Big Epoxy Granite machine, Which frame is better ?

**EZELab** · 14-09-2020

Hello !
I am in the design phase of my new CNC machine. I am aming to 120x60 cm working area, And most of my milling will be in Aluminum, Brass, And some steel!
The frame will be epoxy granite and a welded laser cut steel skeleton inside.
I prototyped 2 design to choose from, Here's my 2 prototypes

Click image for larger version.

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I really like the U base design (#1) its easy to enclose & Chip control is great.
But, Is the design #2 stiffer?
Which one should I go to ?

Thank you all in advance (:

**Voicecoil** · 14-09-2020

I can't help but think that you would get better results with both designs if you rotate your working area by 90 degrees to minimise the gantry length.

**EZELab** · 14-09-2020

Originally Posted by Voicecoil

I can't help but think that you would get better results with both designs if you rotate your working area by 90 degrees to minimise the gantry length.

Thank you for your addition. I'll think about this.

**pippin88** · 15-09-2020

Why put a steel skeleton inside?

If the steel is for stiffness it is better to put it outside. Material at or close to the neutral axis does very little for stiffness.

Save the money you would spend on laser cut steel and put it into more epoxy granite for bigger sections.

**EZELab** · 15-09-2020

Originally Posted by pippin88

Why put a steel skeleton inside?

If the steel is for stiffness it is better to put it outside. Material at or close to the neutral axis does very little for stiffness.

Save the money you would spend on laser cut steel and put it into more epoxy granite for bigger sections.

Are your sure about that ?
Sure I'll put steel outside for screwing and rails, ballscrews .. etc.
Epoxy granite alone is that stiff for a large machine ?

**phill05** · 15-09-2020

Originally Posted by pippin88

Why put a steel skeleton inside?

If the steel is for stiffness it is better to put it outside. Material at or close to the neutral axis does very little for stiffness.

.

How do you arrive at that thinking? most things have the skeleton inside for strength of the main structure and appearance there would be very little point of having steel outside of Epoxy granite other than to cut down on resonance of a steel box frame you could fill the box with epoxy.

Phill

Edit: I would be tempted to go for No2 image

**EZELab** · 15-09-2020

Originally Posted by phill05

Edit: I would be tempted to go for No2 image

Thank you for your replay.
Yes, design #2 have many Advantages, Easier to align, Easier to machine flat, uses 3 motor not 4.
But will it add any more stiffness more than the #1 design ?

**phill05** · 15-09-2020

I believe it will, a fixed gantry has got to be more stable than a moving mass gantry, the only thing is you need more room for the moving table.
I have a fixed gantry and no problem at all.

Phill

**Zorbit** · 16-09-2020

Removed.

**phill05** · 16-09-2020

Originally Posted by Zorbit

Pippin is correct. That's why a piece of box section steel is stiffer than the same amount of steel in solid rod form. It's a cube law - the stiffness is proportional to the cube of the thickness.

Beware of mixing materials with such differing properties, you may end up with a structure that performs worse than either would on their own.

Did you not read the OP's post "The frame will be epoxy granite and a welded laser cut steel skeleton inside" you would not cast an epoxy granite frame then clad it outside with steel Box section Box section was not questioned totally different build.
Come on!!!

Take a look at this project: https://www.model-engineer.co.uk/for....asp?th=139042

Phill