[ckelloug]

Walter,

The national Machine tool manufactuer's association was taken over by The Association For Manufacturing Technology. Their URL is www.amtonline.org

I've put in a request to their general information e-mail for a source of the publication. I also looked it up on www.worldcat.com and found that purdue university is likely the nearest library to you with a copy. . . Georgia tech is the closest one to me. . .

That looks like a good paper.

I've been looking at your samples and musing about the air bubbles and some other factors. I need to redo the calculation for how much stress I think your sample is under at the breaking point so that I can compute the Griffith (critical) flaw size. The Griffith Flaw size is the size of flaw at a given stress level which will cause a crack to propagate through the material leading to failure. From some back of the enveloping, I think the critical flaw size could be as small as 2mm right now.

It also looks like your samples failed with the aggregate breaking. This is a sign that the failure is not necessarily the matrix's fault. A possibly cost impractical test would be to redo the experiment with alumina, zirconia, or agate as they have much much higher fracture toughness values than quartz.

In that the sample that utilized the wide range of aggregates performed better than the one with the large aggregates, I think it supports the generally accepted theory result that the fracture toughness of small aggregates is better than large aggregates. I believe the fracture toughness of the aggregate is right up there with the overall density of the material as a critical factor.

As a note, several items I've read lately indicate that water vapor supports the creation of critical flaws in glass fiber which is chemically very similar to quartz.

Your use of heat and waiting for the material to settle are consistent with all of the epoxy manufacturer best practices sheets I read lately. Kudos for the good work on your technique!! It would also be good to leave the samples overnight in the oven at about 200F for absolute maximum epoxy crosslinking.
I'm still of the belief that the air bubbles have to go to get truly strong material: especially with what I've been looking into about Griffith flaws.

I haven't started experimenting yet and I goofed today and fell asleep when I should have been bidding for this vacuum pump on ebay Larry warned me about. . . Worse yet, I have to go back to work tomorrow Sanity should be limited this week as my Bridgeport and lathe will be here and I need to put up the rollup door in the shop.

While the epoxy we have may not be perfect, it does look at least decent. I still think silyated silica fume or nanosand could bring us the order of magnitude improvement based on the papers I've read. DAK3333 Are you out there?

--Cameron

[walter]

Thanks, good info.

As for the heating, post curing, etc.. I'm purposefully trying to keep this stuff out of my system. I've already had some allergic skin reactions and am not in the mood for breathing the fumes. I did the heating to prove the point but that's about it. Done with heating. I may have to step down to 60F degree epoxy, not sure yet.

Anyway. Thanks for the digging! We've come so far and yet have so far to go.. Unbelievable.

Oops, found another one http://jrp.sagepub.com/cgi/reprint/25/1/17.pdf (sorry, can't help it!)


(Yes, I know about nitrile rubber or nitrile butatoluene gloves).
_

[lgalla]

I am dazed&confused as usual.
If the actual aggregates are breaking,the epoxy and air entrapment are not the problem.Possibly crushed aggregates come with cracks or fractures.the stone should be much stronger than the epoxy.I am scratching my head a lot on this one.Perhaps larger aggregates split along the grain of the granite.Large blocks of granite can be split accurately by pounding in wedges.
High quality granite has 60% or greater quartz content.Some or the machine castors are using 100% quartz.These observations are meant to be questions.
Walter,are you getting epoxy on your skin,resulting in the irritation?ArU wearing gloves?
Larry

[greybeard]

Just a thought about rocks in general.
Would igneous rocks(large glassy mass) be stronger or weaker in general than metamorphic(small crystalline matrix) ?
I'm wondering, after Larry's suggestion that the crushed granite might come with its own micro cracks between the crystal boundaries, if other sources/stone types might be a better bet.
In my location flint is the common "rock", and is available in all sizes and conditions.

John

[Zumba]

I've done about a dozen tests with US Composites epoxy and Home Depot sand, and the aggregates don't break. I would avoid large aggregates completely.

