[jkeyser14]

This was suggested back in Post #200

Ah, yeah, I missed it. It seems like it would work to me. A friend and I threw some large sized garnet in some slow cure resin. We're going to let it cure for a week. I'll post the results. I can crush test it to find young's modulus. I also might be able to ask someone in the vibrations lab here to test a piece for us, but it would require making a mold to get an exact shape. Don't expect results quickly however, I've got a lot of school work going on at the moment.

[martinw]

Walter,



In a composite beam without reinforcement like rebar etc, we will ultimately be limited by the tensile strength of the material.

One interesting idea both to improve tensile strength and to null out static deflection due to the wight of a beams is post tensioning. .

Dear Cameron,

1) Rebars will fix any tensile problems.

2) Who cares about static deflection under self-weight? It's the deflection under shifting and variable loads that matters, isn't it?

IMVVVVHO,

Best wishes,


Martin

[lgalla]

John,allrocks are not created equal.We know from the machinery guys E/Q or E/G is the stuff machines are made of.We are having enough trouble with known rocks.Adding another is adding to the confusion.I am sure you will have good results,but I will stick with granite or quartz which is cheap filler.
Larry

[lgalla]

Cameron here is something to decifer,Its all Greek to me.Found info searching Google"filler particle size vs strength.
) State Research Institute of Structural Graphite Materials, Elektrodnaya ul. 2, Moscow, 111524, Russia

Abstract The effect of the grain size of the filler on the mechanical properties (compressive, bending, and tensile strength and modulus of elasticity) of synthetic graphite is analyzed using data for commercial structural graphites. As the mean particle size of the filler (deltaav) decreases from 3000 to 1 mgrm, the modulus of elasticity increases, on the average, from 10 to 15 GPa, and the compressive, bending, and tensile strength increases by about one order of magnitude. The Griffith equation is used to evaluate the size of defects that initiate fracture (c c) in different types of graphites. It is shown that the factors determining the critical defect size depend on the particle size of the filler. For deltaav > 150 mgrm, c c is comparable to deltaav or deltamaxthinsp. In the range 30 < deltaav < 150 mgrm, c c is equal to or greater than deltamaxthinsp. In graphites with deltaav < 30 mgrm, c c far exceeds deltamax and, presumably, corresponds to the particle size of the molding powder.
From a laymans point,looks like fine fillers offer better specs over rocks.Fine spheres arrest cracks as I assume the micro crack will circle the sphere and end.A large 1/2" aggregate would be a very large crack indeed.I am just trying to guess why micro particulate is better.I am refering to cracking as cast iron has damping qualities as it is full of micro cracks which gives it it's damping properties.
Larry

[ckelloug]

Dear Cameron,

1) Rebars will fix any tensile problems.



2) Who cares about static deflection under self-weight? It's the deflection under shifting and variable loads that matters, isn't it?



Martin

On point 1 In strength terms you are absolutely right. I have a theory however that due to their continuous nature and their higher Q value that they may transmit more vibration. I have not worked out a way to do the analysis yet to determine anything so all I have is a conjecture. The effect is probably minimal anyway.

On point 2 I think you are largely right about static deflection as well. The only case where I think it could matter is in keeping a gantry beam _exactly_ parallel with the table. If you can dynamically adjust this deflection to zero by post tensioning, then I believe it might be possible to size gantry beams a bit smaller and also make the Z axis vary less with respect to the y-axis position of the milling head on the gantry.

In a 12.5 inch square 4 ft long E/G beam with no reinforcement, I calculate the deflection as being about .0004inches at the center with a 200 pound mill head and a 525 lb weight of the beam. If .0004 is an acceptable deflection on the gantry beam then static deflection can be safely ignored.


In short, I'm a metals guy and a theoretician at present so I think in terms of reducing all sources of error in the system for the most accurate possible parts. For a CNC wood router, the two concerns I mentioned here are probably nil.

[lgalla]

Cameron I can't speak for the other members,but .0004 is what they would like,but a 500lb gantry is scary to them.I see posts where the guys are worried about a 100lb gantry and 50lb Z axis.For the wood guys deflection is not a big issue as the spoil board is surfaced to be level with the gantry.At .004 what is the deflection.I don't think anyone here is making opitical accuracy tables.
Thanks for all the input
Larry

[ckelloug]

Larry,

I worked out that a 4.25 inch square E/G beam 48 inches long, with the 16GPa strength from my last calculation, should have enough strength to be within .004 deflection at the midpoint and weight about 60 lbs along with 50 lbs of z axis load for 110 lbs total.

