[GAWnCA]

i don't think it can load high torque .

Why not?

[tivoidethuong]

Why not?

because nut make by copper

[GAWnCA]

So to avoid that just use the same material as the drive rod and you should be in good shape. I do't think I would have drilled through but turned down the rod and then threaded it. Use lock nuts on the ends and it not only hold the planetary rods but the but the bracket together as well. You could even fit bearings over the ends. Check out Posix's idea at http://www.cnczone.com/forums/showth...t=13593&page=2
He's on the right track. Simple and yet very functional.

because nut make by copper

[wjfiles]

with reference to roll threads,
would anyone know the pitch circle diameter of a 12mm threaded rod

[voltsandbolts]

This is an example, sized for ¼-20 threaded rod. I am making a prototype with the ends of the rollers turned down to .125 and will take a photo. You may or may not get any thing out of looking at this.

[Jason Marsha]

Thanks for the drawings. We await your results.

Jason

[greybeard]

wjfiles - according to my "Newnes Engineer's Reference Book" 1st Edition 1946

12mm x 1.75mm pitch.... 10.863mm
12mm x 1.5 mm pitch.... 11.026mm

Hope this helps.
John

[GAWnCA]

This is an example, sized for ¼-20 threaded rod. I am making a prototype with the ends of the rollers turned down to .125 and will take a photo. You may or may not get any thing out of looking at this.

Hey Volts. I got a lot out of your drawing. Now I've got to learn how to draw in AutoCAD. If you could place that in a can type enclosure, you would have the same operation and yet the threads would be somewhat protected from flying chips and dust. Posix's idea was good and so is yours, simple. Let us knwo how well it works out.

[voltsandbolts]

GAWnCA,

Ouch! That hurt. Are you offering a class in AutoCAD, maybe I’ll sign up… Seriously I consider my self a CAD dabbler. If you got something out of the sketch I’m glad. If I get something functional put together I’ll do a more complete drawing and some photos.

[GAWnCA]

Ouch? I didn't mean to ouch you. LOL I do need to learn CAD. I did understand what you drew up. All these types of ideas give me and others ideas to maybe start dabbling and improving. Who knows, we may never need to buy another $35.00 roller nut or spring loaded lead nut.

GAWnCA,

Ouch! That hurt. Are you offering a class in AutoCAD, maybe I’ll sign up… Seriously I consider my self a CAD dabbler. If you got something out of the sketch I’m glad. If I get something functional put together I’ll do a more complete drawing and some photos.

[DesignerDave]

Wow, I just read that whole thread. I think it should be a requirement that anyone who wishes to post in this thread read the whole thing... I cannot count how many times people came to the conclusion that in the designs which involve a pitch on the rollers of this design that any slippage will cause the nut to become inaccurately located. I remember at the beginning of this whole thing I was wondering how anyone was going to make it work unless they cut the rollers with straight grooves.

Anyway, this has all been beaten to death... so I'll try to bring my observations to the table hopefully to further our development on this type of screw/nut homebrew design.

First and formost, I think this design should have at least 4 rollers, and if there are more they should be an even number. Maybe I'm being too picky but if you use only two rollers and preload them laterally you essentially are twisting the nut... (when looking from the top of it, I'll do a FEA model for you guys if you want to see what I'm gabbing about) what probably doesn't matter if the rollers are of any substantial length, such as 3 times the diamater of the leadscrew or more, but still, the forces aren't even on anything in the assembly. A member posted saying that putting preload downward on the threads removes backlash but I question how much preload is required to do that and how easy can it be maintained. With only two rollers I would think that the leadscrew would really want to deflect off of center to relieve its preload, which then would transfer into unwanted friction between the leadscrew and whatever is keeping it on center. Not effecient, but this all comes back to how accurate you want everything to stay, and how cheap you are.

With four rollers the leadscrew would always keep its center location, and the forces preloading the threads would be evenly distributed thus keeping the friction between those surfaces to a minimum. ACME threads have been mentioned a few times and I cannot help but think that would be a nice way to go about this design. I figure the rollers do not have to be all that big, but the allowable pitch of the leadscrew would have to be determined using the diameter of the leadscrew to make sure the rollers wouldn't be scuffing too much as the helical thread started deviating away from the straight cut profile on the rollers.

I think I may not be describing this well. The last couple posts were about CAD. My two cents, programs like ACAD are awesome for what they are good at but if you want to get into real fun you have move onto parametric solid modeling. I design for a living and I'm currently running SolidWorks. I'll never go back. Inventor by AutoDesk is also very similar.

I should model something up and throw it on this thread so I don't get stuck in the desert with peanut butter... mmm... peanut butter.

