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IndustryArena Forum > Mechanical Engineering > Mechanical Calculations/Engineering Design > advice about tolerances milling/grinding for a student
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  1. #1
    Join Date
    Oct 2005
    Posts
    375

    advice about tolerances milling/grinding for a student

    Hello people,

    As you cann see in the jpegs i want to make 2 decks for linear motion and a jig, when finished i want to insert the decks in a E/G L-shape base.

    The decks are made from AW7075-T6 alu
    The jig of a steel xxxx, cast iron (advice please??)

    Decks:
    Cann you tell me something about the possible tolerances between surface
    1, 2, 3. (straightness, flatness, parralel ????)
    Do i need to surface mill or grind, what about deformation after operation?

    Jig
    Think i will waterjet the raw shape and then cnc mill the contact faces
    face 1, 2 has to be perpendicular, face 3, 4 has to be on the same plane.

    If i go to a machine shop with modern machines wath cann i aspect to get.
    wath would be smart to ask for tolerances.

    i will have c3 ballscrews and matching bearing blocks.

    Any advice is welcome.

    regards

    Roy B.
    Attached Thumbnails Attached Thumbnails slide dek.JPG   dekken.JPG   jig.JPG  

  2. #2
    Join Date
    Apr 2007
    Posts
    1955
    Once you define your "reference surface", then it is a matter of tolerances from there.

    Most modern machinery can mill these items easily to +/- 0.003 in (about .01 mm) and a good shop with a skilled machinist can go 10 X better than this.

    The real challenge to go substantially better than +/- 0.01mm is not really the machinery, but your use environment. At this point, your results are increasingly dependent on temperature control and oil film thickness.

    I like 7075 and use it myself. There is another Al alloy called ______ (cannot remember) which is cast and is exceptionally resistant to warping from internal stresses. I am sure someone else here knows what it is.

  3. #3
    Join Date
    Aug 2007
    Posts
    147
    The cast aluminum that Harry's referring to is Mic-6 I believe. Should work well in this application.

    There's no need for a waterjet. A good machining center with Zirconium Nitride (ZrN) coated endmills will plow through aluminum at an incredible rate. No grinding is necessary either. We've built quite a few linear stages out of aluminum up to 60" in length on our Mori Seiki machining center and we've been able to hold +/- .0002" without a sweat.

    That said, you won't want to ask for that kind of tolerance from a shop, as you will get charged heavily for it. A competent shop with good machines should be able to hit +/-.003" without probing. These are dimensional tolerances like the width of the slot. The flatness, straightness, parallelism, depth of the slot, and relative bolt hole locations should naturally be much better than that. You should definitely explain your situation to the shop you'll be working with.

    By the way, you should add a shoulder to the plate on one side. Linear rails aren't perfectly straight, so pushing the rail against the shoulder during mounting will maximize straightness.

  4. #4
    Join Date
    Aug 2008
    Posts
    7

    how good do you really need it

    I would start from the other end i.e. how good do you really need it.

    If you are trying to produce an extremely accurate machine to do something like a racing gearbox housing then a tight tolerance is required but if you are looking to use it to learn construction electronics and codling the a more realistic tolerance could be used. This would save you money to spend on a good anti backlash system that in the long run would teach you more and make a more repeatable machine.

    Yours Paul Boscott

  5. #5
    Join Date
    Oct 2005
    Posts
    375

    Nice

    Thanks a lot for the advice!!!

    Found mic-6 in the Netherlands and its the right one.
    Cheaper then 7075 and also lower tensile strenght,
    but ths is no problem, modules E is still the same.

    Milling all parts will be good enough,
    dimensional tolerances are not that important.
    Te better the machine and strategy,
    the better the flatness, straightness, parallelism
    Last 3 are the important ones as you cann see in the design.
    Do i need shouders on both rails per axes. (think so).

    The machine on jpeg is for alu rc car parts, max. endmill dia 6mm.
    Only prototypes
    High rpm, low forces. The idea is to cast in sections with E/G.
    I already have 3 nice preloaded C3 ballscrews,
    that are imcorperated in the design.

    In the jpegs there is also a new deck xy, Is there something to say
    about the time it takes to machine the 3 decks and the jig from mic-6.
    What cann i aspect to get for a price for machining/programming only???
    the small parts i make at school, on a old machine.

    So, Milling will do for reference surfaces. Dimensional is more than good.
    Thanks guys,

    Regards,

    Roy B.
    Attached Thumbnails Attached Thumbnails naamloos.JPG   onder.JPG   onder jig.JPG   totaal.JPG  

    xy.JPG  

  6. #6
    Join Date
    Oct 2005
    Posts
    375
    for programming i can deliver drawing or step or igus

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