As for the Russian abstract, that was interesting. Maybe not $43.00 worth of interesting but. . . Putting this in closer to layman's terms: as particle size decreases in graphite from about 150 times the width of a hair to the width of 1/20th of a hair, deflection in a beam made from the material goes down 33% for the same load and the ultimate load carried by the beam before it breaks is 10 times what it was before.

The mechanism is this. Imagine a block of epoxy as a bunch of tinkertoys. Each round hub has spokes coming out in several directions and the pattern of a bunch of tinker toys put together at the same angles is repetitive but, like the work of a child, it contains mistakes. These mistakes are called dislocations. If the structure contained no dislocations, aka was a perfect crystal, it would be very strong. When the epoxy is bent, these mistakes in the tinkertoy structure try to move in order to allow the structure to flex. In the tinkertoys, imagine a few missing spokes and spokes that are just barely pushed into the hubs. The effect of adding the tiny particles is like placing the tinkertoy structure into a bucket of pingpong balls. Anywhere you grab the structure and try to bend it, the hubs and spokes hit pingpong balls before they move very far. This effect is called dislocation pinning generally and in this case dispersion hardening. If bowling balls were used instead of the pingpong balls, the sticks in the tinkertoys would be too short and you would get some regions filled with globs of tinkertoys and other regions filled with bowling balls but no regions where large portions of the structure of tinkertoys was connected to itself.

In short, small particles cause keep the molecules of material from sliding past each other on the atomic scale. I would suspect that once a crack decided to form in the material that it would probably just keep going unless it hit a larger aggregate particle which was strong enough to stop it. That's why high strength concrete with pozzolans also has aggregate. Both macro and micro reinforcement are beneficial.

I do expect that the micro reinforcement is probably not needed at the level required to make the stuff turn into vaseline. If it's truly thixotropic however, vibration at the right frequency should make it thin right out.


P.S. I've written the first version of my program for doing crude beam calculations. It's in java so it is portable but I am still figuring out a way that it can be run without a hassle since it is the wrong dialect of java to run from a web browser right now.

[walter]

Walter you have micron silica,sand,epoxy,carbon black,Zeeospheres,vibrator in the works,etc.Do you have a hammer???

Broke the hammer crackin' walnuts. Ordering 2 new hammers!

[walter]

Did I say walnuts?!

I meant E/G !!

[ckelloug]

I called plti as mentioned in my earlier post and inquired about budgetary pricing for compressive strength and modulus tests. Tests are $175 for strength and $175 for modulus and require 5 samples :drowning:

I think I'm going to hit the library and see if we can improvise something. We should be able to get strain with a dial gage so if we can come up with a good way to measure pressure, we can test it ourselves.

[greybeard]

........so if we can come up with a good way to measure pressure, we can test it ourselves.

Bathroom scales ?
John

[martinw]

I think I'm going to hit the library and see if we can improvise something. We should be able to get strain with a dial gage so if we can come up with a good way to measure pressure, we can test it ourselves.

Dear Cameron,

For a supply of pressure, I'd go for weights of a known mass, and a "pad" (or point) with a known contact area. It will save on all that load cell expense.

IMVVVHO

Best wishes,

Martin

[martinw]

For a supply of pressure, I'd go for weights of a known mass, and a "pad" (or point) with a known contact area. It will save on all that load cell expense.



Martin

Dear Cameron,

Actually, there may be another rough and ready way to apply a known force.

Machinists's vices usually have some kind of ACME thread screw to open and close the jaws. They can apply huge pressures if the "pressure pad" is of small area.

If you know the geometry of the ACME screw, you can calculate the torque to force ratio. OK, OK...you will have to assume some efficiency ratio for the leadscrew torque to force calculation (and this is the weak link in this barmey idea), but it could be a way forward.

Any comments dudes......?

Best wishes


Martin

[yugami]

Anyone have a hydraulic shop press with a gauge? place sample on 2 blocks and push down on the center.

Use a dial gauge to measure deflection and read the PSI on the gauge.

[martinw]

Anyone have a hydraulic shop press with a gauge? place sample on 2 blocks and push down on the center.

