What is it with the fit of old slotted screws?!

Hello all. Another beginner question:

Whilst restoring a Stanley plane and similar aged tools I've noticed an annoying tendency of not quite getting a perfect fit flat head screw driver.. I must have about 10 flat heads of various ages (a few with W Germany stamping) up to modern metrics, and as I say sometimes nothing quite fits, with the blade turning a smidge in the slotted head...

was there an Imperial flathead dimension system that I've missed? Any advice is welcome. I'm a firm believer in 'right tool for the job'.

I was taught by my father from an early age how to file a screwdriver to the correct size. Simply take one of your many & file it to a perfect fit for the screw In question. Using a fine good quality file. It is satisfying too.

I have many in my tool collection that have been made to fit a certain item. I even have my old ones for adjusting the clutch springs on the Villers 2T engine, with a slot in the centre to clear the threaded portion of the adjusters.

Steve.

Some old screws seem to have a v shape, whereas modern flathead screwdriver has a flat bottom with parallel sides.

I have to agree with Steve, although I tend to grind mine rather than filing them.

The only 'specification' I have ever seen on a flat blade screwdriver is the width of the blade, which is not usually of much help, although the right width will assist in preventing damage to the slot.

Getting a perfect fit is essential on tools, clocks etc. where the appearance of the head needs to be protected. It is especially necessary on brass screw-heads which are easily damaged and cannot be repaired, apart from by cutting a wider slot that looks just about as bad.

As Steve has suggested, get a screwdriver of the correct width and file or grind to the correct slot width. If it is one you will be coming back to label it and put it aside - possibly keeping it with the tool for the next occasion.

Taper ended screwdrivers are still readily available - but are of the old manual type, i.e. with long tang and a handle, rather than the modern replaceable type.

A similar problem can occur with cross-head screws, usually because the center part of the slot is too shallow. Grinding a small amount from the end of the screwdriver tip will usually allow the driver to fully engage with the screw, minimising the risk of damage.

Pero

I have found dealing with slot head (or any head) screws is one job where my homemade EDM machine excels.

I EDM a square socket or hex socket into any old screw and then drive it in or out properly with a Robertson screwdriver or a hex key!

It is not fast though to use this method. If pressed for time I use the old oxy-acetylene gas axe screwdriver (bit hard on clocks and watches though)

Edited By Jeff Dayman on 07/04/2020 02:48:11

You can buy handplane screwdrives and they tend to have little or no taper to the end or are ground with the round edge of a wheel so the end is parallel sides not tapered. Gunsmiths like a similar end as any form of taper will only fit at the top of the slot but be loose at the bottom. You can also get stubby ones for removing the blade that are nothing more than a flat piece of steel in a handle.

Edited By JasonB on 07/04/2020 07:43:56

I remember our local gunsmiths workshop, he must have had over 100 wooden handled "Turnscrews" (his words) one would be chosen and filed to fit the screw.

Gun smithing is a prime example of using their own standard, if there actually is one. The correct sized screwdriver works.

You can buy sets of those gunsmith-style large hollow-ground screwdriver bits in large sizes with a 3/8 square drive to attach to your socket set. Might be worth a look. I've found the invaluable for old motorbike restoration work without messing up the old straight slot screws.

Edited By Hopper on 07/04/2020 08:10:53

A properly tempered screwdriver should be possible to file with a decent file but the tip will always be slightly tapered, I always make sure that the taper is nice and slow so the end is close to parallel. Grinding freehand is very easy to overheat the tip and keeping the end parallel takes some care. Although I don’t find the various flavours of cross shaped sockets very attractive they are very practical for power driving, a plain slot will very soon have a hole drilled in the job if power driving is used. Screwdrivers are not overly expensive and customising for a perfect fit is a must for screws that will be on show, clickspring made a fitted screwdriver for the highly visible screws on his clock. Most of the time a best fit screwdriver will be adequate and not do any significant damage to the screw head. Slot standards may exist but they are either not adhered to or they are very wide ranging. Some wood screws seemed to have very narrow slots that were thinner than a normal screwdriver tip. Myford Super Seven ball handles are fixed with a chrome plated large screw, worth making a well fitting tip for this or you will see the chewed screw every time you use the lathe.

