[martinw]

DAK3333,



As for gantry top beams, you are right about other profiles specifically rectangle and rectangle tube being better. I did the calculations with square beams mainly because it was simple and gave some intuitive feel for the materials for those who don't have a complete grasp of moments of inertia and flexure.


Deflection is inversely proportional to the fourth power of the depth of the beam so profile does matter much. Thus square is suboptimal for minimum deflection but it's easier to visualize.



--Cameron

Dear Cameron,

Moment of inertia is a measure of any section's property to resisit bending deflection, I.

If b= the width of the section...
and ..

d= the depth of the section

in the particlar case of a square section

I= b^4/12

in the case of a rectangular section...

I=(b*(d^3))/12


That means that deflection is inversely proportional to the third power of the depth, not the fourth power..

IMVVHO, and I have probably got it wrong, yet again..

Best wishes


Martin

[ckelloug]

Martin,

You are absolutely right. That's what I get for not checking the hand book or integrating it out again. Oops!

I'll correct my post.

[walter]

I tested the 14.5% sample from post #1283.

Very poor performance, only 10 ft*lb, but at least we know that epoxy sticks to the aggregates.

This was pretty much expected, these aggregates came from Home Depot and aquarium store- you get what you pay for...
_

[ckelloug]

Awesome Walter,

The picture shows that it broke down the middle of the aggregates like you said. That means the matrix effectively carried the load to the aggregates. I suspect that aggregate much less than twice the void size are probably completely ineffective so I assume that the only aggregate which was particularly effective was the really large stuff.

DAK3333, the materials chemist, thinks that carbon black is probably the wrong dispersion hardener for this material. I'm thinking it's adsorbing some of the hardener. He's thinking silica fume is more appropriate here. I'm thinking along the lines of Cabot TS-530 or TS-610 silica fume or equivalent. Given that you don't have silica fume in stock, I'd suggest dropping the carbon black if you happen to make any more samples any time soon until we can figure out what to replace it with and how much.

The message I take form this test is that either much better deairing is needed either via a vacuum pump and/or additives or much bigger and stronger aggregate must be used. I might also try the technique suggested by harryn and wash the aggregate in 91% grade rubbing alcohol.

One last thing: You take great pictures but if you have a ruler, it would be a great thing to put in the picture so the rest of us can get the scale of features.

Finally, I spent the day trenching for sprinklers but I'm about 75% done and should have sprinklers down and be starting on sod by Tuesday along with the concrete approach for my workshop.

[walter]

Cameron, you are a lifesaver.. That Agsco is a goldmine!

Just check out these gems, my goodness...

(did I mention they are 99.5% Quartz??)

I'm not going to be able to sleep tonight. :cheers:

[ckelloug]

So,

Whatcha got? Itty bitty sand and great big rocks? What sizes are they?

[walter]

What sizes are they?

0.06-0.13mm
0.25-0.50mm
0.80-1.70mm
5.00-15.0mm

(angular shape)

[ckelloug]

That Quartz looks great! Good catch on finding it in the catalog. I missed it. I think I'd like to try some of the #3/0 stuff when I get my act together. The #3/0 is rather close to the NISS profile but it will suck tremendously badly without some serious deairing.

I'm anxiously awaiting your results and furiously digging for sprinkler pipes.

[harryn]

Hi - just a quick FYI. I do some business development consulting for a small material supplier, and had a chance to spend quite a bit of time this past week with one of their chemist. Before you get too excited, language was a barrier that we worked hard to overcome.

In any event, it appears that the pro s consider vacuum degassing of adhesives / epoxy / silicones to be "commonly known normal good practice". Sort of like opening the wrapper of a candy bar before eating it.

Most of the other stuff I learned from him had more to do with resistance to UV light chemistry, so it really does not apply here.

[ckelloug]

Harry,

Let me be the first one to flippantly say "That Sucks!"

It's an excellent contribution to making progress on E/G however. Back to E-bay to search for vacuum ovens.

--Cameron

[harryn]

Harry,

Let me be the first one to flippantly say "That Sucks!"

