View Full Version : Screw You

03-07-2003, 07:38 PM
Okay I've determinded that there's two different size screws (and in the process threaded who knows how many =), the larger threaded ones seem to be for cases, adapter cards, PSUs, and hard drives, and the finer threaded ones are for CD-ROM drives and floppy drives.

Anyone know WHY a CD-ROM and FDD would use a small screw but not a hard drive...?! I can understand why cases and PSUs use bigger ones.

Just out of curiosity.

03-07-2003, 09:02 PM
Actually there are three different sizes used (mainly). Can't answer why tho.

Terry Porritt
03-07-2003, 09:03 PM
It's mainly historical. IBM is American, and they "invented" the PC so they used No. 6 UNC (Unified Coarse), 0.138" dia, 32tpi.

CDROM and floppy drives use M3 metric screws, but I'm not sure why the switch to metric, as the US abhors and cant understand metric, whereas Japan uses metric screws almost exclusively

Terry Porritt
03-07-2003, 09:06 PM
............and yet, I have an old IBM 386 tall tower case that uses M4 screws for the case covers ?:|

Terry Porritt
03-07-2003, 09:14 PM
I keep getting these afterthoughts :) ...........

It is a fact that the ISO metric screw thread system is not as satifactory as the older Imperial and US screw thread systems in respect of having fine and coarse threads.

There is a metric fine thread, but it is very rarely seen in everyday nuts and bolts. The "normal" metric coarse thread series is very much finer than the inch US and british counterparts and is very much poorer for thread applications in aluminium or cast iron. UNC and the Whitworth thread series were much superior in this respect, and also for sheet metal work threads such as computer cases where thread engagement is minimal to say the least.

I expect I'll think of more on the subject as my brain cells re-juvenate :)

Murray P
03-07-2003, 10:31 PM
Keep going Terry, I'm interested.

Its always bothered me why different sized screws and threads are used with no particular logic in PC's as above some components use one type then another similar compnent will use something completely incompatible. It usually only seriously bothers me just after I've stripped the thread with the wrong screw in the right hole :(, or should that be visa-versa?

I'm also interested in a way to avoid being bothered by it :D

03-07-2003, 11:23 PM
The secret is to stop if the screw gives any resistance.

Having learnt the hard way, I now turn the screw using only the shaft of the screwdriver which makes it almost impossible to do any damage.

I also use thumb screws for frequently used ones to prevent stripping the thread.

Murray P
03-07-2003, 11:35 PM
> I also use thumb screws for frequently used ones to
> prevent stripping the thread.


04-07-2003, 08:54 AM
I thought this was interesting on threads.


Murray P
04-07-2003, 09:53 AM
The plot thickens. Its a long time since I've done any tinkering with Brit cars or bikes. Normally we just put in what had come out, hopefully in the same place it came from then, there are VW's they definately had their idiosyncrasies. Jap bikes were much easier to get around and less oil on dad's drive, the odd pool in the garage tho :(.

Back to puters. I think some of my problems have come from collecting screws from all sorts of different machines in a jar for safe keeping. Maybe I should get around to ordering them by type. The thread gage mentioned in the article would certainly help.

Cheers Murray P

04-07-2003, 11:07 AM
Machine Screws have 6 main things about them, diameter, thread pitch, head, strength, type of steel and length. Well that's what I think is important about them, in different industries and different places around the world they use different types of screws, now some industries use the different standards, most common standards is UNC, UNF, and Metric ISO, these standards can still be varied by whether you are wanting a finer or coarse pitch, their a many advantages/disavantages in using certain types of screws. A big disadvantage is if the standard is considered obsolete BSW, BSF, etc, although BSW is the same as UNC except for 1/2" diameter (not likely you'll get a screw for a computer that big though, I use to work in an engineering supply company) because you'll find it hard to replace a lost screw, easiest way is to retap a new thread.

Finer thread screws are more likely to stay on longer than a coarser threaded screw due to vibration, automotive bolts use finer threads.

With computers though, the metal for the case is pretty weak, you can tap your own thread with any screw as long as diameter is similar, just requires a bit more effort to get the screw in but once in, you've probably retapped it.

Terry Porritt
04-07-2003, 12:45 PM
Well...er... BSW and UNC are not the same. BSW has a 55 degree thread angle with radiused crests and roots, whereas UNC and UNF are basically truncated threads with a 60 degree thread angle. They are most definitely not interchangeable.
My brain cells seem to remember there may be a radius option for the roots of Unified thread series.

If you are really old, you will easily remember that UNC and UNF were derived from ANC and ANF, American National Coarse and Fine.

UNC and UNF came about to UNIFY the manufacture and specs for these threads manufactured in Europe ( mainly in UK) and America, driven by the car and aircraft industries, so as to be fully interchangeable.

Whilst on the subject of threads, the other popular screw thread was/is the British Association or BA screw threads for instrument type applications. These are a funny mixture of metric and imperial with a 47 degree thread and radiused roots and crests.

04-07-2003, 02:00 PM
Terry, I'm not that old (20 this month :P), and I only understand the basics required in the sales position of an engineering supply store, not the manufacturing of it. As I understand from 1/4" and up UNC and BSW have the same thread per inch except for 1/2".

If they asked for BSW and as long as it wasn't 1/2" I could supply them with UNC fasteners. Now I don't know if it's because these people were use to calling it BSW when infact what they were using was UNC but my boss was the one who explained to me they were just the same and he's got over 35 years experience in Engineering, I had none at the time.

Explain this 55 degree thread angle and 60 degree thread angle, as if BSW couldn't be used in conjunction with UNC as this degree seems it would at least make it difficult for fastening those two together.