Mike

Has this come up despite reading the same topic on another forum or because of it?

My father always used to hollow grind them. If you get it correct the blade will lock into the screwdriver slot. Colin

.

It ain't necessarily so !!

The tip can actually be any shape that you are capable of filing:

MichaelG.

Apprenticeship as an instrument maker,we were expected to buy via our companies tool club all our tools,all the screws we used were even number BA and and our Screwdrivers with plastic handles were made by STEAD ,and they made a set of 5 screwdrivers to suit 0 to 8 BA screws. Sixty years later I still have them,very good crewdrivers. A cheap way of getting screwdrivers which can be butchered is go round a boot sale or auto jumble and look in the odds and end boxes, you can find old english made screwdrivers,they may be bent a bit or the blade chipped but can be restored,ignore the foreighn ones most are rubbish, Younger readers may wonder what a tool club was ,on pay day you could pay into a tool club run by the company,when the funds accumulated you could place an order for a tool and the company would get them for you at a discount, eg Gordon spanners were around 50% ff list price. Some firms just aranged at discount scheme where you just went along and bought them direct at discount,The longest save of 5 bob a week was for my Moore and Wright toolmakers cabinet which was 146 shillings, its still written on the back of the cabinet in pencil. Its sad to think that the firms who made these fine tools have disappeared or the name has been bought up but not really the same company. Best bit of luck I had was after 9 years of work and two jobs, I got a job with a multi national co,on joining I was informed that I would not need to bring my tools as all tools ,cabinets ,overalls etc were supplied ,plus the toolroom was absolutely brand new. So I had all my tools at home ,to allow me to start serious model making,wages were good and I bought a new Ml7 and new trials bike within a year and have never looked back.

Some well-made firearms use(d) very narrow-slotted screws that look 'wrong' to engineers. Actually, it's a great idea. The turnscrew (approved jargon) used is chosen to fit. Its blade has a very shallow taper. Because the slot is relatively deep, the blade doesn't bottom out, so it wedges itself in the slot and doesn't slip out, ruining all the expensively-engraved action work. Safer than parallel-ground blades.

Hi most VDE flat blade screwdrivers, which are aimed at electricians, have a hollow type profile shown in JasonB's illustration, the two left hand ones. I often grind a very shallow hollow on then end of the flat blade ones, especially after they have had a lot of use, this helps stop the screwdriver climbing out and saves it from rocking about from side to side in the middle bottom of the slot

Regards Nick.

Edited By Nicholas Farr on 07/04/2020 12:30:36

There are standards but they don't help much. The answer is the width of the slot depends on the diameter of the screw and the thread system.

BA slots are specified in BS57-1920 as being 0.2D + 0.1mm, where D is the full diameter of the thread.

For BSF and Whitworth, slot widths are listed in a table. 8 different slot widths are listed for the 12 screw diameters between 1/8" and 1".

For extra confusion, the depth of the slot is related to it's width and possibly also to the type of head. For example, BA slot depths vary depending on whether the head is Countersunk, Instrument, Round, Cheese, Filister Capstan or Connection.

Metric and other standards follow similar rules.

In theory a different sized screwdriver is needed for almost every type and size of slot-head screw; i.e. dozens of them.

In practice, if the correct sized screwdriver isn't in the toolbox, one can either be ground to fit or the job can be bodged by attacking the screw with whatever is handy. Judging by the number of old machine screws with chewed slots, most of the work in the good old days was done by lazy apprentices rather than master-craftsmen!

Dave

The best thing about cheap scewdrivers is how easy it is to turn them into something useful!

I've even got one filed to a 5mm hex on the end for allens screws in very deep holes as well as one that matches that bit above perfectly.

Neil

Thanks - that's a good explanation of wth the difference between screwdriver and turnscrew actually is. I'd just thought it was flannel and mystique.

There is a downside for the home gunsmith though - the thin slot will generally mean that he'll have to use a screwdriver that's much narrower than the screwhead, unless he understands enough to realise he's gotta shape his own. Force is less distributed and burring may result. So firearms that've been worked on by home gunsmiths - some of them quite good engineers but not knowing of the screwhead design 'feature' - can end up looking like they've spent some time in the chimpanzees' enclosure...

Edited By Mick B1 on 07/04/2020 14:22:33