[martinw]

Martin forgot to ask you.Did you see the posts on epoxy telescope mirror's.Looks cool to me.Any comments?
Larry

Dear Larry,

Will do, but must go.

Best wishes

Martin

[walter]

Consider a slab of granite. Find a custom countertop builder and check out their inventory. I live in a small town in upstate New York and there are at least two locations that have ample supplies of granite in various thicknesses and color.

Thanks. This sounds great, but we are looking for cheaper and easily workable materials.

At $0.80 per lb, Epoxy/Quartz is hard to beat. Castable to any shape.

We are far away from granite-like performance, and it'll take a while but that's where we're headed.

In a year or two we should be able to build this frame:



We are going to cheat and use massive steel reinforcements, but this is the look and performance we're after.


Larry,
I don't recall the percentages on the latest samples, but they were mighty tasty.
Now wait till I get my shaker going...

[lgalla]

Victrof epoxy/granite has better damping and stability than solid granite.A slab 4'X8' of1" or2" granite cannot support itself.To be self supporting in this size it would be 12" thick.
Larry

[ckelloug]

Hi all,

My name is Cameron. I'm a newbie here but I've read every post in this thread as well as the thread where Harlow talks about his semi-commercial E/G minimill parts e.g. http://www.cnczone.com/modules.php?n...d=24879&page=6.

Note that Harlow uses rebar both above and below the neutral axis of his tables in the castings.

On the bright side, I am another engineer (though not a PE) for the merry band although I am a touch rusty on structures calculations.

At any rate, I think that there is a critical missing factor in all of these discussions which is the proposed load on any of the E/G members folks want to build. The predicted loads combined with the weights predicted for the members being contemplated will tell you what you need in terms of internal reinforcements.

It is my general impression that conventional machine tools are as heavy as they are not so solely for deflection reasons but rather for vibration reasons since F=Ma. If this assumption is true, the reason that the E/G material works so well is that the intrinsic damping characteristics of the material do away with the need for as great a mass such that the reduced strength caused by the use of E/G is only a factor in Forklift Collisions and other destructive events.

So in short, can those who have posted specs for what's trying to be built here give some estimate of the weight they intend to add to the structure? What does your workhead weigh on the gantry, what does the part weight that you are trying to machine and what does the size of the machine absolutely have to be.

E/G and regular concrete formulations have no tensile strength to speak of: about 2ksi as I recall. As a result, you will have to do something to get tensile strength if your piece actually needs it. If you are building a machine base or a gantry, then your piece is likely to need it but without knowing the loads then you can't know for sure.

The guys who want to make steel parts and reinforce them with E/G may be on the right track as pieces that experience tensile loads of any great degree will deform substantially due to the near zero tensile strength of E/G. It's not cheating to reinforce these things with metal, it's just not necessarily helpful to do so without an understanding of why something is being reinforced where it needs to be reinforced for the most gain.

A beam carrying a horizontal load between two supports looks like this from the end an side. Above the center line, the beam is in compression. Below the center line the beam is in tension. AS a result, E/G in the beam really only has a major effect in the upper half!
Filling the lower half with either air, meringue or diamond will have almost the same effect as filling it with E/G.

E/G in both the bottom half and the top half may qualitatively (Though the effect may be insignificant) promote crack propagation in the whole E/G structure as a crack that started in the tension portion where the E/G is weak could easily propogate upward into the compression portion. If it is a filled metal beam of sufficient strength then the metal will take up the tension load in the bottom of the beam. If it is an E/G beam, then either structural elements like rebar or fiberous reinforcements will have to carry then tension loading to prevent fracture of the beam.

Beam in ASCII art.
End of beam                  Side of Beam   
******                     ***********************
------ Compression         *---------------------*             
****** Neutral Axis       *********  ****  *********
++++++ Tension             *+++++++++++++++++++++*
******                     ***********************

It's quite easily possible that the tensile loading on many of the parts to be made isn't even up to the 2ksi tensile strength of the E/G material in which case the reinforcement isn't an issue.

It is fun to build prototypes of things and there will definitely be a need for good fabrication of what ever is decided upon but I would think that the group might be able to come up with a provably good design if people could provide the loads that they are trying to deal with so that the more engineerish types can do some load calculations.

--Cameron
a.k.a ckelloug

[LeeWay]

This thread has been progressing very well. Glad to see different formula's and testing taking place.
As mentioned before, the bendy cake probably wasn't at full cure. This coupled with the lack of larger aggregate was lacking to achieve the rigidity sought.
This mix seems to have more correlation to mortar rather than to concrete. I think rocks would have helped.
When you add fiberglass shreds to the fluid epoxy, it will thicken of course, but it doesn't take a lot of it to increase the strength. Especially so if you are using some rocks along with the sand.
Rather than using an oven for the cure, you might look at using an IR bulb. That will heat up a bunch with good results. Curing this in an oven you still want to use for soufflé's is not desirable. Trust me on this one. I do a lot of powder coating and you need a dedicated oven. The IR lamp will heat the surface, so it would need to be flipped after one side is cured.

