Drilling my CRS rails for mounting - what size hole?

[Enk1du]

It seems like such a simple question, but it really isn't.

I picked up the cold-rolled steel rails for my machine build this week, and this weekend I'm planning on getting the holes drilled for mounting to my 8020 table. I'm using 4" wide CRS mounted to 3030 struts for the X-axis, so I'm thinking two bolts (one in each T-slot) every 12". The screws are 5/16 - 18, and I'm just wondering how much clearance I should give the mounting holes to account for both the tolerance of getting all 12 holes lined up perfectly and also allowing for a little extra play to have when aligning the two rails once I get the gantry installed.

According the the tap chart I have, a "free fit" for that size bolt is to use a Q drill, which is only about 20 thousandths over the bolt diameter. I'm thinking that I should drill them out a little more - like maybe going a 11/32" drill.

What have other people done on their rails?

[derek]

I used 3/8" for two reasons. One, I have a 3/8" indexable spade bit and two I felt that having a little room on the bolts would allow me to align the CRS to the 8020. I drilled the holes with a CNC mill so mostly it was reason #1

Derek

[brucer]

Using a bridgeport mill I drilled my mounting holes 11/32.. I drilled holes like every 3 inches..

Also in a bridgeport I drilled and reamed 8mm holes in 4 locations and used roll pins to positive locate the rail to the extrusion frame...

Heres a picture of my rails

[RossMosh]

CNCRouterParts

3/8" is what CRP suggests.

[Enk1du]

CNCRouterParts

3/8" is what CRP suggests.

Oh, I hadn't noticed that one. Thanks!