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John Stevenson
09-25-2010, 07:13 AM
New thread as the old one went off tilt a bit but prompted initially by Darrin's thread over on PM a while ago and Evans regurgitation of it I resurrected my trials on it.

First a bit of history to see why I went this way.

I get called on the modify ballscrews for CNC machines, just machining the ends. The ball-screw is dispatched in long lengths with a ball nut fitted and these are cut with a slitting disk to length and machined, often the waste length can be up to 600 - 700 mm long, they don't bother because the ball-screw is relatively cheap, it's the nut that costs the money. So I have a series of cut off lengths that are basically worthless.

First try about a year ago was to grind 3 flutes in a small section and tap it, the end result wasn't very good so it went on the back burner, then Darrins remark prompted a go at doing it which worked but I wasn't happy with the result.

Evan did roughly the same as Darrin but posted his results better laid out for people to follow.

Anyway after a few trials and tribulations I came up with this.

http://www.stevenson-engineers.co.uk/files/plastic%20ball%20nut.jpg

A one piece ball nut in some crappy plastic that was on hand, probably nylon or nylon 66 with no splits in it and a fully formed thread inside.

Can't get a shot inside as the whit nylon fuzzes the picture up, runs nice on the ball screw and my next step is to do a square nut and replace the Y axis standard nut and block on a X1L machine I have here for testing.
The X1L already has CNC on the Y but using standard parts, it will be interesting to see how it behaves.

BTW, none of this is new, Moore international have made round thread screws and plastic nuts for quite a few years, called Rondo Thread.

http://www.moore-international.com/lead_screws_rondo_round_thread.html

On their web site they go on to say:-

Rondo round thread has been developed as a true alternative to Acme / trapezoidal threads. They deliver outstanding efficiency and quiet running due to their round thread profile. In terms of performance Rondo has almost twice the efficiency against standard acme, trapezoidal threads.

More on this method later when I get some process shots.

.

Evan
09-25-2010, 07:47 AM
First try about a year ago was to grind 3 flutes in a small section and tap it, the end result wasn't very good so it went on the back burner, then Darrins remark prompted a go at doing it which worked but I wasn't happy with the result.

Evan did roughly the same as Darrin but posted his results better laid out for people to follow.


Actually John, I never even saw that thread or I would have posted in it.

http://bbs.homeshopmachinist.net/showthread.php?t=30977




Some time ago I made a 3 axis gantry router and used a leadscrew that came from a garage door opener to run the motions. This stuff was 1/2 inch diameter, 3 tpi, and 3 start threads. I molded some nuts around this rod using of all things- coffee table epoxy. I figured I would have some problems removing the nuts from the shaft, which I had treated with some kind of release compound, and I wasn't dissapointed. Once I had a nut removed, I ran a tap through it (made from the same leadscrew of course) and that cleared it out just enough to run easily. Two of these nuts carried the weight of the gantry with the router on it without problem, and they hadn't developed any play by the time I dismantled the machine. I didn't use the machine as much as I thought I would, and it was too big to keep around taking up space. I used the pieces for other things, one of which (using one of these nuts) was to push and pull a machine of about 100 lbs across a pair of steel rails, sliding on uhmw pads. That got a fair amount of use, and there is still no play in the nut. (I just checked that- the mechanism is sitting right beside me, waiting for its next application).

I've also played with uhmw to make nuts from. It doesn't machine out well enough with a tap to make a satisfactory part, not IMO anyway, but if I heated and molded it around the leadscrew it made a nice thread. I didn't get it to fill the groove though, and abandoned the idea.




BTW, none of this is new, Moore international have made round thread screws and plastic nuts for quite a few years, called Rondo Thread.


What is new is that I developed a method to make home shop acetal nuts that even you can follow.

added: What I thought up and what Darryl didn't and what you took from my idea is to heat the screw and use the hot metal to form the threads.

PixMan
09-25-2010, 08:24 AM
I'm just wondering, why make nuts like that out of plastic at all? Isn't it just easier to grind a HSS threading tool and make them out of a much-stronger metal?

