A different (better) kind of screw

[Geof]

Originally Posted by greybeard Geof - is it one of these we need to tune up our servos ?

Who was the cartoonist who created The Acme Pewter Tuning Fork Co? I am under the impression he was British (English)?

Posix;
There are many trite sayings about he who never dares never gains and all that stuff; mostly becasue in general they have a large element of truth. Sometimes after chasing an idea with lots of time and sometimes lots of money you realise not only is it not going anywhere it never could go anywhere and feel somewhat chagrined. However, if you never pursue anything you never get anywhere. I know on balance I am ahead with the return from ideas that did go places far outweighing what went into the ones that stalled. Probably most people who persisted are.

[posix]

ggg: your roller thread is wrong way, both lead screw and rollers have normal, clockwise screw (right-hand?) threads.

geof thanks for the kind words of support but you did hit the nail on the head there by assuming I'm somewhat discouraged by this whole malarky.

It gets very VERY frustrating having to travel 10-20 miles to get something done on a lathe only to discover that the holes are 0.1mm out or somesuch when you need 0.01mm accuracy. Well, ok maybe 0.01 is too much to ask for but 0.05 is the biggest mistake this whole shebang will tolerate. More than that and it begins to wobble allover the place.

Yesterday I gave up completely and ask those guys to just make me 3 stupid acme rods and nuts so I can get this machine finished. Once I'm (hopefully) up and running I'll look into getting a 4th axis (somehow) and experiment myself. It'll be cheaper (at least no petrol costs and wasted materials at least) and quicker (cnc).

I, however, do believe my design works. I have built-in some adjustment to get rid of slip completely obviating need for teeny-weeny pinions but I need parts machined to exact specifications rather than 0.1mm out here and 0.1mm out there. 0.2mm is lots of slip, lots of play, lots of backlash, lots of waste above all, both money and time-wise.

I will be putting this on a back-burner for a while, at least untill I have a working machine to work with. But even THAT is proving difficult. I think these acme leadscrews will prove to be too much for my steppers, 20mm steel rod (their linear support can't take rods less than 20mm diameter!) 500mm long, weigh a ton. They did promise to make me an aluminium lead screw if this one can't be moved by my steppers and I suspect that will be the case...

[ggg]

Originally Posted by posix ggg: your roller thread is wrong way, both lead screw and rollers have normal, clockwise screw (right-thread?) threads.

But if you have both normal , clockwise screw, you will have two contact points not axial aligne . The roller thread will try to get an angle to correct fix with the lead screw thread. is that a problem?
If you have different ways you roller an the lead screw forces work aligned so you will have less friction.

[DavidJSwanson]

Originally Posted by posix
ggg: your roller thread is wrong way, both lead screw and rollers have normal, clockwise screw (right-thread?) threads.
----------------

If i'm understanding this correctly (perhaps i'm not but i think i am), having opposite threads, two equal sized rods whose threads were meshed would result in no linear movement as the driving rod would drive the other rod in one direction, but the driven one would "climb" the driver at the same rate it is being driven. (hope that made sense)

if we used this arrangement with satellite rollers that were smaller than the driving rod it would result in linear motion in the opposite direction of what we're used to. and if the satellites were larger it would go in the traditional direction, but with a speed REDUCTION. Also ggg's "backward" threads will will have forces axially along the rods in BOTH DIRECTIONS ALL THE TIME. Whereas traditional designs adjust out the backlash, this design FUNDAMENTALLY HAS NO BACKLASH.

So in theory, we could set our ratio of rod diameters to give us a very fine feed for a highly accurate machine.

A minor problem that i see with this arrangement would be the slippage between rods as always. I say "minor" because the greater friction would reduce slippage, and if our ratio is set to give us 1/10,000" feed and we moved 1/10,000" we'd have less slippage than that.

the major problem i see is finding someone to calculate the exact diameter of contact between the rollers so we'd know what our "gear ratio" was. And then the tolerances to which they would need to be machined to get anywhere close to that.

I hope i didn't post any wrong information, my head is starting to spin from thinking about how helixes interact

[derekj308]

Hi Guys

What a thread! My eyes are burnt after reading all of these posts. I have been investigating the roller screw for a while now and have considered the following points which may be of interest to those considering making their own roller screw. The attached file was given to me by a friend with about 25 years on me. He knew I was into hobby CNC so he thought it may be of use to me. Sorry I can't quote the mag it is out of but the article was written by John Jardine. Thanks John wherever you are!

Using a roller screw which is as per attached file ie same thread used for rollers and main screw result in a lead multiplication of theoretically 2. Practically if you were to make this design you would use phosphour bronze rollers so as to minimise wear between the roller and main screw.

Using "All Thread" which is commonly available from hardware stores have relatively fine pitches (16mm, 2mm pitch for example) which when used with a simple nut severly restricts your rapid speeds. Having the roller design with a 2 x multiplication factor bring this back to 4mm (as a example using the 16mm, 2mm pitch thread) which gives reasonable rapid speeds.

"All Thread" is not well know for its pitch accuracy.

"Proper" roller screws (likewise for ball and leadscrews) are made from engineered steels made for high surface pressure use and are hardened with processes other than quenching when it goes cherry red. Most of us aren't going to be able to build a decent roller screw which will, performance wise, compare with a rolled ball screw.

You can buy precision rolled ballscrews from homeshopcnc very cheap (I'm not plugging them, probably other sources as well???) which have a lead accuracy of 0.1mm per 300mm which for hobby cnc is more than acceptable.

Go through this exercise and you will realise that the pain of researching this subject is for the fun of it alone (I did it, I had fun and learnt something new) and not a practical cost effective solution if you consider your own time and the finishing of your machine in a shorter time period valuable.

BTW my current machine uses 12mm x 1.75 all thread with 2 nuts in a wood housing separated by a spring. I'm not anti "All Thread" I just know that the next step is rolled ball screw and not making my own roller screw.

Cheers

Derek

[posix]

I was really trying to get away from the bulky rectangular arrangement but that seems to be the only "diy"-able solution. Everything else requires a lathe. Not saying that this doesn't require one either but at least it looks a lot less forgiving!

[NEATman]

I just found these intersting videos of the internal working of SKF roller and ball screws. I thought that others might like to take a look:
http://www.linearmotion.skf.com/en/0...er_screws.html
http://www.linearmotion.skf.com/en/0...ll_screws.html
Under "related links" near the bottom of the page.
Check out the recirculating one. The rollers accually shift a thread each revolution!

Neatman

[derekj308]

Thanks Neatman, cool videos. Don't forget to check out the PDF's as well. Heaps of interesting reading.

Cheers

Derek

[posix]

Is there an explanation for the "jump"?

[ggg]

Originally Posted by posix Is there an explanation for the "jump"?

if you look carefully in the "reciculating roller screw" the roller hasn't thread, only are "V" slots and the external thread have only 1 lead.

Looks like the "jump" corrects the relation between the roller an the external piece thread and the roller never goes out.

[posix]

doesn't make sense. grooved rollers don't "go out" they don't have the gearing problem as threaded rollers. why would they complicate something needlessly?

[Zach_G]

It's so that the outer ring can be threaded instead of just grooved, which would allow gear ratios. There's probably other benefits, but yeah it does seem somewhat overly complex when it could be so much simpler. I'd imagine that design would have problems with high speed operation vibration and noise levels. Kinda neat how it works though.