[jsage]

Thanks Igalla,

Zumba, yeah the steel strikes me as overkill. Having epoxy granite on the outside seems more complicated than just filling steel sections. But this thread is polymer concrete frame. It just has to be on the outside don't it .

At the same time as you strive for that very large outside diameter that gives you unbelievable stiffness doesn't your material cost increase substantially. It would also seem more difficult to work with very large beams. I would lean towards slightly smaller then the steel makes more sense to me. If we were talking tabletop then massive might become unecessary. For the small format verticle mill then massive would not hurt.

I just wouldn't feel comfortable without some steel skins or reinforcement on any span over 4 feet. It would seem to me that you would have to have more supports, more cost, more labor.

Lastly which would dampen better, steel on the outside or epoxy granite. One thought when I did this rough mockup was by keeping the steel structure slightly internal and through mounting through epoxy filler mounting blocks would reduce vibration more than the opposite.

This was all a shot in the dark so I'm not overly committed.

mike

[jsage]

Igalla,

I can give you a couple examples that might help. A good composite does more than add up a bad one subtracts.

From boats. Solid fiberglass is very heavy so if you build a boat with fiberglass then you have to achieve a thickness that will give you the rigidity you need to keep it from tearing apart under repeated forces.

A wood boat is actually a bit better because of its cellular structure and density. You get greather thickness for less weight. Wood also has dimensional qualities which can be used to the boats advantage. It has the strength required for a tree stand straight. However wood requires more craftmanship and labor.

If you build a composite of wood with fiberglass skins of sufficient thickness than you can get the same characteristics of solid fiberglass and solid wood with less weight and moderately reduced thickness. The fiberglass provides direction reinforcement through the alignment of fibers and the glass provides tension under loads. The wood in this case is increasing thickness at lower density. Wood has good compressive strength for weight.

You can use honey comb, balsa, pvc foam to replace wood and reduce weight further. Of course then shear, fatigue, and delamination become more critical with PVC foam and honeycomb. With PVC foam a higher strength laminate reinforcement like carbon fiber can help in high speed applications. The epoxy formulation or similiar needs to be compatible with carbon fiber. The skin enhancement is because the PVC foam and honeycomb don't add as much to strength. The skin has to be stronger to compensate. The gain here is mostly a moderate reduction in weight.

compatibility of when a material will come into load strength versus another material is potentially an issue. If the fiberglass and epoxy in this example had lots of elongation before reaching top strength then the wood would carry much of the loading and fail without the stretchy fiberglass epoxy every really adding much.

There are also issues of dissimilarity including the natural frequency of oscillation of a particular material. Aluminum and carbon fiber are potentially disimilar enough to cause fatigue failure at connection points.

In my mind epoxy granite is an amorphous structure with no directional reinforcement so I wonder over time if it can become distorted.

So now you can see where my biases are derived from.


Anyhow thanks for the link that is very helpful.

Mike

[lgalla]

Hey Mike
I think filling a tube with 16lb density foam will increase the stiffness even if the tube is 1/4" steel.This is similar to a honeycomb or I beam structure,thick stiff skins and a lightweight core.Deflection or compression of the beam should be less as the thick skins spread the load on the core material.Not to worry, a few thou difference at least the foam fill will eliminate cavity resonance.I have filled thin aluminum tubes with 2lbDensity foam and it makes a huge difference in deflection.I have been searching the web for info on filled tubes and have found little.The only thing I can find is on crash resistance of automobiles.Light foam is used in crash tubes in cars and roof support beams.
They claim 3times the bending strength and 3times th compressive stregnth.
It appears a 4 or 5' gantry of steel and epoxy would be to heavy.Just looking at alteranate routes for long spans with vibra damping qualities.E/G is great for bases, frames,tables etc.but we may need to "lighten up"on the moving parts or the machine.
Larry

[martinw]

Hey Mike
I think filling a tube with 16lb density foam will increase the stiffness even if the tube is 1/4" steel.This is similar to a honeycomb or I beam structure,thick stiff skins and a lightweight core.Deflection or compression of the beam should be less as the thick skins spread the load on the core material.Not to worry, a few thou difference at least the foam fill will eliminate cavity resonance.I have filled thin aluminum tubes with 2lbDensity foam and it makes a huge difference in deflection.I have been searching the web for info on filled tubes and have found little.The only thing I can find is on crash resistance of automobiles.Light foam is used in crash tubes in cars and roof support beams.
They claim 3times the bending strength and 3times th compressive stregnth.
It appears a 4 or 5' gantry of steel and epoxy would be to heavy.Just looking at alteranate routes for long spans with vibra damping qualities.E/G is great for bases, frames,tables etc.but we may need to "lighten up"on the moving parts or the machine.
Larry

Dear Larry,

One effect of a foam fill is to greatly reduce the tendency of a hollow section to fail bybuckling . This is not the same as failure in pure bending. I think that the foam fill supports the compressed part of the section and reduces this tendency. Here is a link that suggests this...

http://www.sae.org/technical/papers/960435

I think that the other effect of the foam, as you suggest, is to tie the top and bottom "flanges" of a rectangular hollow beam together, so that the whole system acts as a "stressed skin panel".

