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  1. #31
    Cheers for the help. I suppose it's time to lock down the design and start adding all the fixing points and decide how i'm going to run the timing belt for the X axis.
    Thanks again,
    Charlie

  2. #32
    I would use one belt only, as its easier to align properly. Make sure the idler pulleys not to bend. I would encase them so every steel pin is supported at both sides. Makes one of the idles a tensioner. Find the Gates belts site and there were calculations how to tension the belt using a guitar tuner if you dont have proper device to measure the tension.
    Mistakes here will affect the performance, especially the precision. Belt must be tensioned properly, not by feel.
    Last edited by Boyan Silyavski; 14-08-2014 at 11:42 PM.

  3. #33
    I don't care what the spread sheet says or the calculations it will be Mistake to go with the unconventional Z axis you will be creating a tuning fork and this will show in the finished cut guaranteed.!!

    Regards the belts then problem with one long belt arranged like this is you can't get enough tension on it with out applying pull on the screws and it becomes unwieldy belt length because you have to make sure the motor pulley as enough teeth engaged and this often means longer belt.
    So I'd be more inclined to have 2 short belts going from screws to rotating idler shafts so can get nice tension on screws then longer belt to same shafts on another set of pulleys that you can get good tension on the belt using motor as tensioner. (like doodle in pic) More expense and time but will be better than one long belt.
    Click image for larger version. 

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    To be honest it's far easier and stronger for machine to run a brace across back then just one belt can be used. In practice you'll more than likely find you never put any thing on bed that it gets in way off and you can still pass long boards thru.
    It would also mean you can lift the ballscrews back up closer to gantry which will stiffen it up slightly and make easier to build.

  4. #34
    I was thinking of going that way for the belts but it just seemed a bit unwieldy. Can you tell me why you think it will act like a tuning fork, have you made one like it before? Is it because I am using steel plate or is there another issue going on that i cant see?

    One thing worth noting, at work we grind thin walled exotic alloys like inconel and waspaloy and they really do resonate whilst machining, we get around this by using a glorified elastic band and it reduces the vibration drastically which got me thinking about the rubber floor mats you can get for playgrounds. If I were to use strips of that glued to the metal plate in stategic positions then that should help with dampening if that is the issue, What do you think?
    Cheers
    Last edited by CharlieRam; 15-08-2014 at 12:14 PM.

  5. #35
    Quote Originally Posted by CharlieRam View Post
    Can you tell me why you think it will act like a tuning fork, have you made one like it before? Is it because I am using steel plate or is there another issue going on that i cant see?

    One thing worth noting, at work we grind thin walled exotic alloys like inconel and waspaloy and they really do resonate whilst machining,
    I have made many Z axis but only ounce did I build one this way and that was due existing design issues(not my design), it was for a machine which exclusively cut HDPE so no big deal but the difference between my normal conventional way was clear when pushed hard.

    The differnece between this and your work example is that the work machine didn't vibrate the material did which happens now imagine what would happen to finish if the tool was vibrating as well.! . . How do you stop or deal with that.? . . . . . Cut slower/shallower point being your making compromises you shouldn't have too just becasue of machine design

  6. #36
    Maybe i am being thick but i cant see how flipping the rails will introduce vibration, looking at the conventional method then the spindle is mounted away/lower from the bearing blocks whereas my design is a constant length of zero from the base of the spindle clamp to the bottom of the bearing block. Is it the spindle clamp causing the issue or is the spindle too low in the clamp? How low can the spindle be in its holder?

    Also think i may use a plate across the back of the machine like you suggest, I suppose one open end is enough if I need any overhang
    Cheers
    Last edited by CharlieRam; 15-08-2014 at 01:17 PM.

  7. #37
    The compromise i am making is due to lack of space for the M/C vs. Large enough work area. With this design the total width is about 900mm with the other it was about 1100mm. I havnt even got anywhere to put the machine yet, I have an area of 2m x 5m behind my log cabin which i plan to build a soundproof ish shed with concrete base for the machine, it will then run lengthways leaving me roughly 1m workspace to pass the machine, the other design will only give me about 800mm which is just too small.
    TBH i am building this machine to prove i can and to give me something to do. It may well never even nachine ally but then again i might dream of all sorts of wonderful things to make. Who knows? :-)

  8. #38
    Quote Originally Posted by CharlieRam View Post
    Maybe i am being thick but i cant see how flipping the rails will introduce vibration, looking at the conventional method then the spindle is mounted away/lower from the bearing blocks whereas my design is a constant length of zero from the base of the spindle clamp to the bottom of the bearing block. Is it the spindle clamp causing the issue or is the spindle too low in the clamp? How low can the spindle be in its holder?
    Cheers

    Whatever you do if you dont:

    -use long size 20 bearing blocks
    -space the bearings that the front plate slides on at least 260mm from outside ends/each long block is 90mm long/
    -fix the rails on the front plate back

    you will finish with a machine incapable of working Aluminum and maybe not so precise on hard woods.

    There not so much to understand, the rails +the bars they rest form ribs that prevent long axis bend. That simple.

    Front plate must be 12mm steel backed with steel bars below the rails, or at least or 20mm aluminum backed with aluminum bars. Using 2 spindle brackets would make things even better as the spindle will add stiffness.

    Linear rails are cheap, so that will add very small amount to the build. See 200mm travel axis made same way as we speak that reaps/i can not help say it / fully extended , through aluminum without any chatter . here post #109 and how looks finished from 12mm steel plate a bit up at the same page

  9. #39
    Hi Silyavski, the rails and bearings are 20mm profiled from chai, they are 75mm long plus 15mm for grease nipple which I haven't added to my sketchup model yet. I originally spaced the bearings 250mm apart from outer dimensions but was told 200mm was fine, this then gave me 150mm depth of cut instead of the original 100mm I planned so I have no issue going back to 250mm spacing if it will help.

    The front Z plate is 20mm thick steel with 15mm X 50mm steel spacers glued and bolted to the plate and then the carriages are bolted onto this, the way I see it is the spacers should give me the added rigidity, would it help if they were surface hardened to make them stiffer because that is the only difference I see between having the rails mounted to the Z plate.

    I am just finding it hard to understand why flipping the rails will introduce vibration as dean stated and I can't understand how the plate can bend as it has the bearing blocks on each corner. I am planning on having two spindle clamps as you suggested, in fact, it was there on earlier iterations, I must of deleted it!

    Jazz, yes it is the workpeice that resonates in my example but the rubber absorbs a lot of it and I am thinking if I tensioned the rubber over the front of the Z plate it would absorb some of the resonance created in the steel Z plate if that is indeed the issue (steel instead of ally?).

    Thanks, Charlie
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  10. #40
    Given the short length of your desired travel it will work ok. Bus as you have drawn it, the bit should touch the finished machine bed. Cause lower it will not go obviously. Is it so?

    The case is if say you want it to travel 200mm for example or longer , then the 4 blocks on the Z will go higher than the gantry, hence the tuning fork.

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