Monday, August 13, 2012

Things You Might Take For Granted: The Hard Drive Platter

I could I suppose, have made the header read, Things We Might Take For Granted, but that wouldn't have been, for two reasons in the main, quite true.

The first of those reasons is that I am an engineer, pure and simple. The other reason being that I dismantled an old hard-drive for no other reason to have a look at some of the engineering techniques that went into manufacturing, what by anybody's standards, is a remarkable piece of kit.

What I want to do however, is not concentrate the function of the hard drive, remarkable that it may be, but rather to take a look at the platter itself as a manufactured item.

To describe the platter, or shall we be a bit familiar and call it a disc? To describe then, this three and a half inch, ninety millimetre disc, as a marvel of modern engineering doesn't quite do the thing justice. A more accurate description might be, a miracle of modern engineering; and it's all of that be assured.

To actually manufacture something so big and so thin and end up with something so flat, so parallel, so smooth and so true about its axis, does, without a shadow of doubt, borders on the miraculous.

Let me try to put a little perspective on things. The physical and the physics of slip gauges described below are a good place to start. If you can bear in mind that these gauges or blocks, are extremely stable due to the material from which they are made, hardened steel or tungsten carbide, coupled with there relative small size, approximately ten by thirty millimetres, then it's not such a wonder that they can be manufactured to tolerances that allow them to be used in the manner described.

Slip gauges

Slip gauges are blocks of steel that have been hardened and stabilized by heat treatment. They are ground and lapped to size to very high standards of accuracy and surface finish. They are the most accurate standards of length available for use in workshops. The accuracy and finish is so high that two or more slip gauges may be wrung together. The method of wringing slip gauges together is shown as followings. When correctly cleaned and wrung together, the individual slip gauges adhere to each other by molecular attraction and , if left like this for too long, a partial cold weld will take place. more

But to take those same properties and tolerances and apply them to, what at the end of the day is still basically a ninety millimetre aluminium blank, a couple of millimetres thick, and where when finished, it too will 'wring,' and they do, is as I said earlier, bordering on the miraculous.

And where some might argue that it is the subsequent coatings that are applied to the disk that give it these remarkable properties, which in the case of 'smoothness' would be correct, there is still no getting away from the fact that the blank has to be perfect in every shape, form and plane before any of these processes can take place. And what processes they are! mind-blowing isn't a too fanciful description, but for that it's the text body you need, certainly not me.

But before you move on, let me try and give some perspective to what these tolerances actually mean. Or more accurately, let someone else explain, because one thing I have learned since I started researching this article, I'm not the only fellow who is in awe of the manufacturing process.

. . . . To better understand how data can be destroyed on a hard drive, let's consider what makes a hard drive work in the first place. The ubiquitous hard drive that everyone takes for granted is an absolutely amazing feat of engineering that requires the most demanding manufacturing processes imaginable. For the hard drive to work, the platter surfaces must be absolutely perfect to tolerances measured in Angstroms. An Angstrom is .1 nanometer or put another way, .0000001 millimeter. The surface of the blank, nonmagnetic platters must be flawless in every detail before an equally perfect coating of exotic magnetic material (that holds the data) is added.

Hard drive platters typically spin between 5,000 and 10,000 RPM with some high performance hard drives spinning platters even faster. The spinning platter creates a tiny air current on the surface. The read/write heads are designed to make use of this air current and fly about 2 nanometers above the surface of the platter. For reference, the smallest diameter of a human hair measures approximately 17 micrometers or roughly 8,500 times the gap between the head and the platter. Any platter imperfection will cause the head to crash and destroy the magnetic recording surface. More

With that said, and with that in mind, the article proper.

How the humble hard drive is made

It's like a 747 flying 0.14mm above the ground
By Mike Bedford from PC Plus
January 31st 2010

A hard disk is an amazing feat of electronic and mechanical engineering

The fact that silicon chips start life as nothing more exotic than sand is amazing enough, but have you ever thought about that other important PC component, the hard disk?

Its origins couldn't be more different. The heart of a hard disk – the rotating platter where your data is stored – is made out of an exotic mix of elements including ruthenium and platinum, two of the world's rarest and most expensive metals.