I have heard that hollow aluminium masts on sailing boats are often filled with foam to increase their strength for these reasons.

One last point. I'm pretty sure that you really must use a two part foam, even if it is more expensive. Single component moisture-curing foam is spectacularly useless at filling large voids evenly.

Best wishes

Martin

[Zumba]

Martin, good point on the buckling issue. Similar to how a can filled with soda is is much stronger than an empty one, even though liquid clearly has zero structural properties. In the case of the soda can, the liquid has no place to go, and because liquid is not compressable, it acts like a bottomed out spring (after the co2 fizz has compressed).

The foam technique is very interesting because foam is filled with tons of air pockets. What prevents it from flexing just a like a seat cushion?

When filling a pole with foam, I'm guessing that the foam is basically there to contain air. The tiny pores of dense urethane foam act like damper valves to prevent the air from escaping to quickly. This would prevent the pole from denting (and consquently buckling) as a result of shock loads. But for rigidity under constant loads, I'm not sure how much it would actually do.

[martinw]

Martin, good point on the buckling issue. Similar to how a can filled with soda is is much stronger than an empty one, even though liquid clearly has zero structural properties. In the case of the soda can, the liquid has no place to go, and because liquid is not compressable, it acts like a bottomed out spring (after the co2 fizz has compressed).

The foam technique is very interesting because foam is filled with tons of air pockets. What prevents it from flexing just a like a seat cushion?

When filling a pole with foam, I'm guessing that the foam is basically there to contain air. The tiny pores of dense urethane foam act like damper valves to prevent the air from escaping to quickly. This would prevent the pole from denting (and consquently buckling) as a result of shock loads. But for rigidity under constant loads, I'm not sure how much it would actually do.

Dear Zumba,

I think there is a difference between soda (or beer cans!) and cushions.

With the soda can, there is nowhere for the air to go. With an open-cell foam cushion, the air can escape.

Structural urethane foams are closed cell, so the air is trapped, and I suspect that this is why they work. All the air is doing is supporting a matrix of structural plastic. I agree that the air also provides some damping.

Best wishes

Martin

[brunog]

I believe rigid foam panels can be encapsulated in E/G, someone tell me if am wrong:

Epoxy resin does not melt foam like polyester resin does. Don't boat makers laminate rigid foam with fiberglass to insulate and add stiffness to the hulls?

[martinw]

I believe rigid foam panels can be encapsulated in E/G, someone tell me if am wrong:

Epoxy resin does not melt foam like polyester resin does. Don't boat makers laminate rigid foam with fiberglass to insulate and add stiffness to the hulls?

Dear Bruno,

Excellent point.

Yes, they most certainly do.

Polystyrene foam is attacked by polyester resin, polyurethane foam is not. Epoxy resin works with both types of foam.

The idea of casting in rigid panels, rather than injecting later into "formwork" is a beautiful idea.

Best wishes

Martin

[lgalla]

Zumba the 16lb denisity foam I was refering to is extremely stiff and as stated previously is similar to solid softwood.It is Extremely rigid for only weighing 16/lbs/cu ft.
Martin good point of the two componet foam.The moisture cure is s$*&.The two componet is extremely fast maybe you have one minute to mix.I might add this stuff sticks to anything.The foaming or bubbles clean the surface resulting in superior bonds.If filling a metal tube the bubbles break on the cooler surface causing solid resin against the tube making its own stiff skins with a light core.
Zumba&Martin thanks for bringing up the soda/beer can thing.I have been working with another zone member in Italy on BUD Beams and can core tables.
To make a 4'X8'/beer can/soda can/E/G structural table would require 512 uncrushed cans for the core and some method of emptying the cans.Any sugestions?:cheers:
It may appear to be a bad pun,but really has some merit.
Larry

[walter]

It appears a 4 or 5' gantry of steel and epoxy would be to heavy.Just looking at alteranate routes for long spans with vibra damping qualities.E/G is great for bases, frames,tables etc.but we may need to "lighten up"on the moving parts or the machine.