Evan
09-25-2010, 09:09 AM
Look at the ball screw in the picture. How will you make a tool that is an accurate fit to that profile? It is much easier to use the ball screw itself as a hot forming tap. Further, plastics are superior to a metal part because they are self lubricating and do not tend to wear in use.

djc
09-25-2010, 10:21 AM
If you knew the size of the balls in the original screw, could you wind up a spring* using rod of the same diameter as the balls, twist it onto the screw and then use this to hot form your nut? Bore out the minor diameter of the nut so it clears the major diameter of the screw. Drill two holes** and fill with balls. More thought required on the recirculating channel...

* probably on a slightly smaller dia. former than the minor dia. of the screw, and maybe to a slightly slower pitch so that it's tight when wound onto the screw stock.

** determining the linear and angular position of these holes relative to the axis of the shaft may be a challenge. Perhaps they should be tangential to the screw's helix?

lazlo
09-25-2010, 10:27 AM
Nice job John! I would imagine these are much lower friction than the acetal acme nut?

It's not clear how to tapped the nut? You said you tried to flute the ballscrew, and that you got unsatisfactory results. Did you use heat like Darryl's method?

PixMan
09-25-2010, 11:02 AM
Look at the ball screw in the picture. How will you make a tool that is an accurate fit to that profile? It is much easier to use the ball screw itself as a hot forming tap. Further, plastics are superior to a metal part because they are self lubricating and do not tend to wear in use.

I agree that it's a good way to get a complex shape done. Personally I would never have thought to take apart a ball screw, discard the the whole recirculating ball system and essentially make a conventional lead screw out of it. I would have made (or bought) an Acme screw and fabricated a two-piece Oilite bronze nut first. Also self-lubricating and (theoretically) stronger against loads than many plastics.

What was so wrong with the ball screw in your photo that it made a good candidate for a plastic nut? Is it loose in the middle? I'm NOT questioning your engineering or machining prowess after looking at your website and seeing the level of work you do. I'm just looking for an education because I've always been more inclined to seek conventional solutions.

EDIT: Went back to see these were "scraps" of new ball screw. I still would have gone Acme or tried my hand at making recirculating ball nuts. :D

Evan
09-25-2010, 11:09 AM
John has a bunch of left over ball screw rod from cutting them to size. He didn't discard anything. This is an excellent way to use the remainders for other projects. I know this because he e-mailed me about the project.

Evan
09-25-2010, 11:11 AM
It's not clear how to tapped the nut? You said you tried to flute the ballscrew, and that you got unsatisfactory results. Did you use heat like Darryl's method?


I don't recall Darryl ever saying he used a heated screw as a tap.

John Stevenson
09-25-2010, 11:13 AM
Actually John, I never even saw that thread or I would have posted in it.

http://bbs.homeshopmachinist.net/showthread.php?t=30977

No Evan read the top of the post again, Darrins post over on PM not here where he used softened UHMW or whatever.




What is new is that I developed a method to make home shop acetal nuts that even you can follow.

Correction, What is new is that you developed ONE method to make home shop acetal nuts that anyone can follow.

If there is one method it follows that there are other methods, some may be better, some may be worse.

My method, which isn't new other than being done in the home shop with limited equipment is to do it in one piece as opposed to your two piece so it could be said that this is an improvement.


added: What I thought up and what Darryl didn't and what you took from my idea is to heat the screw and use the hot metal to form the threads.

My method doesn't heat the screw at all, the screw is there solely as a former for the thread.

Evan
09-25-2010, 11:18 AM
No Evan read the top of the post again, Darrins post over on PM not here where he used softened UHMW or whatever.


Are you talking about Darin or Darryl?

I very rarely look at PM and haven't posted there or read it in a long time.

PM is far too slow to load on my connection. The only time I have posted there at all in the last few years was when this board was down for several days. Other than that I don't look at it at all.

John Stevenson
09-25-2010, 11:40 AM
Sorry Darryl, my mistake and it was back in 2005 when you were quite active on PM so i take it you had read it ?

wierdscience
09-25-2010, 12:05 PM
None of you can pin any of this on me:D

I was wondering after Evan posted the melted method if it would work to soften the plastic by boiling it and the screw together and then forcing the thread in while it was soft.
Seems like the screw would be the last to cool down forcing the plastic to set around it.

It might work,or I might be all wet with boiling water:eek:

Liger Zero
09-25-2010, 12:06 PM
Further, plastics are superior to a metal part because they are self lubricating and do not tend to wear in use.