Needless to say, this statement doesn't even hint at the complexity involved in transforming rare ores into gigabytes of data storage. The hard disk's high speeds of rotation and the close proximity of the head to the platter means that the processes must be carried out with the ultimate in precision and cleanliness.

Add to this the strange properties of magnetic media and the techniques required to achieve the optimum capacity, and the story of how disks are made becomes one that encompasses the fields of mining, metallurgy, chemistry, physics and involves the pinnacle of engineering and manufacturing technology.

As a whole, a hard disk is an amazing feat of electronic and mechanical engineering, but two parts – the heads and the platter – stand out for their sheer manufacturing complexity. As the part that actually stores the data, the platter is what many people consider the heart of a hard disk drive – and here we reveal the secrets of its manufacture.

Step 1: Mineral extraction and processing

Platinum is only the 70th most abundant element in the Earth's crust, making up just three parts per billion. Ruthenium comes two places lower with an abundance of only one part per billion. By way of comparison, silicon – the raw material from which microprocessors are made – accounts for around 27 per cent of the Earth's crust.

It's no surprise then that platinum is hugely expensive – today's market price is more than $1,300 per Troy ounce. Turning to ruthenium, the total annual production is just 27 tonnes, an amount that would fit in a 1.3m3 cube. Both are mined predominantly in South Africa.

Platinum is one of the noble metals, which means that it's relatively unreactive. Unlike metals such as copper – the main ores of which are compounds – platinum is normally found in its metallic form. This doesn't mean that extracting it from its ore is simple, though, as platinum is normally found mixed with other metals.

Obtaining pure platinum involves separating it from the iron, copper, gold, nickel, iridium, palladium, rhodium, ruthenium and osmium that it's invariably found with. Let's just say it's a complicated multistage chemical process that can take up to six months to complete. Fortuitously, though, the ruthenium that's also needed in disk manufacture is a by-product of the process.

A deep mine in the Bushveld Complex of South Africa might seem far-removed from a finished hard disk, and in this sense it's an ideal place to start our investigation. But we're not going to need the platinum or the ruthenium until well down the line, so for now we'll put them aside as we move to something more down to earth – and considerably more common.

Step 2: Making aluminium blanks

The manufacture of a hard disk platter starts with the fabrication of aluminium blanks, which are disks of aluminium alloy onto which the magnetic recording layer will eventually be deposited.

High-purity alloy that contains four to five per cent magnesium plus small amounts of silicon, copper, iron and zinc to give it the necessary properties is cast into an ingot weighing seven tonnes. The ingot is then heat-treated, hot-rolled and cold-rolled in multiple passes to provide a sheet of the necessary thickness (usually 0.635mm, 0.8mm, 1.0mm, 1.27mm, 1.5mm or 1.8 mm – just enough to provide adequate stability while rotating at high speed) from which the blanks will be punched.

GETTING STARTED:Hot rolling mills process aluminium ingots into thin slivers of metal from which disks will be punched

Punching takes place once the alloy sheet has been coiled into large rolls so that a single stamping process produces lots of blanks. This is then followed by a stacked annealing process to reflatten the blanks. Finally the blanks are ground to a high level of precision to achieve the necessary surface and edge finish. Bear in mind that this and all subsequent steps are carried out on both sides of the platter so that it ends up with two recording surfaces.

Step 3: NiP plating

The aluminium blanks are now precision-ground using 'stones' that are composed of PVA and which contain silicon carbide as the abrasive agent. However, even with all the care taken to produce a good finish, the surfaces of the aluminium blanks produced in Step 2 are not yet nearly perfect enough. Because there's a limit to the degree of smoothness to which aluminium alloy can be ground, the next step is to apply a hard coating that will take a better finish.

PERFECT FINISH:The soft aluminium is plated with a hard NiP layer so that it can be polished to an incredible degree of smoothness

This hard coating is an amorphous alloy of nickel and phosphorous (NiP). It's applied by an electroless process in which complex supersaturated solutions containing compounds of nickel and phosphorous react on the surface of the disk to leave the required NiP layer. This layer can now be further refined in the next step of the process. Go to page two of three.

A bit of nonsense I found on YouTube.


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