Gantry design and FEA analysis

[eci22]

I know you are designing on the basis of 50 x 50 material but rather than the complexity of the braces in the designs above have you considered two pieces of 100 x 50 welded together to give a flat face 200 high? Your overall height looks to be about that anyway. I'm really asking because that's what I've built and I'd love to see the deflection calculations.

Hi Kitwin, do you mean like this ?


This seems like a very stout design and is better than the bracing I used. I have used it to generate some numbers below- hope they are of interest. But the analysis suggests the biggest component for deflection is the material of the Z plate, it seems to contribute the most. If you are interested let me know your exact materials ,including Z plate and I would be happy to run the numbers for you

I've changed all model calculations to use 100 Newtons of force. Also the Z plate is now 200x300x12mm Aluminium



My main question is where did you perform the welds for the two pieces of 50x100?, A seam weld covering the entire length would lead to significant warp id imagine and tack welds wouldn't hold up, bolts would work I suppose. How did you make yours ?

Agree with Voicecoil.
Analysis 2 is closer to worst case as this puts a twisting moment on the gantry as the force is applied at some distance away from the structure and will usually cause the biggest deflection.

Hi routercnc, can I just say the videos on your CNC build is really inspiring. I learnt so many subtle things the info is worth its weight in gold to me. It's something I come back to time and time again. Thank you for putting it out there !

I wrote a spreadsheet a long time ago just to play with the numbers and I looked at 3 load conditions.
Vertical load at the centre of the gantry due to the mass of the Y and Z axis and estimate the sag.

(All simulation diagrams use 100 newton force)

Vertical Load like this ?


Presumably the only force is gravity and the weight of your cutting tool, which I haven't included.

Lateral load applied at the tool tip as this will want to turn the gantry into a parallelogram- one reason why raised bed sides is stiffer than tall gantries.

lateral load


correct ?

Foreaft load at the tool tip to twist the gantry. This usually causes the most deflection and you need to make the gantry have a shape with material around the outside not 2 isolated beams. So yes plate them together is an option.

Is that the same as moment force around the Y axis of the Z plate ?

Remember this is all to see relative performance and get a feel for what aspects are important as it will assume perfect joints and you would need to model the whole machine and the cutter to get close to the actual deflection and even this is the steady cutting force not the deflection caused by vibration which will add some more.

Yes definitely, I just find this really interesting to do. It's a good way of comparing different designs. I'll use this for guidance only and for learning FEA which is something I have not done before. I also appreciate not having the Y axis, the feet, basically the rest of the machine modeled will skew the results somewhat

The stiffest gantries in terms of numbers are raised bed sides with a large box section as this gives good stiffness for all load cases. But practical considerations mean other options are good including the famous L shape gantry with 2 good sized rectangles joined in an L shape.

Would you mind directing me to a picture of this gantry design ?

Setup 2 is much more realistic since you want to design for decent performance when the cutter is hanging down a fair way below the gantry, 'coz that's where it will often be used. Having an equal horizontal force on each beam really isn't going to happen as that would mean the workpiece being considerably higher than the bottom of the gantry. 1000N cutting force is maybe a bit much unless you're using big cutters, I've heard figures of 200N tops bandied about before for typical machines such as people on this forum build though I haven't seen the sums behind that. I did find this reference a little while back though:

https://www.ctemag.com/news/articles...e-when-milling

Putting some numbers into the formula for a 1.5mm DOC on a 3 flute cutter at 0.1mm feed per tooth I get a tangential force around 90N for the best 6000 series ali.

It would also be interesting to do an analysis with a plate fixed on the back of your 2 bits of box section as that will make kind of a super box with the Z plate.

PS looking at your pics again, there might be a decimal point adrift in the text for the second set of results.....

Thanks for the link Voicecoil, much appreciated. I updated the the force to 100 newtons in my above simulations, I also made my Z plate thinner as I realised 20mm thick tool plate is out of my budget. If I have done the simulations correctly then the numbers still look promising. I will look to add the fixed plate at the back tomorrow. Thank you very much for your input