At the moment my new machine is in a very early design stage: it was quite far advanced until I started asking questions on this forum and found that many of the things I was going to do wouldn't work, so at the moment even the basic architecture is in the air.
The eventual aim is to make a milling machine to make very small parts in anything from machinable wax to steel. The main requirements are accuracy, accuracy and more accuracy. Something slightly bigger than my existing Proxxon MF70 would be nice - a Y axis limited to 40mm is a bit of a drag, but 200mm by 200mm with enough space in the Z axis to allow 4th and even 5th axis work (eventually) would cover everything I want to make.
I am impatiently waiting for some linear guides that I bough on ebay to arrive so I can start serious design work - the manufacturer's site does not have datasheets for the ones I bought. Meanwhile I have attached some JPEGs, A formation of components done in Turbocad - mostly not applicable, I am (probably) not using round rail, the toothed pulley data was taken from an RS datasheet which was just plain wrong. The second photo is of a 218W brushless DC motor from Zapp, along with a controller that I built - PCB made on the Proxxon. Since that photo I have had the motor drive a spindle at 55,000RPM.
Last edited by Lee Roberts; 03-05-2010 at 07:43 PM. Reason: Removed the large images from the post
Start off with a large chequebook, a large bed plate about 3 tonnes to mount it on and build a temperature controlled workshop.
Forget steppers with their inefficient micro stepping and go to servos to start with.
.John S -
I think the point that both Robin and John are making in their own inimitable ways is that accuracy = rigidity and rigidity = mass, and damped mass at that. You haven't said what accuracy you want/expect... a few spec's would be a good start. It is quite feasible to make a reasonably accurate router/gantry style machine that can do 0.05mm or better accuracy (NB accuracy is NOT the same as resolution) in engineering plastics and aluminium... especially if the depth of cut (DOC) is small (.1- .2mm) It is another thing entirely (by a factor of 3 or more) to do the same thing in free-cutting mild steel, and another jump up in stainless steel...
O.K., I will rephrase. Accuracy, accuracy, accuracy within limits, within budget and within practicability. My target is 0.001" throughout the range, but I know that this is hard. Hopefully the same will be achievable on a small part. As far as rigidity goes, I am the fortunate owner of an old (1960s)sensitive drill that weighs about 300lbs and teaches you that a big lump will always beat stylish when it comes to accuracy.
For rigidity I am hoping to make the frame and non-moving gantry in 15mm steel plate but may drop back to 20mm aluminium with some form of temperature control - possibly peltier heater/cooler elements in the vertical and cross members.
Be careful as high accuracy will come with a high price tag, what are you making that needs to be manufactured to that degree of accuracy???
Not so sure about your cooling/heating idea for the aluminium frame, aluminium will expand at twice the amount of steel but for a machine as small as you are describing then not so sure it would be necessary.
For instance, a 100mm length of steel will increase or decrease in length by 0.00126mm for every degree C of temperature change so a temperature change of say 20 degrees C will change the length by 0.0252mm, aluminium will be approx twice this with a change of 0.048mm.
Buy yourself a good heater for the winter and open the doors in the summer then you should not see much difference!
Why do I need accuracy? I am afraid the best answer I can give is, in the words of Sir Edmund Hillary, "because it's there". In constructional hobbies, and this is my hobby, most people strive for some principle in what they make: elegance, simplicity, beauty etc.. For me, the principle is accuracy.
As far as expence goes, what is a reasonable price to pay for a CNC milling machine? If you regard Arc Euro Trade's KX1 at £2350 as reasonable, then I think that I can build something quite special for less than that - especially as I have controllers, stepper motors etc.
What will I make with it? Clocks, small but accurate tools, a co-axial brushless DC motor for a model aircraft, wax models for investment casting, you name it. How long will it take? A couple of months if I work on it all of my spare time, a year ot two if I only spend the time "She Who Must Be Obeyed" allows me, but probably somewhere in between. Will it be worth it? If you are in business the answer must be "Hell no!" but if you are a hobbiest then the answer is "Damn yes!"
Regarding the idea of using Peltier temperature control. After the normal suspects, backlash, vibration etc., thermal differences in the workpiece and machine contribute a lot to errors. Controlling the temperature of bits that get hot such as the spindle and table, and bits that are long such as the upright bits of the gantry with Peltier devices is cheap - about £5 for a 90W device and somewhat less than that for the bits to control it, a few pounds for heatsinks and tubing and perhaps £20 for one of the water cooled radiators sold to computer modders. Including fans and concertina hose to dump the stepper and spindle motor heat outside the case I think there should be some change from a couple of hundred pounds.
Perhaps confidence is that feeling you get just before you understand the problem? :whistling:
I don't think you have to worry about heat quite yet. I suspect you are losing the plot and zooming off on a tangent.
You need a lot of iron and the cheapest way to get it is an older machine that you tart up. Scrape the slides, add ball screws etc.
It may seem a good idea to buy a shiny new wonder mill aimed at the CNC hobby market, but an older industrial or (better) ex-school machine is probably a much better bet, not so pretty but considerably cheaper and with more possibility for improvement.
I suggest you aim for 5 micron accuracy, the resolution of your everyday digi-caliper. You will then get 25 microns fairly easily. You will then realise 25um it isn't enough.
If you shopped with 5um in mind you don't wind up replacing the whole works like I had to.
Well good luck, since its for hobby use then you will probably find that you are continually tweaking and changing things anyway
If you are worried about thermal effects then you will definitely need the consider dynamics as well, beam/column frequency and moving masses etc. just adding more mass can lead to resonance not to mention problems with accel/decel.
As with the thermal effects I suspect that the small size wont be to much of an issue but might be some thing to look at on your pursuit of perfection.....
Have you ordered the C1 precision ground ball screws yet?
I just don't think it's possible using commercial bearings, there simply isn't enough contact area to take the load.
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