It's an excellent contribution to making progress on E/G however. Back to E-bay to search for vacuum ovens.

--Cameron

Hi, just to clarify my post a bit. Vacuum removal of air bubbles prior to curing is recommended. I am not at all clear about doing the cure in a vacuum - there are some tricks related to doing the cure under pressure that might be better for the highly motivated.

[qroger]

CKelloug
I think that vacuum is one the places where the 90/10 rule works. You can get 90% of the vacuum by spending 10% of the money. The other 10% will cost the 90%. A single stage home size compressor creating low pressure via venturi, will get you lower pressure, and at fairly hign speed. Actually your garden hose might work. Probably won't give you enough though.
roger

[brunog]

0.06-0.13mm
0.25-0.50mm
0.80-1.70mm
5.00-15.0mm

(angular shape)

Nice aggregate Walter,

For starters I suggest you go with the following mix:

0.06-0.13mm: 9%
0.25-0.50mm: 9%
0.80-1.70mm: 15.5%
5.00-15.0mm: 66.5%

this is based on Fuller's parabola.

this mix might be a little heavier (higher density):

0.06-0.13mm: 19%
0.25-0.50mm: 11%
0.80-1.70mm: 16.5%
5.00-15.0mm: 53.5%

Actually both mixes are based on Fuller's parabola:

V= 100*(d/D)^n

V= %volume
d= sieve dimension; in this case I have used .13mm, .50mm, 1.70 mm and 15.0mm
D= largest sieve dimension; in this case 15.0mm
n= .5 on the first mix and .35 on the second mix


Note: the results of the calculation is a subtotal of the total volume, the reason being that .006mm to .50 mm aggregate will result in 18% of the volume which includes the 9% volume used by .06-.13mm aggregate:

.13mm will give you 9%
.50mm will give you 18% including the 9% of .13mm
1.7mm will give you 33.5% including the 9% of .13mm and 9% of .50mm
15.0mm will result in 100% bcasue it includes all of teh aggregate sizes.

Best regards

Bruno

[brunog]

CKelloug
I think that vacuum is one the places where the 90/10 rule works. You can get 90% of the vacuum by spending 10% of the money. The other 10% will cost the 90%. A single stage home size compressor creating low pressure via venturi, will get you lower pressure, and at fairly hign speed. Actually your garden hose might work. Probably won't give you enough though.
roger

An old refrigerator compressor can generate up to 28hg inches of vacuum

Best regards
Bruno

[ckelloug]

I agree with harryn's post 1332 http://www.cnczone.com/forums/showpo...postcount=1332 that curing under pressure is good and perhaps better than curing under vacuum. B.W. Staynes in his 1975 paper in the London proceedings suggests that curing under pressure produced much better results than curing without pressure although it did little for the air content itself. He also cited two other papers as saying the same thing but I will have no luck getting a hold of them.

As for my comments about high vacuum, I found a manufacturer of commercial vacuum deairing equipment back in post 1249 that had a chart of vacuum levels required for various degassing processes.
http://www.cnczone.com/forums/showpo...postcount=1249

Their chart indicates that 29.72 inches of vacuum is required for adequate deairing of most resins and that virtually no deairing occurs at 29 inches or less vacuum.

The 90/10 rule applies to most things and trying 28 or 29 inches of vacuum is perhaps a start. Based on the chart I found however, it looks to me like it's a case where 90% of the needed vacuum turns out to provide 0% deairing.

[Servo Wizard]

Hi All,

I have built machines for various applications using weldaments and box tubing and I used hydrocal B11 and concrete sand mix to fill the voids. B11 actually expands a small amout when it cures so shrinkage is not a problem. It easy to work with because it has more pot life and the viscosity is thinner then epoxy. Dampening will equal or out perform epoxy and aggregate mix at a fraction of the cost.

I borrowed that technology from my old iron Top Fuel motors, yeah that was several years ago.

