Alu vs CRS tube for rails?

[MasterMaker]

I am planing to build a moving gantry cnc using a beefed up version of the "Little Brute" slider (basicaly twice the number of bearings and a longer distance between the points where the bearings contact the rails), 32 bearings per axis...I love overkill.

Anyway, I see that a lot of people here have used alu as a rail material and I am wondering how it is holding up when it comes to wear.

I realize that since the bearing are rolling on the alu that the only force(idealy) is compressive but still.......

I am also wondering about the rigidity of the rails when it comes to repeatable precision but I could always epoxy a round tube inside the square one to increase rigidity.

Alu is easier to work with but does anyone here know how precise the extrusions are and how does this precision compare to thick walled square CRS tubing.

And also I am currious about which is more dimentionaly consisten/precise, thickwalled or thinwalled tubing(square).

The size of the machine I am going to build will be approx 40cm(16")X - 100cm(40")Y - 15cm(6")Z, and I will use an electric die-grinder for a spindle and I will be using it on everything except steel( I have a chester champion that I will make new Z axis for using linear slides that I will use for steel).

[ahren]

Cold rolled steel flat bar is extremely accurate dimensionally. While it varies based on the size you get, it is usually +0.000 / -0.005" or better. Aluminum is typically much worse than this, and is on the order of +/- 0.020". That's why I recommend CRS for the rails used with my linear motion system.

As for tubes, I'm not sure, but if these are made by bending and welding, you will lose the advantage of the dimensional accuracy from the rolling process.

Ahren
www.cncrouterparts.com

[FandZ]

I'm useing aluminum tubing on my X axis and so far the wear isn't bad but I only have about 10 hours of actual run time on the machine. I guess I'm useing one of the cheaper aluminum grades of tubes bought from lowes. You can tell the track the steel bearings leave behind because it's almost a mirror Finnish down that stretch. Also when I made my rollers and mounted them on the tube I used my finger on top of the bearing to try and find any low spots as it rolled and the bearings resistance was even through out. It's not the same on hot rolled steel though. I could feel spots where the bearing could almost spin freely due to dips in the metal.

Cold rolled steel would be ideal though. I haven't found any cold rolled steel tubes. Only hot rolled. I didn't think they existed. Do you have a link?

I'm working towards a build down the line where I'm going to use 1/4inch walled aluminum tubing with cold rolled steel flats mounted on all four sides to get those excellent tolerances Ahren is pointing out. But if cold rolled steel tubing with tight tolerances can be had I'm going to have to rethink it.

[FandZ]

Well looky here tolerances for CRS tubing. If I'm reading it right. .020 +/- for 2-1/2 or smaller tubing. I wonder how bad that is.

http://www.speedymetals.com/information/material59.html

[MasterMaker]

Fanz, thanks for the link .

From what I can gather the tolerance are way to sloppy for my use but what is listed is total tolerance not relative tolerance(how much a dimention is allowed to deviate over a given lenght) but the twist tolerance is waaay more than I can live with....bummer

This leaves me with more questions than answers since I now have to figure out how much sag there is in solid square crs when suspended by the ends and spanning 40-60".


For the life of me I can not find the info on speedymetals...

[ahren]

You don't have to suspend it from the ends if your CRS overhangs a support bar such as a piece of aluminum extrusion. I happen to know a very clever (and handsome) guy who designed a linear motion system around this idea .

You're right about the "within" vs. "between" tolerances -- what is listed on Speedy doesn't tell you how much a piece of steel varies within itself. However, from experience, CRS rarely varies more than 0.001" within a single piece. I have had great success with it as a bearing rail.

Here's a piece of it in action here:

http://www.cnczone.com/forums/showth...t=77372&page=6

Ahren
www.cncrouterparts.com

[MasterMaker]

From what I see in your shameless plug there is no way of adjusting the botom bearing to aleviate play.

This is the very reason I was leaning towards the "littlebrute" desingn of slide and the rail being suported at the ends only makes the straightness of the mounting surface a non issue.

If money was a non issue I would just use 8020 or similar and linear components from INA to make the machine....

I want the machine to be to built along the ideal that overkill is barely enough and I have an inherent disstrust in the whole concept of "just about right" so I could never quite get to grips with mdf and such materials.

Having discarded the crs tubing what I am leaning towards now is solid rectangular crs 30x40 mm with the shorter side being where I support it on each end.

Anyone know how long a span I can use with this before sag becomes an issue....

[ahren]

Continuing the shameless plug, I'll say that there is an adjustment to the bottom bearing -- the bolt it rides on is in a slot, and is positioned by a cross-drilled set screw. This system is running well on dozens of machines.

That being said, if you want to calculate deflection in a beam, it's fairly straightforward. You just need the section moment of inertia, length and load. Here's the calculator you need for moment of inertia:

http://www.engineersedge.com/calcula...are_case_6.htm

And here's the calculator for beam deflection:

http://www.engineersedge.com/beam_be...flection_2.htm

FYI, the default value in the second calculator of E is correct for steel (in psi), so just plug in your dimensions in inches and you can play with load, span, and deflection.

Ahren
www.cncrouterparts.com

[amishx64]

Originally Posted by FandZ Cold rolled steel would be ideal though. I haven't found any cold rolled steel tubes. Only hot rolled. I didn't think they existed. Do you have a link?

Home Depot. I was talking about cold rolled steel square tube in my build thread. Nice, high quality, expensive stuff. They also have the flat bar type

[FandZ]

Originally Posted by amishx64 Home Depot. I was talking about cold rolled steel square tube in my build thread. Nice, high quality, expensive stuff. They also have the flat bar type

I'll have to go double check at home depot, it could be just my location, but as I remember the only tubes I saw were the ones with holes in it that had a great finish. The regular tubes were the same stuff as Lowes and has a rougher finish. The flats all seemed to be hot rolled to me because they had the ends done in red paint, cold rolled is blue I think. I know the aluminum tubes and angles at home depot were much thinner than at lowes. But stock materials could very vastly from one store to the next. I'd imagine it isn't like screwdrivers where they buy them in bulk from one place. They probably buy the metal from a few manufactures around the nation to save on shipping.

[MasterMaker]

Ahren, thanks for the calculators it would apear that the deflection of the center point of a 1500mm long bar of 30x40mm crs is so small as to not be an issue with any reasonable load

Looks like I will have to start a build tread...

[FandZ]

Originally Posted by amishx64 Home Depot. I was talking about cold rolled steel square tube in my build thread. Nice, high quality, expensive stuff. They also have the flat bar type

I just got back from Homedepot, I had to fo see...lol You are right, it is cold rolled sq tubes. Nice stuff. They also have some really smooth 1/2inch round bars. Makes me want to sell out and buy a pair of these to use on a z axis http://cgi.ebay.com/Japan-Linear-CNC...d=p3286.c0.m14

I really liked the 5/8 steel tubes... With those I could drill my bearing holes straight down the middle for the roller asemblies. This will simplify some modifications I want to make to my machine. Thanks for the heads up about homedepot.

MasterMaker, Sorry for jacking your thread

I'm looking forward to your build thread. Do you have any drawings of how you plan to go about it?