some opinions about 20mm linear rod

[blau_schuh]

What would be the longest length possible with a 20mm diameter rod before I'd need to worry about the rod deflecting?

I'm thinking of using some short (< 15") ones for my Z axis, but im not sure the 40+" for the y axis is going to be a wise decision, or even the 30" for the x for that matter.

any suggestions would be appreciated.

[Madclicker]

It really all depends on what you need the machine to do. When I started I thought I could deal with .050" accuracy......soon found out even .010" looked large when I tried to fit parts together.

For the accuracy I need I wouldn't use unsupported rod for anything but the Z....as long as it's short like yours.

[blau_schuh]

Originally Posted by Madclicker It really all depends on what you need the machine to do. When I started I thought I could deal with .050" accuracy......soon found out even .010" looked large when I tried to fit parts together. For the accuracy I need I wouldn't use unsupported rod for anything but the Z....as long as it's short like yours.

mind telling me what you meant by your accuracy comment? do you mean that the accuracy of the things you were making was better than you expected... or that things just werent fitting together when you started building the machine?

[Madclicker]

Before I finished my machine I thought I could get away with .050" or so accuracy. My brother and I both thought this. We were wrong. For one thing the dados in the cabinets I make need much closer tolerances than .050". .005" is closer to what is needed for a good ply to dado fit.

I didn't get the accuracy I needed until I tweeked and shimmed my machice. I also slowed down the cuts for accuracy. I've learned much from my first machine. I have been cutting very accurate stuff lately.

[monte55]

My machine is 60"x60". My rails for X is 1" round cold roll steel. I could deflect them down enough to see the difference without that much pressure. I made support blocks to fit under the rails mid way..........I'm sure glad the bearings were the open type. It made all the difference in the world.

[Reef Guy]

I tried using 20mm rod on a longer 30"-40" run and it's more trouble than it's worth. I think you would have to support it and have open bearings as in Monte's case. I tried using 2 double bearing blocks per side (not open type) and it still pretty loose. I ended up getting Hiwin linear rails and blocks and it's wayyyyyyy better. I am going to attempt using the 20 mm for my y axis since I have everything already so I'll see how that goes. It's not as long of a run though.

One thing is the way you attach the rod has a big affect on how it flexes and it you have 2 mounts on each end of the rod it improves it quite a bit. There's a post about that here that gives a lot of specifics.

There was another post I remember reading where a guy ground off spots on the rail and then made them supported by attaching them.