A good test to prove this point is to get some of those aggregates and pound them with a hammer (against a piece of wood or metal plate). Then try pounding a small piece of Epoxy-Sand with a hammer. I can guarantee you that it'll be a lot easier to break the large aggregate.

My favorite mix so far has been 4:1 by weight, of sand to epoxy. 5:1 is okay, maybe a bit thick. 6:1 and 7:1 have been difficult to pour and somewhat weaker. 8:1 and 9:1 were very thick, very dry, and weak. Not enough epoxy to bind the sand particles.

I did some strength tests using Epoxy-Sand packed in PVC pipes, 1-1/4" ID. I sprayed the inside of the pipes with LPS-3 and let it dry to wax. I probably could've used liquid car wax as well.

I proceeded with some rudimentary test by pressing on the E/S cylinders with a mechanical arbor press (1-ton). The cylinders were supported on the ends. It is interesting to note that there was some plastic deformation going on with the cylinders. In one instance (perhaps because the cylinder had only been curing for 24 hours), the cylinder bent a bit but didn't break. Like a banana but not as curved.

I will conduct more formal tests later with a hydraulic press w/ pressure gauge. I'm currently building a simple vibrating table out of 2x4s and will see if that helps with the vacuum degassing.

I'm pretty confident that E/S in a 4:1 ratio can be made to be as strong as softwoods, especially with a rebar core.

[RotarySMP]

Zumba, this is great experimenting you are doing, but I think you are missing the point of polymer concrete a bit here. You are aiming for maximum tensile strength with these tests, which is of little relevance to a machine tool frame.

The aims of a polymer concrete machine tool frame include adequate strength, maximal rigidity, low shrinkage, thermal stability, and lowest cost.

With a 4:1 aggregate:epoxy ratio you make a test sample with unnecessary tensile strength, excessive ductility (it bent), low rigidity and excessive cost (epoxy is far more expensive than aggregate).

I would guess that you have become focused on tensile strength because is the easiest characteristic for you to test in the home environment.

Since the material is cheap, and the process allows variable feature thickness in a single pour, strength is pretty irrelevant. You just add thickness to the feature until it supports the design load times the factor of safety.

Epoxy is only added to lock the aggregate together. The strength and rigidity come from the aggregate.

[icando48]

Hey guys.. didnt think I had any thing to add but maybe..

as a sculptor I use resin and .... marble dust... to make sculptures, and also the back up for molds. I have never tried to make a self leveling bed for something but if you use the marble dust $6.-$7. a bag........at your local place that makes counter tops.. and the resin is a bit expensive.. $99. a gal I use the polytec resin #2 or #3 .. (harden in 3-5 min)

when pouring I eyeball it and use the marble dust as filler.......they say 10%.. but I use a lot more than that I also use microbloons a very fine filler .. so it doesnt get too heavy. I use the uneven ones..they are a lot cheeper.. bout a bunch years ago and havent even dented the bag. . any way you can add enough filler to the resin so you can mold it by hand. like play dough.

Must use precautions as always masks and gloves when handeling.

If any one is interested in more information and specifics they can get back to me.

thanks for all of the information you all are willing to share.. I just sit back and read and enjoy.. hope this helps some one.

I.Can.Do makes resin copys for artists and has a spin casting foundry that makes jewelry from pewter. and other things.

[Eson]

I hope you guys have gloves, good clothes and gas mask and beeing outside while doing this. You can easilly get allergic against epoxy and you can never be near uncured epoxy again. I know a guy whos dad repaired boats for meny meny years and now hes paralyzed in his hands because of polymer allergy.

I have one request, what if you had a layer of carbon/glasfibre on both sides of the E/G sample making it an E/G sandwich. Wouldnt that make it strong enough?

