ok, first attempt using sketchup 8

[routercnc]

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.

[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.!!

[JAZZCNC]

Dean yes 6" x 3/8" (150x9.5mm) plate would do fine and the Cad models I drew used them dimensions except for gantry sides and bearing plates which where 19mm plate and I'd recommend you use.

[Jonathan]

It's only a suggestion, take it or leave it. I post these things to benefit other people who may want more from the machine than Dean.
Adding the plate to make a box is very simple and should make a big difference but if Dean never wants to cut aluminium regularly there's no point. 0.01mm tool deflection might not sound like much, but when you start cutting metals it is - surface finish, material removal rate and tool life are affected. Running the calculation with cutting force for aluminium gives 0.058mm.

I used 8mm web thickness for the RSJ, not 6mm, in all the calculations as that's a standard size - with 6mm tool deflection is about twice as much.

[JAZZCNC]

It's only a suggestion, take it or leave it. I post these things to benefit other people who may want more from the machine than Dean.

Great Jonathan but in a thread like this surely the target is to help the person who's building come to a design that suits his needs not others.? Thats how approach every thread.
First and foremost I try to advise with clear uncluttered information and suggestions to help them arrive at a design that will suit the intended purpose and do it the best it can. . . No more no less.!! . . . . If I can help them achieve this in a cheap and easy way then I most certainly do and get a warm fuzzy felling doing so:naughty:

Over complicating and over building past the usuall built in safety margin is just wasteful in both time and money.!

The simple FACT to DIYCNC is that it's NOT POSSIBLE to easily and cheaply build a machine that can cut every material like a purpose built machine optimised to cut the same material . . . . Belive me because I've tried very hard.!! You either build the machine to do the very best it can at that intended job at a sensible price and accept it's short falls in other area's. . . . OR . . . . Throw lots of serious money and time massively over build and have it perform in all areas average.!

Now back to the job in hand.

Dean for your MAIN need of cutting wood the RSJ will perform perfectly well without any extra bracing, welding or what ever. It will cut Ali but obviosly with restriction and greatly reduced performance compared to if it was built for that use.
Accept it's limitations and you'll love the machine which will become your faithfull servant but push it past them and you'll hate the bloody thing because it will try to kick you in the bollocks at every inopertune chance it gets.!!

Personally I'd use the RSJ but want to fill in the front and rear with thin covers, these would be serving 2 purposes with a frindge bennifit of adding strength.
Firstly functional, Secondly cosmetic, on the front they would be protecting the screw from debris with the slot idea on the previous design and on the rear it would be mainly cosmetic possibly houseing wires etc.!!. . . . but heres the fringe bennifit.?
To do this I would cut plywood braces that get bolted between the flanges and thru the web of the RSJ the covers would be just thin plywood screwed to the blocks. The bolted blocks would increase rigidity and combined with the plywood increase strength.

I've attached another pic to help explain.

Edit: Added coverd in pics for completeness.!