[lazlo]

I think he's being overly critical.

Wait a second! That's Concrete not EG. Nice setup though!

Ah, that explains a lot. romihs said it was "EQ" which I presumed to mean Epoxy Quartz. The aggregate in the core sample appears to violate the packing rule of 1/5ths that's been posted here (and analyzed to within an inch of it's life ).

In any event, I was comparing to Walter's pour on like, page 3 of this thread, which I believe is Cameron's simplified formula

[romihs]

Lazlo,

You are correct, I initially though it was EQ.... but then edited the post (on the bottom)..

It is still an interesting project.

Regards

Sandi

[J13]

howdy,
I've been lurking for a while on this thread & the Zone in general; I've decided to take the plunge & build my own CNC mill (metal cutting). After contacting Asgco and getting sticker shock for their materials I've been attempting to find a reasonably local supplier (within 4 hours or 200 miles one way) for abrasive-graded aluminum oxide, silicon carbide or quartz and am not finding success. Does anyone know of a small quantity (300# or so) abrasive supplier within that distance of Buffalo, NY with a web presence? (I'm an over the road truck driver and prefer to window shop while im not home - then make the purchase in person when i get home)
TIA
J
nevermind - found a msc branch office in buffalo

[ckelloug]

Lazlo,

The original mixture of using 1/5th or each aggregate is prone to segregation when vibrated. I can't think who posted that they had observed this in the last hundred or so posts. I don't have the post number at hand but the post I recently made about the improved mixture is likely to behave better under sufficient vibration.

J13,

I've had the folks at Washington Mills act quite responsive about aluminum oxide but I haven't bought anything yet. They're in Birmingham, AL. I don't know about suppliers in the new york area but I seem to remember finding one a long time ago.

My General status update:
I just received my surplus 400lb 3HP vibrator capable of generating 7000lbf. The adjustable eccentric rotor is so heavy, I can barely turn it by hand. I need to get a VFD to run it off the 220V 1Phase in my lab as the $5000 my utility wants for the transformer upgrade to get 3 phase power is out of the question right now. I hope to get the vibrator online in May and start getting some samples tested.

I'm heading to the UK on Friday to visit greybeard John however so I won't be getting a lot of E/G work done for a few weeks. Lab work is a lot slower than theory.

I have also noted recently that Jeffamines D-230 and T-403 are such slow hardeners that are required in such quantity that they dilute thick epoxy down to almost the viscosity of the reactively diluted epoxy.

Regards all,

Cameron

[J13]

Thanks Cameron, I've spoken with the folks at Washington Mills, they directed me to a reseller, called them and I'm going to be using Washington Mills aggregates... I've already gotten the surface plate and vac pump, air compressor is already here and when i get home again im going to be attempting a few trial batches - has anyone done anything with either silicon carbide or aluminum oxide? (or both?)

SiC or Al2O3 being the sole type of aggregate... no quartz...

[jhudler]

I just received my surplus 400lb 3HP vibrator capable of generating 7000lbf.

<O</O
<O</O
Gordon Bennett!! 7000! Bit of overkill ehh??
LOL! Bolt that to the workshop floor! Ready made earthquake!!<O</O
<O</O
(Well... it would be in the local vicinity)<O</O

[craigl2]

J13,

I have been considering experimenting with EG and looking at the Washington Mills web site brings up one question out of many I have been considering.

Most of the Silicon Carbide and some of the Aluminum Oxide aggregates are characterized on that site as friable or semi-friable. Throughout the messages in this thread I have had the impression that the innate "hardness" of the material comprising the aggregates is being equated directly with the strength they impart to the final EG mixture. I have wondered if other properties of the material (shear strength? etc.) should not also be considered. Apparently the samples that have been made and tested have failed by fracturing cleanly through the aggregate and not by crushing the aggregates as I would expect if "hardness" (compressive strength?) were the only factor involved.

I would like some one with more understanding of this than me to give their thoughts on the subject.

Also I would appreciate hearing which specific aggregates from Washington Mills you choose to use and why.

Craig

[J13]

The aggregates I'm looking at right now are silicon carbide, Al oxide, and either carbon black or silica fume. The largest size I'm thinking of is 60 micron; mostly for cost reasons. I'm also thinking that I'm going to need to use some type of non-welded steel reinforcement to assist with placement and securement of the forms.

I purchased a 2x3 foot surface plate that will be used first as a pour surface and then as the main structural member of my moving-table mill.

If anyone sees a potential failure point, please speak up.

[greybeard]

The largest size I'm thinking of is 60 micron.......
If anyone sees a potential failure point, please speak up.

Is 60micron a typo ?
If not, I don't understand how this is going to add any improvement in the structural strength of the epoxy. It will certainly reduce the amount of epoxy you need for a given casting, but I can't see it doing much more than making it opaque.

John

[J13]

it wasn't a typo - the way im planning on doing this is to vac-bag within a 100 psi chamber + vibrate the whole mess

[greybeard]

So, that will give you a very compact casting, but I fear your selection of such a uniformly small size of aggregate will give you no improvement in strength.

The point of the large range of sizes is twofold. Firstly the smaller particles can pack in the spaces left when a group of large particles touch each other, and then secondly, any crack that starts to develop, externally or internally, is continually having to change direction as it goes round each particle. This continual change of direction absorbs energy from the force that's causing the crack to travel through the material, until it has not enough to go any further.

With such a small size, any crack will hardly need to change direction, but simply cleave the epoxy.
This is why gravel is added to the sand/cement mix to produce stronger concrete.

John

[J13]

in that case i could add in some 400 micron aluminum oxide or playsand - how concerned should i be about crack propagation when im planning on using steel reinforcement?