Adding steel for strength would be desirable in thicker pours. It could be rebar for a high load/ stress part. Hardware cloth is steel mesh that could be used on smaller pours that would greatly improve strength. Keep up the great work, guys.

I must have missed it, but where are you guys buying your epoxy from?

[walter]

Awesome post Cameron!

We are definitely in the early stages and not wanting to limit ourselves to just one platform. Here are the considerations:

1. Damping.
Some folks will use E/G for damping only. High-density composite base is said to stiffen the machine and provide ten times more vibration dampening than traditional materials.

2. Large Base/Table.
Also for dampening purposes, but as a self sustaining structure which forms the body of the device. Possibly with use of large I Beams. Weight 1000-5000 lbs(?).

3. Gantry.
Picture in post #942 shows another possibility- It seems more challenging and rewarding. Some tricky reinforcements will be needed. (Let's add 50-100 lbs for Z Axis).

4. Hybrid.
Low cost, barebone hobby structures encapsulated in E/G. Cost $0.80 per lb.






LeeWay,
The IR bulb sounds interesting.
Here's the stuff I would use in thicker pours:



Our epoxy store: www.uscomposites.com
_

[LeeWay]

Thanks a bunch, Walter.
Here is what I plan to fill. I had bought some hydraulic cement to use, but I'll use that somewhere else. This stuff looks more like the ticket now.

http://www.cnczone.com/forums/attach...8&d=1174145162
Thread linky

[lgalla]

Welcome Cameron and thank you for reading all the posts.I am the flat table,steel framed don't want a forklift collision guy.I have a few questions for you or our other engineers not related to my purpose.
Looking back at articles and pictures of commercial machines, no where do they
mention use of steel reinforcments.Where a cast iron table top may be 1" thick an equivalant E/Q may be 4"thick.I assume this is to try to componsate for the low tensile strength of E/Q????
The huge E/Q gantrys we have seen may be hollow.Would this help the deflection issues?Thinking back to the pictures,the E/Q gantrys appear to be for inspection machines and other low load purposes.Sorry for puting a [dampining factor]on the issue,but I can't see a solid E/Q gantry being useful.
May be I missed or forgot some pictures.Can someone correct me on this.Anyone seen a E/G gantry with a large spindle?
Larry

[romihs]

I must have missed it, but where are you guys buying your epoxy from?

LeeWay,

I ordered my Epoxy from a German company; R & G.

Regards

Sandi

[lgalla]

Lee make sure you get the 635 thin epoxy.Walter has had problems with too slow a cure.Lets wait,perhaps he can give some tips.It may be necessary to use the medium cure.I think Walter has the slow cure.
Just found this on the US Composites site.
NOTE: Our slow hardener should only be used at temperatures above 80degrees unless an extremely long curing time is desired. Temperatures below 70F during the cure can result in a 2-3 day drying time.
Larry

[walter]

Ok, here's something new. The E/Q leftovers in this pot are very concrete like. Hard and edgy (after only 12 hrs).



I will try to replicate that in a larger sample.
(Epoxy curing may have something to do with it).

EDIT:
Yes Larry, slow cure seems to be the problem. 4"x4" E/Q beam might take months in this setting.
_

[ckelloug]

The thread about Harlow and the minimill parts led me to the fact that he had rebar in his tables. It was either in the thread or on Harlow's site IIRC.

You can however predict the need for some kind of tensile support in the bottom for any E/G beam supported in two places with an unsupported span in the middle from qualitative application of materials science.

As the ascii art in my last post showed, the bottom of any such beam will always be in tension. The only question is how much.

[lgalla]

Walter have you tried heating the mix by putting the mix container in a sink of hot water?Your viscosity will half or better and allow higher aggregate ratios.The pot life will also shorten.I still think one should experiment with slow epoxy and add heat during mixing without the pot gelling to fast.Try it you will like it.
Larry

[ckelloug]

What is the maximum length of member for which the the consensus of the folks on the thread is reasonable? Are we talking 8ft or are we talking more? I don't know the dimensions of the picture in post 942 as I've never used any of the machines you all are building.

[walter]

What is the maximum length of member for which the the consensus of the folks on the thread is reasonable? Are we talking 8ft or are we talking more?

I'd say 4ft for unsupported member and up to 20ft for supported. Just shooting from the hip. People here are building all kinds of stuff...


Larry,
I tried it all - hot water sink, heat gun, low viscosity, singing, etc. Works good but I'm still worried about resin sitting in the mold (and not curing 100%). I need to be sure of curing. There's a lot of things that go into casting- can't add another worry to the process. Epoxy will simply have to take care of itself (lol).

The gummy-shoe-sole was not pretty at 2lbs - imagine it at 100lbs...

No big deal. I'm sure we can work this out, just need a little more time. I've got some new samples and will continue during the weekend. Taking a few days off for now.

You guys have fun!
:cheers:

[romihs]

Walter have you tried heating the mix by putting the mix container in a sink of hot water?Your viscosity will half or better and allow higher aggregate ratios.The pot life will also shorten.I still think one should experiment with slow epoxy and add heat during mixing without the pot gelling to fast.Try it you will like it.
Larry

Hi all,

I agree with Larry using slow epoxy for this type of application.
But I do feel that if one uses hot E/G, or it heats up during curing, when the curing process slows and the E/G cools, it will shrink.

