ok, first attempt using sketchup 8

[deannos]

I would of also thought with the end plates it would be more than up to do the job i require. But i have to bow to knowledge of you guys who know more than i ever will. Was poking around on another forum and found this, but unfortunately i don't think there is a build log.

[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.!

[Swarfing]

The torsional stiffness of I section is very poor compared to RHS or box section and clearly the ganty twisting directly leads to tool deflection (proportional to the extension of the Z-axis) parallel to the Y-axis.

Jazz i'm with you on this one, i would love to see Jonathan try and twist an RSJ mounted on his machine? you are still talking 5/6mm minimum steel plate.

[JAZZCNC]

I would of also thought with the end plates it would be more than up to do the job i require.

Yes Dean it will be more than up to cutting wood.! . . . . There are issue's which arn't Ideal like easy setup of parallel rails like Jonathan touched upon. . . But if like I said with carefull selection of material they will be very close and then It's not difficult to manipulate them parallel with filling/grinding and spacers/shims.
My experinece with Cheap chinese round rail is that it's not straight or consistant in dimesions along it's length, just as the bearings mounting surfaces are rarely level, flat or true so shimming is nearly always required to fine tune. . . BUT AGAIN . . . It's down to intended use and for machineing wood then the tolerences are much lower and forgiving so can be tolerated.
If you want to wear belt & braces (excuse the PUN!!) and just add some bracing along it's length.!!


Regards that Pic you posted I'm pritty I've seen that before and it was actually built to cut stone.?? . . . . But don't quote on that!!. . . . But I've certainly seen one built the same that was cutting stone and it actually came up for sale on Ebay.

[deannos]

Regards that Pic you posted I'm pritty I've seen that before and it was actually built to cut stone.?? . . . . But don't quote on that!!. . . . But I've certainly seen one built the same that was cutting stone and it actually came up for sale on Ebay.

Jazz,
I found this on cnczone and reading his posts i think you are correct and it was used for cutting stone, he was in Indonesia. If i chose the earlier option of 2 box sections sandwiched between plate, would it be ok to use 10mm plate and use 20mm on the Z axis only. Just want to weight up all the options at the moment.

Just out of interest, i'm looking at using 1605 and 2005 ball screws, belts and pulleys if possible. I had a go at the weekend cutting the infamous Aztec/Mayan calendar, approx 12" size, after 3 hours i had cut less than a 1/5th. How much faster roughly would using ball screws make a difference.

[Jonathan]

20mm is a good size plate for the Z-axis.

The efficiency of ballscrews is roughly twice that of ACME screws, so it will be significantly faster. For the Aztec calender acceleration is much more important as it's lots of tiny moves. The bearing nut thingy you're using will make the difference slightly less. The main advantage is 5mm pitch versus 2mm pitch - the kinetic energy in the screw is significantly less if it's rotating 2.5 times slower.
The most accurate comparison you're going to get is comparing with my machine, as I sold you the screw. I got 6m/min with the screw your using and 42:13 pulleys on Y. I ran it at less than that with smaller pulleys as that was too close to the critical speed of the screw (see my build log for actual values). With 42:14 (I think) and a 10mm pitch ballscrew I got 60m/min. That's much more than you'll ever need so I've left it on close to 1:1 to get better resolution and acceleration.