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  1. #21
    Quote Originally Posted by JAZZCNC View Post

    Regards that Pic you posted I'm pritty I've seen that before and it was actually built to cut stone.?? . . . . But don't quote on that!!. . . . But I've certainly seen one built the same that was cutting stone and it actually came up for sale on Ebay.
    Jazz,
    I found this on cnczone and reading his posts i think you are correct and it was used for cutting stone, he was in Indonesia. If i chose the earlier option of 2 box sections sandwiched between plate, would it be ok to use 10mm plate and use 20mm on the Z axis only. Just want to weight up all the options at the moment.

    Just out of interest, i'm looking at using 1605 and 2005 ball screws, belts and pulleys if possible. I had a go at the weekend cutting the infamous Aztec/Mayan calendar, approx 12" size, after 3 hours i had cut less than a 1/5th. How much faster roughly would using ball screws make a difference.
    Last edited by deannos; 26-01-2012 at 03:35 PM.

  2. #22
    20mm is a good size plate for the Z-axis.

    The efficiency of ballscrews is roughly twice that of ACME screws, so it will be significantly faster. For the Aztec calender acceleration is much more important as it's lots of tiny moves. The bearing nut thingy you're using will make the difference slightly less. The main advantage is 5mm pitch versus 2mm pitch - the kinetic energy in the screw is significantly less if it's rotating 2.5 times slower.
    The most accurate comparison you're going to get is comparing with my machine, as I sold you the screw. I got 6m/min with the screw your using and 42:13 pulleys on Y. I ran it at less than that with smaller pulleys as that was too close to the critical speed of the screw (see my build log for actual values). With 42:14 (I think) and a 10mm pitch ballscrew I got 60m/min. That's much more than you'll ever need so I've left it on close to 1:1 to get better resolution and acceleration.

  3. #23
    Quote Originally Posted by Jonathan View Post
    20mm is a good size plate for the Z-axis.

    The efficiency of ballscrews is roughly twice that of ACME screws, so it will be significantly faster. For the Aztec calender acceleration is much more important as it's lots of tiny moves. The bearing nut thingy you're using will make the difference slightly less. The main advantage is 5mm pitch versus 2mm pitch - the kinetic energy in the screw is significantly less if it's rotating 2.5 times slower.
    The most accurate comparison you're going to get is comparing with my machine, as I sold you the screw. I got 6m/min with the screw your using and 42:13 pulleys on Y. I ran it at less than that with smaller pulleys as that was too close to the critical speed of the screw (see my build log for actual values). With 42:14 (I think) and a 10mm pitch ballscrew I got 60m/min. That's much more than you'll ever need so I've left it on close to 1:1 to get better resolution and acceleration.
    Ok thanks Jonathan,

    What is your thinking on using 10mm plate on the Y gantry, or would it be better using 20mm because of the length

  4. #24
    Quote Originally Posted by routercnc View Post
    Hi Deanos,

    I've compared the following 1500mm long steel gantry shapes for tool deflection due to gantry twisting (when cutting wood in the X direction):
    _____________________I beam__RHS____RHS
    Width _______________100_____100_____100
    Depth _______________180_____180_____180
    Flange/wall thickness ___6________6_______3
    Mass (kg) ____________26______38______19

    Deflection (mm)______0.021___0.00037__0.00068

    Clearly RHS is better, but I beam is also 'useable' for cutting wood. In the end it is about what is sufficient for your needs and what you can comfortably make.
    Ok didn't see this or Jonathans last post so this apply's to both post's.

    First I'm not knocking the Calculations or saying they are wrong but your both missing the POINT. . . .!! IT WILL DO THE JOB AND DO IT GOOD.!! . . . IT'S CHEAP AND EASY.!!

    It's easy to build with absolutly minimal machining or drilling, far less Jonathan than my first design which yes is stronger but much more expensive.

    Jonathan good design is about building the best machine to do the intended use accurately and effiecently. A good designer will always design a machine to do the required job with a certain safety percentage designed in and no more. Any more is just wasted expense and ineffiecent.

    Working On your design criteria then you FAILED because you could have used far heaver gauge box section or steel plate rather than Aluminium or why not go the whole hog and Cast some Iron castings.!!. . You had the knowledge,equipment and abilty so why not.? . . . . . Because it didn't warrant the extra cost and effort to accumplish what you intended to do with the machine.

