encoder so I could control the grip width. Add a rubber pad on each finger and you get a rudimentary approach to grip-force.


Since it is "end of arm" tooling, and you don't want to add weight, can the motor be reversed to "ungrip" the load and return to a fully open position and then shut off and wait in the "fully open" condition until it is positioned properly to "close and grip" again?

Yes, I would be using a reversible motor.

brian Rupnow
04-04-2014, 08:02 PM
Sansbury--I had sent you a private message to ask those questions, but you didn't respond to it, so I assumed that you did not receive it. The type of gripper most often favoured for end of arm tooling is pneumatic, simply because you can "stall out" either a pneumatic cylinder or rotary actuator without causing any harm to it, while still applying full "grip force". I'm not sure about "stalling out" a stepper or servo motor without overheating and damaging it. If you have compressible "pads" on the gripper, and if the parts are all of very uniform size, then you may get away with it, however the hysteresis will be a very small range, equal to the range of compressibility in the pads.---Brian

sansbury
04-05-2014, 12:54 PM
Thanks Brian--I did get your message but been very busy lately and missed the notification. The ideas you've posed have been very well taken. As I've moved along in this I've come to realize that the gripper is really a full robotic system by itself. I've been avoiding pneumatics because that would mean I'd need a compressor to run the full system. While that would be fine for just me in the shop, I'm hoping to build something that can live in a variety of settings so I'd prefer to have an all-electric solution.

Now here's an interesting critter--a force-sensitive resistor: http://www.phidgets.com/products.php?product_id=3104

Heard about these the other day as someone was using them as part of a 3D printer. At $7.50 each they're cheap enough, and sensitive to .2-20 Newtons of force (so roughly an ounce to several pounds of force). Not super-precise, but sounds like they could differentiate between one ounce of force or four, which would be better than I was expecting to be able to do easily. Probably use an Arduino or PIC-type setup so that the user can say "apply X ounces of force" and the motor runs until it senses that. Add a home switch at the end of the travel to control "full open" position. That means I could go back to using a simple DC motor which saves me money up and down the stack.

darryl
04-05-2014, 04:12 PM
There have been countless times I see a mechanism of some kind, then relate that to how the parts of the human body work. Look at how fingers work- tendons basically pull against the knuckles and draw the fingertips inwards. Something similar could be implemented, and pulling against a set of cables could easily be done using a motor driving a threaded rod. With a direct drive and fine thread you would get both torque and self-holding- with a coarser thread you'd probably need a speed reduction as part of the drive, and you'd have some control over the curve - you could still make it self-holding, or set it up such that it is not self-holding, but requires only minimal motor power to maintain a grip on something.

Actuating gripper fingers this way allows you the option to streamline the mechanics of it all fairly well. This could be simply a bonus, or it might actually be a requirement. I'd suggest that you wouldn't want to create something that is 'fumble-fingered', or couldn't reach to grip something because it's too bulky.

sansbury
04-05-2014, 05:23 PM
By the way, here's a video I made today of the first test of the thing. This was to verify that my motor power calculations were somewhere in the ballpark. I have a water bottle strapped to the back of the small arm to simulate motors and such I expect to mount there, and about 2 pounds in the bag hanging off the end. Steppers are 425oz-in size taken off my old X2 benchtop mill which I replaced with a Tormach last year. Any more weight and the rear motor starts to lose it. Since my goal was to have at least 2 pounds to work with at the end, I'm pretty happy to see this work finally. Now back to the gripper design!


https://www.youtube.com/watch?v=1gIIH7j2leo