# Anyone interested in DIY UHPC (Concrete) CNC Mills?



## dboe (Feb 27, 2014)

I am working on building a CNC mill out of UHPC (Polymer) Concrete, which has 5X the strength of typical concrete (150mpa). 

I understand the rigidity is awesome allowing for milling harder materials at home for minimal build cost.

There is a product in Germany called Duracrete and in North America one by LeFarge called Ductal, which is a "Premix" where you add water, Superplaticizer and Steel Fiber. 

The basic recipe for UHPC if you did your own goes something like this:
Portland Cement    28.50%
Fine Sand            40.80%
Silica Fume            9.30%
Ground Quartz        8.40%
Superplasticizer    1.20%
Accelerator            1.20%
Steel Fibers            6.20%
Water                4.40%    

CAUTION: Silica Fume is nano size particles and requires special safety precautions to prevent inhalation, etc. You have been warned..

Here is an example of what they look like although this one is Epoxy base, which is much higher cost than UHPC: https://www.youtube.com/watch?v=C8vlOa2Y34s

What I have seen online people tend to pour it in two pieces, the base and "C" column with steel plates embedded into each to allow for an easier pour, assembly and adjustment. It looks like the weight comes in around 550-600lbs total.

Has anyone attempted this and can offer additional info/tips?
Anyone else have an interest?

Thanks
dboe


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## John Hasler (Feb 27, 2014)

dboe said:


> I am working on building a CNC mill out of UHPC (Polymer) Concrete, which has 5X the strength of typical concrete (150mpa).
> 
> I understand the rigidity is awesome allowing for milling harder materials at home for minimal build cost.
> 
> ...



I have nothing to add but I'm interested.  I plan on filling the core of my homemade lathe with magnetite concrete this summer.  How do you plan to deal with shrinkage?


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## dboe (Feb 27, 2014)

It seems a vibrating table or vacuum is required during the pour to remove the trapped air and UHPC is more like a fluid than concrete and does not shrink as much. The German product claims it self evacuates air to minimize shrinkage and improve strength. It is also "self leveling". Check out this video: https://www.youtube.com/watch?v=ftopsaGUZzs

Also the base is poured upside down so settling just makes the base a bit thinner and can be compensated for by building the forms a bit higher.

I am working on getting some test material and do some test runs to determine these factors can compensate, which you should be able to work into the build. 

There are plenty of example cheap vibrating tables: https://www.youtube.com/watch?v=XHGnGuWf8s8
Although the example is WAY to light to hold a 325lb base pour.

My biggest concern is keeping things straight as I don't have a granite standard to build it on. Have to get creative.

Slill lots to work out..


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## cjtoombs (Feb 27, 2014)

Besides making machine bases for machines, I think this would be interesting to use to make solid stands for conventional bench type machines.


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## dboe (Feb 27, 2014)

Another good use. I need to build the machine first before I have to consider a stand. :LOL:

I worked out a rough material requirements:

Component    Lb/Yd    Percent    Amount    Cu Yard    46656    0.1265432099
Portland Cement    1200.00    28.50    151.95    Cu Meter    61024    0.0967488201
Fine Sand    1720.00    40.80    217.53            
Silica Fume    390.00    9.30    49.58            
Ground Quartz    355.00    8.40    44.79            
Superplasticizer    51.80    1.20    6.40            
Accelerator    50.50    1.20    6.40            
Steel Fibers    262.00    6.20    33.06            
Water    184.00    4.40    23.46            
Total per/yd    4213.30                    

So it works out to about an 1/8th of a cu/yd, which is about 550 lbs and should be reasonably cheap as 370ish lbs is Portland Cement and Sand, which is dirt cheap.
You would need 1 x 50lb bag of Silica Fume, 6.5 lbs of Sperplasticizer and 33lbs of steel fibre. The Accelerator is needed in some quantity, but I don't know how much because 6.5 lbs shown would make it set in 1 hour, which may be to short for a home build.

dboe


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## cjtoombs (Feb 27, 2014)

One warning about designing with concrete.  Strength of concrete is only ever listed as compression strength, because its strength in tension is abysmal.  Concrete structures are designed with this in mind, and use steel or other reinforcement to give it tensile strength.  Deflection is very important in machine tool design as well, and one would need to make some samples of the mix that was planned and test it to determine it's modulus, or just rely on the reinforcement for deflection.  Of course the nice thing about concrete is that it is cheap and you can cast it at home, so you can use a lot of it with relatively cheap reinforcement to overcome those weaknesses.


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## dboe (Feb 27, 2014)

Ductal is rated at 3850 psi tensile strength for a 4 inch cylinder. Given you have some flexibility on column size I would think we could do better than that.

Shrinkage is 0.025% after 28days

The Modulus of Elasticity figures I am still trying to understand the charts 

Do you know the kind of forces we need to deal with?

dboe


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## cjtoombs (Feb 27, 2014)

3850 PSI for tensile strength compares with cheap steel at 36,000 PSI.  I think Modulus is going to be the biggest factor, since it controls deflection.  I think you will have to cast a test bar, check it's deflection under known conditions and back out the modulus.  It's easily doable at home.  I think there will be so much deviation from one formula to another that testing will be the only way to know for sure.  I think for the members that need stiffness, you will probably need to either use a lot of steel reinforcement or just make them out of steel and aluminum.  I would think the forces would be highest in the Z direction, for drilling and plunge end milling.  It depends on the size of the machine (what size drills and mills you will use) but I would think 500 lbs. would be a safe bet for the small machines we are talking about here.  Maybe half or less for X and Y forces, and torque will likely be negligible.  That would be for deflection.  It will probably be 3 times that for safety to keep from breaking things under a crash condition.  And believe me, you will crash it .


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## dboe (Feb 27, 2014)

Apparently they are doing this already commercially. Product looks amazing..

http://www.durcrete.de/BE-DC-system/BE-DC-system.aspx

If I could even get close to that I would be laughing. 

Keep in mind we are talking about the comparable price to a 8020 aluminum frame, which is going to flex, ring, etc a lot and is not really good for steel milling.

Check out this Polymer Cement mill: https://www.youtube.com/watch?v=84iP10KShvY&feature=c4-overview&list=UUuQTJrCuFUnrQM0JI5R4r9Q


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## cjtoombs (Feb 28, 2014)

I would just be careful about where the loads from the cutting tools are going to go.  Those guys doing it commercially know the properties of their materials and the loads their parts are going to take, so they can add reinforcement where it's needed then cast around it.  Once you decide on the layout of the tool you want to make with it, examine how the loads from the X, Y and Z directions will go into the structure.  Anywhere it looks like you will get significant tension loads, put reinforcement there.  On a bed type mill, the column would need reinforcement on the front and sides.  The back may need a small amount, just to support the weight of the mill head, but not as much.  Using some glass fiber or something like that in the mix may be enough for the back.    Carbon fiber would make a good tension reinforcement for something like that, if laid on the surface, but that may defeat the low cost aspect.  A rebar structure with all the nuts you want to use to attach things to the mills welded on would do the trick.  Just don't think those structures those guys are showing off are that simple.  I doubt they use the epoxy to take significant tension loads.


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