[ckelloug]

RotarySMP,

Your observations are good but I think they are lacking in scale. One of the reasons that a lot of optimization is going on on this thread is that the materials produced thus far (I'm not as familiar with Zumba's tests as Walter's) are not strong enough to hold even their own weight in many cases if used in a serious machine part. The problem is that to get deflections in the ten thousandths of an inch, with the low moduli involved, pieces reach the point where their weights grow faster than their strengths. These pieces could be augmented a great deal if they were supported with metal but metal has a higher Q value than Epoxy Granite and some here would like to see it possible to make a machine without having to cut any large pieces of metal. The strongest E/G possible will probably have a lower material cost than weaker stuff with too much epoxy and be only marginally harder to process so why not engineer a good mixture?

The second problem being dealt with is the fact that intuitive mixing and construction techniques handle the drier mixes rather ineffectively resulting in materials that don't perform nearly as well as something like the rule of mixtures would portend.

Finally, we have the result from Gupta that a 30-36 um shell of epoxy around each piece of aggregate according to Gamski's model is optimal in terms of maximum strength. Generally speaking, the mixtures should both have more tensile and compressive strength until the the epoxy amount reaches the 30 um level where strength begins to fall off.

I think all of your observations are good points qualitatively however it seems to me that quantitatively, production problems with the E/G material rather than mis-guided optimization are leading to a lot of experimentation for a mixture that overcomes the problems.

Thanks for posting your thoughts on this. I remember that you have made many good contributions to this thread over time.

--Cameron

[ckelloug]

Eson,

That is an excellent idea. It slightly complicates modeling the structure but it does give you much higher tensile strength on the bottom which is a key problem with E/G. If our epoxy bonds better to the glass or carbon than it does to the granite, this would work quite well. I'd want to check and see if it increases costs too. I am luck on this in the respect that Toray Carbon Fiber's factory is about 30 minutes from my house. I was thinking of using 1 inch randomly oriented carbon fiber strands for the bottom of my castings.

icando48,

Are you talking about acrylic resins or epoxy resins?


Thanks for Posting,

--Cameron

[walter]

Regular gloves are no good- epoxy penetrates that in 5 min. Use nitrile rubber or nitrile butatoluene gloves.

[RotarySMP]

Cameron,

Thanks for the update, I must admit I missed a few hundred posts a while back

I do remember that you guys were talking about a pretty big gantry though. I an see your point about strength becoming an issue for unsupported/poorly supported parts with design tolerances for deflection in the tenths.

You are talking about extremely specialized machinery there. I think it is great to set such goals, but when you get to the point when you need to machine metal parts measured in meters, with tolerance under thou, you have to question every part of the home CNC chain.

From Spindle bearings, to driver software, to toolpath optimization etc.

One thing is sure though, if you set a goal to make a large gantry to hold tenths in metals, and prototype it, you will probably end up with the skills to make an outstanding smallish hobby mill, or a great large gantry for softer materials, even if you miss the original goal.

[Mazaholic]

Don't know if this helps but.
I think a vacuum will be required for the epoxy to adhere to the aggregate..Here's my thinking.
Almost all aggregate that i know of will be porous to some extent,the traped air inside the porosity will not be able to escape and will cause problems when trying to get the epoxy to adhere.
A vacuum will do two things.
1 remove any air traped in the epoxy.
2 remove any air traped in the porosity of the aggregate and allow it to be replaced with epoxy,giving you a stronger bond.
I don't know if it will increase the bond enough to get the results your looking for.
We impregnate alluminum parts where i work.
It seals the parts by removing air and replacing it with resin,without a vacuum the resin would just dry up and fall of the parts...no adhesion at all.
I know alluminum is not the same really, but it seems like the same principle.
Don't know if it helps...just my .02

[PlasticWorker]

I have read most of the posts in this thread with interest. If I might add a couple of observations.

