[bensaad]

hi all
iam about fo finish my cnc router frame build,but now iam stucked in the gantry
I am now faced with two options
usign :
1- one shs square tube 120mm
or
2- two shs squaretube 90mm brazed together

the max shs dimention i found in the market is 120mm square.

my mind goes for the second choice beacause i have alreday the shs 90mm ,plus the advantage the space between the linear rail would be large enough

any suggestion is welcom and thenks in advance

[jaguar36]

What is the wall thickness on the tubes? I'd say you're better off with 1x120mm as it'll be stiffer horizontally which is the direction you'll most frequently be loading. Plus it'd be alot less work. How are you going to keep the tubes straight? Steel tube is not particularly straight to begin with, you'll need to have some way of dealing with it in either case. Brazing two tubes together will make it worse though.

[bensaad]

hello jaquare and thanks for your reply
the 120mm shs thickness is 5mm and the shs 90 i own already is 3mm

[peteeng]

Hi Ben - If this is a 8x4 machine the 90x3 will be just stiff enough for timber and plastic. It would deal with aluminium but maybe a bit springy on aluminium with a heavy cut. Are you going to machine the rail lands? If so how are you going to align the two cuts as they are on different surfaces so will be different set ups? I'd go bigger then 90x3 for the gantry... 120x5 SHS is a good size. Peter

[bensaad]

hi peter and thanks
Now that I am convinced to use 120x5mm shs as a gantry ,
Which confuses me now is how to install the linear rail ,top bottom,in the same face or as 90 degree as shown in the previous posted photo
and thanks in advance

[peteeng]

Hi Ben - I have done saddles and gantries a couple of ways. I now place both rails on top and make a saddle that's has the Z bearings and gantry bearings on two planes, this means I can get to all the bolts easily. This separates the bearings, allows the z axis to be mounted very close to the gantry face and if machined the gantry rail foundations can be done in one setup. The saddle is bolted aluminium and these are the stiffest best saddles I have ever made... Have a think - Peter

Re: round rails - I hadn't built with round rails before so this machine I did to see how they went. All OK but the price difference wasn't much compared to square and I get square in medium preload which is much better then the rounds. So I don't think I'd use round again.

[bensaad]

hi peter looking great and well done
but in my case i dont think i could install the rails in the manner like you did
the space between the rails will be narrow for 120mm shs,i am using the square sliding rail block HGW20CC wich is 63mm width

[peteeng]

Hi Mohamed - The drive does not have to go between the rails. It can go nearly anywhere and it will work. Could go above the saddle, behind it etc etc.. Or use a 150x120 section, don't let the size of a section drive the design, make the best design and then pick what's needed to achieve it. Other wise your compromising, compromising compromising..... I personally don't like SHS or RHS as it drives the design vs the designer designing the part... Peter

[bensaad]

hello peter
based on your idea i come up with this gantry design showen in the pictures attacehed


what you think

[bensaad]

The drive does not have to go between the rails. It can go nearly anywhere and it will work.

hi peter i dont mean i need space to put the drive between the rail
i mean a 120mm shs is narrow to put two linear rail in the same face.
Regarding 150mm x120mm shs section, I searched for it before and did not find it. The maximum size I found available is 120mm square 5mm thick
thanks peter

[peteeng]

Hi Mohamed - Do you intend to machine the rail foundations? SHS and RHS surfaces are not flat. They are cupped or crowned as they are cold rolled. This means the cars will jam as the rails are not true to each other. If you intend machining the foundations then that's OK but if you use opposing sides this means its two setups and the second set up is tricky. Its easier and more accurate to machine the foundations as one set up ie have the rails in the same orientation.... What is your philosophy behind this design? There are a lot of things to get aligned and its overly complex. Every machine component has to have a function plus it has to be able to be made to the required tolerance. In CAD space this is very easy to do, in real world there are several hurdles to jump to get things right. So even if your rail foundations are perfect the U has to be perfect (& match the foundation spacing which is now a variable) to not compress or stretch (jam) the cars....or you have to shim or relieve the U saddle somehow. Making things fit properly between things or across multiple surfaces or multiple machining operations is a tough gig... Peter.

120mm spacing for 20mm cars is OK by the way.

[bensaad]

Hello Peter
Would you please advise me regarding the foots count of the machine? I have some piecs of shs 90mm left and I want to add them as an extra foots to make the machine eight foots instead of six, the frame size is 3x1.5 meter, is that going to add some rigidity or stay with six would be better
thanks in advance

[peteeng]

Hi Mohamed - 6 is fine for this size machine. The issue will be that the frame will not be straight after welding so you will have to correct it somehow. So are you going to machine the lands post welding? To improve the frame rigidity you are better to add diagonals to the frame. But then its harder to use the base for storage....Peter

[bensaad]

Hi Peter
Here is a photo of the welded frame
I added 8 cross member to it
And yes for sure i will machined the land after finish welding
Take a look

[peteeng]

Excellent - Its real. Peter

[bensaad]

Excellent - Its real. Peter

All this thanks to your valuable guidance and advice, Peter. Thank you very much