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Re: Large Epoxy Granite Vertical CNC Mill
As I mentioned in my previous post everything begins with the casting of the base. And for that one needs a mold.
Attachment 269534
This is the mold fully assembled. |
Attachment 269536
Here you can see the side wall anchors, the linear rail anchors at the bottom and one of the six feet. |
Attachment 269538
The three brass pipes are the holes for the column mounting screws. The nuts are glued to the top of the pipes. |
Attachment 269540
The steel plate in the middle is for mounting the Y-ballscrew and stepper motor. |
Attachment 269542
And this is 20 feet of 5/8" reebar to be placed in the EG for strength. |
The base is going to be cast upside down. That's why the six steel steel feet are mounter on the aluminum extrusions at the top of the mold and the anchors (bolts) for mounting the linear rails are at the bottom. Equally spaced anchors were placed around the mold to have places where you can mount something with a simple bolt if the need comes.
The mold is made of melamine covered particle board that's glued and bolted together. The glue is needed so the mold seals and does not leak the epoxy which is very thin.
The red interior of the mold is simply adhesive tape that happened to be red. The white areas are covered too but with white tape (no particular reason). Epoxy seeps into the wood right through the melamine layer and you get a total mess. The tape is there to form an impenetrable barrier for the epoxy. It will also be waxed prior to casting to make it easy (or easier!) to separate the mold from the casting.
The six steel pads are super glued to the aluminum extrusions and the extrusions are covered with tape to prevent epoxy from sticking to them. After solidification the extrusions can be yanked off leaving the feet inside.
The mold will most likely not survive the de-molding process. Even with tape and wax its still difficult to cleanly take these molds apart so you revert to hammers, chisels and pry bars to break it apart piece by piece. That's why it helps that the mold is made of particle board which crumples easily.
There will be two layers of reebar, two bars for each straight segment of mold. In total 10 pieces of reebar per layer.
Notice the bolts on the six feet and the Y-ballscrew plate. They are there to serve as anchors for these pieces and prevent them form ever moving of being pulled out under heavy loads.
The arrays of bolts sitting at the bottom of the mold are for mounting the linear rails. They hang at 7-8mm from the bottom so they will end up submerged by this much under the rails (the casting is upside down). The plan is then to drill through these 7-8mm of EG and them drill the bolt and tap it so the rails can be bolted down. The bolts sitting to the sides of the middle row are there for mounting alignment edges so the linear rails can be straightened out.
Oh yes, there is also a cover (not shown in pictures) that will be placed in the middle to cover those two holes.
The next steps are to prepare the aggregate and to make a vibrator to be mounted under the mold. The mold itself will be placed on two "soft" supports that will not dampen vibration.
Re: Large Epoxy Granite Vertical CNC Mill
If you want to keep that square shape in the middle, perhaps fill it with sand? this way it should have some sort of equilibrium? between the pressure of the EG mix on the exterior, and the sand fighting it. sand makes great aggregate as well if some falls in the mix.
Re: Large Epoxy Granite Vertical CNC Mill
Quote:
Originally Posted by
petruscad
What's the rationale for this?
I remember it was the recommended setup in THK design guide. Its meant to prevent binding in case of uneven force loading. On second thought its not something you should worry about.
Re: Large Epoxy Granite Vertical CNC Mill
Great work! Keep the updates coming!
#MoarPictures
Re: Large Epoxy Granite Vertical CNC Mill
Thanks for your suggestion with covering the mould with tape. I used it on a test piece, and it demoulded perfectly, even without waxing the entire surface.
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Re: Large Epoxy Granite Vertical CNC Mill
Here is the source of vibration for the mold. As I mentioned previously this motor will be bolted to bottom of the mold and the mold will be supported on soft cushions.
This is a 220V, 3340 rpm motor for which I made an eccentric weight. The weight has two pieces bolted to each other which makes it adjustable! When the mold is empty a little eccentricity will make it shake violently but when you fill it with aggregate you get a lot of dampening and you need to make the weight a lot more eccentric.
2 Attachment(s)
Re: Large Epoxy Granite Vertical CNC Mill
<br>This is the aggregate composition that I will be using. I came up with it by following the Fuller curve used for asphalt aggregates. But unlike other formulations on that ungodly long EG thread I wanted an aggregate that was cheap and obtainable from local stores. All components are either from Home Depot, from garden supply stores or from Harbor Freight.
Attachment 270320
From left to right the components are:
- 27% - Quartz Rock (10mm)
- 20% - Salmon Bay Rock (6mm)
- 14% - Olimpia #2 Sand (#10, 2mm, only the largest particles)
- 8% - Coarse Sand (#20, 0.8mm)
- 9% - Medium Sand (#25, 0.7mm)
- 6% - Fine Sand (#45, 0.35mm)
- 16% - Glass Beads (#80, <0.2mm)
The percentages are fractions by volume.
To this you add 20% by volume of epoxy resin.
