Thread: New CNC Mill on the horizon
I have been lurking around on the site for quite some time now reading all the build logs for various sized machines and the advice people have been offering and it has been extremely eye opening and has change my plans of what I have wanted to build several times over I am finally at a stage where I beleiev I have a reasonable budget to not be scrimping on parts to build something half decent and with the advice of others I can only imagine it will get better!
a bit of background.....
...... my previous experience with CNC was with a 3 axis chruchill matrix cnc lathe which was an enormous beast in comparison with what I am intending to build. I want the machine to be capable of machine hardwoods, aluminium, plastics etc at a reasonable DOC, the mill is not for one specific task as it were as I am always up to all sorts so it needs to be big enough to allow for a wide variety of parts to be made but I am limited in space so it really cant be too big.
Current thinking :
Rough dimensions: 120 x 800 table size
Steel box section construction
2.2kw chinese spindle
x\y running on fully supported rails, z on hiwin profile rails
16mm ball screw (twin drive x axis) singles on y and z
Potential for manual adjustable bed height (bolts out move it bolts back in)
At the moment I am working away in solidworks to develop the design a bit further before sticking it up on here but I thought I would air a few ideas first incase I need to make some changes straight away.
My main issue at the moment is to do with the fully supported rails vs the hiwin style bearings. I know this is always the case and a lot of the deciding factor can be down to cost which is not the only reason for asking this as if I can find a way to use them I will do.
First off making a machine of this size, what size rail would I need in terms of the hiwin rails bearing in mind the main gantry will be a large piece of steel box section at the moment.
I know some peoples opinions of the open ended bearings are that they are terrible, other than using hiwin is there anyway to make them any better? using more carraiges under the gantry 3 per side? 2 above 2 below connected by a plate on the x axis as you would do on the y axis so the openings are opposed?
I would like this machine to be pretty accurate if I am going to the effort of investing time and £££ into it and am I going to be utterly dissapointed with these linear bearings over the profile rails.
Thanks for reading and I hope this thread will come togther to form an interesting build once I get going soon.
Since you want to machine 'aluminium at a reasonable DOC' then an adjustable height bed, or very small Z-travel is critical. Clearly a small Z-travel is quite limiting for machining non-metals, so the adjustable height bed is preferable.
You should be able to get the profile rails on eBay for a good price - certainly a lot less than new. 15mm rails will be good compared to any size supported round rail since it is the pre-load that makes the profile rails more rigid. If you find bigger (20-25mm) rails then that's good, but it's not worth spending a lot more on them. Since you seem to have waited 5 years to post I guess waiting a bit longer for the right price isn't a problem!
You could try 4 rails on X as you suggested, I considered it for my machine. The problem with the supported round rails is that the bearing blocks are not preloaded, so even though load ratings are plenty high enough adding more bearings doesn't magically pre-load them so it will still be worse.
Thanks for the response, yes it was a typo 1200 is correct. I am not sure on the z travel at the moment I am looking to keep it reasonably short this was the reason for the adjustable bed. What would you consider a reasonable z travel based on machining aluminium?
So I could go as small as 15mm ok that helps I was expecting to need 20mm dont ask me why just thought that would be more the size.
I see you use those bearings on your machine, by saying they will still be worse than the profile rails what exactly am I likely to notice between using the two? (thats not me posing that questions as if im not expecting the profile to be better because I am just wanted to know what key differences in the machined end product)
If you have been lurking long enough you will have probably seen my dislike of round rail/bearings and complete louthing of unsupported rail.!! . . IMO really is no comparison between round rail and profile rail.! Profile rail makes the difference between DIY machine and Pro machine that will out last the machine and out perform round rail by magnitude of 10X.
To me if your serious enough to invest couple of grand and several months of hard graft then the extra cost is just not worth skimping on and like Jonathan says if you've waited 5yrs then don't spoil the broth for a pinch of salt and wait untill you can afford them.
Regards size then 15mm will more than handle the load but I find 20mm is the better option due to bearing size being slightly larger offering better support and easier mounting. 25mm is just over kill unless really large/heavy machine and waste of money plus has well has being inefficeint and stressing the motors more.
To be fair I expected that comment and yes I do agree that profile is the way to go just got to find a good supplier!
When I made my machine a few years ago I bought profile rails from this place on ebay. For example:
They are second hand parts, so you take a chance, but I ordered a pair at 700mm (Y) and a pair at 300mm (Z). 700mm was about £140 for pair of rails and 4 blocks, and 300mm was around £80 for pair of rails and 4 blocks. Vaguely remember paying import tax on one of these orders. They had a very smooth action and work very well on my machine although I only really cut plywood, liteply, and balsa.
Routercnc, thanks for the link I will keep an eye out for something that fits the build. Did you find them to be a good seller? How long did they take to arrive?
After sleeping on it I am 100% behind profile rails so am going to hunt some down.
I will get the design outlay up over the next few days and let the critics at it!
Last edited by fvfdrums; 19-05-2012 at 09:36 AM.
Many moons ago I bought a class 1 ballscrew from the same guy routercnc suggested and was very happy with it, still are.! Thou it did need a strip down and clean to be fair so be prepared that you may have to get slightly dirty to get best from a used components.
Still it's a good cheaper way to have profiled rails and worth the effort and risk. Even if there's a bit of play or slop then it's not difficult to replace balls in both bearings or ballscrews so they can be brought back to life or certainly far better operation.
If I remember correct they didn't take long to come roughly about 2-3wks and was packaged ok.
Thanks for that Jazz,
I was looking at the prices on there and to be honest by the time I have added up the cost of shipping the lengths I am looking at and added the chance of VAT on top its looks like I could buy the same from Zapp for about £25 more but new (thats based on what they have on their eBay store at the moment) probably better to go new I would have thought.
I am assuming 4 blocks on the z axis is a must for a machine like this? This was the original plan but i thought i better check.
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