Epoxy-Granite machine bases (was Polymer concrete frame?)

[anarchos]

From the first study, I found the conclusion to be interesting:

"From the tests, it was observed that Granite Epoxy specimen with 12.5% resin content has excellent damping compared to other specimens containing different resin percentages. It has a damping ratio which is around 5 times more than cast iron. Also, it can be concluded from the vibration tests on Granite Epoxy with CI powder, that addition of CI powder to Gr-Ep composites increases the damping properties by a considerable amount. "

I only briefly read the article, but it seems like 12% resin content with 50% coarse granite and 50% fine granite is the way to go. Maybe I skimmed past, but I didn't see what they are defining as coarse and fine (I saw 10mm maximum granite size). They also didn't seem to mention any ratio for the cast iron powder. They also mentioned using 40hz vibration as that's close to granite's resonate frequency.

[probinson]

Granite's longitudinal wave propagation is roughly 5950m/s and transverse is roughly 3450m/s. Without looking up the crystal size or doing any math my estimate puts the molecular resonance closer to a gigahertz. Where does the figure for a natural resonance of near 40Hz come from?

[CaptainVee]

It seems to me the article is not really scientific but primarily using a scientific path to bring structure to a question.

"50% coarse granite and 50% fine granite" can by no means be repeated by others.

Somewhere it is mentioned that excess hardner is not used in the matrix but that is only half the truth. The excess hardner takes away strenth from the matrix that it would have had if there had been no excess.
Epoxy ratios need to be kept as the manufacturer states, using other ratios will weaken the mix.

[RotarySMP]

Did you use a casting epoxy or a laminating epoxy ?? =)

It is a laminating Epoxy I got of Ebay.DE

[RotarySMP]

My first test sample with 10% epoxy cured, so set it up and measured deflection vrs load, and calculate the youngs modulus at 40 GPa. Since all the errors in clamping would lead to excessive deflection, and the various surface defects result in a lesser cross sectional area than I roughly measured, I am confident that 40GPa os a conservative value for this recipe.

I first clamped it across bottom and top surfaces, which is not optimal, as the top has a strong miniscus, so I repeated the measurments with the test sample on it's side, clamped across two nice flat surfaces. The result was 38GPa

I have also cast a second sample with 8% epoxy, which has only cured for 3 days so far. It wont reach final strength till Sunday. Today it measured aabout 50GPa. This time I used a pneumatic rivetting gun to vibrate the mould. This recipe was stiffer, and barely responded to tapping into the mould with a stick. The vibration made it slump, but quite a bit slower than 10% epoxy, and it still had some air inclusions.

If you don't have access to vibrating equipment, I would suggest using 10% epoxy.

[garyfivell]

So it's all about the mix?

[dan_the_welder]

Hey Everyone. I had the control panel off of my Light Machines Prolight mill and took a couple of pictures of the inside.

This is the back of the Z column.

Thought there might be some interest!

[shedbob]

Hallo dan_the_welder

Thanks for the pictures of the prolight milI .I have looked at pictures of this mill before to see what could be learned.Never seen a rear view before and not what I expected.Always thought it would be solid.Seems they cast the CI (?) rail supports/plate in the eg then machine rail mounts and bolt rails from the rear ? Or do they grout rail plates into the frame while held in prealigned jig ? Or ? Looks like it compromises column (and base presumably the same) stiffness mostly in torsion ? I didn't find a lot of user reports on these mills when I looked at them.Do you think the eg offers any advantages in use over similar CI frame ? Always wondered what the saddle and head were made of presumably CI not eg.

Robert.

[eloid]

I didnt have the time to read all the pages on this thread,
But has anyone had succeed machining this material,
I understand the general intent is for casting bases and frames
but what type of finish are achieved before and after machining
the E.G. material that has embeded steel or aluminum?

[castinite]

I didnt have the time to read all the pages on this thread,
But has anyone had succeed machining this material,
I understand the general intent is for casting bases and frames
but what type of finish are achieved before and after machining
the E.G. material that has embeded steel or aluminum?

The material can be ground or milled, if you know what you are doing. We have precision surfaces routinely milled to very precise tolerances

[probinson]

The quality of the surface finish for a bare casting is pretty much dictated by the quality of your casting. A well compacted aggregate can be polished to a mirror finish without any fillers. The EG material, however, is typically easily bondable so surface treatments can also be used to apply a surface material that can be polished on top of an otherwise rough casting.

As for grinding verses milling, composite materials are usually composed of both very hard and very soft materials so, as a generalization, composite materials need to be ground with an interrupted abrasive cutting wheel.

Hard materials like quartz, aluminum oxide, carbide, etc. quickly take the cutting edge off of a milling cutter. Grinding, on the other hand, wears away both the workpiece and the cutting tool simultaneously. This continuously exposes new cutting edges so the cutting tool remains sharp. Soft materials like aluminum and the polymer binder come off as long shavings and need space between the cutting edges. These longer shavings tend to stick to the cutter, gumming up the abrasive surface, and preventing the abrasive surface from refreshing it's self. By having gaps in the cutter, the interrupted abrasive cutter can grind away the hard materials and expel the debris though the gaps before the accumulation plugs up the abrasive.

These are, of course, just generalizations. Your choice of materials will dictate the machining methods that are most effective. The bottom line usually boils down to deciding how much effort is it worth for a given surface finish.

[eloid]

Jack,

I made my own "plate" using junk&rocks. Now topping everything with TT epoxy.

