# Thread: simplest/cheapest possible linear slide

1. ## simplest/cheapest possible linear slide

Hey guys, I've been ghosting around so long I figured I should register and ask some questions I haven't been able to find answers to.

I'm working on a design for a 3D printer (like the Makerbot and Reprap) but I'm trying to see how far I can push the simplicity of the design to take advantage of the near total lack of weight that needs to be moved.

At the moment I'm running up against my lack of experience with certain materials. Specifically, UHMW-PE.

Lets say you took a structure made out of MDF and slides of aluminum angle (like the design at build your own cnc Build Your Own CNC Router Machine ) but instead of using vee-groove bearings you just cut a 90 slot into a chunk of UHMW. From what I've been able to find the co-friction of bearings is, like, 0.001 or better, and the co-friction of UHMW is more like 0.15. Also, UHMW creeps and deforms under constant pressure.

HOWEVER, it seems like 3D printers are small (think microwave sized), don't have to move a heavy router around, and aren't subject to any side loads (like in milling). So, if the block of UHMW is the appropriate size it should have enough surface area to keep the lbs per square inch low enough it won't deform from being squeezed against the aluminum angle and the higher friction still shouldn't be enough to affect the speed or accuracy of the printer since the NEMA motors could easily be slightly upsized. It would take a bit more juice to move, due to the order of magnitude increase in friction, but the weight savings in mounting hardware might make up for that.

For example, lets say I had 1/2" MDF with a 90 degree angle routed into the edge so it points outwards, and a strip of 1/16" thick aluminum angle laid on top of that. Then two pieces of UHMW with concaved 90 degree channels would be fastened in place on opposite sides of the linear rail and tightened towards each other just enough to take the slop out of the movement. Would that produce useful linear motion? Would it get sloppy quickly? What about sticking strips of UHMW anti-friction tape to the outer surfaces of the aluminum angle so the movement is UHMW on UHMW?

2. Aluminium is a poor choice for a sliding surface, it's soft and, once the slider wears through the thin oxide layer, very reactive, so it tends to stick and weld itself to the slider material.

A better option would be something like chromed steel tube (as used for clothes hanger rails) or chrome edging trim.

A block Nylon 66, Acetal or PTFE would slide very well.

Bill

3. Aluminum sliders used on small machines, like printers are anodized.
The anodized surface is much harder.

4. How about drawer slides? Dirt cheap, and simple to use.

I have an engraving machine made with drawer slides at work, and it can even cut aluminium.

5. I don't suppose the aluminum angle you pick up at home depot or lowes is anodized? I tried scraping some with a penny and the penny won, so I suppose that settles that question. How to Tell the Difference Between Polished & Anodized Aluminum | eHow.com

Is it practical to anodize at home? Anyone done that?

What about laying strips of adhesive backed UHMW on the aluminum so it's just a support structure?

I definitely like the drawer slider idea, and even had a design drawn up for a while but ultimately it's just a bearing that's been laid flat, so it doesn't really satisfy the attempt to avoid bearings.

I found a guy on the router forums who replaced his vee groove bearings with UHMW slides but I haven't yet found out how well the design held up over time. UHMW Slides - Router Forums

6. Here's an article on DIY anodizing: Aluminum Anodizing at home - DIY aluminium Anodising

7. UHMW inserts would certainly not be the best choice, if for no other reason then you will not be able to keep that small of pieces flat or straight. UHMW has virtually no columnar strength and will bow upon tightening AND you would be very hard pressed to find an adhesive that will adhere to UHMW. PTFE is a great choice for lubricity, however again PTFE has little to no columnar strength and will not tolerate tightening without distortion and again, you will not find an adhesive that will stick to it. To keep price and weight down and still use a polymer, HDPE would be a much better choice. The surface finish is much slicker, it has a higher durometer (plastic hardness) rating, and the machinability is better. I also agree that the aluminum must be anodized for this type of application. Once thru the oxide of an aluminum extrusion such as angle, aluminum in fact is very abrasive. An alternative to the store bought aluminum plus machining plus anodizing may be to use Corian, yes the same stuff counter tops are made of. Easier to machine, no cost in anodizing, and no wear when sliding against HDPE.

8. I have been thinking of curtain tracks with plastic sliders for hot wire cutting.
There are many types. Aluminum Curtain Track | Aluminum Drapery Hardware - Curtain Tracks

Lars

9. That's a cool idea. There are so many different aluminum profiles floating around it seems like one of them should be exactly what you're looking for.

I can't find something that allows me to calculate how much UHMW would deform, but I did find this Cope Plastics | TivarŪ UHMW where they say, "It is recommended that the length of a TIVARŪ bearing be equal to the shaft diameter, unless under a high load, where more surface area is required to resist creep." So, it sounds like keeping the psi low enough will prevent the material from deforming (also temperature).

Since the application here is very low weight, it seems like keeping the surface area up there should reduce the psi below whatever the threshold is to squish UHMW. For example, say the sliding surface is 1/2" x 6"; that would be 3 square inches. Assuming there are two of them, that makes 6 square inches.

I don't know if I'm reading this chart correctly, but if it means that UHMW will deform 2% when it's subjected to 400 psi, then a total of 6 square inches of surface should be able to support 66 lbs without changing shape significantly.
UHMW PE1000 Sheet products, buy UHMW PE1000 Sheet products from alibaba.com
I doubt the whole machine will weigh that much with the power supply and motors.