Its my first post guys... I love this site. I've been restraining from doing the typical noob "I wanna build a mill what should I do first" post and I've been researching this site and others (such as 5bears.com) for as much as I can learn before I start requesting anything!

So far this forum has posts from some really good minds out there. I look forward to reading more of your ideas people!

Dave

[handlewanker]

Hi Dave, good show old chap, having read the whole lot you at least know what direction our thinking is gravitating to.
One point you forget from the posts, if you try to mesh Acme screw thread with Acme form roller, the helix angles being of opposite hands at the mating faces will squeeze each other apart and only the tips will be in contact.

However if you use vee thread they will mesh full depth, allowing for tip rounding to prevent bottoming, and so will lend themselves to full screw thread face contact, as they rely 100% on friction of the flanks to cause rotation of the rollers.

There is a certain amount of friction, even though the thread and rolls are rotating in opposite directions while meshed, but this is of a low order compared to the sliding face contact of the Acme thread and a bronze nut.
The problem lies in getting precision Vee threaded rod as opposed to off the shelf readily available Acme transmission screws.

To use Acme threaded rod, for accuracy as opposed to transmission, would mean you would have to thin the flanks of the roller to allow penetration into the thread, which means the roller would have to be split to allow squeezing together for anti backlash application, see my sketch from an earlier post.
All of this becomes simple when you only want to design for frictionless transmission, as the vee screw and threaded roller will work very efficiently and simply together.

BTW, you only need 3 rolls to centralise the screw, 4 or more are irrelevant, but might become necessary when the rolls are made a much smaller diam.
Ian.

[wjfiles]

I have watched this tread with interest hoping to see a simple and easy to make linear motion assembly.
The best in my opinion is the first photo posted by POSIX.
Three rolls with a series of grooves the same form as the thread on the lead screw.
I believe that one of the "planetary" rolls would have to have NO axial movement. This would eliminate backlash.
The other two would need to float a distance at least equal to the pitch of the lead screw.This would make sure that all rolls constantly meshed correctly.

The Planetary rolls would need grooves with a form slightly modified to allow for the helix angle on the lead screw if you want a perfect match.

One roll would need to be adjustable radially to maintain a constant meshing of the threads.
I hope to make one over the next two weeks. I will use an M16 x 2.0
High tensile threaded rod and remove the points from the thread so that it cannot bottom.

WJF

[handlewanker]

Hi Wjf, sounds good, I would make all three rolls a close fit, end wise, and add thrust bearings at both of their end faces to prevent backlash.
The rolls will rotate to get position for all three to engage quite freely, and then they will each share the end thrust generatd by the screw.

Whatever thread you use, and for a trial run Allthread would be OK, the rollers only need to be a grooved vee form of the same thread angle and pitch.

Acme thread could be used, as it's a pretty accurate thread off the shelf, but the rollers would have to grooved to Acme form and the flanks thinned a bit to allow thread engagement.

With the flank thinning it means that the rolls would have to be split and tensioned endwise to grip the flanks of the thread, otherwise bottoming will occur, resulting in backlash.

I am looking at the possibility,( at his moment in time on paper), of making right hand Acme thread to mate with left hand Acme threaded rolls.
While they will rotate freely as the helix angles match perfectly, no motion will occur as the left/right hand mating will cancel out any forward movement of the nut, and so the screw will just rotate in the nut without going anywhere.

However if an internally threaded shroud is then made to mate with the OD of the rolls, we will have a sun and planet type of set up, whereby if the shroud is held stationary and the rolls allowed to rotate round the screw, it will cause the assembly to advance forward as the screw rotates.

The same would happen if the shroud were to be rotated, and the rolls were held stationary, the screw would advance or retreat also, similar to a ballrace and cage.

The necessary fits would add a lot of complication, when simplicity is really what is being aimed for.
I would be interested to see what progress you make.

At the end of the day, there will still be some friction as the faces of the rolls/screw will be a partial sliding and rolling contact, but not so much as a total sliding bronze nut/Acme thread set-up.
Ian.

[Bobbyr70]

I agree if it has more moving parts it will cost more and wear out faster.
I am sticking with the KISS method for motion.

[handlewanker]

Hi all, I just had a stroke of luck.
I am a great believer in "one picture is worth a 1000 words", and while scratching in the "too valuable to throw away" box I came across some goodies that I've been hoarding for years, knowing that someday they'll come in handy.

I have always been under the impression that Acme form will not mate with Acme form, when placed alongside one another, due to the helix angles being different, seems that was a wrong conclusion.

