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  1. #11
    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?

  2. #12
    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?????

  3. Not a problem, you just need to tell me what and the quantity.

    Quote Originally Posted by Ross77 View Post
    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?????

  4. 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...

  5. Quote Originally Posted by Ross77 View Post
    ...
    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?

  6. #16
    Tom's Avatar
    Location unknown. Last Activity: 30-11-2016 Has been a member for 7-8 years. Has a total post count of 172. Referred 1 members to the community.
    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...

  7. #17
    If you need PCBs designing, I can do that.

  8. Quote Originally Posted by Tom View Post
    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

    Quote Originally Posted by Tom View Post
    * Linear bearings (unsupported rails -> skate bearings -> v bearings -> supported round rails -> recirculating ball carriages)
    Supported round rails or carriages
    Quote Originally Posted by Tom View Post
    * Drive method (chain/allthread -> belt/trap screw -> ballscrew))
    Ball screw
    Quote Originally Posted by Tom View Post
    * 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
    Quote Originally Posted by Tom View Post
    Probably loads i've missed, so copy paste and add/change bits...)))
    How about bed construction?

    Quote Originally Posted by Tom View Post
    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

  9. #19
    Plenty of interest then.

    Defining mid range could be a problem. linear rails, ballscrews and vfd spindle seems quite high spec to me....

    theres enough flexibility for people to use what ever they need to suit their requirements. Although I dont think it wouldnt be very economical to mount a dremel on a machine designed to take a heavy vfd spindle?

    Thanks Irving. thats perfect, knew I'd seen that somewhere. Is that the rotational force of the motor or the force required to push the axis?

    If high speed is needed then would it be better to use belt drive? or would that cause problems with higher powered motors?

    For the drivers has any one else seen the "CPLD tutorial" on cnc zone? from memory :whistling:

    80V @ 7A with 5 micro steps and works out at 10 to 15 an axis. The only drawback is the chip is a tiny SMD so not very diy friendly. It was proposed to mount a preprogramed chip on a separate daugther board that could plug in to a 40 pin wide DIL and be sold cheaply so people could then build their own but it was never realised. any takers?

  10. Quote Originally Posted by Ross77 View Post
    Plenty of interest then.

    ....

    Thanks Irving. thats perfect, knew I'd seen that somewhere. Is that the rotational force of the motor or the force required to push the axis?
    No thats the effective linear force pushing back on the cutter. For deflection calculation purposes assume its both perpendicular and parallel to the direction of cut . For leadscrew/stepper torque calcs assume it adds to the load opposing the motion...

    Also note in the previous calcs, a screw rpm of 360 means a minmum of a 12mm dia leadscrew for a 1m screw supported drive end only, or 16mm for a 1.5m screw assuming supported both ends (class B support). For bigger 2m screws you're into 20mm+ ballscrew class C support territory and big bucks.

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