My machine will probably have a moving table so I would still consider a heavy E/G gantry (with steel core or some kind of reinforcements).

BTW, I'm learning a lot from these posts- you guys are awesome and the effort is much appreciated.

[lgalla]

Boy I am slow at posting,3 while struggling with my post typing is a one finger operation for me.
Brunog yes ridig urethane panels can be used in E/G.You were actually refering to polystyrene foam which does melt i n polyester.
The density of polystyrene and strength is of little use in E/G,urethane is the way to go.If you laid down 1"ofE/G and put in 6"squares of P/U foam separated 1/2" and poured over this would form ribs connecting the top and bottom forming a lite stiff pannel.
Larry

[martinw]

.
To make a 4'X8'/beer can/soda can/E/G structural table would require 512 uncrushed cans for the core and some method of emptying the cans.Any sugestions:
It may appear to be a bad pun,but really has some merit.
Larry

Dear Larry,

Speaking of bad puns, I'm "working on the case".

Best wishes

Martin

[brunog]

Larry,
what about foaming epoxy to fill a light steel of aluminum structure?
I the product difficult to handle without having to line every square inch inside the garage, before working with this stuff??
I've seen some pictures showing panels made of 2 steel plates laminated with half inch thick epoxy foam in the middle. It's basically a very similar structure to steel or fiberglas of domestic exterior and garage doors.

It could also be used alone or as a second cast to fill cavities in a lighter E/G structure.

Best regards to all

Bruno

[brunog]

Larrry,
I am as slow as you are in posting, or maybe there is are so many posts tonight that it's hard for us to keep up.

Bruno

[brunog]

To make a 4'X8'/beer can/soda can/E/G structural table would require 512 uncrushed cans for the core and some method of emptying the cans.Any sugestions?:cheers:

Larry,
Drink the beer and/or soda and then fill the cans with 2 part foam !!??!!

You don't have to drink all 512 cans tonight, keep a few for tomorrow

Bruno

[lgalla]

Bruno beleive it or not I have tried epoxy foam 25years ago.If memory serves me correct,which does not happen often at my age,"Its boring"Man this stuff took 24hrs to rise.It is used in deep sea submercibles as it can be 50,000lbs crush resistance.I costs more than I make in a week[month].
Larry,
Dring the beer and/or soda and then fill the cans with 2 part foam !!??!
WOW I just learned to cut and paste.Actually I am a rather lazy guy and foaming the cans would be time consuming and messy.Expanding urethane foam has 20psi pressure.Doesn't sound like much but I have blown apart cabinets from overfilling.When I finnish the can I am working on I will try the crush test with the top down.In theory if the air is confined the vessel should have greator crush resistance.Give me 5 minutes i will let you know the results.
You don't have to drink all 512 cans tonight, keep a few for tomorrow
Actually I am 2 beers Larry.It would take256 days to have enough material to finish.This is longer than waiting for 8' thk's to appear on E-bay.Perhaps a party or two would help.:cheers:
Martin the British are renouned for their puns.From the colonies I hear by challenge you to a duel on the best revelent pun that has revelentcy to E/G
Larry

[brunog]

Larry,
I know you need to bulid a large table. I don't know what type of material you are going to process with your machine. If you need to hold your pieces with vacuum, you should consider including the vacuum circuit pipes to be encapsulated in the E/G table, this will somwhar reduce the overall weight and E/G volume and likely still have enough vibration damping efficiency.

best regards

Bruno

[martinw]

Martin the British are renouned for their puns.From the colonies I hear by challenge you to a duel on the best revelent pun Larry

Be warned Larry,

My pencil is now razor-sharp.

Best wishes,

Martin

[jsage]

canned laughter.

You folks decide to have a dry, offbeat humor contest, let me know!

P.S.

I like the idea of high density 30+ psi foam but again not cheap. Like I felt fiberglass would require to many laminations to be useful on the outside, I wonder if the lower psi stuff would help the deformation problem.

Despite my earlier mistatement this stuff is not likely fail but have compression oriented distortions for lack of any real structure over time. Everytime I go to Lowe's or Home Depot and see all those synthetic decking pieces I cringe a little. You want to prevent the distortion from ever occuring by counteracting those forces with something of similar resistance. Or perhaps resistance is futile

[lgalla]

Bruno I have considered the vacuum equation and piping in the base is or can be benifical as reducing the weight can result in stiffer structures with less weight.The downside is if you want to vacuum clamp through a MDF spoil board 4'X8' the vacuum pump must be 15hp or confined to purpose confined hold down areas,resulting in slower production rates.Hope you understand my blurb,I didn't.
Larry
Thanks for the input,gets me thinkin