Y-axis design question.

[Drools]

I'm trying to figure out the optimal distance between the bottom of my Y- axis cross member and the top of the table. Of cource the design of the Z-axis will play some part in the design. Is there a rule of thumb to follow or similar?
thanks

[ger21]

I make mine high enough to clear the thickest part you'll have mounted. Imo, for a router cutting wood, that's probably about 3". But I'm making it ~5" to allow for jigs or fixtures. The lower it is, the more rigid the machine will be.

Once I pick the gantry clearance, I then determine Z axis travel. I want to be able to reach the table with a bit sticking 1" out of the collet, and clear the gantry with a bit sticking out about 3". For 5" clearance, that's 4" down, and 3" up, for a Z travel of about 7". Then add 1 more inch or so for rapid clearance.

[Drools]

Thanks Gerry, I was looking at 4" or 5.5". I didn't figure in the height of the benchdogs.

[johnmac]

For the most part, the greater the distance between the table and gantry, the greater the chance of frame flexing.

John

[Drools]

I'm about to take my Aluminum to the machine shop to have the gantry sides made (holes drilled). I was hoping the elite here would give me some comments.
I will be using 2 x THK25A bearing blocks per side. The HSR25 rail will be mounted to the lower slot of a piece of 80/20-3030. I have made sure all measurements comply with the measurements on the FLA machines as I want to use their bearing blocks with my design. The Y-Axis will be a piece of 80/20-3060.
The distance from the top of the 3030 to the botom of the 3060 will be 6.75" and with a 1.5" spoil board on the 3030 the clearance will be 5.25". The Aluminum is .75" thick by 8" wide and 18.5" long. I'm drilling .3281dia clearance holes for the 5/16 bolts, is this the correct diameter?
The THK25A blocks call for an 8mm bolt so I made the clearance holes .3125dia.
Thanks everyone