ok, first attempt using sketchup 8

[JAZZCNC]

Hi Deanos,

I've compared the following 1500mm long steel gantry shapes for tool deflection due to gantry twisting (when cutting wood in the X direction):
_____________________I beam__RHS____RHS
Width _______________100_____100_____100
Depth _______________180_____180_____180
Flange/wall thickness ___6________6_______3
Mass (kg) ____________26______38______19

Deflection (mm)______0.021___0.00037__0.00068

Clearly RHS is better, but I beam is also 'useable' for cutting wood. In the end it is about what is sufficient for your needs and what you can comfortably make.

Ok didn't see this or Jonathans last post so this apply's to both post's.

First I'm not knocking the Calculations or saying they are wrong but your both missing the POINT. . . .!! IT WILL DO THE JOB AND DO IT GOOD.!! . . . IT'S CHEAP AND EASY.!!

It's easy to build with absolutly minimal machining or drilling, far less Jonathan than my first design which yes is stronger but much more expensive.

Jonathan good design is about building the best machine to do the intended use accurately and effiecently. A good designer will always design a machine to do the required job with a certain safety percentage designed in and no more. Any more is just wasted expense and ineffiecent.

Working On your design criteria then you FAILED because you could have used far heaver gauge box section or steel plate rather than Aluminium or why not go the whole hog and Cast some Iron castings.!!. . You had the knowledge,equipment and abilty so why not.? . . . . . Because it didn't warrant the extra cost and effort to accumplish what you intended to do with the machine.

I've just checked price with my supplier and I can get 1x1800 & 2x1000mm 178x102 RSJ 8mm steel for £75 no vat cut to length delivered.
That will do both the gantry and the X axis. Or Just the gantry would be £37.!!

So Profile/plate way:
Ali plate 4mtr x 150 x 9.5mm = £86inc vat + £10 delivery = £96 (400mm left over)
Profile (as per my other design)1800 x 90x45H = £41.50 x 2= £83 + £10 delivery = £93 . . .or 1800x45x45 (As dean's design) =£43 +£10 del = £53 . . .OR. .
80x40x3 steel box section £39
Plus lots more Nuts and bolts so another £10

Total £199 OR 45x45 £159 OR Steelbox £145

OR

RSJ way:
RSJ 1800x178x102 £37
Nuts bolts £5 (far less required)

Total £42

Also a lot less drilling work which will also add to the cost because of worn/snapped drills.!


Now taking the RHS route and regards the Calc's and price stand point theres also the issue that like for like dimensions where used. Now 180x100x6mm RHS is not a common size and the nearist would be 160x80x6mm Which is £160 so no saving there and it won't be as easy to work with as RSJ.

Also the calculated strength of the I beam is going to go up because of 8mm thickness not 6mm and the RSH will drop slightly due to lesser dimensions, it will still be stronger but the difference is less but more the point the Ibeam which is already more than suitable will increase.!

This is my POINT.!!