Servo

[ckelloug]

I googled up what Servo Wizard was talking about since I had never heard of it. It's a high tech version of plaster of paris:

http://www.gypsumsolutions.com/brand.asp?brand=hydrocal

Hydrocal B11 expands when it sets and has compressive strength of about 4500psi. For comparison purposes, it is about 70% as strong in compression as the weakest E/G mixture in the NIST paper. A major differentiator between E/G and hydrocal is that hydrocal does not appear to have significant tensile strength. The datasheet warns that normal hydrocal does not bond well to 10 mesh and larger aggregate. I also could not find thermal expansion numbers or info on whether it promotes corrosion but it is an alkaline material which would lead me to believe that it does. With the exothermic setting and the high expansion, I am a bit afraid that filling a large cast iron part with it might induce cracking and destruction of the part.

I believe that Servo Wizard is right that it makes a good weldment filler for steel anyway and depending on the design of the weldment, it might produce a bit of a post tensioning effect leading to lower static deflection of the weldment.

For Servo Wizard,
Near as I can tell, most of us still hanging on this thread after 1300 posts are trying to figure out whether and how we can make Epoxy Granite or Epoxy Quartz composites with sufficient strength to make machine parts without weldments.

If I understand the consensus, it was decided that weldments could be filled with many materials: poorly mixed E/G, portland cement in a rubber bag (Dr. Bamberg's thesis). It sounds like hydrocal is another excellent methods of filling weldments that has come up on this thread.

My personal opinion is that without more information, E/G is lower risk to the machine being filled as there aren't any circumstances or issues that would cause E/G to misbehave when filling the machine and damage anything. There is no corrosion issue with E/G and it won't expand and possibly crack an iron casting. If these two questions can be answered satisfactorily for hydrocal then it would be a more effective and much cheaper material than E/G for machine filling.

$.02 <--Cameron

[Geof]

Couple of comments: If the hydrocal is alkaline it will actually tend to reduce corrosion; acid promotes corrosion in steel alkaline recduces it.

ckelloug; in a couple of places in different post you mention pre or post tensioning reducing deflection...are you sure? Deflection depends entirely on Young's modulus, are you modifying the modulus with the tensioning? I thought that all tensioning did was increase yield strength by putting tensile stresses into the tensioning medium instead of relying on the tensile strength of the bulk material.

[ckelloug]

Geof,

Good comment regarding alkaline generally inhibiting corrosion in steel.

I just remembered breaking up plaster of paris over chicken wire as a kid finding rust and assumed neutral pH is what was likely to be good. I also note that the concentrated KOH solution around my lab tends to do bad things to most everything it comes in contact with. . . Corrosion isn't my cup of tea although I do know that if you put steel feet on an aluminum leg that it won't pass mil standard 810F environment tests for corrosion. . . (This was not my fault.)

As for post-tensioning, what I was thinking of is this:

Take a simply supported beam. It will sag in the middle. The bottom is in tension, the top is in compression. If you add a bit of compression to the bottom of the beam by tensioning a tension member below the neutral axis, you can reduce the deflection of the beam. If you adjust this additional force below the neutral axis of the beam, you should be able to reduce the static strain on the beam to zero. As additional load is placed on the beam, the additional tension member will elongate as the beam tries to bend and you will be fighting both the moment of the beam and the tensile modulus of the tension member.

In highway projects with pretensioned concrete, a beam is often bowed upward by a significant amount due to pretensioning in the no-load case so that it lays down flat when the full load is applied.

All I'm trying to say by "null out the deflection" is that you can reduce the strain to zero in the beam under static loading by adding a tension member and tension such that it creates an opposite deflection to that of the beam under gravity and cancels out the beam's own static deflection. A static tensile load is thus carried by the tension member instead of the beam.

In essence, the flexural strain is transferred from the beam (which we would like to have remain flat) to a tensile strain in the reinforcement member whose exact length doesn't matter to us.

I hope that explains what I was thinking.

[Servo Wizard]

ckelloug,

I see that corrosion has been addressed, so let me assure you that broken castings will not be a problem due to B11's 1% per cubic foot expansion rate. As stated in my original post, I used the B11 mix to fill the water jackets in my old cast iron Top Fuel motors and it did not crack the water jackets nor did it distort the cylinder walls. B11 is an excellent fill with a price tag that will work with any ones project budget.

Servo