Design Help Pt2 Required for CNC design/Build

[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

[JAZZCNC]

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]

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. . . .Simplizzzzz.

JazzCNC - this is a brillant suggestion :tup: !!! Improve what I have but design in an upgrade path so that the existing investment is not totally lost!!!. A question springs to mind for everyone - in a twin ballscrew setup with 2 motors on the X-Axis (and I presume 2 stepper drivers) how do you ensure equal power output and speed to avoid crabbing the gantry? The Solsylva solution uses a single motor and belts so you don't get the problem.

Now that you guys have planted the seed of doubt in my mind :confused:, I will work up a twin screw solution as you've all suggested and do the numbers to see if my budget and timelines can take it !!!

Thanks All for your patience and interest

Mike

[MikeyC38]

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:...

[m.marino]

Mike,

Jazz and I designed a single ball screw machine and when it all came down to it the cost for the pulleys and belts where actually more then going the twin screw route. Now I have to get a second parallel port card to allow me to bring the 4th axis on line and set up a second BOB and driver set up. But outside of materials this is not really a problem when using Mach3. Good luck and hope all things go well.

Michael

[routercnc]

Hi Mike
Good to hear someone getting use out of my spreadsheet. Nice to put something back into this forum.
Just a comment on the x axis bearings. When mounted on their side it does make for a neat installation but be aware that because the blocks are open (sort of C shaped) that they will open up slightly due to the gantry mass and to a less extent the cutting forces. I've not done the calcs but suspect the effect is small so don't panic. Interested to hear if anyone has worked out how much they deflect for a given side load (or vertical in this case) or has experience of this orientation.

As for twin screw - I have a gantry about 600mm wide with a single ball screw and I can rock the ends of the gantry back and forth by hand up to about 1mm if I push hard. Having said that I can happily machine balsa, liteply and ply and they come out at the right sizes. But I would still say twin screw is better and would like to upgrade when funds allow because I could then up the feedrate.

[MikeyC38]

Hi Mike
Good to hear someone getting use out of my spreadsheet. Nice to put something back into this forum.
Just a comment on the x axis bearings. When mounted on their side it does make for a neat installation but be aware that because the blocks are open (sort of C shaped) that they will open up slightly due to the gantry mass and to a less extent the cutting forces. I've not done the calcs but suspect the effect is small so don't panic. Interested to hear if anyone has worked out how much they deflect for a given side load (or vertical in this case) or has experience of this orientation.

As for twin screw - I have a gantry about 600mm wide with a single ball screw and I can rock the ends of the gantry back and forth by hand up to about 1mm if I push hard. Having said that I can happily machine balsa, liteply and ply and they come out at the right sizes. But I would still say twin screw is better and would like to upgrade when funds allow because I could then up the feedrate.

Thanks routercnc for the spreadsheet and the comment re the X axis bearing placement. I've seen them placed like this on another design, but somehow felt that vertical placement would be better for carring the vertical weight of the gantry and resisting racking. For the gantry I will be using 20mm plate sides with 120x40mm heavy extrusion (8.8 kg/m) for the y-axis on the basis that the box section resists twisting from the cutting loads better than plate and is lighter, reducing the forces required to accelerate and stop the gantry.

BTW I too am a model flyer (Gliders and power) and have in mind to build a 1:3.5 scale (4.25m) "Reheinland" glider with all the bits CNC cut - once I have done my brother's guitars!

Kind Regards
Mike

[Jonathan]

I've not done the calcs but suspect the effect is small so don't panic. Interested to hear if anyone has worked out how much they deflect for a given side load (or vertical in this case) or has experience of this orientation

It's going to be tricky to calculate as it relies on the strength of the bolted joint between the round rail and the support. The aluminium surface at the joint is a V-shape, so the rail only makes two line contacts. There's not many bolts (and if you don't threadlock them they can work loose) which overall means the round bar can, and do, bend in that direction upon the rail. Check it with a dial indicator and you should be able to see the effect.

For the gantry I will be using 20mm plate sides with 120x40mm heavy extrusion (8.8 kg/m) for the y-axis on the basis that the box section resists twisting from the cutting loads better than plate and is lighter, reducing the forces required to accelerate and stop the gantry.

See you mention making the frame very strong, which is great, but with only one ballscrew you're kind of defeating the object of doing that. Not saying don't make the frame strong, as you can't change that later (without wasting material/money) unlike adding an additional ballscrew which is obviously easy enough.

BTW I too am a model flyer (Gliders and power) and have in mind to build a 1:3.5 scale (4.25m) "Reheinland" glider with all the bits CNC cut

I bought a 500 size helicopter recently, so I'm also almost a model flyer. Not dared actually fly it yet! Couple of bits to cut from carbon on my router first. So far I have just stuck to model cars.