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JoeHarris
25-04-2012, 11:27 PM
Hello.

When I first decided I wanted to build a cnc router I was going to make a cheap mdf / threaded rod affair and now things are getting out of hand - mainly thanks to you lot, (thanks I guess!)

I am now intending to build a mainly aluminium frame driven by 3 ball screws. I intend to buy the rails, ballscrews and nuts as a job lot from china - "missmycar" on ebay seems very reasonable - does anyone have any experience of this seller?

whilst the project is very much in the design stage and is a work in progress I have uploaded so screen shots that I would really appreciate some honest comments on. I can barely afford to do this once so need to get it right first time around.

Many thanks in advance.

Joe

m.marino
26-04-2012, 02:15 PM
If I understand the drawing correctly, you are looking at this being 2m by 800mm in overall dimensions. That might be a little wide for a single central ballscrew, as racking of the gantry can start to be a problem there. I see a good bit of reinforcement built in, but you might want to look at adding some along the Y axis direction as well to help decrease racking of the frame possibilities. I don't know that seller on ebay as I dealt with Chai of linearmotionbearings also out of China. That is about all I feel able to mention on and I am sure those with more knowledge will pipe up shortly.

Michael

JoeHarris
26-04-2012, 02:38 PM
If I understand the drawing correctly, you are looking at this being 2m by 800mm in overall dimensions.
Michael

Thanks for the reply m.marino, the overall dimentions will be about 1500 x 800 - I want to be able to cut quarter sheets of ply - 1220 x 603. Do you think I'm still over-egging it?

JAZZCNC
26-04-2012, 05:11 PM
Thanks for the reply m.marino, the overall dimentions will be about 1500 x 800 - I want to be able to cut quarter sheets of ply - 1220 x 603. Do you think I'm still over-egging it?

Joe Micheals correct your on the very edge of being to long and too wide.?

I have built a few very similair machines in the past and actually still have one half built see pic (It's still sat in that same place and has been for over a year, just can't find the time or inclination to finish it.!!)
5862

Your potential problems will be 2 fold.! Potential Racking (In some materials) because of single screw and Whip from length of ballscrew.
To do justice for this kind of machine you will need 10mm pitch screws and the only financially credible option is 16mm Dia and at 1400-1500mm your on the edge of being troubled with whip at high feeds and seen has your mainly going to cut ply wood then you'll want high feeds for best finish, short job times.

If the gantry is built strong then I can tell you from experience that racking isn't an issue at this width in materials like ply wood,MDF with sensible depth of cut(DOC) but can be in harder materials like Aluminium or really hard woods unless low DOC are taken.! . . That said twin screws are far far better in all materials because it really allows you to ramp up the DOC and feedrates and give a much more robust machine. . . They are my prefered choice every time if budget allows.

If your needs are just ply wood then I'm confident you'll be ok on the racking front so long has your not expecting to cut 19mm in 1 pass. .!!


Regards Ballscrews etc then I'd recommend Chai at Linearmotionbearings on Ebay you won't find a better guy and 110% trust worthy, certainly worth paying a few quid more just for peace of mind your stuff will arrive(usually within 10days) and if wrong or bust it will get sorted.!!

Jonathan
26-04-2012, 08:03 PM
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
26-04-2012, 10:18 PM
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?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649#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/mdg20-ball-nut-housing-p-464.html)

Jonathan
26-04-2012, 10:52 PM
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?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649#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/mdg20-ball-nut-housing-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:

586558665867

JAZZCNC
26-04-2012, 10:52 PM
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.!

motoxy
26-04-2012, 11:17 PM
I have used miss my car and got good service. Linearmotionbearings is good to.

JoeHarris
26-04-2012, 11:20 PM
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 (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
26-04-2012, 11:22 PM
I have used miss my car and got good service.

Does she charge by the hour and how much for the whole night. .:emmersed:

JAZZCNC
26-04-2012, 11:39 PM
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 (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.!!

Chris J
27-04-2012, 12:29 AM
(It's still sat in that same place and has been for over a year, just can't find the time or inclination to finish it.!!)


5862



Looks great

Chris J
27-04-2012, 02:28 PM
If anybody is replying specifically to me I have started a build thread here (http://www.mycncuk.com/forums/showthread.php/4493-The-Dark-Satanic-Mill?p=30256#post30256) so maybe a reply there might be best - cheers

JoeHarris
28-04-2012, 09:29 PM
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 5893

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

JAZZCNC
28-04-2012, 11:49 PM
Just so I'm clear do you advocate one stepper driving the two x axis screws via a belt like the image attached 5893

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
30-04-2012, 09:51 AM
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?

Jonathan
04-05-2012, 01:28 PM
Sorry for the late reply...


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?

Tensioning the screw helps, but you need a lot of tension to make a significant difference to the stiffness and most CNC router frames are not strong enough to apply that much tension. On mine I had the ballscrew machined the same on both ends with a thread. Nut on one end clamps the screw and the other end can be used to tension it a bit. It works well...
I do sell the rotating ballnut mounts, 1500mm is about the threshold where I would say it's worth considering a rotating ballnut, fixed screw setup. You'd certainly get a higher feedrate, but the question is do you actually need it? If you're mainly cutting wood/plastics then you can certainly take advantage of higher feedrates, but if the machine is mainly cutting aluminium then the difference in overall machining time is extremely slight.


Jonathan have you drilled your housings with a holesaw?

No, I milled those housings and bored the hole for the ballnut in the lathe.