Alan Carpenter
04-07-2003, 04:11 PM
There's a simple answer to all this, know as the "Microsoft Solution"

Put your screws in place with a

H uge
A ll-purpose
M assive
M ultipurpose
E ntropic
R ammer

(All those who didn't recoil in horror must now stop reading this thread. There is no hope for you.)

Now, my point (part one). Paste this into Google.

BSW BSF thread diameter metric pitch flat history

Who's copying whom?

My point (part two).
I know there's much, much more on this topic on the web, because I've been there, read that, got the Alzheimer's T-Shirt, but I can't find it again.

I'm VERY interested. Good links, anyone?

(REALLY off topic)
alt.binaries.sounds.mp3.classical and other classical music newsgroups are currently enjoying a flood of Bach's Chacconne and the Goldberg Variations done by different performers.
(That's the Chaconne from the Partita for Solo violin in D Minor BWV 1005 in case you need a driver for it.)

I'd never heard Kurt Rodermeyer play it on guitar before. Now I have, and have a compulsion to share with otheres who haven't heard him.

I still like Radiohead, Filter, Placebo... others, bat Bach is KING. (Or not. You listen to whatever you like.)

I must not go off topic.
I must not go off topic.
I must not go off topic.

Alan Carpenter

Alan Carpenter
04-07-2003, 04:32 PM
(... I must typo-check my posts.)

The links on this page, to do with Screw Threads and Tapping Drills Pages


may be of interest.

Alan Carpenter

Alan Carpenter
04-07-2003, 04:38 PM
At last!


Alan Carpenter

04-07-2003, 04:41 PM
So Bardin,does this help at all?

Terry Porritt
04-07-2003, 06:08 PM
A couple of good reference sites there Alan. Most is explained.

The thread angle of a screw is interesting as it is defined as measured along the axis of the screw, but screw threads have an helix angle and if large the flanks can obscure each other.... read on.

If you are all sitting comfortably, here begins a little lesson on thread measurement, it is not as straight forward as it may appear, for example how do you measure the thread angle inside a nut?

Except for very special screw threads (eg certain aircraft screws like ACME screw jack threads), every day screw threads are checked against GO/NOT GO screw thread gauges which are certified to be within the size tolerances specified in screw thread gauge standards. These gauges are plug and ring, and can be parallel or taper.

So how is a gauge measured? Classically, the profile is measured with an optical projector with the screw tilted through its helix angle so as to see each flank of the screw. A small arithmetical correction is then made to the measured thread angle to get the angle in the axial plane. The root and crest radii and sizes are also measured optically and also sometimes the pitch (distance between consecutive threads in the axial direction) though pitch measuring machines are usually used.

The pitch diameter of the screw which is the diameter of the screw measured at half the height of the theoretical full form of the screw, ie no truncation etc, is measured using calibrated precision thread wires sitting in the thread nominally at the pitch diameter, and using a high resolution "floating carriage" bench micrometer.

Ring gauges which have an internal thread are checked for angle and form using plaster casts, and the casts are measured optically. Axial pitch and pitch diameter are measured using thread measuring machines as above but with internal measuring probes or "feelers" instead of external attachments.

So, where does all this come from? Well I used to work in one of my incarnations for Horstman Gauge and Metrology, a now defunct, but once maybe the leading screw gauge manufacturer in the UK (together with Coventry Tool and Gauge). In another incarnation more recently thread measurement was part of the job at DSIR.

As those references show there are heaps of thread forms for all sorts of different trades, model makers, horology(clocks and watches), oil drilling threads (API, very very special), hydraulic and pipe threads, etc, etc.

04-07-2003, 06:18 PM
>Okay I've determinded that there's two different size screws .....
So you see bardin,aren't you glad you asked;)

Graham L
06-07-2003, 03:56 PM
And cycle threads.

05-09-2003, 01:38 PM
Bah!!! *Recoils in shock* Um...yeah, thanks guys :) I like the HAMMER option.

Graham L
05-09-2003, 01:57 PM
And brass threads. All 19 tpi. :D

Before Whitworth made a leadscrew for his lathe (by wrapping a string around a length of round bar and filing the groove alongside the string) there were no real standards. Blacksmiths made bolts with a file, too.

BSW and UNC in the class you buy from the ordinary retailers are "interchangable" (except for the 12 and 13 tpi at ½") because the sloppy "fit" allows it. (And mixing UNC and Whit at ½ might give "self locking" :D)

I think the size of the hex heads, and hex nuts are different, but your "crescent" (or Kiwi Hammer) will handle that. If you are working on aircraft or racing motorcars (and you wouldn't be using Whit, or UNC :D) where the fits are tighter, the 55° and 60° would matter.

05-09-2003, 02:56 PM
The hex size of the head and nut should be the same outside, although I can't quite remember, all I know is a 1/2" UNC Nut is 3/4" AF. Well you would use a 3/4" Socket to fit on it and I'm sure the hex head of the bolt was the same. I know that Imperial sized bolts will require imperial sized sockets and metric bolts require metric sockets although, you can get away with some, but you'll round off the edges of the bolt or socket.

I discovered that 55 deg and 60 deg do matter with UNC/BSW, a BSW nut would only get to about 1/2 way down a UNC bolt before you have difficulty.

Although we stocked different types of bolts/setscrews most people only needed enough to fasten the bolt to whatever you were trying to fasten it to, so the nut didn't need to go all the way down. I guess it's different if you were going to screw the bolt into an already threaded hole, but if it was BSW we'd tell them to tap it if possible because I remember I spent all day looking for BSW and BSF threaded bolts and the suppliers basically told me to give up while I was ahead, they also asked me if I called them first, and always say yes but I may have called about 3 or 4 different places at the time, they just like to be known as the first supplier you tried :P