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Thread: Large Epoxy Granite Vertical CNC Mill

  1. #241
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    Default Re: Large Epoxy Granite Vertical CNC Mill

    We use granite/eg bases on our grinding machines... from 1 tonne to 40 tonne.

    We don't use any steel re enforcement, it causes issues, Same as it does with concrete.

    Concrete buildings would last 10times longer with out rebar, (assuming no natural disaster etc hit them)

    The rebar corrodes even when incased in another material, grows in size and causes cracks and weak spots.



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    Default Re: Large Epoxy Granite Vertical CNC Mill

    Quote Originally Posted by RotarySMP View Post
    I didn't understand the plan with the inserts. In photo 5.jpg above, they look like they are above the surface of the mold, and will therefore be completely buried in E/G?
    Yes, the inserts are completely buried inside the EG, ~10mm under the surface. This leaves the surface plain EG without steel islands sticking out of it. This is important because I will hand grind the EG to make it flat for the linear rails. Then I just drill through the EG and drill and tap the insert in precisely the right place. That's how the X and Y axes rails were attached.

    - - - Updated - - -

    Quote Originally Posted by LeeWay View Post
    Not to be too dumb here, but wouldn't all thread have been just as good?
    I have seen it used in anchors for concrete before.
    It would work but the rebar is cheaper...



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    Default Re: Large Epoxy Granite Vertical CNC Mill


    Joe mentioned counterbalancing the head assembly and that reminded me that I did not say anything about it. I mentioned in an earlier post that I will be using a pneumatic counterbalance because I don't like the idea of having actual weights moving up and down. Instead I will use an air cylinder with enough travel and force to balance part of the weight of the head. Here's the actual cylinder on top of the mold. It will mount about in the middle of the column and extend down in the back of the mill.

    Large Epoxy Granite Vertical CNC Mill-jpg

    I can use steel cables to connect the cylinder shaft, via pulleys, to the head assembly. Here is the bracket to which the cylinder will mount and the inserts that will go into the EG to mount the bracket to. It's assembled like that so the inserts end up in the right places when casting.

    Large Epoxy Granite Vertical CNC Mill-b-jpg Large Epoxy Granite Vertical CNC Mill-c-jpg

    The hole in the bracket where the cylinder will attach is not machined yet...




  4. #244
    Activation process G59's Avatar
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    Default Re: Large Epoxy Granite Vertical CNC Mill

    The rebar corrodes even when incased in another material, grows in size and causes cracks and weak spots.
    Different story when in EG mix. First the Epoxy ensures no oxygen. It takes oxygen to OXide... The EG grows pretty much at same rate as steel. None of us will be around in 70 years and by that time all these machines will no longer be needed so I wouldn't worry too much about developing cracks..



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    Default Re: Large Epoxy Granite Vertical CNC Mill

    I have seen epoxy coated rebar in various sizes too. They used it in an elevator shaft at a Hospital I helped remodel back in the 90's. They had to use special concrete too because the bottom of the shaft was below the water table. That is where special engineering comes in. I have no doubt it was very expensive.

    Lee


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    Default Re: Large Epoxy Granite Vertical CNC Mill

    Thanks for explaining your insert plan. Not sure what benefit you are expecting verse the way the industry does it, but good luck with it. I like your air cylinder plan.

    Mark

    Regards,
    Mark


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    Default Re: Large Epoxy Granite Vertical CNC Mill

    Balancing out the weight of the Z axis is a very good idea, but using an air cylinder seems way too complex. Just get some decent air springs as used on car bonnets etc. They do work: I have one on mine.

    Cheers
    Roger



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    Default Re: Large Epoxy Granite Vertical CNC Mill

    Very interesting build, though I have to admit I skipped through to see the first castings, now I have to go back through and read everything to get up to speed.

    Cheers.

    Russell.



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    Default Re: Large Epoxy Granite Vertical CNC Mill

    Quote Originally Posted by RCaffin View Post
    Balancing out the weight of the Z axis is a very good idea, but using an air cylinder seems way too complex. Just get some decent air springs as used on car bonnets etc. They do work: I have one on mine.

    Cheers
    Roger
    Air springs tend to wear out, that is why pros (commercial machines) use air cylinders. The system of course needs to be filled with Nitrogen, because of the thermal expansion of air.

    Additionally, air springs used in car bonnets have "brake" inside, which is totally unacceptable for CNC machine.

