EMT rigidity

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This topic contains 7 replies, has 4 voices, and was last updated by  Barry 4 months, 3 weeks ago.

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  • #104220

    William McCart
    Participant

    I know this may have been asked a thousand times but here it goes again. I have seen on here people asking about filling the conduit with concrete to increase the rigidity but people saying the added weight would outweigh any increase in rigidity and it would sag under its own weight.

    So my question is this. Has anybody tried printing 6in cylinders with a 50% infill and epoxied them into the conduit. I would think that would add a substantial strength while at the same time not adding a ton of weight.

    Thoughts?

    #104221

    William McCart
    Participant

    Let me clarify cylinders as full rods with 50% infill in all the pipes other than those requiring wiring to go through them

    #104222

    Dakota Jim
    Participant

    On my build the only conduit without wires inside them are the perimeter tubes. If I need to remove the flex in those than I will install supports as needed, there are several different options available and are listed in older posts. If extreme accuracy is needed then you need to build a very small foot print MPCNC or go with a ridiculously priced commercial cnc. There is no really cost effective way to eliminate 100% of the flex on the center carriage due to the design. You could use SS tubing on a small foot print MPCNC to increase the accuracy but remember the MPCNC was designed as a low cost starter hobbyist cnc but so far has exceeded all expectations from all but the most ardent purists (people that believe that any cnc should be accurate to within .00000001 )

    My MPCNC is still 1000% more accurate then my trying to free hand with my router. I think it is great and a fantastic value as designed.

     

    #104226

    Jamie
    Participant

    I have contemplated this problem and my take is that to really help with stiffness you need to add a material with similar stiffness as the steel.  Printed parts have very low stiffness and won’t help.

    If you have a piece of string and you add a rubber band in parallel to make it “stronger” you aren’t really helping because the rubber band won’t contribute anything until the string has already broken.

    Ignore the yield strength and look at the modulus of elasticity: https://www.engineeringtoolbox.com/young-modulus-d_417.html

    Concrete may seem very “stiff” in practical experience because it doesn’t bend, but it’s not because it has a high modulus of elasticity, it’s because it’s brittle.  The actual modulus of elasticity is much lower than steel, which is why it won’t help.

    Your best bet is to epoxy more steel inside the steel tube.

    #104227

    William McCart
    Participant

    My plan is to add supports along one side of the perimeter pipes but if added to the other side would make it difficult to get material in and out. I was going to build it at 48×36 and add multiple supports along the 48in length. I’m not looking for extreme accuracy just trying to think of ways to reduce flex if possible on the 36in length. I will most likely add supports to one side of the 36in rails and leave the one side open for material.

    #104242

    Barry
    Participant

    When mine was 5 foot square I didn’t attach one of the support feet to the board.  I could remove it if I needed(I never needed).  I used allthread for the supports with a nylock to support the tube.  Didn’t bother with holding down the gantry, just up.

    #104289

    William McCart
    Participant

    So you are saying put in the supports and on the side I plan to feed in my work put in a support but do not screw it down so on the rare times I need the full width it can easily be removed.

    #104293

    Barry
    Participant

    So you are saying put in the supports and on the side I plan to feed in my work put in a support but do not screw it down so on the rare times I need the full width it can easily be removed.

    Yep!

    These are the supports I used.  I just drilled a hole in the bottom of the outer rails then spun them to face down.  The allthread sticks up into the conduit maybe a quarter inch.

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