Open source group project, anybody interested?

[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

[Wotsit]

Deleted by wotsit to keep the thread on track

[Wotsit]

Deleted by wotsit to keep the thread on track

[Wotsit]

Deleted by wotsit to keep the thread on track

[Ross77]

Im sorry but I didnt read all that because it is a subject in its self that I'm not interested in it and it sounded like salesmen speak.....:whistling:

My initial thougths were a cheap workable solution to DIY cnc. Stuctural epoxy in the Uk is expensive and as far as I can tell has a poor modulas of elasticity( it still bends) and dosnt solve the high density required to reduce vibration. (Im talking about the machine frame and not the bed)

Yes it has excellent sticking qualities but so dose a better known natural product:heehee:

Srinkage of concrete is a problem but is controllable and the comments on strength laughable. Yes concrete dose get stronger with age but its characteristic strenght is at 28 days. after that it get stronger.

The shrinkage is due to its water content, but I propose to use minimal water and lots plasitiser and as the concrete is only used for the compression then any skrinkage can be zeroed out with shims ect.

For a stucture to work you have to undersand all the forces and all the reactions......................................... .............................................and if you want cheap you have to make compromises:heehee:


As a final note. When I started using my lathe I soon realised that the quality of the finish was proportional to the type(quality ,shape) of tool and speed of the spindle

So dose this mean we should be focusing our efforts on the spindle and operators capabitiy?

[Ross77]

Not to put you under any pressure Gary but would you concider a group order? may be for X#linear rails and X# ballscrews? What would the min quantiy be?

Just thinking the option of a cheap ish reliable cnc machine would ulltimatly require the support of a willing suppller?????

[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?

[Tom]

Irving, that's a most useful post - I'm going to bookmark those calculations for future use..., thankyou.

I recon Irving's hit the nail on the head with

The question I suppose is...

The meaning of mid-level could be very different depending on who you're talking to. Although both of these chaps has built/converted a machine to run with EMC2, you'd probably get a different answer about "mid-level" from each... :)

[ame="http://www.youtube.com/watch?v=LA5qCIB0j6k"]YouTube- Fresa CNC MDF[/ame]
[ame="http://www.youtube.com/watch?v=mxxdq6y8z8M"]YouTube- EMC2 5 axis cinci at MPM[/ame]

So how about a list of variables, so that we can discuss pros and cons?

(low-spec -> high-spec):
* Working size (min A4 -> max 4ft x 8ft flat sheet)(Z depth?)
* Linear bearings (unsupported rails -> skate bearings -> v bearings -> supported round rails -> recirculating ball carriages)
* Drive method (chain/allthread -> belt/trap screw -> ballscrew)
* Target cutting speed vs. target resolution (trade-off)
* Rigidity & strength (just cut alu, or rip through it in a breeze?)
* Spindle type and power (dremel -> 43mm router -> handheld router -> VFD spindle)

Probably loads i've missed, so copy paste and add/change bits...

On my wishlist would be the ability to add a 4th axis (parallel with x) at some future time, without hacking the machine to bits. Designed in expansion capability if you will...

[Wotsit]

So how about a list of variables, so that we can discuss pros and cons?

(low-spec -> high-spec):
* Working size (min A4 -> max 4ft x 8ft flat sheet)(Z depth?)

For me 2 x 4 is sutable

* Linear bearings (unsupported rails -> skate bearings -> v bearings -> supported round rails -> recirculating ball carriages)

Supported round rails or carriages

* Drive method (chain/allthread -> belt/trap screw -> ballscrew))

Ball screw

* Target cutting speed vs. target resolution (trade-off)
* Rigidity & strength (just cut alu, or rip through it in a breeze?)
* Spindle type and power (dremel -> 43mm router -> handheld router -> VFD spindle)))

Cut alu at a resonable feed rate, spindle down to end user but designed for VFD

Probably loads i've missed, so copy paste and add/change bits...)))

How about bed construction?

On my wishlist would be the ability to add a 4th axis (parallel with x) at some future time, without hacking the machine to bits. Designed in expansion capability if you will......)))

This is important to me