Newbie- Determining Screw Length?

[georgeburns]

I'm making a "plan-less" design for my wood router.

I have the following rail lengths from components I have bought off of ebay.

55" 20mm fully supported rails. Y
34" supports take up a half inch on each side. X
15" THK fully supported rails. Z

Of course that is the max length of the rails.

Should screw lengths be the same as rails minus support.
Should screw lengths match the rails, minus support and bearing blocks...

or is there some way ya'll determine the length of screws.

i.e. pay for what is standard and make fixed material fit what you get? or is there a nice way to figure out the screw lengths?

I'm a skilled woodworker and can make some nice stuff... is there a certain calculation here that just makes life easier?

-gb

[steven newcombe]

Being a lurker who has not actually built a machine as yet (my 305oz steppers are on the way ) I am definitely no expert. However, regarding acme thread, I found this page that has a calculator (critical speed calculator link at the bottom of the page) that calculates the screw length once you decide on the max rpm, method of support and the root diameter of acme screw.

[ger21]

You're design is the only thing that will dictate screw length. Just figure out where you'll mount it, and how you'll mount it, and you'll know how long it needs to be. Just make sure that what you do will give you the travel you're looking for.