Use a dial gauge to measure deflection and read the PSI on the gauge.

Dear yugami,

Thanks. I didn't even know those things existed!

Best wishes

Martin

[Geof]

.....I think I'm going to hit the library and see if we can improvise something. We should be able to get strain with a dial gage so if we can come up with a good way to measure pressure, we can test it ourselves.

This assumes you have a press, screw, hydraulic or rack.

Take a piece of cold rolled steel. Support it on two pieces of round stock a known distance apart. The round stock will have to be blocked up a few inches because space is needed underneath the center of the cold rolled.

Now you have a simply supported beam for which you can easily determine I and can make a plot of beam deflection versus load.

Mount a dial gauge under the center of this beam; apply pressure from the press; again via a piece of round stock at the center. Now you can relate deflection of the beam to the force being applied by the press.

If it is a hydraulic press it should have a gauge; you can now calibrate the gauge in terms of force applied by the press. If it is a screw press you will need to use a torque wrench and if it is a rack driven press you just need a spring scale between you and the end of the lever.

You will need to choose the dimensions of the cold rold and the distance between the supports to get the calibration rig somewhere close to the range you want to measure. Just remember you have to be in the elastic region during calibration. If the dial gauge does not return to zero when the force is removed you have move into plastic deformation.

EDIT; Obviously I was reading Yugami's mind, or vice versa.

[martinw]

This assumes you have a press, screw, hydraulic or rack.

Take a piece of cold rolled steel. Support it on two pieces of round stock a known distance apart. The round stock will have to be blocked up a few inches because space is needed underneath the center of the cold rolled.

Now you have a simply supported beam for which you can easily determine I and can make a plot of beam deflection versus load.

Mount a dial gauge under the center of this beam; apply pressure from the press; again via a piece of round stock at the center. Now you can relate deflection of the beam to the force being applied by the press.

If it is a hydraulic press it should have a gauge; you can now calibrate the gauge in terms of force applied by the press. If it is a screw press you will need to use a torque wrench and if it is a rack driven press you just need a spring scale between you and the end of the lever.

You will need to choose the dimensions of the cold rold and the distance between the supports to get the calibration rig somewhere close to the range you want to measure. Just remember you have to be in the elastic region during calibration. If the dial gauge does not return to zero when the force is removed you have move into plastic deformation.

EDIT; Obviously I was reading Yugami's mind, or vice versa.

Dear Geof,

I will probably have completely misunderstood, in which case, apologies.

You are suggesting that this is a way to calibrate the force applied by the "force applier"?

If this is a calibration procedure, the "knowns" are the sectional dimensions of the "calibration" beam and its Young's modulus, and the dimensions and positions of the standard loading forces and reactions. Is that right? Simply supported point load at mid-span? Your idea is to use the steel beam as a force reference by measuring its deflection and using book values for material properties?

I'm being slow...

Best wishes

Martin

[Geof]

.....I'm being slow...

Best wishes

Martin

No you aren't. Your interpretation is correct. You know the beam length, you know I, you measure the deflection and back calculate for the point load.

Actually the way I phrased it was calculate the deflection for various loads. Then apply the load until you come to your first calculated point and record your gauge pressure, or whatever. Then go back to zero and up to your next deflection point, repeat, repeat, etc. If you find you have not gone back to zero you have gone to far.

To improve precision it could be done with different cross sections and different lengths.

Provided you stay in the elastic region the deflection is very repeatable.

[martinw]

No you aren't. Your interpretation is correct. You know the beam length, you know I, you measure the deflection and back calculate for the point load.

Actually the way I phrased it was calculate the deflection for various loads. Then apply the load until you come to your first calculated point and record your gauge pressure, or whatever. Then go back to zero and up to your next deflection point, repeat, repeat, etc. If you find you have not gone back to zero you have gone to far.

To improve precision it could be done with different cross sections and different lengths.

Provided you stay in the elastic region the deflection is very repeatable.

Dear Geof,

Got it. Thanks.

I quite take the plastic issue, and the desirability of a number of samples.

Best wishes

Martin

[yugami]

Dear yugami,

Thanks. I didn't even know those things existed!

Think I saw it on Mythbusters

Basically a hand pump connected to a hydraulic cylinder with a pressure gauge in between.

That will tell you what PSI the jacks pushing down with, from there the smarter people in this thread could work out the calcs