When making fibreglass parts the strongest stiffest part has the least amount of resin with adequate wet-out. That is why the vacuum bag is a popular technique. A quick explaination for those not familiar with the process. Glass lay up on mold with resin worked in. Then a BARRIER film which has very small holes in it. Then sometimes a layer of coremat to absorb excess resin. Then a nonporus plastic bag is sealed to the edge of the mold Area. ( or the whole mold can be placed into a bag and sealed) What this does is use atomospheric pressure to force the resin/glass together while pulling the excess resin through the very small holes in the BARRIER film. When cured the barrier film acts as a mold release between the fiberglass part and the excess cured resin, which can be peeled away.

What if you used the same process on the open side of the mold. This would compact the mixture into the mold with a force of 2000 psf, while allowing the excess epoxy / air to come out. Just pulling a vacuum above the mixture surface ( as in degassing ) only expands the air bubbles to make them "lighter " in relation to the rest of the resin and / or mixture so they try to float to the top. But that only works when the mix is liquid so the bubbles can move through it. Because the bag is flexible and can move down onto the casting it applies a 14.7 psi pressure ( approx 2000 psf ) to the whole casting open face. On a 4' x 4' casting that is almost 17 tons! If you are worried, you can always use a lower vacuum.
Vibration tries to get the mixture to settle and help the aggregate to "fit" into each other, which it good but the only pressure is the weight of the casting itself. It seems to me that a combination of the two techniques would give the highest density casting with the least air entrained.

CKELLOUG
If you put random carbon fiber into the bottom surface of the part it doesn't have anywhere near the effect of continuous fibers running 90 degrees to the bending monent, that is why we use cloth instead of mat. I believe you can get carbon cloth with a majority of the fibres running in one direction.

Without trying to hijack the thread, one last observation. While a rigid, vibration damping base might be great, the weakest design element I have seen in most of the home built gantry machines are the uprights for the bridge side. They are much too weak and flexible. If you think about the cutting forces involved.
When you plunge the force is up which puts the gantry sides into tension and tries to bend the crossbeam. This is OK since the Z depth will go to where it belongs as the Z stops going down and the beam straightens. Most of the cutting forces are generated when cutting in the XY plane. The resultant force is 90 degrees and opposite to the cutting edges at any given instant. This puts all the force into flexing the gantry side. And since the gantry sides are normally longer along the direction of travel there is a constantly variable force shaking the gantry. That is why if you look at any well made commercial router, the supports are massive square tubes.

Regards,
Will

[speed33317]

Decomp Composite www.decomp.com of Clevland Oklahoma.

They are also located in Dunnelon Florida.

I just remembered these guys.
I bought High Temp epoxy to mold some carbon fiber
valve covers. They were super strong, with only three layers
of cloth I was able to stand on them. They weighed about 6 oz. !!!

I vacuum bagged and post cured.

Had good deals on carbon fiber also, but this was 4 years ago.

[ckelloug]

PlasticWorker,

Thanks for the insightful post. This is the most extensive description of vacuum bagging to date. Your thoughts about the imposed loads on the router are priceless and by no means a thread hijack. The guys who are doing the serious router construction will undoubtedly comment more. (I'm the thread's lose cannon engineer/theoretician). B.W. Staynes' 1975 paper in the London Symposium Proceedings suggests that setting under pressure (as you have observed) is the most effective method of compacting the mixture. The biggest obstacle to vacuum is that nobody I know of on the thread has bit the bullet and obtained a vacuum pump yet.

As for the random carbon fiber in the bottom of the part, I am aware that it isn't as good as continuous, I believe it is much cheaper and probably easier to do however which is why I've been considering it. According to the materials engineering book I have been working from, this can be expected to provides isotropic reinforcement at 1/5th the strength of continuous carbon fiber which is still pretty substantial. The important point is that the fibers must be longer than the critical length which IIRC is about 3/4 of an inch. Were the suggested inch long short fibers in the base aligned 90 degrees to the bending moment rather than randomly, they could be expected to perform almost as well as continuous fibers since 1 inch fibers are longer than the critical length for carbon reinforcing fiber.

