Hello

[Jonathan]

Whilst it's true that as it stands that machine will cut plywood at a reasonable speed it would improve it no end to have two ballscrews. I'm confident most people make a CNC router then ultimately end up using it for all sorts of things they didn't expect, so why risk compromising it later. True it's not cheap to have a second ballscrew, but as a percentage of the whole machine it's not so bad?

If you do decide to stick with one ballscrew then space the linear bearing blocks out as much as possible to compensate. Ultimately your material removal rate will be proportional to the spacing of those bearings, along with a number of other things of course.

Also another vote for linearmotionbearings2008. When the time comes send him a message asking for an invoice for the exact parts you need since that's generally a little cheaper than the eBay listings.

The stiffness of your gantry will be greatly improved by simply adding an aluminium plate (10mm-1/2" ish) thick to the back of the gantry. Bolt the extrusion to it all the way along. It will virtually eliminate bending of the extrusion thus reducing bending of the rails.

[JoeHarris]

Thanks for the advice it is very mush appreciated, I am getting the message that if buying from china I should go with Chai but for your information the listing I having been looking at is http://www.ebay.com/itm/300652998890...ht_3739wt_1037

£400 seemed good for this lot and the ball screws for the X and Y axis are 20mm so whipping should be reduced?

Another question, to make the ball nut housing - if I cut a 36mm hole with a holesaw on my pillar drill into aluminium plate will this be accurate enough or am I best to fork out on ready made ones at £40 a go? (http://www.zappautomation.co.uk/mdg2...ing-p-464.html)

[Jonathan]

Thanks for the advice I is very mush appreciated, I am getting the message that if buying from china I should go with Chai but for you information the listing I having looking at is http://www.ebay.com/itm/300652998890...ht_3739wt_1037

£400 seemed good for this lot and the ball screws for the X and Y axis are 20mm so whipping should be reduced?

1) RM2005 is more expensive than RM1605 and you only want 20mm screws on one axis, not all.
2) RM2005 has to spin at twice the rpm as RM1610 to obtain the same feedrate and the critical speed of a 20mm screw is less than twice that of a 16mm screw so you'll actually end up with a lower feedrate.
3) Inertia of 20mm screw is significantly greater than 16mm screw so the acceleration and feedrate will be reduced.
4) RM2010 would be worthwhile, but you can only get it in England which is far too expensive particularly for two.

Another question, to make the ball nut housing - if I cut a 36mm hole with a holesaw on my pillar drill into aluminium plate will this be accurate enough or am I best to fork out on ready made ones at £40 a go? (http://www.zappautomation.co.uk/mdg2...ing-p-464.html)

It's easiest to make the hole a little oversize then bolt the nut in the right position so it rests on the flange. That way you get a little adjustment of the centre height to ensure the nut is aligned accurately. Here's some I made earlier:

[JAZZCNC]

Whilst it's true that as it stands that machine will cut plywood at a reasonable speed it would improve it no end to have two ballscrews. I'm confident most people make a CNC router then ultimately end up using it for all sorts of things they didn't expect, so why risk compromising it later. True it's not cheap to have a second ballscrew, but as a percentage of the whole machine it's not so bad?

While I agree with everything Jonathan says here let me just say that because of the style of machine you have choosen it's not difficult to add another ballscrew etc at a later date.!
The pic of the machine posted earlier can be changed from single to twin screw setup by just adding another motor mount bracket and repositioning of the screws in just a few hours so if budget is tight then it's not major rebuild issue like on some machines.!

It will eat thin plywood all day long on a single screw setup but Jonathans correct you WILL want to try other things which will find it's limits but if finances are tight then it will be ok and should pay for it's own upgrade if put to good use.! . . . So don't be put off using a single screw setup and if your mindfull of it's limitations then it won't be a problem.!

[JoeHarris]

OK, so will whipping still be an issue with 2no. 1610 screws at 1500mm long - as they will still both sag? Jonathan have you drilled your housings with a holesaw? if so can you recommend a make, I bought some makita ones a while back and they are rubbish.

