Planning my first CNC machine

[Sharp909]

I'm by no means an engineer but I find myself wanting to make my own CNC machine as I cant find anything within my budget that I feel confident will do the job I want. I want to make fairly simple aluminium parts with a 0.1mm or better accuracy. In my budget it's nearly all CNC router type machines but I see many comments about the rigidity of the gantry performing 2 motions. So I came up with this idea where the gantry is only providing Z motion with the spindle supported from both sides rather than the usual 1 side. The frame work is 160 x 80 aluminium extrusion and some 20mm Aluminium plate. I could also bury the base of this design in cement/epoxy granite. I plan to use a MASSO controller. Any advice welcome.

[routerdriver]

I think building a machine is absolutely the best way to have a full understanding of what makes it tick and wish you every success with your efforts.I do wonder if having a relatively long and quite slender axis moving across the machine as a result of the input of a single central stepper may lead to disappointment as it may tend to rack.The more usual disposition of steppers would be to have two slaved together on the gantry base and one traversing the gantry with the other driving the Z.How much Z axis travel do you need?

[Sharp909]

I think you've misunderstood the design. The gantry doesn't move. It only provides Z axis movement. The Y axis is fixed in line with the cutting tool as it is on a normal cnc mill. This should make the machine more rigid than the typical router set up.

[routerdriver]

I may well have misunderstood.Could you provide images with the axes,preferably labelled. at both extremities of their range?

[Sharp909]

I may well have misunderstood.Could you provide images with the axes,preferably labelled. at both extremities of their range?

If you look at the image the x axis is parallel to gantry and has about 350mm of travel. The Y axis has about 500mm of travel. Z can be anything yet but around 150mm should be enough for my needs.

[routerdriver]

I fear that you may be headed for difficulties with racking.If you were to place two steppers on the X axis and only one on the Z axis,I believe things might improve.Just imagine the forces on the workpiece if the cut is at one end of the table.It would be a lot closer to conventional thinking to have a full width table with the Z axis traversing the gantry.There is a downside in that the rather generous Z axis travel will encourage deflection of the Z axis with that rather generous amount of travel.In short,I would advocate that you consider a moving table machine with a full size table and a bit less headroom for rigidity.Obviously you have a greater understanding of your requirements than anybody else but I would ask you to imagine that you have been able to machine a part of some height and the end of the sequence is reached.With maximum retraction,can the cutter return to the home position without leaving a groove through the job?

[Sharp909]

I appreciate you being devil's advocate. It's the best way to check a design and people's thinking. I think we see things in a very different manner as by your thinking all CNC mills would have racking issues with their single point of contact on the X and Y tables. On a mill the X and Y axes are 'wide' and heavy plates of steel which I've swapped out for 4 linear rails on the X and 2 on the Y. The Y axis load is supported across a wider area than a typical mill. Granted a decent mill has weight on its side for strength and rigidity so with a lighter weight rig you have to think differently. CNC routers with Y and Z travel on the gantry are notorious for poor accuracy. I haven't seen any sub £5k routers cut aluminium with any degree of accuracy or surface finish. By fixing the Z so it only does Z surely helps with rigidity? Where you would normally have to make a very sturdy single column for a typical mil, by utilising the gantry set up you make the Z a much simpler set up.

I'm only cutting small parts so wont be cutting at any table extremes. The longer Y is to get access to the bed to set it up because the gantry is in the way. This is the only reason I can see that mills are made with a single column - easier access. It makes more sense to me to have a gantry to support the Z axis. With the Z being the weakest link why would you only have 1 ball screw and not one each side of the spindle? I did think to have 2 ball screws on the X axis to support the longer Y table, but then dont you want the drive right where you are cutting? My design has you cutting right above or very close to the X and Y ball screws at all times.

I've not seen a design like mine, which is either because there's a fundamental flaw with it or it's a case of 'we always made them this way, so why change?'. I cant come up with a flaw when comparing to similar spec, sub £5k routers.

[m_c]

I have seen a similar design in the past (pretty sure it was actually on here somewhere).

It does work, but the Z-axis is a lot of work to get everything spaced correctly, and restricts access to the spindle.

My concern would be the twin stepper setup, as ideally for twin steppers, you'd want to be able to home them independently which requires a certain amount of flex in the design. With too rigid a setup, there is a risk the motors might end up fighting each other.
I'd probably just go for a single Z-axis screw, as if the gantry is stiff enough, racking under load shouldn't be a problem, especially on a Z-axis.
The biggest load on a Z-axis is going to be doing things like drilling, where accuracy isn't a major concern. The bigger concern is rigidity in the X and Y axes, which twin screws isn't going to benefit.

Generally moving gantry designs are preferred as they take up less space for any given table size, but for compact machines, it's not so much of an issue.

[pippin88]

Lifting Z is a good stiff design.

But having the X and Y down the bottom is weird.
You have 4 rails on the X axis - this is a lot of added expense, but worse will be the difficulty of aligning 4 rails.

The X axis should travel across the lifting Z beam.
Down the bottom should just be one axis, the Y axis (moving forwards and backwards under the Z beam)

[Sharp909]

I'd only add the Y to the Z beam if I could support the spindle from the front as well a the back. The twisting force of the spindle hanging so far away from the rails puts me off the typical router set up.