Choosing linear motion system

I've got a good chunk of my Gantry style router made up in CAD, but I'm worrying a bit about the linear motion. As it is in my machine, I'm using unsupported rails, which are always said to be less than satisfactory. I'm trying to build this machine for as little money as possible, but I understand spending more initially is cheaper than doing things twice; but still, price is a consideration to make.

The lengths are not very long, so this method may work, but I just want some more opinions.

The X-axis will have about 18" of the rod supported, with X" supported at the ends (didn't bother figuring out how to support them yet, as I haven't decided I'm using them). The Y-axis will have about 12" unsupported, same deal as the X-axis for the supported length. Z-axis will have about 6" unsupported, but I don't think that will be a huge issue.

So with those lengths, I'm wondering if I could get away with unsupported rails, or if I should go with an alternative method. I did a quick search but couldn't find anything really definitive for calculating deflection. I will mostly be machining various plastics, but I would like to be able to machine aluminum; I understand I'd have to use a very low DOC, but that's fine. The rails I'm using in my design are 5/8", and I've also been looking at 20mm, but that can be changed easily enough.

Thanks for the help, lemme know if I left any info out.


- Jesse

Defiantly go at least 20mm, with proper end supports those lengths should be ok.

I am running 30mm on my ~60" X and 20mm on my ~25" Y, the X really is too long to run unsupported and bounces around alot. but the Y is nice and stiff. I have 1.75" of support on both ends.. Use the rod supports that are made for your rods, they will work the best. take a look at my machine In this forum.

There are many deflection calculators available online, mostly from the manufacturers of the linear rods website. I even found a deflection calculator program online somewhere.

Thank you for your input. I think that 20mm should be adequate for my machine, but I'll just go do a bit of math to ensure that it is. Also, your machine looks amazing.

I also noticed you live in BC, so do I! :p

I use 18" long 3/4" shafts for my Z axis, so the 20mm should be OK.

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Originally Posted by ger21

I use 18" long 3/4" shafts for my Z axis, so the 20mm should be OK.

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Awesome, thanks! :)

Here's a quick rendering of what I've got so far. I'm by no means great at CAD, but I'm good enough for how much I use it. I don't have a lot of the smaller parts made, just as I'm too lazy to make them. Just looking for some input/suggestions.

http://i36.photobucket.com/albums/e1...am/CNCMill.png

Quote:

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Originally Posted by Jesse B

I did a quick search but couldn't find anything really definitive for calculating deflection. I will mostly be machining various plastics, but I would like to be able to machine aluminum; I understand I'd have to use a very low DOC, but that's fine.
- Jesse

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For calculating the deflection you can use this:
http://www.aps.anl.gov/APS_Engineeri...sticBeam2.html

The moment of inertia used for this calculation can be found here:
http://en.wikipedia.org/wiki/Second_moment_of_area

You can find the relevant material properties (young's modulus) on wikipedia as well.

The cutting forces will depend on many factors but if you want to cut aluminum I'd say 10um deflection under 250N force should be in the right ballpark (I'll take a correctionion on that if someone has one). Your worst case scenario is in the middle of travel.