View Full Version : Where to start?
26-11-2012, 11:49 AM
I would like to build a small CNC machine to drill and mill prototype PCBs. A maximum of 360mm x 360mm working area would be generous for what I want to work with.
Occasionally I need to cut out shapes in cases, sometimes mild steel, to take connectors etc. and it would be nice to be able to use the same machine for this.
Being able to place components on PCBs would be nice too, speed is not an issue I don't care about switching tools etc. by hand or selecting components manually. (I do this at the moment with the assistance of a tiny Proxon mill using either a mandrel or bent pin combined with blu-tack :-))
Given that I know very little about CNC machines, but can learn quickly, can someone please point me in the correct direction to get started? What should I be looking at first?
Thanks in advance,
26-11-2012, 02:28 PM
Jon, not saying it can't be done but you are asking a lot from one machine.
Best bet would be to start by drawing up some designs & posting them, that's generally the best way forward with this sort of thing. Once you have a design posted some of the more experienced guys will give you pointers to help improve it if it needs it.
26-11-2012, 04:10 PM
Thanks for the response. To be honest I thought that I was asking too little from one machine given that it's nothing that I can't do with a far less sophisticated manual machine :) Albeit in a ridiculously slow and idiotic way :)
What's the bit that bothers you? Surely I could at least do drilling and milling of PCBs and small panels?
I'll try and draw up plans as you suggest.
26-11-2012, 04:22 PM
Making a machine to make PCBs is not difficult since although the accuracy required is high, the cutting forces are low. However for mild steel you need substantially higher rigidity, which is plausible, but the main problem will be the requirement for a much lower spindle speed for cutting steel - probably 1/10th of the speed you use for engraving the PCBs. That's asking a lot from a single spindle since the power tends to be limited at low speeds, so having two spindles which are mounted such that they are easy to swap is the easiest solution.
26-11-2012, 05:22 PM
Apart from what Jonathan has said the other thing I was thinking is what sort of size are the cases you want to cut panels out of, if you are talking about a flat piece like the removable side of a computer case then there wouldn't be a problem but if you were talking about a box then you may need quite a bit of Z axis travel which may affect rigidity. or another option might be a hight adjustable bed but things like that would have to be sorted out in design stage obviously.
27-11-2012, 10:01 AM
Brilliant information, thank a million, that's exactly what I needed to know.
I was researching the rails and its the spindle that would be the primary issue, that's saved me a lot of hassle :-)
Would VFD or PWM help at all?
Martin, I'd be very happy with just flat panels but of course boxes would be amazing but probably two bridges too far.
27-11-2012, 12:22 PM
Would VFD or PWM help at all?
The spindles controlled by a VFD are induction motors and the VFD aims to get constant torque down over the full speed range. It doesn't quite manage that, particularly at low speeds. The problem is Power=torque*angular velocity, so as you reduce the speed, with torque constant, the overall power available reduces linearly, hence at 1/10th the rated speed you'll have hardly any power left. The same sort of thing is true for other types of motor since fundamentally the torque is proportional to the current in the stator and you can't increase that above the rated current.
One option is to just get a much higher power spindle than you need for just PCBs, then it could have enough power at low speeds for cutting steel. That may work out more expensive than two spindles though...
Another option is to use tools which tolerate being run at a higher rpm than one would normally use in steel, such as these:
X-5070 Blue Nanograin Coated Solid Carbide (Hardened Steel HRc 50-70) - Cutwel Products (http://www.cutweltools.co.uk/home/milling-cutters/x5070-blue-nanograin-coated-solid-carbide-hardened-steel-hrc-5070)
If you set up flood coolant then that will help use a higher speed which might be enough to get into the range for normal carbide tooling.
You could make two spindles with RC motors (https://www.google.com/url?q=http://www.mycncuk.com/forums/diy-machine-building/3096-using-rc-brushless-motors-spindle.html&sa=U&ei=C7C0UIOYBdSQ4gSIzYHYAg&ved=0CAcQFjAA&client=internal-uds-cse&usg=AFQjCNFodOgdUyw1mYAS9ei105wvW-2lwA) as they're relatively cheap and can be made for very high speeds which is ideal for cutting PCBs.
27-11-2012, 01:35 PM
Jonathan, thanks for your continued help and patience. The equation helps a lot, I was thinking of i*v*cos phase angle for power, now I get it.
From a cursory reading of the post to which you have linked I'd say that that probably will be the solution.
I think that someone should sticky any and all threads that you've started :-)
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