    I've just checked price with my supplier and I can get 1x1800 & 2x1000mm 178x102 RSJ 8mm steel for 75 no vat cut to length delivered.
    That will do both the gantry and the X axis. Or Just the gantry would be 37.!!

    So Profile/plate way:
    Ali plate 4mtr x 150 x 9.5mm = 86inc vat + 10 delivery = 96 (400mm left over)
    Profile (as per my other design)1800 x 90x45H = 41.50 x 2= 83 + 10 delivery = 93 . . .or 1800x45x45 (As dean's design) =43 +10 del = 53 . . .OR. .
    80x40x3 steel box section 39
    Plus lots more Nuts and bolts so another 10

    Total 199 OR 45x45 159 OR Steelbox 145

    OR

    RSJ way:
    RSJ 1800x178x102 37
    Nuts bolts 5 (far less required)

    Total 42

    Also a lot less drilling work which will also add to the cost because of worn/snapped drills.!


    Now taking the RHS route and regards the Calc's and price stand point theres also the issue that like for like dimensions where used. Now 180x100x6mm RHS is not a common size and the nearist would be 160x80x6mm Which is 160 so no saving there and it won't be as easy to work with as RSJ.

    Also the calculated strength of the I beam is going to go up because of 8mm thickness not 6mm and the RSH will drop slightly due to lesser dimensions, it will still be stronger but the difference is less but more the point the Ibeam which is already more than suitable will increase.!

    This is my POINT.!!
    Last edited by JAZZCNC; 26-01-2012 at 04:26 PM.

  5. The Following User Says Thank You to JAZZCNC For This Useful Post:


  6. #25
    Dean yes 6" x 3/8" (150x9.5mm) plate would do fine and the Cad models I drew used them dimensions except for gantry sides and bearing plates which where 19mm plate and I'd recommend you use.

  7. #26
    Hi Jazzcnc,

    I totally agree with you that I beam would be sufficient for cutting wood. I just wanted to show Deanos the improved performance of RHS to explain Jonathan's point about how it could be better if you wanted to make it do more in the future.

    I also can't fault your logic and argument on the cost side because that is very important for a home build.

    Just for completeness, and not to start a war (!) I've done the calcs for the sizes you mention:

    ___________________I beam____RHS
    Width _______________100______80
    Depth _______________180_____160
    Flange/wall thickness ___8_______6
    Mass (kg) ____________34______32

    Tool deflection (mm)______0.009___0.00067

    [all assuming 150mm between gantry centre and tool cutting tip - the moment arm trying to twist the gantry]

    So the I beam still looks reasonable.
    Last edited by routercnc; 26-01-2012 at 09:38 PM. Reason: format
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  8. The Following User Says Thank You to routercnc For This Useful Post:


  9. #27
    Quote Originally Posted by routercnc View Post
    Hi Jazzcnc,

    Just for completeness, and not to start a war (!) I've done the calcs for the sizes you mention:
    " For to win one hundred victories in one hundred battles is not the acme of skill. To subdue the enemy without fighting is the acme of skill." (Sun Tzu)

    It's Aussie day so no worries Mate.






  10. #28
    Looks like bloody good odds to me especially from such a simple and quick build
    If the nagging gets really bad......Get a bigger shed:naughty:

  11. #29
    Quote Originally Posted by 2e0poz View Post
    Looks like bloody good odds to me especially from such a simple and quick build
    I agree, that's why i've decided to go the RSJ route, especially as i don't have any fancy tools. But i can weld and this will be far the easiest route for me, and there won't be a lot of drilling and tapping either. The only thing that worries me slightly is the weight of the gantry, i know weight is a good thing, but would this be to much to get decent speeds. Now i'm not looking to break any speed records, just to get a reasonable feed etc. The motors i have at the moment are nema23-3NM and MSD542 drivers with 50v power supply.

    Dean

  12. #30
    Quote Originally Posted by JAZZCNC View Post
    " For to win one hundred victories in one hundred battles is not the acme of skill. To subdue the enemy without fighting is the acme of skill." (Sun Tzu)

    It's Aussie day so no worries Mate.





    I count myself part Aussie, having done nearly all my schooling there, :tup:

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