RE: [CAD_CAM_EDM_DRO] Re: Linear motion design Q

>-----Original Message-----
>From: stratton@... [mailto:stratton@...]On Behalf Of ccs@...
>> What I have been working on is a way to use roller bearing (from
>> Roller blades), they cost $15 for 16 bearing. I am thinking of running
>> them on a piece of colled rolled steel. It's cheap and will not
>> deflect as a round shaft would. I priced a piece at onlinemetals.com
>> and it runs about $10 for a 4 ft section. I will post a picture of my
>> idea later. Mind you, it's only a ruff draft.
>What makes you think CRS will not deflect? All steel has nearly the
>same modulus of elasticity, and simply being square in cross section
>will actually hurt compared to being round, for a given mass of metal.

Actually it can work either way "for a given mass of metal". You can have two
tubes with the same cross-sectional area, one twice the diameter (and half the
wall thickness) of the other. The larger thinner-wall tube will be stiffer.
Even for solid materials, solid square stock on the flat (or, strangely enough,
also on the diagonal) is pi/3 times stiffer than solid round stock of the same
cross section (about 5% stiffer).

The advantages of square or rectangular CRS are that it is cheap, easy to
replace, easy to mount, easy to hold for machining during construction, ...

I have built the gantry beam for a CNC router setup using CRS rails. The main
body of the gantry is an aluminum hollow box section (from a scrapyard, I think
it is 5" square with 1/16" walls). On each of three faces, I have mounted
2.5x0.25" CRS rails. They bolt to the box section with numerous bolts. Each of
these rails has three roller bearings running on it (two on one side, one on the
other), and each only provides positioning in one plane. So if the CRS flexes
across its thin dimension it doesn't matter, and the thick dimension is both
intrinsically stiff, and also further stiffened by being bolted to the box
section.

Unfortunately time constraints mean the thing has stopped at that stage and has
been gathering dust for over a year.

The CRS may not be straight (in fact, it definitely isn't--the 30" rails have
.010" bow in them!), but they will be stiff, and thus the carriage will have a
*consistent* crooked motion, which can be cured by compensation in the CNC
software, or someday I can machine the rails straighter. I don't think taking
.010 off a rail 2.5" wide will cause much warpage; if it does I can just keep
straighening it, and it should eventually settle down.
-Kevin Martin