    Regards
    Lubos



  10. #250

    Default Re: Large Epoxy Granite Vertical CNC Mill

    Any updates? This thread is an inspiration. One other option for counterbalancing the z-axis that I don't think anybody has mentioned is the use of simple constant force springs (aka clock springs). I have used several of these on my large CNC router and they work great. The benefit is that there are no seals, pneumatics to contend with, low mass so they add little to nothing to the inertia the motor must overcome. The problem is the biggest one I could find is ~33lbs (Century Spring). I believe you said you needed to offset nearly 90kg? that would require quite an array of springs. Anyway, looking forward to seeing the next update.



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    Default Re: Large Epoxy Granite Vertical CNC Mill

    Has this build gone silent. I don't see that there have been any posts at all from petruscad in over 6 months?



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    Default Re: Large Epoxy Granite Vertical CNC Mill

    Is there need to go to such length ensuring squareness of axis on CNC machine? Can't it be compensated in software, leaving only task to buld bind-free and slop-free linear motion?



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    Default Re: Large Epoxy Granite Vertical CNC Mill

    Quote Originally Posted by shadowjack View Post
    Is there need to go to such length ensuring squareness of axis on CNC machine? Can't it be compensated in software, leaving only task to buld bind-free and slop-free linear motion?
    No, you can make up for some elements such as play in axial (along the ballscrew) with software but if the machine is not square, you will always have issues.



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    Default Re: Large Epoxy Granite Vertical CNC Mill

    Quote Originally Posted by Chazaxl View Post
    ... but if the machine is not square, you will always have issues.
    Can you elaborate, what kind of issues?
    If machine is out of square in XY plane for example, when you command movement in X direction software can add small movement in Y direction and vice versa.



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    Default Re: Large Epoxy Granite Vertical CNC Mill

    Quote Originally Posted by shadowjack View Post
    Can you elaborate, what kind of issues?
    If machine is out of square in XY plane for example, when you command movement in X direction software can add small movement in Y direction and vice versa.
    Suppose it could be done but I dont see the value of taking the risk.

    As it is, good machines struggle to do 'perfect' circles. This will add another dimension of issues and tuning. I think its difficult enough to make a good machine work well and reliably if it is square (think of high speed machining), I cant see the value in taking something not square and then somehow mathematically trying to fix it.

    Plus, not all errors are linear. If the X 'snakes' a few micron left and right, how would you compensate? You would need a massive calculation to know, your time is better spent just getting the mill square in the first place. Within reason, its not that difficult if you have the correct tools.

    My own mill will probably have 0.05mm error only as I have a limitation on what tools / ability I have to do at home. But for me, that's fine.



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    Default Re: Large Epoxy Granite Vertical CNC Mill

    Mostly, yes. But, most machines aren't great at moving near zero velocity, due to friction and backlash. So, lets say you are moving back and forth in the Y direction. The X will have to make very small moves with changing velocity direction, causing motion problems. These will be very small problems, but will likely show up as strange surface finish issues. If tiny surface finish issues are ok, then compensation will likely work for you.



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    Default Re: Large Epoxy Granite Vertical CNC Mill

    nice



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    Default Re: Large Epoxy Granite Vertical CNC Mill

    Quote Originally Posted by petruscad View Post


    Yeah, I did not post for a long time, did I? I've been very busy lately with... moving to a new place.

    The column mold is completely finished now with all bells and whistles required (well, it was actually completed before the move). Here's a picture of the bottom side of the mold being glued together.



    The holes in the bottom (there are six of them) are where the bolts mounting the column to the base will go. You can see I'm trying hard to make all angles be 90 degrees! And here's the mold almost finished.





    You can see the shiny tubes that are going to create the column to base mounting holes. I also added thick metal plates on the top side to distribute the pressure so the EG does not crack while tightening. Here's another view of that.



    There are two rebar assemblies that will be embedded in the column and the rebars also hold the bolts to which the Z-axis linear rails will be mounted to as you can see from this picture.



    I will post on the detail of the rebar and the mold in later posts. I hope this quenches you curiosity for the time being...

    Been over a year... Didn't see an update from this pour. Not nice to leave us hanging man...



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    Default Re: Large Epoxy Granite Vertical CNC Mill

    Quote Originally Posted by nateman_doo View Post
    Not nice to leave us hanging man...
    Nagging about updates is also not nice.

    Sven http://www.cnczone.com/forums/diy-cnc-router-table-machines/320812-aluminium-1250x1250x250-router.html


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    Default Re: Large Epoxy Granite Vertical CNC Mill

    nagging? its been a year, and I asked once. How is that nagging?



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