# Thread: MASSIVE 2.5M x 4M BED - GANTRY DESIGN HELP NEEDED!!

1. But say 600mm to be on the safe side

2. Originally Posted by woodhouse
We could probably get this down to 500mm
Now next is my common sense, later you could prove it in the calculator.

-Then you will need a gantry vertically wide so that 500mm spacing between bearings is achieved.

-2x 150x150x4mm /18kg/m/ separated 200mm and connected every 500mm with braces from 100x100x4mm that all form a leather like structure. Similar to my gantry. Could be reinforced with bolted later steel plate of needed in some direction, though i doubt that you will need for wood.

or simply weld together 2x 200x200x4 or 5mm, though careful engineering here is very important

- You must decide at this point how you will move Z left right, RP or rotating ball nut, how you will move the gantry on long axis, all that before continuing with the gantry design, cause you will need space in between for the rotating nut to pass and so on.

3. Well, I can get my hands on a 50mm ball screw and nut from the same machine the rails came off but the components I will need For it I feel will probably cost more in the long run than RP. So I am tending towards RP at the moment.

4. Originally Posted by woodhouse
I know this is a very ambiguous question, but does anyone know the maximum horizontal force that can be expected from a spindle?
From this I can obviously work out the resulting force on the gantry rails for different designs and keep the gantries deflection below spec

There are three maxima for the largest forces on tool tip :

(1) The thrust load provided by the ball screw et al . Stall torque of axis motor referred through drive mechanism and ball screw and corrected for any leverage effects .

May have components in more than one direction .

(2) The worst case cutting load . This is usually a max depth cut on one side of cutter only . Stall torque of drive motor referred through drive mechanism and any leverage effects .

May have components in more than one direction .

(3) Crash load . Can be estimated but if you design on (2) that is probably ok .

Simple quasi static loads are not the complete answer . Any cutting is unstable and vibration problems can be more troublesome than the simple loads .

Ned to consider masses , geometry and elastic stiffness of all components to make sense of this .

Generally apart from being just stiff a simple support structure needs to be relatively heavy to bring natural frequency of structure down into a safe zone .

Other methods are available to make lighter structures vibration insensitive .

Beam needs in any case to be stiff in three senses - vertical , horizontal and in twist .

Generally a beam structure which is of large cross section dimensions is desireable . A fabrication of relatively light sections at big spacings is a way to go .

Hollow sections may be filled with structural foam to aid damping and add a little stiffness .

An alternative way to go is to use space frames made of relatively light components arranged very much like a roof truss or lattice bridge .

An unorthodox alternative - which is already being used in some real machines - is to cast a hollow lintel in either concrete or epoxy concrete .

The above all assumes relatively slow movements of tool/table . If rapid moves are likely then there are further design considerations .

michaelw

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