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  1. #1
    I'm trying to design a Z-axis for the CNC router I'm building to have 400mm travel. My current thinking is to use two of the 20mm linear rails from Zapp with two bearings on each rail...not the supported rails. I think the deflection of the rails themselves should be fine with this size?

    The rails will be mounted to 15x160x600 alu plate so the bearings are 200mm apart and the rails spaced at 150mm (ish). I then intend to use another 15x160x600 aluminium plate to mount the router. This works out at over 50 in aluminium...ouch

    Is this going to be strong enough? Using the 'gantry stiffnessv4' spreadsheet I've found on the forum and putting in these numbers I get a deflection of 0.58mm with 100N which is clearly unacceptable.

    That's made me think instead of using aluminium plate for the router/bearing mount thingy I should use say an I section. I'm currently thinking of using 6x160x600 aluminium and fixing some 2"x0.5"x600mm 'strips' onto the edges of the aluminium plate to make an I cross section. I would mill a 6mm slot in the strips to fit the main sheet. According to the spreadsheet this reduces the deflection to 0.027mm which I think is fine....it's also a lot cheaper!

    So, which option is the best - am I missing something here?

    Thanks in advance!

  2. For a wood router 100N is much to big... I take it this is the horizontal cutting force you are refering to... for general wood cutting 5 - 10N is more reasonable, 100N is more steel-bashing territory...

  3. #3
    I was intending to use this for milling aluminium, steel would be nice but I understand that might be pushing it. I choose the 100N figure as a worst case...
    Sorry I should have put 'spindle' not router in the first post.

  4. #4
    Just done a quick model to clarify the plan:

    Click image for larger version. 

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  5. #5
    Hi Jonathan,

    I posted that gantry spreadsheet so am pleased someone is trying it out. However, it is not configured to do the calc you are trying directly, unless you are moving things around and doing some extra calcs. Can you tell me a bit more about which of the above pieces you are modelling in the spreadsheet and how you are doing it? I think there are moments which are not being considered with your approach. Don't forget that the cutting is taking place some distance away from the Z axis which will lead to moments in 2 directions.

    Maybe even post the spreadsheet with it set to your parameters (you need to zip it to attach it).

    How far down is it from some reference point on this Z axis down to the cutting tool?

    Are you doing the calcs on the rail, the I beam or the plate etc?

    If you are really looking at 100N, then can I suggest using profile rails instead of unsupported rail. I think this will always be a limiting factor no matter what sections you use elsewhere. I bought a used pair of 15mm profile rails and carraiges, about 400mm long, from Ebay China for around 80 if memory serves.
    Last edited by routercnc; 20-08-2010 at 09:03 PM.
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  6. #6
    I have another look at profile rails. Is it possible to machine the standard hardened rail? I'm thinking if I drill holes into the rail at say 100mm intervals and use them to support it then that should be pretty much as good as the proper stuff? Certainly a lot better than nothing.
    What about using one or two of these:
    Perhaps I could use two mounted perpendicular to each other to better take care of the forces in both directions.

    Those numbers I got with your spreadsheet are a bit of a bodge. I just entered 1200mm for the width (to simulatie the I section bit) and put a low value for the supports - I figured that'd be near enough reality! I was more trying to compare the different cross sections than get absolute figures for deflection. I think I'll modify the spreadsheet to better fit my needs. By the way thanks for making it!

    I've tried using the spreadsheed for the 20mm rails aswell and figured that they would be fine.

  7. #7
    Argh, just set fire to the microwave whilst i was posting that!

  8. #8
    Hi Jonathan,

    Hope the microwave is OK - this CNC stuff can draw your concentration!

    I still can't quite picture how you are using the spreadsheet, but if you are happy with your calcs then that's fine. If you have any more detailed dimensions and layouts then post them up and I'll try to help further if you like.

    I briefly looked over those IGUS bearings. 21 and the low moment capability were concerns, but I'll study them a bit more and get back to you. Someone else might have used them and be able to comment? I'm a big fan of profile rail and over that short a span (400mm), and in say a 15mm size, are potentially affordable.

    Drilling hardened rail - I'd worry about stress relief from the holes causing deflection of the nicely ground straight section. You'd have to switch to open bearing pillow blocks as well. Don't know if it's worth the effort, over buying the real thing. Anyone tried this?
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  9. #9
    Luckily the microwave survived - some pretty impressive flames though!

    So do you think the price of those rails implies that they are probably not up to much?

    16mm supported rail and bearings works out at 93.60+VAT from Zapp compared with 45+VAT for the 20mm unsupported.

    The open bearing pillow blocks are only a few more, so that's fine. I hadn't thought of stress in the rails. If it does work I reckon it's worth the effort to save about 40!

    I'm currently dimensioning the drawing - I'll post it as soon as I'm done. But basically the 4 bearings are currently spaced as a square, 100mm between centres - hence the rails are also 100mm apart. The rails are 600mm long (the above prices are for 700mm just to be safe) which leaves a nice 400mm or so travel.
    Thanks for the help so far - much appreciated!

  10. #10
    Here's the dimensioned drawing:
    Click image for larger version. 

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