Wednesday, November 07, 2007


MCB Bearing Issues Solved!

Recently, I've been working on building my McWire Cartesian Bot. This design is based on a really cool, simple design posted on instructables about a month ago. It uses standard iron pipe + aluminum rails + roller skate bearings + acrylic stages to create a simple, elegant 3D positioning system.

I took it upon myself to build this design, and improve upon it if I could. There were a few major flaws I found with the design:

1. it used three aluminum rails per axis to keep it aligned. This created alot of friction, and aluminum rails are sort of pricey. I replaced one of the aluminum rails with roller skate bearings. This serves the same purpose of keeping the stage aligned, except it has less friction, and its cheaper to boot!

2. the next major problem i found was that the bearing arms to keep the stage up tight against the rail was only kept in place by a screw. If it worked loose, you basically had to keep tightening it down to keep it in place. I fixed this by adding a second bearing arm, facing the opposite direction and pulling them together with a spring. You still tighten them down, but if the arms loosen, the spring will keep the stage somewhat in place. It also helps with attaching the stage.

3. one of the things that really bugged me was that the acrylic stage just sat directly on the aluminum rail. even after just driving it for a short while, there were lines and scratches directly on the acrylic. seeing as how getting the acrylic stages lasercut is rather expensive (not to mention replacing a stage is time consuming). also, this was a source of a decent amount of friction in the system.

#3 was the toughest nut to crack. I tried a variety of things, that all failed for different reasons. here they are:

* roller bearings inside the rails. the idea was to have the stage ride on top of roller bearings. it was really hard to mount them, and really hard to get them all to the same height. failed.

* next, i decided to mount some sort of bearing on the stage itself. i thought about things that were soft, and came up with the idea of felt and/or ribbon. a trip to the fabric store and $7 later, i had some stuff to try. i had to glue them to the stage, but when i put it on the aluminum rails, it actually glided very smoothly. the problem with these is that i dont think they'd work very smoothly.

* after a couple days of mulling over it, i was thinking about what types of things have low friction when I remembered PTFE (aka Teflon) which has one of the (if not the) lowest friction ratings out there. i pulled up my favorite website EVER, and looked at their PTFE offerings. It turns out, they offer strips of PTFE for sale!!! I even found the perfect item for this: it is an ultra low friction grade PTFE strip: 1/2" wide, 1/8" thick, and sold by the foot. Fearing the worst, I checked the price: only $3.31 a foot!!!! Since my bearings only needed to be 2"-3" long this was absolutely perfect!

i ordered the PTFE strip and it arrived yesterday. i rushed over to the lab, cut it into strips. i then drilled holes in the ends of each strip for mounting. after that, i marked on the stages where each strip was to be mounted. i also numbered the strips and where they were to be mounted (i didnt really measure the strips much.) i then drilled the stages, and mounted the strips with self-tapping screws.

after that it was the moment of truth. i put the stages on the rails, and tested it out... it glided like butter!!!! i havent even sanded/polished the rails yet, but it seems like this is going to work really well.

the cool thing is that these bearings are *super* easy to replace. they are each just held on with 2 screws, so it would be a very simple process to buy another $3.50 strip of PTFE, cut and drill new strips, then screw them back on. simple, easy, awesome.

sorry about the novel, but i'm pretty psyched about this!

Hi Zack,

2 questions;

1: Do you ever sleep????
2: Could I get the Mouser part number for the PTFE strip from you? (I looked about on the Mouser site with little luck)

I built one of the McWire CNC machines the day after it appeared on Instructables. My biggest problem seems to be with friction both metal on metal with the guides and the groove that are slowly appearing in the plexi. I used a little silicone grease for the rails but the PDFE strips for the plexi should do the trick. Good work that!

Thanks in advance
oops! i wrote Mouser when I really meant McMaster. here is the link:

you might as well get 3-4 feet of it since its cheap and then you'll have backups!
and no, i do not sleep, i live in NYC :P
LOL That explains my lack of search results. Thanks again, I'll post the results of my "upgrade"

and as always backups are good! ;)
Looks good but I can't help thinking that by the time you have added rails, rollers, springs and bearings you might as well just used a pair of draw slides mounted at right angles.

Mind, I have only ever played with them in the shop, but they seem very smooth, not much friction and remarkably stiff in one direction. That's why you would need two at right angles for each axis, or perhaps three.

I am sure people started using them earlier in the project but I didn't see anything come of it.
The ones that seemed the most suitable were intended for mounting a keyboard shelf under a desk.
Hi Zach,

... i'm actually at building the fixing- and gliding-parts for my tripod and use a PTFE-plate with 10 mm thickness, from which i mill glider-fins and threaded blocks for the feeding and moving parts, which should have lesser friction ...

If you didn't need the tickness of your PTFE-stripe, then there are paper-thick self-adhesive sheets of PTFE-coated glass-fabrics, mostly used as glider-pads under pc-mouses, coffee-cups or toys.
You can cut them in form with a knife or scissors, press them on the place and haven't so much adding in heighth ...


Yeah, drawer slides are definitely a viable option. They do have some drawbacks as well though.

1. they arent standard (ie you're dealing with regional variances and such, so its hard to make a standard design based on them

2. they are sort of hard to align. if they are off, it will cause more friction. its possible, but with this system only one rail keeps it aligned. the second rail is for support.

3. they have practically 0 height, so you have to mount them on something. pretty minor concern.

i've acutally used them, and they do work quite well. if for some reason this doesnt work out, i'll definitely re-investigate that path.

thats a good idea. i'm not sure if i need the thickness yet or not. i'm not sure how fast they will wear down (it may take years, months, or days... i dunno yet!)

the 1/8" height is pretty small, but it does help out with spacing because its rather tight between stages as it is now. sort of an added benefit.
Hi Zach,

... the white stripes seems to be bulk-PTFE, same as my gliders milled from a massive block.

The yellow/brown coloured PTFE-coated glider-sheets are optimized for gliding without to much wearing off and have a much harder surface, then the white PTFE - i think, the thin pads glued on a surface could last a bit longer without wearing off, then the massive blocks ...

On the other side - with the thick stripe and my 5 mm-blocks there is much more material to wear ;)

Hi Zach,

concerning the "drawer slides aren't standard" point, I thought about
IKEA modding/hacking.


Could it be worth investigating into
IKEA parts for repstrapping?


an ikea based repstrap would be AMAZING. seriously cool. i dont think i have time for it (since i'm ready to start trying to print) but if you want to attempt it, we can hook you up with access to post here or something.
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