Design Help Pt2 Required for CNC design/Build

**MikeyC38** · 28-12-2011

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** · 29-12-2011

Originally Posted by MikeyC38

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** · 29-12-2011

Originally Posted by MikeyC38

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** · 29-12-2011

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** · 29-12-2011

Originally Posted by 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.?

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** · 29-12-2011

Originally Posted by JAZZCNC

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** · 29-12-2011

Originally Posted by wiatroda

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.

Originally Posted by wiatroda

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** · 29-12-2011

Originally Posted by Jonathan

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

**JAZZCNC** · 29-12-2011

Mike I agree with the others completely that 2 ballscrews is much much better and prefered. . .BUT . . . I do know from experience with pritty much the same size machine it will handle wood, even hard woods no problem. Yes you won't be able to be has aggressive as if you had twin screws regards DOC etc but it will do the job ok. It will cut Ali but very light duty with shallow cuts.
You will need to widen the distance between bearings, more the better, and make the gantry as stiff as possible. The gantry width on the pics I've shown is 220mm with 240mm of actual bearing spread because it use's profiled rails not round and the bearings are slightly longer than the mounting pads so effectively 240mm wide gantry.

If you can stretch to twin screws then it's a no brainer just do it.!. . . but if not then I recommend you design the end plates for twin screw upgrade in future. The machine I showed in the pics is done this way.
The pics don't show it.!. . . but because I use Timing belts to keep the motors on the inside, the end plates are drilled at either side on the inside to accept twin screw setup and can be upgraded at any time.
It's a simple case of removing the existing single screw remounting the BK/BF blocks on the inside of the outer profiles and mounting the motor bracket setup in it's new locations on the end plate. Obviuosly then you add the extra screw along with a extra motor brackets etc. It also means a new ballnut mount for both sides so the existing one is not re-used but that's no big deal or great expense. All in all it takes 2 to 3 hours to change from single to twin screws.

In your case with direct mount motors just have the end plate machined at either side to accept the motors then cover with blanking plates untill ready, then just swap when ready and cover single screw mount with one of the blanking plates.!! . . .Simplizzzzz.

**MikeyC38** · 29-12-2011

Originally Posted by Jonathan

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.

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

Well Jonathan, I think you managed to plant the seed of doubt in my mind :confused:...