[prestondocks]

Hi All,

I have been running a CNC machine that I built from MDF about 18 months ago. I would now like to upgrade the machine by replacing the MDF with an Aluminium frame.

The total span of my X Axis is about 1100mm. Cost is the biggest issue for me. I have 10, 16 and 20mm thickness as options from my supplier.

The dimensions of the gantry will be 1100mmx250mm.

Could I get away with 10mm over that span?

Thanks
Simon

[prestondocks]

Well since I did not get any replies to my post I decided to give the Stress Engineers where I work a try. I work at BAE systems and I got all the info that I needed to help me as well as a great calculation tool in the form of an excel spreadsheet. See attachment.

When you open it you may feel a bit overwhelmed but if you follow these simple instructions it wont be so bad.

The only cells where you need to put in any information are the yellow cells and even then you don’t have to touch all of them.

rows 32, 33, and 34 are where you specific y the size of your cross section.
If you are only using a vertical gantry with no back supports you don’t have to fill in rows 33 or 34. If you are going to be using a box section for your Gantry you could also fill in row 35 to provide measurements for the back wall of the gantry.

I will use an example to try and demonstrate better how to use the tool.

I decided I want a gantry that is 1000mm in legnth. I am going to use 20mm Aluminium and it will be 300mm wide.

There will be two supporting members on the back of the gantry. They will also be 20mm in thickness, 1000mm long but only 100mm wide. These form the U shape of the gantry.

In Row 32 I put the data for my main gantry wall.
In Cell D32 I put 300mm, if you look on the diagram you will see this relates to span B1
In Cell E32 I put 20mm, again if you look on the diagram this relates to span D1.

In row 33 I put the measurements for the first of my gantry back supports.
In Cell D33 I enter 20mm, this is the thickness and relates to span B2 in the diagram.
In Cell E33 I enter 100MM, this relates to span D2 on the diagram

In Row 34 I enter the measurements for my last second gantry back support.
In Cell D34 I enter 20mm for the thickness - Span B3 on the diagram
In Cell E34 I I enter 100mm - Span D3 on the diagram.

Now we need to specify the the width of our gantry. put this value in Cell G74

and last we specify what the maximum force in KG that will be transferred from our cutter when it is in the material to the gantry. We do this in Cell G65. this is the only bit where I am guessing here, because even for my own machine I am not sure what force is applied. I assume it is less than 40KG. So if I I can get 0.0xxxmm of deflection with this value I know that my gantry will be fine.

That’s it folks.

The value in cell M78 is the maximum deflection our gantry will show if 40KG of force was applied at it's centre. In this case it is 0.016mm which is fine for anyone.

If I remove the values for the Gantry back supports and put a value of zero in Cells D33, E33, D34 and E34 The deflection increases to 1mm. Now that could get to be a problem. Which shows the benefit of using an I beam configuration for your gantry.

Please note that the calculations only work for a symmetrical U shape. I.E you could not specify 100mm width for the top gantry support and 50mm for the bottom.


Hope this helps and I hope that my instructions are clear enough for you to follow.

[JBaptiste]

Thanks