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CNC Mill I-Beam Concept?

Discussion in 'Concepts and Ideas' started by Josh Mascote, Sep 1, 2014.

  1. Josh Mascote

    Josh Mascote Journeyman
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    Anyone have any thoughts on using an aluminum I-beam sandwiched between two 20x80 rails (for 1500mm size span)? This is for a modified 1500x1000 OX.

    The I-beam I have in mind is 0.349" thick x 3" base width x 2.509" Leg Length.

    I saw someone used an aluminum I-beam on some site, but orientated in 90 degrees from what I am proposing. My understanding is that the most stress comes from when the bit enters the material and when drilling/plunging. The I-beam's strength is mostly in the vertical direction when it looks like an I and not sideways.

    My idea would be to drill holes in the two horizontal parts and use tee nuts to marry the I-beam to both 20x80 rails. My Z carriage would ride on the top and bottom 20x80's.

    Something like this:

    XXXX
    __|__

    XXXX
     
  2. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    The only potential problem I see would be even the slightest bit of waviness in the flange alignment of the aluminum beam could cause looseness in the grip on the wheels. You might consider sandwiching a 2x3 tube between a pair of vertically oriented 20x80s. Yes, this will be more difficult to connect but you will be able to use standard wheel spacings gripping the 80mm depth and you don't have to worry about misalignment between the bearing surfaces. This will also provide good stiffness in both directions.
     
  3. bobt

    bobt Veteran
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    Josh - You idea has some merit but what you should use is a "fletch beam" which is 2 pieces of extrusion with a piece of steel between them with the whole assembly bolted together. The resulting beam is a strong a a beam of steel that same size because the steel carries the weight for the extrusions.

    Bob
     
  4. Josh Mascote

    Josh Mascote Journeyman
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    Bob,

    You have my attention. So I would I drill through holes on one of the extrusions or through both so I could use actual nuts instead of tee nuts? Trying to understand how one would effectively bolt the piece of steel in the middle.
     
  5. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    Actually, it's a "flitch" beam. The easiest route would be to drill through the first extrusion and the steel plate and then drill and tap the second extrusion.


    Flitch Beam.jpg
     
    silopolis likes this.
  6. Josh Mascote

    Josh Mascote Journeyman
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    Great illustration Rick! Thank you! Spacing at maybe 150mm, something like that.
    This looks totally doable.
     
  7. John Ellis

    John Ellis Well-Known
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    Josh, I'd like to offer the following alternative...
    upload_2014-9-1_22-14-48.png

    The designed fastening strength of extrusions is between faces, not through the profile, which can cause deformation of this part in both the 20mm axis and elongation along the 80mm axis. Instead, save the tapping and just drill a 4mm access hole (not visible in my diagram) in the bottom of the slots of one rail (approx every 150mm) to allow your hex wrench to tighten a blind M5/pre-assembly nut combination. This keeps the strengths in the extrusion, doesn't affect its dimensional tolerances, is nearly invisible, does not require precise alignment of the drilled holes, and to top it all of, does not impede on use of any of the V-Rail slots for motion!

    Consider also the following, using some standard HFSF5-4040 Misumi extrusions in combination with V-Rail. The same clamping and access holes described in the above paragraph would apply here.
    upload_2014-9-1_22-28-12.png
     
    #7 John Ellis, Sep 2, 2014
    Last edited: Sep 2, 2014
    silopolis likes this.
  8. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    @John Ellis, While I grant that your solution is far more elegant, the comment you made:
    is neither true nor even remotely possible. Screws at any reasonable spacing cannot be tightened enough to cause any measurable deformation to the V-slot. V-slot is far stiffer than you give it credit for.

    BTW, your solution will require a washer beneath the screw head otherwise it won't stay centered in the slot during the last bit of tightening and may damage the edge of the slot as it tries to crawl out.
     
  9. Josh Mascote

    Josh Mascote Journeyman
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    Very interesting views on this, love the innovation on this subject! This is what Openbuilds is all about. Both have merit, it will come down to cost at this point. I might kick these two around and see what sticks. I have to see if my local recycling place has a suitable piece of steel or even the aluminum piece.

    FYI: The entire purpose of this concept is to have a 1500mm span suitable for milling aluminum.

    *Edit: Thank you Rick and John for your input!
     
    Mark Carew likes this.
  10. John Ellis

    John Ellis Well-Known
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    Rick, I am glad this is leading to a discussion, hopefully I did not offend in my comment though.

    I have yet to use any washers under the screw heads because there was less than .1mm of designed clearance for a washer/low pro screw combination, instead opting for clamping the pieces against a common flat surface prior to tightening. I will try the washer again and see if it helps me any.

    I would like further information on the comment, "V-slot is far stiffer than you give it credit for."

    I cannot find any design criteria for V-Slot, so in this case I assumed (and I know what assuming does) that the V-Slot 20x80 properties are similar to that of the Misumi HFS5-2080. They are both made of the same material, 6063-T5 Aluminum, are of similar construct and dimensions. The Misumi's, with its 10% thicker members/sectional area, has a cross sectional moment of inertia of 25600mm^4 in the 20mm dimension (for the discussion of screw compression). I also cannot find any ISO property class of the low profile screws, so I assumed worst case scenario of 8.8. An M5 8.8 has a recommended torque specification of 4.88Nm. Its thread length to pitch ratio gives it almost a 20:1 mechanical advantage, providing a recommended 96 N of clamping force per bolt. Two bolts at 150mm (Josh's suggestion and I agree) create a calculated compression of the 20mm axis using the through bolt method of .008mm, resulting in an elongation of the 80mm axis by .032mm versus the face clamping/access hole method of 0mm in both directions.

    Nobody has ever accused me of being too flimsy in my calculations, so I probably oversize to maintain rigidity and accuracy more than is necessary/generally acceptable, but in this case I found it to be easily avoidable and affordable and therefore offered my alternative solution. I am new to V-Slot, but not to mechanical properties of similar extrusions.

    So I would love it if you could publish the modulus of elasticity and cross sectional moments of inertia for each V-Slot profile as well as list the grades of the low profile m5 screws that you sell. Then I can accurately do my calculations with a whole lot less assuming and worry.
     
    shinybro, Serge E. and Jestah like this.

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