27 replies to this topic

[Pretty green flames]

Certain grades of SS are sparkless.

[pudi.dk]

Copper free and high nickel-chrome content, such as 310S24 or 316S16
(from Passfire)

[Andrew]

Andrew, Stainless steel is sparkless, you must be confusing it with steel.
I'm going to use SS as ball milling media as it is one of the best.
Sparkless, hard and dense.

It's not a case of does it or not, the real world is very complicated.

Almost all Stainless Steels spark horrifically. Check out this page that details the results of spark tests. These tests are very similar to the sort of interaction you'd get while using a star plate.

Most SS sparks folk as well (forking increases the chance of igniting a composition), it is not an ideal material at all. Even so called "sparkless" stainless does in fact make small sparks (usually a blue hot gas forms close to the impact site but sparks still form). Even ceramic milling media forms these blue hot gases upon extremely hard impact, but sparks are almost unheard of with ceramic. SS milling media is just asking for trouble!!!

There are several factors governing the evolution of low velocity derived sparks. Hardness is not a factor that really affects the generation of sparks. It is related though. In general the SOFTER a material is, the less likely it is to generate sparks. The biggest affecting factors are to do with how the material interacts in a collision and how energy is distributed. A hard material collision will mostly be of an elastic type (many factors here as well you have to consider the conservation of both momentum and energy). Even so, what plastic deformation that does happen, takes place in a very small layer of the material thus heating far more than if that deformation would have occurred within a larger body of matter. The next biggy is heat capacity and conductivity. LOW heat capacity and LOW thermal conductivity leads to a tendency to form sparks. That energy imparted as previously mentioned will get that material up to a greater temperature. Then you have the ability to break into small shards that polishes off the ability to spark. Again this is a factor of hardness.


Within normal confines:

Lead does not spark because it is very soft.
Hardened lead does not spark because the antimony content increases the thermal conductivity.
Copper does not spark because it is soft and has a high thermal conductivity and heat capacity.
Aluminium does not spark because it is just like copper.
Acrylic does not spark because of the extent of plastic yielding at an impact site.

Steel sparks because it is hard, it splinters on impact and has a low thermal conductivity.
Stainless Steels spark because it is even harder and has a stupidly low thermal conductivity.

Aluminium WILL spark in the presence of rust (and any other metal oxide where the cation is less reactive) because of the thermit reaction. This makes Aluminium generally unsafe for tooling over copper based alloys.

Edited by Andrew, 17 August 2007 - 02:10 PM.

[Andrew]

Copper free and high nickel-chrome content, such as 310S24 or 316S16
(from Passfire)

When it says recommended as a non-spark material, it does not guarantee that it is non-spark. Hit it hard enough and it will spark. Stainless Steel's losing point on sparking is it ultra low thermal conductivity. You can improve it's characteristics but you will never get a truly spark free Stainless.

What you have to concentrate on is whether or not it's threshold impact energy to spark is many orders of magnitude higher than the expected energies. I suspect that the safety margin with ANY Stainless will not be amazing.

Added to this, Stainless is ridiculously expensive at the moment, getting a specialist grade will really hit your wallet hard. Especially for 12mm plate or there abouts!

[Bonny]

[quote name='Andrew' date='Aug 17 2007, 08:58 AM' post='37088']



Acrylic is fine! It easy to work and is not brittle if you get good material. It does undergo plastic yielding if it is struck. It can crack under certain strains but it will never shatter, it will only ever break on a fault close to the point of maximum stress. It is easy to drill. Just don't drill at 16 million revs. Drill as slow as you can go and apply pressure; allow the Sharp Bit (little hint there) to do it's job... CUT.

My company is in the plastic industry,including an acrylic fabricating shop. Sharp bits are fine, however,when dealing with "simpleton" workers, a negative rake on drill bits helps prevent cracking when drilling, as it stops the bit from grabbing the material.Drillbits that have been filed will easily and cleanly drill acrylic. As you said above though, slow speed is very important as well as only enough pressure, allowing the drill bit to cut and not trying to force through.

A softer material such as polycarbonate/Lexan is more forgiving when drilling.

[pudi.dk]

How about acrylic tube for milling jar (along with 12mm SS cylinders)? I have thought about it because I got ~1 meter of it, it would be pretty cool to be able to see your BP while milling, of course it would turn dull because it might get scratched, but still...

About SS as media, I trust Passfire when they say:

Stainless Steel Media:
While not very commonly used, stainless steel is perhaps the best media available. Stainless doesn't spark, has a very smooth surface, is heavy, is resistant to wear, is resistant to corrosion and can be used to mill all types of chemicals. There are many different types of stainless, but the ideal grade should be copper free and high in nickel-chrome content, such as 310S24 or 316S16. Having a machine shop produce 140 pieces of 3/4" stainless media about an 1" long and rounded on both ends might be an expensive way to go, but the resulting media would last a lifetime and perform quite well.

Edited by pudi.dk, 17 August 2007 - 02:42 PM.

[Bonny]

[quote name='pudi.dk' date='Aug 17 2007, 02:42 PM' post='37104']
How about acrylic tube for milling jar (along with 12mm SS cylinders)? I have thought about it because I got ~1 meter of it, it would be pretty cool to be able to see your BP while milling, of course it would turn dull because it might get scratched, but still...


It would probably work,but would definately scratch. Making a removable lid might be a pain though without machining equipment. I use empty peanut butter containers, clear plastic.They are scratched up quite a bit though, but can still see through a little. I wouldn't ever look at BP being milled though, could be the last thing you ever see!! Maybe through a video camera.

[Jerronimo]

Finished my new starplate a few days ago:
pic 1
pic 2
finished stars

It can press 240 10mm stars in 4 pressings.
The pin plate and hole plate are made of to pieces of 12mm trespa glued together with contact clue, the pins are aluminium.

Edited by Jerronimo, 25 September 2007 - 07:08 PM.

[leosedf]

That's a very nice star plate mate!
Can you get more pics?
What is trespa?

[Jerronimo]

Thanks Leo,

I will make more pictures soon, but I have to clean the starplate first , have been pressing a formula with a very high bright fine flake Al content.

Trespa is a very hard and durable material, it has many uses including lab furniture and outside wall covering.
Actually it's cellulose fibre reinforced phenolic resin.
It's chemical and water resistent, very strong and heavy but quite expensive.
I'm lucky that I know someone who works at the production facillity not far from where I live, so I get it for free
Very tough to machine though, but if you got the right tools it's not that hard.
You realy need a very sharp and high quality drill bit to drill it and a fine tooth ''widia'' sawblade to saw it.
But it's worth it, this starplate will probably outlast me, and I'm 26....

I think I 'll make another one soon, in a different size probably 8mm.

Edited by Jerronimo, 26 September 2007 - 04:39 PM.

[leosedf]

I think it's the same material my chinese starplate uses.

[Pretty green flames]

Just got one of those food chopping boards. It's 10mm thick and drills WONDERFULLY, very easy and very clean cuts, just perfect. Now i got a question, what size hole should I drill in these, i was thinking 8mm because the stars get compressed in it and i'd like to have "Cubic" stars.

So does 8mm sound about right or can i go 10mm?

Thanks

[icarus]

i would clamp it to a wooden board then drill through it and the board with a drill that matches dowelling then glue short dowels into the wood fill the nylon board with comp with a tile grouter then use the wooden pegged board to push the stars out