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.
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.
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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.!!)
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.!!
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.
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)
Last edited by JoeHarris; 26-04-2012 at 09:40 PM.
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.
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.!
I have used miss my car and got good service. Linearmotionbearings is good to.The more I know, I know, I know the less. (John Owen)
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!