Mazaholic,

I think your remarks about vacuum are dead on and I hope that they serve as impetus for more folks who are serious to start hunting for vacuum pumps.

RotarySMP,

Most of the folks whose designs I understand are trying to make machines about 1 meter square. Even at that level, the going is pretty tough. With E/G of 2ksi flexural strength a 4x12 inch cross section unreinforced gantry beam is required hold a few ten thousandths of an inch of Z deflection over 36 inches with the weight of the E/G and 100 lb axis load. Nobody who has posted any kind of test results seems to have a mix anywhere near that 2ksi flexural strength even though it is half the value of published figures for commercial E/G.

Nobody I know of on the thread has tried vacuum deairing, silane based bonding agents, deairing agents, or high density aggregate designs yet though.

All,
Thanks again to the new posters and the returning old ones. Slowly but surely we are making progress. . .

<B>If anybody has any informed opinions on dispersion hardening agents, they would be greatly appreciated here.</B>

--Cameron

[Mcgyver]

wow this thread has marched along! holding tenths over 36" gantry's? maybe, but not with EG imo, modulus of elasticity is just not up to it. to the threads beginning, professionally EG is used to reduce vibration which does because its made of two difference materials impeding the wave transfer (imo what type of rock doesn't matter much, its not intended as a strong structure, its a vibration dampening structure) I always believed it would make a great filler for a fabrication - ending up with something a little better than cast iron for both vibration dampening and elastic deformation. it depends of course on what the load is, but if the fabrication is normalized and EG filled, a very rigid & stable structure would result. I do admire the efforts made but continue to be a doubter on EG as machine material unto itself - if you want the rigidity, better learn to weld boys

[ckelloug]

Mcgyver,

I agree with you totally that E/G is much easier as a weldment filler than as a structural material. I've read enough of the literature however to believe that it should be possible to get material with a 2000ksi flexural modulus since this is at the bottom end of commerical products. It might not be trivially easy and it is surely not easier than a rationally designed metal machine.

The compressive modulus of a mixture using epoxy available in 1975 cited by B.W. Staynes was 4000ksi. This formula used Huntsman GY250 with HY 830 and HY 850 hardeners and 10-12 mm rocks and 52-100 mesh sand along with fly ash. The large rock was Diabase Hornfels which as near as I can tell is similar to the stronger class of quartz like minerals. The same epoxy and hardeners are made today and this formulation is the first one on the Araldite GY250 epoxy data sheet. (It is a thick 18,000cps epoxy IIRC).

I have to concede to you Mcgyver that making a machine mostly out of E/G is swimming upstream. I've run through a fair number of engineering calculations however and I believe that if the 2000ksi or better flexural modulus number is reached then it will be possible to make most parts on a machine out of E/G. Results of the thread members doing lab work appear not to be there yet on modulus and I haven't started doing my wet lab work yet. As a reality check, E/G with good enough modulus was done before I was born so I think that it should be possible to make it in the 21st century especially considering the progress in bonding agents, additives and cheaply available vacuum pumps.

That's my .02 worth. Thanks, Mcgyver for dropping by this thread again despite thinking we're not on a successful path. Polite criticism is welcomed by me and others. We might be crazy but I don't think so. I'm at distraction junction this week as my Bridgeport mill and lathe should be here Wednesday.

Regards,

Cameron

[Mcgyver]

no smugness intended, on the contrary, excellent contributions and my hat's off to all the experimenters and sources of knowledge. thats the only way to get a break through and who knows, may happen yet. i don't think that if EG fails on its own that that means 1) its not a excellent candidate for a diy structure in conjunction with other materials and 2) its use for diy has been substantially advanced by the thread. I've learned a bunch by watching it; there is a huge amount of info and I'm appreciative of the contributions

[Geof]

....We might be crazy but I don't think so...

Ah but would you trust the judgement of a possibly crazy person?