Large Epoxy Granite Vmc Project

gt40

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Exceeding the performance envelope in every way of my Rf45 clone is the base line goal while being on a budget.

I have been accumulating components over the past year. I have come to hate dovetails and am going with linear guides:

THK SHS30 for X axis 18”/700mm travel
THK SHS 20 for Y&Z axis 20”/780mm travel
I am going to be using either THK or NSK -32mm ground ballscrews
Epoxy granite for all major components and use of precision ground steel plates cast in place for the linear rails, servos, ballscrews etc.

Here is the basic design. I am using simple straight forward forms for the molds and incorporating plumbing in the casting to provide for lubrication and cabling:

mill%20frame%20sketch.jpg


THK Linear Guides-

thk-shs.jpg

more to follow...
 
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sgisler

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Interesting......watching


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gt40

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A key point to the design is using 2 steel bars ground to .001" on all sides and 1/2"x3"x36". Each bar weighs a lot. I am aligning the bars and bolting them to each other with the cross members embedded in the epoxy for strength. That way I can pre drill the holes and fix them precisely by bolting them through the mold using coupling nuts which will serve as anchors for them. They will form a coplanar mounting platform for both the linear rails and the mill column.

steel-log.jpg



I am aligning the two 3x36x1/2 precision ground bars parallel using a 12" granite surface plate. Then I will align them on the same plane by cross bolting 1 x 1 precision ground bars to fix them in place aligned.. Once cast into the epoxy base this assembly forms the basis for both the y axis and the vertical column for the z axis which will also use 2 ground steel bars aligned the same manner as the base.

Finally, I am going to cast in place a 15x15 ground plate "frame" on the bottom of the z axis. After reading about using grouting for alignment, I will use this method and have jack screws to do the final adjustments and then grout.

All the ground bars are being drilled and will be through bolted in the mold using coupling nuts to hold them in place.when I cast the epoxy granite. This should provide a precision ground and aligned surface to do final indicating of the rails.

I managed to drill the steel bars for the 30mm linear rails. I was worried about the prospect of drilling 18 holes through tool steel all aligned but it worked. I had to do 3 different setups on my rf45 clone for each rod. I bolted one of the linear rails direct to the table and indicated it straight and then used that as a straight edge for steel bars. Really made me wish I had a bigger mill! At the end, every single hole witnesses the linear rail holes.
drilled%20steel.jpg

As far as the size of the linear rails, they were selected on the basis of the largest I could afford and fit in the design. I got all of them on Ebayand they were new stock. I averaged 200 per axis for rails and guide blocks.

The Y axis has the most forces on it because it has the x on top so I went with the extended version THK SHS30. They are going to be 15" for maximum rigidity. I can get a decent amount of deflection cutting steel on the RF45 and this is one area I wanted to really improve.

The X and Z have less force and going with the 20mm THK SHS to provide more range of motion.

To get an idea of the footprint of the machine, The base is 22"x42" x 9" and will weigh over 800 lbs by itself before adding linear rails, ballscrews and servos.

I expect the whole machine to come in around 2700-3300 lbs(1225-1360 kg)

Envelope for each axis is:

X: 22-24"/560-600mm

Y: 15"/380mm

Z: 18"/460mm

Trying to find a 7-8 hp spindle.

I got a 40mm THK ground screw and another ground ballscew for Harding that is a double nut. Both are larger than my original design so everything got supersized.
 
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gt40

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Here are some more pics:

Length of mold:
length.jpg

width of base:

THK 40mm ballscrew- this thing is really heavy, new old surplus-. It must weigh 30 lbs/14kilos:
thk%2040mm%20ballscrew.jpg
Harding 36mm ballscrew for the y axis:
harding%20ballscrew.jpg
Here is a pic of the layout for the mold base. It is almost ready to go- I will be radiusing all corners in the mold with grout. The pipes are going to be cast in place lift points- stick a pipe or lift strap through them. Keep in mind all of these pics are from the perspective of the bottom of the base looking up or upside down:
layout%20with%20tubes.jpg
Final shot showing the x axis linear rails laid over the mold to gauge foot print- The wider base will support the x axis better.
x%20axis%20rails.jpg

I will post more but would also like to ask for help- I should have the mold finished and be ready to pour the base by next weekend. If anyone local to Santa Monica would like to come down, I could use the help. Mixing 7 gallons of epoxy and 700lbs of aggregate should be interesting.
 
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gt40

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Update:

I have the mold for the base almost finished and the ground steel bases for the linear rails aligned and bolted to the mold. The two 1/2" thick 3" wide bars also have the benefit of supporting and keeping the mold really flat. Pic of the mold with the ground steel bars bolted and aligned:

ry%3D400
My epoxy just arrived and I should have the "flour" portion of the aggregate mix tomorrow.

FYI, the "flour" is from a pottery supply house and is Custer Feldspar, so names because it is a type of mineral mined in Custer, SD, is made up of silica and alumina combined with sodium and potassium oxides, It is finer than their 320 grit stuff they had and the price was right at 50 bucks shipped for 55lbs. There are studies showing adding an extremely fine flour like mix of powdered quartz/aluminum oxide to the blend of aggregates can increase strength by as much as 30%.

I have 3 size sands, decomposed granite to pea sized gravel also.

I plan on casting next week now that I have everything together.

Anyone not familiar with "epoxy granite" may find this link of interest:

http://en.wikipedia.org/wiki/Epoxy_granite

There is also a 4000 post thread on the zone but basically you gain as much as 10 times the vibration dampening capability of cast iron and other benefits:
  • Flexibility: custom linear ways, hydraulic fluid tanks, threaded inserts, cutting fluid, and conduit piping can all be integrated into the polymer base.
  • Inclusion of inserts etc. allows greatly reduced machining of the finished casting.
  • Assembly time is reduced by incorporating multiple components into one casting.
  • Does not require a uniform wall thickness, allowing for greater design flexibility of your base.
  • Chemical resistance to most common solvents, acids, alkalis, and cutting fluids.
  • Does not require painting.
  • Composite has a density approximately the same as aluminum (but pieces are thicker to achieve equivalent strength).
  • The composite polymer concrete casting process uses much less energy than metallic castings. Polymer cast resins use very little energy to produce, and the casting process is done at room temperature.
The idea is to mix various size hard quartz, granite etc aggregates in a mix that minimizes the space between them and 8-15% epoxy as a binder only. you ram it into a mold and if possible vibrate it to further compact things. The material will take accurate impressions of the mold and can cast flat to .001" if done correctly.

While I have worked with concrete and composites a bit, this is my first effort with this material besides casting a sample.

Should be interesting....
 
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jbolt

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Great project!

Just out of curiosity, are you going to add additional support around the perimeter of the mold?

Jay
 

gt40

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Great project!

Just out of curiosity, are you going to add additional support around the perimeter of the mold?

Jay

I going to use strap clamps around the mold and bar clamps + the 2 pipes will be through bolted to prevent 800 lbs of mix from finding a way out hopefully. I have screws every 6" on the mold seams + construction adhesive.

Part of the reason I want some other people around is in case it gets interesting :p
 

Cobra

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With mold walls that thin I would be tempted to run "edge-on" rings around the mold to keep the sides from flexing outwards.
 

MikeWi

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More work and expense, but it seems like embedding the whole mold in a larger box with tamped sand would offer complete support to the mold walls and keep everything where you want it. That's a lot of weight pushing on that wood, and the packing process will want to push it out as well.
 

gt40

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Thanks for the suggestions- I think you are both right about reinforcing the perimeter. I haven't seen any builds this size so better safe than sorry. Any other suggestions are appreciated...
 
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