[greybeard]

In several places on the thread, you'll find a rule of thumb that you start with aggregate that is ~1/5 of the size of your smallest wall thickness, i.e. if you are casting a piece that is 2" thick, the biggest aggregate should be about 3/8" diameter.
Then you should have at least four other smaller sizes, so that each is roughly 1/3 the size of the previous one, whatever they are made of.
Other posts in the thread give a more accurate suggestion, this is meant as a guide only.
This sort of mix will not only give improvement in strength, but greatly improve the sound absorption of the structure, if you have steel reinforcing or not.

All the information is buried in the thread , but I freely admit it takes a lot of reading.
John

[ckelloug]

Hi Craigl and J13,


I worked out the stiffness vs. percentage of fill for Aluminum oxide back in post <A href=http://www.cnczone.com/forums/showpost.php?p=546814&postcount=3441> 3441</A> and in an earlier post for quartz. I haven't worked out which Washington Mills material I would like to use however to make an alumina mixture. I did have an eye for Duralum Special White as it was their purest product and is thus the most likely to meet published numbers as I see it.

I think we can agree that things that are exceptionally friable aren't good but I'm not sure how to react to their numbers as most aluminum oxide I've seen is pretty fracture resistant. According to the wikipedia entry on <A href=http://en.wikipedia.org/wiki/Fracture_toughness>Fracture Toughness</A> Aluminum Oxide has a fracture toughness of about 4 MPa*m^1/2 whereas pure aluminum has a toughness around 24 MPa*m^1/2. As a result, it is 6 times as difficult to fracture aluminum than aluminum oxide. Any hard ceramic material will be a lot less tough than metal. What I can say is that both the theory and observation in the literature dictate that smaller particles have a higher toughness than larger ones due to there being smaller volume for flaws to be present.

In general, Alumina or silicon carbide will make nice materials however they have higher thermal expansions and lower thermal conductivities than quartz which could be an issue in an extremely high precision machine. I'd like to use alumina or silicon carbide but I haven't looked far into it.

J13,
With a maximum size of 60 microns, I'd expect that it will be fairly hard to get a high packing density and stiffness is almost solely dependent on packing density. The stiffness can be accurately predicted from the above cited post derived from the equations in Kinloch's book <u>Advances in Structural Resins and Primers</u>. My formulas go from about 4000microns to 1 micron since my modeling efforts suggest that 4 orders of magnitude and 10 fractions is likely to produce the best density if vibrated solidly enough.

You need to make it to 82% alumina to be equivalent to commercial products in stiffness. If you make it to 68%, you'll have about the same stiffness as wood. Any less than 68% mineral and the result will be fairly similar to pure epoxy.

As long as you're not planning on loading the material heavily, I probably wouldn't worry about structural failure via crack propagation. The ultimate tensile strength is likely to be around 4ksi and the compressive strength much higher. I did some calculations that demonstrated (to me anyway) that if the part is being designed thick enough to hold sub-thousandth tolerances for a respectable machine tool that it shouldn't be in danger of breaking under reasonable loads. I thus stopped worrying about the parts breaking a long time ago. I'd be careful with the steel as it has a large difference in thermal expansion characteristics from E/G and will have a tendency to want to warp a precision structure as the temperature changes. I haven't quantified this effect however.

Carbon black and silica fume are excellent materials but they are likely to require much more sophisticated dispersion equipment than any of us have available. The talks I had with Cabot suggest that one must break the agglomerated particles up into the tiny particles they are composed of to get them to work as nano-materials. I'd say they are unlikely to increase modulus or strength very much in homebrew E/G but I haven't run tests. Given ultrasonic or high shear mechanical dispersers, I expect great things from silica fume and carbon black.

The other thing I have learned lately is that BYK A-525 deairing agent's dosage level is about 2grams per kilogram of epoxy and like they say in the BYK book, it tends to give the epoxy a milky appearance. I had to add it with a small plastic eye dropper (pipette).

While making E/G stiffer than wood and casting it accurately are both difficult, I'm hoping to actually get test parts made in may or June.

If anybody has any more questions, feel free to keep posting as somebody here is sure to answer.

Regards all,

Cameron

[J13]

I've read thru what you posted yesterday + am wondering, aside from the expense, why there has been no suggestion of using low-grade (low - grade as in dosent quite measure up to bearing quality) steel, brass, or even bismuth shot as part of the matrix? they've got a MUCH higher fracture toughness ect...

[ckelloug]

J13,

I've thought about your question. For the home shop builder, it is quite possible that a metal filled epoxy would be better. It should require a somewhat lower fill percentage to get reasonable stiffness albeit at higher cost. Take a look at http://www.ptm-w.com/ as they have off the shelf products that may be suitable.

E/G is good for cheapness and production products but if you can find a source of grease-free metal filings and the packing density is reasonable, it should be great. I'll do a graph on the likely stiffness of iron filled epoxy when I get back from lunch.

Regards all,

Cameron

[the4thseal]

i have been working on the steel shot and steel fiber ect having success. i am going to post pics ect. there is a process but it is looking good.

[J13]

I'de LOVE to see some results with steel shot, am wondering if a composite using both ceramic and metal grits as well as the balls and fibers couldn't push the density and strength of the finished casting quite a bit beyond the commercial products

anyone working on a homebrew electromagnetic vibrator for compaction/dispersion?

[the4thseal]

that is what i have been working on. i can get my hands on steel shot and bearing easier than graded stone. i have been using much of what has been discussed here. with the addition of steel fibers...i will try to give a a better idea with pics soon,

[the4thseal]

that is what i have been doing i just have not had the time to post my results. though i have used fine steel fibers. i will post my stuff soon.