Now this is not so bad if all you have is E/Q as the the whole mass will shrink equally, but if you have any other kind of material embedded in the E/Q, the CTE's of the two materials being different, will introduce stresses into the structure as it cools.
Again, this could be neglected if the dimensions of the inserts are small, but if one adds rebar to a 4' beam of E/Q, the differences in CTE could cause the entire E/Q structure to buckle,ever so slightly, under the stress induced by the cooling.

Several posts back, someone noted that no forms of reinforcement was ever seen in any photos of these machines. I remember a photo of a huge mold being filled with E/Q and the only 'additions' to the structure were inserts and no reinforcement. It is entirely possible that we have not seen any reinforcement in these structures because the manufacturers want to keep that secret.... but it is also possible that there is no reinforcement within the E/Q because it could cause deformation of the final product with changes in temperature.

That was my 2 cents worth. I have no mechanical engineering background, but I wanted to get my thoughts out there....

Regards

Sandi

P.S. I used an epoxy that has a pot life of 400 minutes and requires 15h curing at RT + 10h at 70C (which I still have not tried yet)...

[LeeWay]

I agree in theory with what you are saying about the heat, however heat has been discussed and dismissed as an issue. Evidently it just does not get that hot. I imagine when poured very thick (over 4") then there exists that possibility. I think steel would have a comparable shrink rate when cooling provided they both were cooled at the same time, which would be the case here.
Rebar wouldn't be used for any thin pours (less than 4"). In that type pour if you needed reinforcing, you could use the mesh or the fiberglass particles. The mesh does help with tension as well by spreading any stresses across the mass rather than in one spot. A machine base with anchor points may not even need any reinforcing, but a stand alone gantry certainly will.
Think of a concrete header over a door or window. These have rebars for a reason. That reason is to spread any weight on the top of the header across to the supports or sides of the window. This is basically the same type of structure and load spread that you would want.......no, that you would need for any machine gantry other than perhaps a plasma cutter.

[romihs]

Lee,

I understand what you're saying.

I agree that a beam needs to be reinforced so that it can distribute loads applied to it.

I was just taking a shot at the reason why reinforcement has not been seen in E/Q machines yet.

It's really late, my gray cells are probably half asleep already..... so I'm not thinking clearly...

Regards

Sandi

[brunog]

Sandi,
There a many ways of reinforcing E/Q, and I believe that large manufacturers will not use rebar specifically but the pipes for lubricant and/or vacuum, conduit for wiring, air lines... all of them can and are used as reinforcement purpose in addition to their main purpose or vice-versa.:idea:

I you look back at the casting done on the german site, I am sure that what looks like 1 x 2 fb was not used only for fastening the linear slides on the X and Y axis, but also as reinforcement because the flat bars are also anchored in the E/G. Not only that but all major fastening points are reinforced with anchored steel plates.

Best regards

Bruno

[ckelloug]

The basic principle governing reinforcement is this:

To first order, an E/G beam supported at the ends without reinforcement has to be at least twice as heavy and likely twice as deep as a reinforced one because all of the material below the neutral axis contributes virtually no strength since E/G has a negligible tensile strength. See my ascii art a few posts back for the meaning of neutral axis. If the doubled mass(weight) is an asset in your application rather than a liability then reinforcement may not be necessary.

Harlow's commerical machine at http://www.cncbridges.com has rebar however:
http://www.cnczone.com/forums/showpo...9&postcount=51

This is not to say all machines need rebar but Harlow is selling a lightweight mini-mill and it's certainly the way to make it easily in the size it is.

See also
http://www.cnczone.com/forums/showpo...59&postcount=9 for a bit more info.

Generally, the heavier a gantry, the harder it will be for the machine to change direction. The lighter the gantry, the more it will vibrate. There is likely a happy medium somewhere. Likewise, there are some designs that will deflect more than others depending on how the materials are used.

As for the thermal issues, the E/G has a higher coefficient of expansion than reinforcing steel. In an extremely thin member, or one that is not symmetrically reinforced on the top and bottom, the reinforcement will tend to bow the member concave down. This effect will constrain how thin composite members can be made.

Anybody have a good sense of the temperatures differences we might see across a baseplate or a gantry beam?

Here's a food for thought: If you use a metallic tensile reinforcement and can preferentially heat the reinforcement material by some method like induction heating by a reasonably large amount just seconds before the epoxy sets, you could prestress the member and get much greater strength with much less reinforcement and less worry about the exact Granite content. You might have to heat the material from it's current temperature by something like 100 degrees C after everything is done and right before the epoxy sets. If you use a reinforcement that exceeds the thermal expansion of E/G then this might be a bit easier (steel does not.) I don't remember the numbers but aluminum might work.
This may not by possible in the HSM environment but I may work more on the idea after I get out some info on beam bending. You could also induction heat metallic reinforcement to help cure recalcitrant epoxy.