Design Help Pt2 Required for CNC design/Build

[Jonathan]

About the Z-axis... It is foolish to do anything other than what Jazz has suggested, namely putting the rails on the front plate. With the rails on the front plate the distance from the tool to the support (i.e. the bearings) is some constant (overhang of spindle plus length of tool) plus how far down the Z-axis is. So the minimum is the constant, and the maximum is the travel plus that constant. Whereas with rails the other way round the overhang is always the same constant plus the travel - i.e. it's always at the worst case scenario for the other orientation!

I don't really see the point of going above 20mm on the Z-axis, the gain is absolutely minuscule compared to the added cost (especially when you're using just 16mm round rails) as I'm sure you'll have found from routercnc's spreadsheet. I used 20mm for this reason.

Aluminium warehouse sell plate cut to size which is a good choice, as unlike the flat bar, it should be flat! It's a little more expensive.

http://www.aluminiumwarehouse.co.uk/cutting_calc.php

However, I urge you to consider not having gantry sides and using the style, like mine and Jazz's bigger machine where the cutter operates below the level of the X-axis rails. That eliminates the flexing of the gantry sides, as there aren't any, and puts the X-axis ballnut(s) in a much more optimal location in terms of the forces they receive.

Y-axis of 750mm sounds a bit much for just one ballscrew on X, especially considering you want to cut aluminium.

[JAZZCNC]

However, I urge you to consider not having gantry sides and using the style, like mine and Jazz's bigger machine where the cutter operates below the level of the X-axis rails. That eliminates the flexing of the gantry sides, as there aren't any, and puts the X-axis ballnut(s) in a much more optimal location in terms of the forces they receive.

Y-axis of 750mm sounds a bit much for just one ballscrew on X, especially considering you want to cut aluminium.

Seen as Jonathan agree'd with me I'll agree with him. . Lol . . . . Thou Ive actually built the same type machine as shown, it was with the specific intension of cutting just Wood which it does very very well and not to disagree with jonathan it also cutts Ali ok.
That said If building an allround machine then I'd go with what Jonathan says and actually I will be building just such a machine over the next few months ounce I've got other commitments out the way.
I've attached the first ruff models thou it may change in dimensions before it's built with ballscrews covered and rear of gantry enclosed along with Cable chain etc. The bed will be adjustable because a forth axis will be needed at some point also allows for milling vises etc.

Edit: Oh by the way It's going to sit on a bench not floor mounted.!

[MikeyC38]

Thanks Jonathan for the additional comments and feedback. More than 1 person saying the same thing is always encouraging!
I am pretty much committed down the existing gantry route because I already have the ballscrews in hand and can't afford to buy anymore at the moment. My earliests thoughts on this was a metal version of the twin ballscrew- belt drive Solsylva design. I accept that the machine may have some limitations with aluminium milling but as this is a tertiary requirement, then I can live with that compromise for now. Wood, GRP, Plastic and CFRP are the priority at this time, hence the size of the machine in the Y axis. I am looking to source aluminium toolplate (6082 T6) and have found a reasonably priced source in Milton Keynes (MetalFast) but that was awhile ago so I will need to recheck.

Kind Regards
Mike

[Jonathan]

I am looking to source aluminium toolplate (6082 T6) and have found a reasonably priced source in Milton Keynes (MetalFast)

Aluminium warehouse has tooling plate which may be cheaper? It's on the website in the same place I linked to earlier. They call it EcoCast.

[JAZZCNC]

I am looking to source aluminium toolplate (6082 T6) and have found a reasonably priced source in Milton Keynes (MetalFast) but that was awhile ago so I will need to recheck.

AW is good like Jonathan says but you really only need tool plate quality in just a few key places, like the Z Axis front/rear plates or for the Bed etc.

Things like gantry sides and general end plates or bearing brackets, motor mounts etc can be got from cheaper flat bar stock, still 6082 T6 just not machined perfectly flat.
The wider width's of flat plate stock can some times come slightly cupped but for none critical stuff or if machining several parts or brackets out of the width then it's not really a problem. . . Often just a quick surface will take away any cup. . . .Thou I must say the last few 300mm wide lengths I've had to deal with have been quite bad. The one I'm using at the moment is particulerly bad and if it wasn't for the fact I was cutting it up into smaller components then it would have been going back.!!

[JAZZCNC]

Mikey now Knowing a bit more about your plans to use a single screw and looking at your design again then I think you may want to reconsider the width of the gantry and make the bearing spacing wider.! . . .That looks awfull narrow for a single ballscrew design making racking a big possibilty.? . . .How wide is it.?

[MikeyC38]

Mikey now Knowing a bit more about your plans to use a single screw and looking at your design again then I think you may want to reconsider the width of the gantry and make the bearing spacing wider.! . . .That looks awfull narrow for a single ballscrew design making racking a big possibilty.? . . .How wide is it.?

It is currently 125mm. I have updated the Z Axis design as you and Jonathan suggested and I was coming to the same conclusion as well. So the gantry sides are up for redesign. Perhaps 175 - 200mm width and up the material thickness to 20mm should do the trick. Eliminating the 'S' shape to make manufacture easier. My thoughts are that the router tool bit should ideally lie between the gantry bearings to reduce racking. Is this true? In the current design it does not (see side shot).

Thanks for the tip on using the toolplate in the accuracy-critical areas - should save costs

RegardsMike

[wiatroda]

I think you may want to reconsider the width of the gantry and make the bearing spacing wider.! . . .That looks awfull narrow for a single ballscrew design making racking a big possibilty.?

Everything what Jazz said plus I would insist two ballscrews. It is worth to invest a bit more ££ and have machine free from any distortion/ racking. You may regret one ballscrew some day.
One stepper per ballscrew, perfectly one stepper with its own driver but I run my machines with one shared driver per two parallel steppers on X axis and never had any problems with this at all.

[Jonathan]

You may regret one ballscrew some day.

No, you will regret one ballscrew some day. So many times people start off saying they'll use one ballscrew and a couple of weeks later we manage to persuade them to get another!
Just imagine what happens when you apply a cutting force with the spindle near the gantry side (i.e. at limits of Y-axis travel). It's going to be at least 300mm from the support (the ballnut) so you've got a large turning moment which is only resisted by the stiffness of the rails. Combined with having the X-axis bearing blocks close together the deflection will be quite a lot, especially since the fixing of the supported rails to the rails is relatively weak. A 0.2mm deflection (for instance) may not sound like much, but that will affect the finish you obtain with wood and stop you machining metals.

Two ballscrews also lets you use the design I suggested earlier, without gantry sides, which saves a bit on materials I reckon and is definitely much stronger.

I run my machines with one shared driver per two parallel steppers on X axis and never had any problems with this at all.

Ooh that's interesting / unexpected. Are the steppers mechanically linked (timing belt or whatever)? What steppers and drivers are you using?

[wiatroda]

Ooh that's interesting / unexpected. Are the steppers mechanically linked (timing belt or whatever)? What steppers and drivers are you using?

Now it does sounds like one stepper is shared between two machines. Does it?? or it just me. One stepper per ballscrew per machine on X axis. I have 2 CNC machines and each one have it's own set of motors and drivers. One is for "wet" another is for "dry" job. I use PM752 from zapp. Motors on X axis are not linked together by any mechanical way apart of Yaxis on my wet machine- 2 steppers, one driver common timing belt. Please see my post no.23