Long story short, I found a 300mm (1ft) length of 20mm diam acme left hand threaded screw rod, reclaimed from a dead vertical spindle molder, and a pair of short worm screws, from a worm drive gear boxabout, 40mm long and 20mm diam with right hand Acme thread forms.

So what? well the short worm screws are Acme thread form, the same size and pitch as the length of Acme thread, but different hand, how convenient.
Placing the rolls and the shaft together showed that the Acme left and rolls right hand threads mate perfectly for their full length, which is understandable as the helixes match on opposite sides.

However when the rolls, (both right hand threads) are placed together they also mated, almost to full depth, probably 90% engagement, which means if a roller is made to Acme form, but GROOVED not threaded, it will engage the threads 100%, good enough for the purposes of a very practical roller nut, without anymore than a pair of thrust races at each end to take the forward and back load.

For general transmission purposes, where accuracy of position is not an issue, the rolls can be made from the same off the shelf Acme form thread that they will mate with, but for high resolution requirements the rolls MUST be a straight grooved not threaded form.

As a picture is worth a 1000 words I'll add a few photos next post to illustrate the match of threads.
I think this is a major breakthrough as the Acme threaded off the shelf screw threads are much more accurate than the Allthread rod.
Ian.

[technomage]

In the interim of posix getting his thing done, I present unto you a roller screw design I've been working on, and plan on prototyping if I ever come up with a CNC frame I like (don't hold your breath). While it looks complex at first, I tried to design it from the aspect of as little complex machining as possible. The three "planets" around the central 1/2-13 UNC rod use a straight grooving instead of a thread/helix, so that there aren't positional errors due to minute slipping between the planets and central screw. Of course, you don't get the speed multiplication that threaded planets provide. The planets have inlaid bearings (cheap from VXB) which ride on 1/4" rod. I couldn't find inexpensive thrust bearings, so I figure take a pair of nice metric washers (the bore on an M6 washer is a perfect fit for 1/4 rod) and sandwich some small BB's, while fabricating a retaining ring out of thin sheet metal, ideally brass. Finally sandwich it all between some angle and it's good to go. If the holes in the angle supports are oversized, it should be possible to bring the planets into contact with the screw with an additional light preload.

How about Zach_G's design done on a drill press with a Thread "repair"file $13.99at ace hardware they usually come with 8 thread sizes ( thread Pitches: 11, 12, 13, 14, 16, 18, 20, 24 T.P.I. I say "thread size" as a thread repair file held at exactly 90 deg to the shaft makes thread sized grooves with no pitch. In normal practice they are held at the pitch on the thread that is being repaired. Chuck the "roller" in the drill press add a vertical guide rod up from the table to each side to steady and control depth. Set the press for a low speed.Then work it like Knurling, file in 6 or 7 grooves overlap by two or three grooves repeat down the roller then back up until you get to where you are flush to both vertical guide rods. holding the file by hand is actually a advantage if you get a good set of grooves to start as the file wants to follow the previous grooves.
http://www.acehardware.com/sm-genera...i-1292851.html
I would imagine the same thing is available in metric.

[handlewanker]

Hi techno, I reckon if lathe facilities wern't available you would get by using the thread repair file, but at the end of the day you would be better off getting a lathe, any lathe, even a woodturning lathe with one of those X-Y tables for a DIY lashup.
Ian.

[Zach_G]

I got bored one day and made this. It turns rough (similar to an abec1) but then I made it using a 3 jaw without worrying about runout. Plus the bearing surfaces are the shoulder of a 1/4-20 screw and some flat washers. The flat washers are definitely very perpendicular to the screw axis either. Oh yeah, it also doesn't engage the 3/4-10 threaded rod because I seeemed to move the cross slide .11 for every groove, oh well... Despite all these issues it feels pretty smooth when loaded from only one direction and the total runout is about 0.005". For prototyping purposes it demonstrates how to make a cheap roller supported with angular contact bearings that could function decently for DIY if care was taken.

[handlewanker]

Hi Zach, I agree with you completely, the grooved rollers will work very well and accurately as a roller nut.
If it's only transmission you need, without accuracy, then the same thread can be used to make the rollers.

I have a collection of worms from worm drive gearboxes, which I used as a test set-up, and although they are an actual Acme thread they mate with one another very well, but for a roller nut where accuracy is required, the threaded form needs to be a grooved form as you have shown.

All you need now is to make a set-up accurately, running true with proper thrust bearings either side and you'll be home and dry.
The principle is pretty simple, and the beauty of it is it can be made by the very average shopworker with little or no skill.

The end result would be a screw drive with no backlash and no friction for very little outlay.
I'm contemplating this for a half nut set up for a lathe, running on the existing acme form leadscrew.
Ian.