Also what are the implications when buying drivers if you are going to add another stepper into the mix later? (I was thinking of getting the electronics from http://www.diycnc.co.uk by the way)

I do want to be able to cut more than just ply! everything from aluminium/ply/mdf for furniture making to paper/card with a drag knife for architectural model making/graphics. not sure if I am being unrealistic as I am sure one machine can't properly fit all tasks?

Apologies for all the questions!

Joe

[JAZZCNC]

OK, so will whipping still be an issue with 2no. 1610 screws at 1500mm long - as they will still both sag?

They will just about be ok but will need very carefull alignment has any miss-alignment will be exagerated at high speeds and this is when whip will occur.

Jonathan have you drilled your housings with a holesaw? if so can you recommend a make, I bought some makita ones a while back and they are rubbish.

I have a Bosch quick realease hole saw and it's brilliant, not cheap but good.

Also what are the implications when buying drivers if you are going to add another stepper into the mix later? (I was thinking of getting the electronics from http://www.diycnc.co.uk by the way)

No problem adding at a later date but does need a bit of initial planning regards PSU to workout best way and reduce cost's further down the line.

Regards DIYCNC then personally I'd say no because he doesn't do any drives powerfull enough for what you'll need to get best performance.
My advice would be buy individual drives and pay the little extra and get 80V drives with 60-65v PSU this will give you the best performance for the 3nm motors your going to need to drive this machine.!

I do want to be able to cut more than just ply! everything from aluminium/ply/mdf for furniture making to paper/card with a drag knife for architectural model making/graphics. not sure if I am being unrealistic as I am sure one machine can't properly fit all tasks?

Then absolutly 110% go with twin screws from the get-go.!!

Apologies for all the questions!

No need thats the whole point and why where here.!!

[JoeHarris]

Then absolutly 110% go with twin screws from the get-go.!!

Just so I'm clear do you advocate one stepper driving the two x axis screws via a belt like the image attached

or two separate motors and drivers? if one is used presumably the motor needs to be more powerful?

[JAZZCNC]

Just so I'm clear do you advocate one stepper driving the two x axis screws via a belt like the image attached

or two separate motors and drivers? if one is used presumably the motor needs to be more powerful?

Personaly yes I prefer one motor using just the system you have drawn.
My machine is done this way and it completly eliminates some potential issue's that come with slaving motors together. I have used both methods and by far prefer this method.
It's not pritty and slightly more work intially but the peace of mind your not running out of sync and that if one motor stall and the other keeps going the machine not going to try and destroy it's self is well worth the trouble.
The cost is not much less due to belts/pulleys and has you say bigger motor and drive but again there's plus's in that it's less electrical components to intigrate and go wrong plus less Inputs/outputs required so more spare for other things like Probes, etc.

The other BIG differance comes when you you tune the motors.? When slaving motors you effectively have to leave a large safety margin regards motor tuning and velocity/accelleration than when using a single motor.
By the very nature of how steppers work and the fact they lose torque has RPM's rise then the chance of a stall increase's massively and on dualscrew setup stalling at high feeds can be devastating to the machine and your heart plus wallet.!! . . . . With single motor you can push the edge without fear of killing the machine or your self.

[JoeHarris]

Personaly yes I prefer one motor using just the system you have drawn.
My machine is done this way and it completly eliminates some potential issue's that come with slaving motors together. I have used both methods and by far prefer this method.

Thanks Jazz, that's very helpful. Do you have any pictures of your machines that have used this method or could you point me in the direction of other builds so I can get a better understanding of how this will work in practice.

Jonathan - if you are still keeping an eye on this thread - I have seen on other threads that you have successfully developed a rotating ballnut system. I was wondering if fixed ball screws held in tension along my x axis would make sense here as it would eliminate whip? Do you sell your spinning ball nut mounts?! :) presumably the arrangement shown in the previous post could be used to drive two ball nuts mounted on the y axis gantry?

Does anyone have any pictures of a good twin x-axis (fixed) ball screw mill?