Here are the beam elements:

any photos of the mix or detail casting tips on this beam finish

[eloid]

Has anyone tried Rust-Oleum TurboKrete with other aggregate mixes

[lcvette]

Wow, very interesting thread for sure! Has there been a determination on the best epoxy? I saw several variants used, but not a lot of comparisons. It looks like West 105 and epon 813 from Heliox are being used a few times.

Out of curiosity, what would you estimate the required volume of epoxy to be for a 10-12% by weight per cubic foot given a fine rock powder or silica carbide through 8-10mm aggregate? Just trying to do some cost analysis on a fairly large machine. Of anyone had some quantity per volume that would be great!

Sent from my XT1080 using Tapatalk

[konobi]

Hi all,

So I recently spent 2 days reading through the whole thread to get a background and understanding.

I'm looking to build a lathe soon, so I'm brushing up on lots of things as well as this materials science. Can I assume that Cameron's recipe on page 404 is the current "recipe" ?

Also, when making a base/machine, has anyone used a concrete vibrator during their cast? It seems like I can rent one fairly cheaply/easily... but will this be sufficient for the packing?

thanks!

--
Scott

[RotarySMP]

Hi Scott,

Can you actually source the ingedients of Camerons ideal packing density recipe? If you can, it would be cool if you could do a test peice and measure the Youngs modulus. Based on academic studies which , this sort of mix should produce a cured material stiffer than 80gPa.

I wouldn't get too hung up on a certain recipe, unless you are certain you can get exactly the same ingredients. There are plenty of examples (at least on the German CNCEcke Forum) of E/G builds.

So far I have only done test peices, but can already see that a vibrator with variable frequency is the essential tool in this process. Does the concrete compactor have variable frequency? 8% epoxy E/G is way stiffer than concrete, and needs to be hit with the right vibration to get it to sag. Nucky and Thomas, who started a company in Germany making E/G gantries have reported needing strong vibration at between 68 and 72 hz, with the exact frequency set within a few tenths of a Hz based on the form, mix and the temperature. They describe their set up here...
Peters CNCECKE

The other thing which my tests made clear is that you want a spread of aggregates down to really fine powder, not just stopping at sand. For my recipe, the sand and gravel is extremely cheap, but the only fine powder I have been able to source in sensible quanitities is powdered alumina from a pottery supply shop.

Thomas and Nucky moved on from making their own recipes to Silamix. In their words it was simply easier and quicker to work with.

I bought the aggreagates I could easily source first, than plugged them into Thomas's spreadsheet and got a recipe (see page 405) :

From the local Hardware store
5-8mm pebbles @ €4/25 kg
2-4mm crushed granite@ €2.50/25kg (unfortunately I can't fins rounded gravel in this size, so this is sharp broken Stone which is not ideal)
0-2mm quarze sand @€4/25kg
0.1-0.3mm quarze sand @€3/25kg

I end up at 0.12c/ Kg for the bulk of the aggregate.

The expensive bits for me are:
Powdered Alumina at €16/5kg from the pottery supplier
Casting Epoxy resin €40/2.5kg from Ebay.

Alumina 34c per kg E/G
Epoxy €1,28 per kg E/G

Total cost of E/G =€1.68/kg

Epoxy gets cheaper in larger qualities. If I buy 7.5kg over ebay, the total E/G Price per kg Drops to €1.15 If I could find a source of quarz flour willing to sell in quantities of less than a ton, It should be pretty easy to get the price below €1/kg (about $0.40/lb)

[konobi]

Hey Mark,

Unfortunately I don't have any way to do any sort of load testing myself. In terms of the concrete vibrator, it'd likely be an oztec electric unit.

Cameron,

I'm not sure if you're still testing away or not, but have you thought about trying a piece using a 1-3mm gelcoat with re-enforcing fibers (of some sort) and then using the normal filler mix? I would have thought that that would make a reasonable improvement to tensile strength.

thanks!

[konobi]

Hey Mark,

So... I've actually found an ebay seller who deals in silicon carbide grit at a reasonable price and who also ships worldwide.

I contacted him to see if they were interested in providing a pre-mixed version of the "recipe". He seems rather keen... http://stores.ebay.ca/mineralabrasives/

"First of all, since I make the mixtures to order, it would be easy for me to create custom mixtures from the grits I have available (60, 90, 120, 220, 500). If you think there is a demand, I would be happy to offer such mixtures as products on E-bay, or on my website."

So if we can get a good mix together for him, we should be able to have a source for the mix available by the lb.

@Cameron

The person seems to have some questions about spherical vs jagged media. We want the jagged variety, correct?

Thanks,

Scott

[CaptainVee]

Why would you want to use silicon carbide for epoxy concrete?

I have used it for filling gaps after setting the gantry straight because it was the finest dense powder I had on hand.

Once set you can not do much more then heat it to change the result.
I removed some edges using a chisel and it was blunt in one stroke.

[RotarySMP]

For best packing density you want rounded aggregate, not sharp/jagged.

Ordering aggregate mixes in useable quantities from a long distance vender is normally an expensive proposition due to freight costs. When I contacted Euroquarz about buying Silimix 282, which is an Aggregate graded especially for this purpose, they offered to gift me the 60kg of aggregate, for my hobby purpose, but the freight from Germany to Vienna was a show stopper.

Remember the whole E/G thing works because you can reach adequate stiffness despite the low Youngs modulus, as you can cast thick sections without the shrinkage issues which make thick metal castings difficult. Thick sections = weight. Not sure what you want to make, but getting hundreds of KG's of aggregate sent from Oregan to anywhere is going to cost a lot.

Unless you can get silicon carbide for the price of sand, I wouldn't bother. It's Youngs modulus is 8x that of granite, but I doubt it would make a measurable difference once mixed and cast as E/G.