Fixed z axis to gantry design

[Lachlan]

Hello everyone,
This is my first post here but i have been a long time reader of this forum and the likes of Jazzcnc and Jason and many, many others have inspired me to take the plunge and build a simple cnc design. I have limited hand tools plus a dropsaw for this build so simplicity is a major factor. I don't have a whole lot of experience working with steel but I can weld.

I will be milling aluminium so the machine needs to be rigid. i will be building it out of 75mm square box steel 5mm thick.

After perusing many designs i think (not carved in stone) I will be basing my design on the following machine.

http://www.youtube.com/watch?v=4Bj6er322Dg
With the following alterations:
rail size will be 900x600by300
Gantry will not be fixed but will move on 20mm Sbr linear rail as will all other axes. The cutting area will be too small if its fixed and i realise this will impact on rigidity. I know profile rail is preferred but its just out of my budget.
Will be using pulleys with HTD5 15mm belts on all axes.
Nema23 425oz.in motors on each axis driving 1605 ballscrew thread.
Reasons i chose this design:
Simple to build
Rigid z axis that i know I can build accurately.
Seems like a very strong design.

So any major design faults with this design?
Am I on the right track?
What would you change about this design to improve upon it?
Lastly, thanks so much for any help given.
Regards,
Lachlan

[routercnc]

The design in the picture is a nice design and would work well as it uses a fixed gantry and moving bed, but if you want to have a fixed bed and moving gantry then I think you will run into problems if you make it like that.

The gantry is quite tall, which it needs to be to allow enough Z movement. When this becomes a moving axis you now have a long cantilever from the bearings at the bottom which is difficult to brace without having widely spaced X axis bearings and significant loss of travel. Getting the stiffness back could be a problem as the geometry is now against you.

Personally for a machine used mostly for aluminium then I would either go with the fixed gantry and beef up the gantry support a bit more than the one in the picture, or go with a conventional moving gantry with Y axis and then Z axis. I think it will end up stiffer and more effective than your in-between design.

Nema 23 would be fine, and 1605 ballscrew works OK (that's what I have), but many choose 1610 and then drive it with a 2:1 reduction pulley or 1:1 pulley depending on what they are cutting.

[JAZZCNC]

I agree with Rob completely increase the size and stay with this design or switch design completely.

Personally I'd stick with this design and increase size because it's a very strong design and ideal for cutting alumnium.
If you do need moving gantry and switch design then stay away from high gantry sides and go with frame that has high sides with rails on top and Gantry sat directly on bearings. It's the only design to use of your wanting to cut aluminium.

Also I'd re-think those SBR rails if your cutting Aluminium has you WILL REGRET it down the line.!! It's alot of work and money to spoil with cheap rails that WILL show there weakness in the finish and accurecy you achive.!

[Lachlan]

Thanks for the feedback.
Both comments make a lot of sense so I will have a think about it and come back with a better design. I will change to 1610 ballscrew. Still just learning Solidworks so it might be a while. If going with the fixed gantry design how best to beef up the gantry? Triangulate, gusset or just larger box section? Also does a design like this really need 2 ballscrews on the z axis? Would it be ok to have one in the middle?

[routercnc]

The fixed gantry would benefit from stiffening in the fore/aft direction. Start at the top of the gantry in each corner and run a brace down to the front or rear corner of the machine. If you look at this machine from the side you would see the gantry sticking up from the base with little support.

If you go with only 1 ballscrew on Z then there is the risk of racking when cutting off centre. Imagine doing a plunge cut at one side of the table, the Z axis will try to rotate, possibly bind the bearings, but certainly loose some accuracy. To combat this you have to space the bearings apart (vertically) but then you start to limit Z travel, or have a much taller gantry and this then needs to be braced still further.

Small ballscrews at this length are not that expensive on e-bay, and you can use a single motor as per the design in the picture to save costs on two motors & stepper drivers.

[Lachlan]

That makes sense, I will use two ballscrews on the z axis and brace the sides of the gantry front to back.