The numbers in brackets are the sizes of the particles in mm and/or the grit size for sands.
Now, I measured the compaction ratio of this mixture and it turned out to be 20%. That means if you want to fill a mold with volume V you need aggregate with total volume of 1.2*V. When mixed and compacted its volume will be V. Also note that epoxy plays no role in volume calculations since it is there to simply fill the gaps left in the aggregate after it is fully compacted.
The glass beads are from Harbor Freight and they are actually made for sand blasting, but should work well here too.
Everything else is bought either from Home Depot (fine and medium sand) or from garden supply stores (everything else). Everything needs to be clean though, so I washed and dried the quartz and salmon bay rocks.
Here are the bags with the various components of the aggregate.
Well, that's it, the next step is casting!
Re: Large Epoxy Granite Vertical CNC Mill
CNC Milling machine in a bag. I love it. :)
Re: Large Epoxy Granite Vertical CNC Mill
Re: Large Epoxy Granite Vertical CNC Mill
Interesting thread. Good luck with the pour. I'm looking forward to seeing the results.
Re: Large Epoxy Granite Vertical CNC Mill
I did the same, buying the aggregates in the gradings which the local hardware store has, with the finest sand also being 0.1-0.3mm like yours. The only thing you might like to consider is some super fine quartz flour, fly ash, or alumina. All the academic articles I have read point to about a 30% increase in strength and rigidity if your aggregate is graded done into a fine powder. I got some Alumina from a pottery supplier. In making cement concrete they stop the grading at fine sand, because the portland cement is the fine powder. Since we don't have that, it makes sense to add a replacement. Using Thomas Zietz XLS, I ended up with 12% alumina in my mix.
Re: Large Epoxy Granite Vertical CNC Mill
Just a thought, and maybe too late.
The Z axis bearing blocks come in two sets: upper and lower. The two sets look a bit to close together to me (picture, page 1). I would prefer to have another 4-6" between uper and lower sets, as the distance from the blocks out to the spindle axis is largish.
Cheers
Roger
Re: Large Epoxy Granite Vertical CNC Mill
Why are there no pictures yet? Its been days.
Re: Large Epoxy Granite Vertical CNC Mill
He's up to his elbows in wet epoxy. :)
Re: Large Epoxy Granite Vertical CNC Mill
Quote:
Originally Posted by
nateman_doo
Why are there no pictures yet? Its been days.
Patience, all good things come in their own time!
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Re: Large Epoxy Granite Vertical CNC Mill
Ok, here's how a big rock is made!
Mix all components of the aggregate really well. You can see the distribution in size of the particles. The glass beads are not really visible in there, its as if they disappear once you put them in. All you get is a slightly whitish color and a smooth feeling when you mix the aggregate.
Attachment 270902
Next, measure the epoxy components, mix them together really well, add the to the aggregate and mix that for half an hour or until its really uniform. It came out pretty good, not too wet not too dry, flowing yet no epoxy separates and floats to the surface. Time to transfer it into the mold, shove it between all the metallic inserts, add the reebar, pound it, vibrate it, dance around it, etc.
Attachment 270904
Put the cover, fill it to the top then add the extrusions with the feet glued to them. The volume calculations were almost perfect. Another liter of mixture would have fix into the mold but its no big deal without it. It means the feet are sticking out of the epoxy by a few millimeters more. The mold was bowing out slightly so I added a clamp and brought it back to being straight.
Attachment 270906
This is after a while. Just a little bit of epoxy came up to the surface, just enough to make the surface smoother.
Attachment 270908
Now its waiting time again. It has to stay in the mold for at least several days for it to solidify enough. Complete curing takes a couple of weeks but it can be removed from the mold without any risk in a week or less.
Re: Large Epoxy Granite Vertical CNC Mill
Cant wait to see the result. Great job Petrus!
Re: Large Epoxy Granite Vertical CNC Mill
Thanks for posting this- the pics and work flow descriptions are great.
I am curious how much epoxy you mixed and for how long before adding it to the aggregate? I saw you have US composite 636 with the slow hardener. I have to mix 6.5 gallons worth for my build and am a little wary about exotherme and running out of time mixing it- Any suggestions or observations are much appreciated...
I can't wait to see when you take off the mold!
Re: Large Epoxy Granite Vertical CNC Mill
Quote:
Originally Posted by
gt40
I have to mix 6.5 gallons worth for my build and am a little wary about exotherme and running out of time mixing it-
The polymerization reaction is exothermic but very, very barely so. I have not noticed any temperature rise at all, either while mixing or in the mold. As to how much time you have, US Composites lists the pot life at 35-40min after mixing. However, I would say you have at least one hour and maybe even 1.5 hours before any solidification begins. So you can mix the epoxy for 5 minutes, mix the aggregate for 10min, fill the mold in 30min and you still have time left. US Composites lists the set time at 5-6h which is about the amount of time after which the EG seems to gel a little. 2-3 days is the minimum time you need to keep the part in the mold.
