Anonymous User
cartisian robot frame smooth rods
December 25, 2008 11:26PM

I'm just starting to source parts for my first reprap. I was looking at the BOM for the cartisian robot frame which calls for McMaster #88625K67. This appear to simply be drill rod. Enco offers what looks like the same thing #505-0240. The question I have is whether the rods will need to be hardened? Anyone that has used drill rod before knows that the metal is annealed and bends easily until it is heated and then quenched in oil. I didn't see any mention of hardening these rods. I would think that you would have a problem with the metal warping unless it is hardened. Any thought on this?

Re: cartisian robot frame smooth rods
December 29, 2008 12:34PM
I didn't have to harden the rods from McMaster, although I guess it wouldn't
hurt to do it if you want to.

If you want to look at the specified rod from a bending point of view, the rod
is well over specified for what it is supporting. I think the decision for the rod diameter was based on availabilty and ease of use instead of load. If it was based on loading, then it would probably use a 4.5mm or 3/16" rod - which I would
hazard a guess to say that it would need to be hardened to prevent bending.
Re: cartisian robot frame smooth rods
December 29, 2008 08:59PM
Keeping them straight might be tricky. I used a box of surplus 8mm printer guide rods I found at a surplus shop. They're very smooth, which is good for the X and Y axis movement, and they're also very tough and stiff. I had a hell of a time cutting them though. I'm not sure what the exact alloy is, but it's some sort of steel, not stainless (it's magnetic) although it is very polished.

I did have problems with slight bending of my threaded 8mm rod which I used for the Z axis drive screws, even though I tried pretty hard to keep them straight. Just handling it seemed to introduce some bending that becomes apparent when the Z stage moves, but it's been working OK so far. I used stainless steel though, so it might be weaker than most other alloys.

The X axis guide rods do flex a bit under normal operation; I believe the new BfB design uses 4 instead of 2 8 mm rods. That's about the only place where it's noticeable, and it's not a problem unless you're trying to do some light milling or use a heavy extruder.

Those annealed rods should work fine, as long as you're careful.

Re: cartisian robot frame smooth rods
December 30, 2008 11:32AM
Those surplus printer rods were probably chromium steel - the stuff they make hand tools out of and chromium is tough to cut through by hand.

As for your threaded rods bending, if you get the stainless steel rods they
do not bend as easily. If you got your threaded rod from a hardware store it
is probably not stainless and it will bend pretty easy - of course you could anneal it...
Re: cartisian robot frame smooth rods
January 02, 2009 12:18PM
Wade: Interesting that you used surplus guide rods. From what I can tell, your printer is very reliable (noting that you've managed to print off a Darwin set with very little trouble). The fact that they're highly polished probably reduces friction quite a bit. I've had a lot of trouble with friction on the Y axis and was able to mitigate it a bit using some steel wool. I wonder if some additional polishing might help.
Re: cartisian robot frame smooth rods
January 02, 2009 12:49PM
Polishing will help, ideally you want a guide rod to be within +/- 0.0001 of an inch in diameter (can't remember how many microns that is). An appropiate sized bronze sleeve bearing will also eliminate alot of the friction. A chromium steel rod with a bronze sleeve bearing and a little lithium grease has very very very low friction - that's why they are used so much in printers.

If you are using a plain steel rod or stainless steel rod then you really need to polish it well and keep it greased with grease, not wd-40 or some other light oil.
Re: cartisian robot frame smooth rods
January 02, 2009 02:03PM
I looked up my order, and the threaded rod was 316 (A4) stainless steel metric DIN 975. I had to special order it, as metric hardware is hard to find in Canada. I was more concerned with corrosion than strength, so I opted for the stainless. 316 stainless seems to have a lower yield stress than most steel alloys in my book; it certainly seems softer than the smooth rods I was using.

Steve, I think I'd call it "relatively little trouble", as it took me almost 5 months to go from a completed Darwin assembly to actually printing useable parts. smiling smiley Most of my issues were software, extruder and pulley problems; I never did have any trouble with those smooth rods, other than trying to cut them. I also greased the rods well, as the Darwin design has the X rods riding directly on the Y rods.

So I suppose that's a vote in favor of stiffer, harder guide rods.
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