Open source group project, anybody interested?

[Wotsit]

Are any of the forum members interested in designing an open source cnc project?

I have some initial ideas for design criteria:

The project should be for owner / users of a basic mdf machine wanting to upgrade to a mid level machine

The build cost should be less than £??? (not including steppers or drivers) and as little as possible

The machine should be capable of a tolerance of +/- 0.MM

The final design should be available as GCode DXF and PDF and open source
:feedback:

[Tom]

As someone nearing the end of my first build - that's a good idea.

What's the aim though? An MDF machine more solidly constructed, or something more fancy (alu sections, etc)? For £200 it's probably either sticking to MDF, or possibly alu with leadscrews rather than ballscrews? But alu would need decent rails to get any benefit - is that achievable with £200?
What are you thinking?

Anyway, I'm happy to contribute my learnings so far...
1) Leadscrew whip is bad.
2) My next router will be more rigid (although the secret aluminium skinning stage for this router isn't complete yet).
3) Ballscrews would be great, but would also be ruined (and pointless) on an MDF machine.
4) Cutting alu takes a properly sorted machine.

[aventgps]

this is my arduino mega controlled cnc machine....







and a video of my machine working...

[ame="http://www.youtube.com/watch?v=ksq8hsMoPJ8"]YouTube- CNC TESTING GIRL using arduino mega[/ame]

it is all open source........ if you want i can post codes and stuff

[Wotsit]

Ok, I think i think the budget should be left open but lets try to keep costs down.

What constitutes a mid level machine? I'm thinking ball screws, linear bearings a rigid frame and sold / flat bed.

What are our material options considering it will need to be cut with a home brewed MDF machine?

[Wotsit]

Might also be an idea to choose a supplier that people are happy with so we can use off the shelf parts that are available in the UK, they might even help us along the way.

How about ZAPP automation? (I have no affiliation with this company)

[Ross77]

Im on board.

If this is mid level then its a good idea to go with linear rails or at least supported round rail. Ive not had a good experience with leadscrews and delrin but many have, and espesially if the rest of the machine is well designed. :whistling: might be a way to compromies on the cost......

My machining skills are limited and cnc knowledge is growing but I do understand structures and forces so could help here if required.

I'm currently looking at a composite beam design that uses mateials to their best and combine to form an effecient structure.

As I understand it Cast iron is the choice of a professional machine as it is rigid, strong and dense enought to damp vibration. and it is the small movements (deflection)from the vibration that cause poor surface finish or in worst cases tool breakage.

But the high density has the problem of being heavy so requires large motors to move. I propose to use an aluminum skin as a former, and for asthetic reasons, but filled with concrete and steel of the correct proportions and place to make a cheap beam that is much stonger than ali extrusions, able to provide moderate damping but not be to heavy.

I know this sounds similar to the epoxy granite idea but will be much cheaper and hopfully stronger. Its still only an idea and I havent done any calcs yet but the principle has been used in buildings for many years and yeilds a 50% saving in materials for the same strength.

Sorry if this is off topic but seems a logical approch to an affordable mid range machine.

Any one have any data on cutting forces? I seem to remember 20N for Ali for some reason :whistling:Probably wrong tho.

Ok wake up. I've finnished :heehee:
Later

[Wotsit]

I like the epoxy granite idea and have followed the thread for a long time but it seems to me a bit to involved for a mid level machine, and has some complicated problems. Do you agree? - Would make a great bed though.

I think the composite idea is a good one, alu box filled and reinforced? Sand in polyester resin / concrete with rebar. Thoughts? Calcs? I wonder what 5kg of resin's exothermic reaction is like?

I agree that linear rails or supported round rail are the minimum for a project of this type

What is a good enevlope? For me it would be 2' x 4' ish

[irving2008]

...
Any one have any data on cutting forces? I seem to remember 20N for Ali for some reason :whistling:Probably wrong tho.
....

Are we talking high-speed machining or conventional cutting? With high-speed machining you need special cutters and high feed rates if you dont want them to burn out... If we are designing this machine to be 'aluminium capable' we need to work that through and be realistic about our goals. Here is an example working as a starter for 10... dont take it as gospel, there are people who will say " I can cut a 5mm deep 10mm wide slot with my 30k rpm spindle on an MDF machine" maybe they are right, maybe not... but I go by my own experience...

The cutting rate for ali is ~100m/min. With a cutter of 10mm diameter you need a spindle speed of 320 * 100 / 10 = 3200rpm. (the 320 is a contant) (10mm assumes a 1/2" router based spindle)

Aluminium needs about 0.3mm/rev feed rate per tooth. If we assume a 2-flute cutter (1 flutes are hard to find) then we need a feed rate of .3 * 2 * 3200 mm/min = ~1800mm/min. With a 5mm pitch lead screw thats 1800/5 = 360rpm, 6rps = 1200 step/sec, thats going to need top end drivers... or we'll need to gear up 2:1 or 3:1. 1mm or 2mm pitch trapezoidal is probably not going to work. TR12x3 might just.

Aluminium is 17W per cc/min removal rate. So taking a 2mm deep cut 5mm wide with a 10mm cutter at 1800mm/min = 0.2 * 0.5 * 180 * 17 = 306W, say 600W input on the spindle.

Power = torque * revs -> torque = power/revs = 306/3200 = ~0.1Nm, which at a 5mm radius = 20N cutting force... (and thats where the number came from Ross). Remeber the torque must be available at that spindle speed, for many variable speed routers the torque falls off quickly as the speed is reduced.

Dont want this to be a blocker... just so people are aware of some of the design issues and recognise the limitations.. There are some good examples of router-style machines cutting ali.. they are all capable of high traverse rates and have adjustable spindle speeds. A high spindle speed isnt always a good thing, it can burn out cutters if the feed rates are too low... rubbing isnt cutting!

The question I suppose is... do you want the ability to do the occasional bit of small ali work or a machine truely capable of doing it as a matter of course?

[CraftyGeek]

I'm interested in this as an idea. I'm still refining my first mdf machine to get it to a point that I'm happy to start playing with it to cut 'proper' projects on. I can already see that I would like to use this one to build another machine in the future.

I think an important point here has to be that it uses easily available materials that are still cheap, without using any complex construction processes - I'm sure that epoxy granite is great...not sure how many people would actually make that though.

I think that a combination of mdf & aluminium angle/box/U-channel can be assembled as an I-beam structure that would create a suitably strong & rigid framework.

I also think that it is a good idea to have a ballpark materials costing in mind - £200-250 is a good figure I think. The cutting area, I feel should be no smaller than 2'x4' - & it would possibly be a good idea to make it scalable/adjustable to a point to suit different people.

I'll be watching this thread to see where it goes...

[tribbles]

If you need PCBs designing, I can do that.