Furthermore, with the right information you can find modified plastics that have different properties. For example I used to mold a part at *arbec Plastics that had a lubricant compounded into the material.

If you know you are going to be using a certain type of coolant because of your process requirements, you can select from modified nylon, acetal or even UHMPE. If you know you are going to be putting a severe load on the machine, Torlon or a reinforced material may be a good choice.

There is no such thing as "crappy plastic" the reason plastic fails in service is because people choose materials based on cost, color and "shiny-ness" rather than material properties. If you're careful, and you ask the right questions, you could (and it has been done) design a plastic replacement part far superior to a metal part.


...and guess what? I have some of this information! And if I don't know, I know who to contact. So don't hesitate to ask.

lazlo
09-25-2010, 12:23 PM
I very rarely look at PM and haven't posted there or read it in a long time.

Evan, you were a very active participant on PracticalMachinist in 2005, when Darryl posted the heat-formed acme nut technique.

http://www.practicalmachinist.com/vb/search.php?searchid=2814591

Evan
09-25-2010, 02:49 PM
Evan, you were a very active participant on PracticalMachinist in 2005, when Darryl posted the heat-formed acme nut technique.


That was when I still had my computer store and high speed access. I don't recall the thread at all. The link you posted doesn't go anywhere. I closed my store a year later.

darryl
09-25-2010, 04:26 PM
Time to chime- firstly, my method was to heat the plastic, then form it onto the leadscrew. I had thought to heat the leadscrew as well, but didn't because I figured it would expand with the heat and make the plastic threads slightly stretched out. Over a couple inches, and with only 300 degrees F or so, maybe this wouldn't have been any problem, but in any event I just squeezed the heated plastic over a cold leadscrew. My other effort at that time was to mold the nut using epoxy, over a coated leadscrew. I made several of these nuts and put them to use in my 3 axis machine, where they worked fine. One pair, the z axis, had constant pressure on them because they were carrying the overarm with the router assembly on it. No problems arose from that. The machine has long since been dismantled because I didn't use it enough and it took up way too much floor space in my small shop.

Evan and John have both probably gone into it deeper than I did, and I'm finding it interesting to follow this thread.

Also this is the first time I've heard anybody else come up with the idea to wind a coil spring and fit that onto a ball screw, molding over it to make the nut. I've wanted to try that for years already, but haven't done it. It would basically be the spring material (music wire or drill rod- whatever it's called where you buy it) bearing against the faces of the leadscrew, so whatever wear factor would be involved with the two materials in contact- well that's what you'd be dealing with mostly, I think. You're going to go through a lot of spring winding tests as well, to get the fit just right, but still I think it's worthy of experiment. You could use a more rigid casting material in that case, because you're not relying on it to give a long-life sliding wearing surface- it's just a binder. I always have thought that idea had merit.

Of course with all of this being shop made parts, you feel freer to add whatever features you might to the project. One of those in my mind was a lubricating port, right in the middle of the threads. Even if you don't run a lube, the port would be a good place to blow dry air to help keep the leadscrew clean.

whitis
09-25-2010, 06:39 PM
There is no such thing as "crappy plastic" the reason plastic fails in service is because people choose materials based on cost, color and "shiny-ness" rather than material properties. If you're careful, and you ask the right questions, you could (and it has been done) design a plastic replacement part far superior to a metal part.
Not quite any part. Sometimes plastic doesn't have the strength, rigidity, temperature handling, conductivity or other properties required in a reasonable amount of space. But you can use plastic for a lot more than people think and sometimes make plastic parts which are better than a specific metal version. All too often, "crappy plastic" parts don't use enough plastic or use enough in the right areas as well as not using the right plastics. Plastic often isn't that cheap, either, as anyone who has priced engineering plastics knows but plastic parts can be mass produced cheaply.

But a plastic 3/4-10 ACME nut can have a rating of 1400lbs (http://www.roton.com/Mating_Components.aspx?family=7060206) - about half of an identical brass nut.

Forming threads in plastic around a hot screw is probably about as old as plastic itself but oft forgotten and reinvented. But it is good that Evan and Daryl have reminded us of it and shown that adequate lead screw nuts can be made using this method.

Like Sir John, I have thought of shop made plastic nuts that mate with ball screws. Specifically, ones which would be interchangable with ball nuts.

