Steel Frame 8' x 5' CNC - Advice Sought

[BrentonSpear]

Hi All,

I have been reading this forum with interest and busily designing my CNC machine.

It will comprise of a steel frame, steel gantry and aluminium Z axis.

I want it to be as accurate as possible and will be cutting acrylic, plastic, wood and hopefully light aluminium.

Over all the machine size will be 2600mm x 1500mm (8' x 5') with a cutting area of 2200mm x 1150mm (7.5' x 4').

I have already purchased 2500mm linear rails for the X axis and will mate them to dual HGH20 linear blocks on each side.

The Y axis will use linear rails with HGH20 blocks also.

I have read with interest the threads that use bolted top rails for the X axis and I am thinking of using epoxy to level the top of the rail between the linear rail and the top rail.

I will have a rack and pinion system on either side of the X axis mated to NEMA34 stepper motors. I am going to use the R&P drive systems from CNCRouterParts.com

Here is the frame design as it stands at the moment:





The Gantry will be made from a 250mm x 150mm RHS steel beam and I am going to add a flat plate inside the bean to strengthen it.

I am working out whether to use a ballscrew on the gantry or to use a R&P system here as well.

Here it is:





I would like to hear peoples opinions on the frame as this is what I will weld up first.

[Boyan Silyavski]

Hi,
IMO:
- Dont add plate inside, add it at the back and bottom if you feel that its necessary
- Use ball screws as much as possible if budget permits it. So maybe even rotating ball nuts for both sides of the gantry.
- Add some more short supports left and right, 3 each side is too low for such long rails

Good luck with the build!

[BrentonSpear]

Ok, here is my updated table design with the extra supports in place.

[Boyan Silyavski]

Hi,
when i said above for the plates i meant the gantry.

What would be the square steel box profile size? I calculated it many times and for steel machine/this size especially/ is best to use bigger profile 100x100x3 or 4mm and simplify the structure. I say always 100x100 as it is the max that could be handled and cut at home using dry saw or small band saw.
The benefits are:
-simpler
-easy to cut at home
-easier to solder
-stronger
-less metal for the same strength so cheaper

You could read more here and see changing the table design leads to substantial savings http://www.mycncuk.com/threads/5302-...highlight=tony

In fact if you have the means the bigger is better here.

[Clive S]

Ok, here is my updated table design with the extra supports in place.

I personally would not put a rail all around the bottom and I would add triangulation in.

I would have it sitting on say six legs, that way it would be a lot easier to set up on an uneven floor. Also make it out of 4mm or 5mm box section then you will have enough thread depth for fixings.
.
It might be worth thinking about having heavier cross rails and ditching the centre supports to be able to use the underneath to store sheet materials. ..Clive

[BrentonSpear]

Hi Guys,

I was looking at using 50mm x 50mm x 4mm SHS steel tubing for the frame.

Silyavski, that is an interesting thought. I suppose I could do it in 100x100x4mm SHS. I will have to do the price calculations. I am planning to get all the pieces cut to the required lengths at the place I buy the steel so the size of the SHS is not really an issue.

Clive S, I had always planned to add feet to the frame.

Here is the updated design with the feet in place.



For the gantry I am going to look at how I get a ball screw between the rails of the Y axis.

I think I will put an I beam inside the RHS beam and then remove material to make a void. I have seen other person do this and it looks doable.

[BrentonSpear]

What sort of span will I get with a 100x100x4mm beam? Could I get the full 2500mm?