Z Axis OVAL

I am needing to order the final parts for my build and have be concerned about the length of the ball screws and linear rails I need to get...
So, I have wondered if there is a general rule of thumb in what should work with specific dimensions...
let me say here I have no CAD experience to draw this up ... I'm in school learning know...so be kind...
I know my distance from the sacrificial top surface to the bottom of the Gantry will be 6"
The Gantry thickness will be 4" high...
I failing to see the total travel I would need to get me in the ideal realm of what will be needed to order to ensure the most rigidity out of the "Z"
So, with the motor in line with the ball screw...
with a 6" + 4" = 10" 10" is 254mm (actual Ball Screw thread) + 51mm for the lock nut end + 10mm clip end = 315 total
So, a 350mm ball screw work?
250mm linear rails work?
or should I add more to both or one of them? and if so which one?
Should I figure that the (A) bottom of the "Z" (portion the spindle mounts to) come into contact with the spoil board or (B) allow for the bit, spindle nut, and portion of the motor be figured into this equation and subtracted from the OVAL length? with the (B) visually it would mean that less material below the Carriages Gantry would be needed...
All conversation on this would be appreciated ...
Paul

Hi,
the maximum travel you can get from a linear rail is (with two cars separated by S mm, S being measured between centres of the cars):
Max Travel = total full length of rail - the length of one car - S (the distance between cars measured on centres)- a little clearance at each end, say 5mm each end for 10mm overall.

If you have a 400mm rail with a 70mm long cars and the cars separated by 200mm (centres) and 5mm clearance at each end:
Max Travel= 400 -70-200-10
=120mm

So you can see that the separation between cars is a very significant contributor to the calculation. To have the most stable axis you want the cars to have wide separation, but to get the most travel
for given length of linear rail you want the cars close together. You have to balance the two requirements.

Strictly speaking the length of the threaded portion of the ballscrew less the length of the ballnut is the max travel of the ballnut. Naturally you have that a little more than the max travel of the linear axis.
It would be wise to allow a few mm either end so that te ballnut does not EVER wind off the end of the thread, so the max travel of a ballscrew is:
Max Travel = length of threaded portion of screw - the length of the ballnut - a little safety margin say 3mm at each end

Double ballnuts are desireible because they give the best control of preload and are the stiffest arrangement but are longer, and therefore require a longer (threaded portion) ballscrew for a given travel.
Short ball nuts give best travel but reduced stiffness, another balancing consideration.

From a design perspective I suggest that you decide what the overall stiffness of the machine is to be, and then work backwards from there. For instance if you are planning to have a machine to mill steel then
you will want to space the cars widely for best stability and probably want a doublenut ballscrew for stiffness. It means that you'l need long rails and ballscrews but that is what the machine requires.
If the machine is for wood and/or plastics then you can relax that a bit and save some money. What you do not want to do is try to save money but end up with a substandard machine, so decide early
want sort of stiffness you need and don't compromise on it. If you do you'll spend a swag of money and be disappointed, when for an extra 25% you could get the result you hoped for.

Craig