Your Old Dog
09-25-2010, 07:36 PM
...........................................
Forming threads in plastic around a hot screw is probably about as old as plastic itself but oft forgotten and reinvented. But it is good that Evan and Daryl have reminded us of it and shown that adequate lead screw nuts can be made using this method........................................

I expect this idea will be used by quite a few. I've been dealing with a sloppy setup and hope to pull this off myself.

dp
09-25-2010, 08:15 PM
My method, which isn't new other than being done in the home shop with limited equipment is to do it in one piece as opposed to your two piece so it could be said that this is an improvement.


So what is your method?

KiloBravo
09-28-2010, 09:50 PM
any updates on your method Sir John ?

John Stevenson
09-29-2010, 05:17 AM
I need to do another and take pics, been busy playing with gears but it's caught up with me and it's close to the end of the month and some stuff has got to go out the door - bloody customers - why won't they leave you alone.

I'll try next week, but we will be real busy getting setup for the Midland Model Engineering show on the 12th.
Big show, not the biggest but this one runs over 5 days so a lot of work. It takes three 17 tonne trucks now just to get everything there.

Rick K.
01-29-2011, 08:57 PM
Have you gotten a chance to get back to this?

lane
01-29-2011, 10:10 PM
Should work real well. Years ago we used to work with lots of different kinds of plastic and built lots of things out of it and we learned to not tap the holes but drill smaller than tap drill size and just run the bolts in . They held better and were stronger than taped threads

PeteF
01-29-2011, 10:21 PM
...and guess what? I have some of this information! And if I don't know, I know who to contact. So don't hesitate to ask.

Ok :D

In your post you pasted a quote that "plastic tends not to wear in use" and I'm curious about that. Wouldn't plastic wear just like any other sliding surface? Whether it's at the same rate as xxxx metal is another factor but to "not wear in use" seems like a big call.

Pete

ligito
01-29-2011, 10:41 PM
Re: Threading plastic.

I seem to remember from a few decades back, that heating plastic rod and running it through a nut to thread it, was another method, albeit, just opposite of this.
I used to heat the pointed Nylon gizmos that were used in letter paper binders and threading them, then melting the ends and flatten them against something cold.
After that, you could machine a slot in the face, for a screwdriver.
This was just to make screws and bolts.

Duffy
01-30-2011, 12:04 AM
As far as winding spring wire on a length of ball screw to make a mandrel for forming a ball nut, why use spring wire? ANY wire of the right size would do, even aluminum, (or if you prefer, aluminium.) It just has to get hot and hold its shape while the plastic forms around it. Soft wire would be a lot less of a struggle to wind.

darryl
01-30-2011, 03:01 AM
The idea of the spring wire was to actually be the surface riding in the ball screw threads. Molding plastic around it would serve to give a method of applying torque to the spring piece, in other words a way of mounting the resultant 'nut'. This was just another idea, and not related to using the threads directly in the plastic to be the mating surface to the leadscrew.

Back to molding the plastic over a leadscrew- I have been wondering if it wouldn't be easier and possibly better to mold over a ball screw as opposed to an acme or other thread form. Seems to me the plastic would flow and fill the round groove better than it would other thread forms.

dp
01-30-2011, 03:32 AM
Why not use plastic wire? It doesn't even have to be round. It can be extruded (and rolled to diameter) with a spine built in to better integrate with the plastic added in the final molding stage.

darryl
01-30-2011, 06:58 AM
Hmm, interesting. How about a teflon wire, and mold a body of some other plastic around that- I wonder if you could get any decent adhesion to the teflon. Maybe you would wind the teflon rod into the threads, then rough up the outer surface before molding. I'm also wondering if the molded body would compress the teflon into the threads as it cooled- that could be a good thing, or it could make it tight- this might be a good test to do. I wonder where you'd get teflon 'wire'-

Rick K.
02-03-2011, 10:26 PM
Still no word from John S. on further developments. Please update.

Thanks

.RC.
04-16-2011, 08:54 PM
A good power point presentation in this thread

http://www.practicalmachinist.com/vb/general/epoxy-bearing-method-material-223826/

Elninio
04-16-2011, 09:01 PM
how's the friction? What kind of force does it take to see elasticity of 5 thou in the axes of the machine the ballscrew is installed