Screwdriver Profile

[deloidtoc]

I use and have never liked the better Bergeon screwdriver set.  I love watching Mark ease in working the screwdriver. he never slips and there always seems to be am excellent engagement with the slot. It looks like there is a concave grind and a proportionately slightly wider tip.

 

Who sells screwdrivers like this and, I guess more importantly, how would you maintain them? 

 

Thanks,

Dean Lapinel

[Marc]

Horotec make a jig for hollow grinding screwdrivers for about £250. Presumably they do the screwdrivers as well but I haven't found anyone selling them. I did see a set marketed by Cas-Ker, equally expensive though. To be honest if you take a little time to work out what angles best suit your own use, and then carefully maintain them properly, straight sided blades are just as good.

There is a lot of debate on the internet about the relative merits of straight versus hollow ground, and I reckon there are as many opinions about it as there are watchmakers.

One thing that I have found useful is to keep two sets of screw drivers (at least the most common sizes), one set ground slightly thinner than the other. Not all screw slots are the same width although they tend to be consistent within a given movement. A quick appraisal of the screws before I start work and I can select the screw drivers that best fit the situation. The result is less risk of slipping and less risk of damaging the screw. Add to that making sure that you always use the right size blade for the screw, and of course plenty of practise, and don't rush, and straight ground blades work fine.

[Geo]

The perfect answer Marc! :-)

[Lawson]

Totally agree with Marc. 

One caveat I will add is firstly buy a few basic screwdrivers to make sure it's a hobby you want to pursue in-depth before investing too heavily, as professional drivers are not cheap. 

 

But if you love working on watches, then buy the very best, as they last a lifetime and make a HUGE difference to the quality and performance of your work. 

 

Horotec are the way to go, in my humble opinion.  It's the ones I purchased (Horotec Screwdriver Set MSA01.205) after a LOT of researching, asking suppliers, and reading copious forums.

 

 

As Marc said, get both in the commons sizes ... here's the extra ones I brought to supplement my main driver set.

 

0.8mm (Gold - Hollow Ground), 0.9mm (Pink - Straight Ground), 1.0mm (Black - Hollow Ground), 1.2mm (Red - Hollow Ground), 1.4mm (Greg - Hollow Ground).

I brought the 0.9mm because I found that there are some screwhead that are annoyingly inbetween the 0.8 and 1.0mm driver.

 

 

Here's a pic of the two grinds on the 1.4mm drivers

 

[colinh]

I suppose if watchmakers haven't switched to philips, pozidrive or torx by now, then they're not going to :)

 

Anyway, what would really make screwdrivers not slip, for people who can't be bothered to hold the things vertically (like me) is a very slight hollow grind on the tip itself. I.e. the bit which contacts the bottom of the groove. I suppose that's a bad idea for a variety of reasons though...

[colinh]

Ooh! I just discovered something which I haven't heard mentioned:

 

If you keep the side of your index finger (which is applying light pressure to the top of the screwdriver) touching the side of your nose/eyebrow/temple then it doesn't wobble—and the screwdriver is much less likely to slip :)

[PadraicB]

Now there's a tip to remember!

[mlfloyd1]

Please explain the difference between a "hollow" and "straight" ground?  I'm not familiar with the terms in a screwdriver.

[bobm12]

mlfloyd1, the difference is that the straight ground one is flat on both sides, chisel like. The other one is not, sort of concave on both side as in the picture Lawson had above.

[mlfloyd1]

Got it.  Makes sense.

 

I've seen Mark use the hollow blades in a his videos.  it seems they provide more torque and less slippage.

[Charlie]

A hollow grind makes more sense as the blade has more contact with the slot and is ground more parallel with the sides of the slot as well. It would seem to me that more torque can be applied with a hollow ground blade verses a straight ground blade.

[PadraicB]

A hollow grind makes more sense as the blade has more contact with the slot and is ground more parallel with the sides of the slot as well. It would seem to me that more torque can be applied with a hollow ground blade verses a straight ground blade.

 

Time for physics ;). Torque is the product of force applied and the force’s distance from the axis of rotation.

 

τ = r x F

 

τ: torque

r: displacement (i.e. distance between torque point and force point)

F: force

 

An increase in either translates into an increase in torque. The same principle underpins how levers work, or why wrenches with a longer handle are easier to turn. For screws, this means that torque is the product of the force used when rotating the screwdriver and the blade size which determines the distance of the applied force from the axis of rotation (being half the blade length as force is applied on both sides of the blade). You can further modify this by reference to the angle between the force applied and the axis of rotation, e.g. a screwdriver not held perfectly vertical won’t be as efficient.

 

So, as far as I can tell, the shape of a screwdriver’s profile will have zero effect on torque unless it modifies either a) the force applied or B) the distance between that force and the axis. A hollow grind changes neither directly.

 

Personally, the benefit of a hollow grind would be that it better fits the screw and helps prevent slips and screw slot damage. A screw slot is cut parallel so a hollow grind blade will engage for the fill depth of the slot maximising friction on the sides (i.e. grip). A flat angled blade will only contact the top of the screw slot’s vertical sides so engagement is minimal, with reduced friction, making it more likely to slip. A hollow grind would also reduce the distance between the screw slot and blade at the bottom of the slot – which also reduces the chance of slippage or weird angled rotation axes which might cause damage to the screw slot or reduce torque (i.e. hollow grinds can indirectly increase torque if you were holding screwdrivers off vertical habitually – the gain would be minimal however).

 

The above also explains why using the right screwdriver size is important, the longer the blade, the greater the torque you create, for the same amount of force applied. Screw slot size is doubtlessly chosen in watches to take advantage of this.

 

I now return you to your regular programming… ;)

Edited October 15, 2014 by PadraicB

[Charlie]

Excellent description Geo, you must be an engineer. Hollow ground blades can increase torque, as you wrote. It really makes a difference with screws that are stubborn, akin to a comparison of a professional tool vrs. an amateur tool in the field of mechanics.

[Geo]

Charlie, you had better credit Padraic for that description, all l I did was say that I liked it. I am an engineer though! :-)

[Charlie]

It's just too early here! Outstanding description PadraicB !!

[PadraicB]

Excellent description Geo, you must be an engineer. Hollow ground blades can increase torque, as you wrote. It really makes a difference with screws that are stubborn, akin to a comparison of a professional tool vrs. an amateur tool in the field of mechanics.

Bear in mind I'd expect the torque increase to be very low, all other things being equal. An experienced professional who keeps their screwdrivers upright perfectly would measure no torque difference in theory. Most of the benefit appears to be avoiding slips, screw damage from poorly fitting blades, and therefore being able to increase force (leading to higher torque) more safely than with a chisel shaped blade.

 

That's very very similar to saying a hollow grind blade "gives you extra torque", but it's more accurate to say it "increases the torque you can apply before something goes wrong". ;)

[Charlie]

Exactly...more torque can be applied before slippage than with a straight ground blade. That is what I meant by it giving you more torque.

[Marc]

As I mentioned in my post above there are probably as many opinions about this as there people to express them and in the final analysis it comes down to personal choice and experience, but for the record, here's my two pence worth.

 

Whenever the subject of hollow ground versus flat ground screwdrivers is discussed in on-line forums (fora?) in regard to watch making (it seems to be one of those perennial topics on the net) the discussion inevitably comes around to the issues of torque and screw head damage, and I always seem to end up wondering about how much force others are using when dealing with screws.

 

I have only ever used flat ground screw drivers and although I can't claim to have never had a screw driver slip, I can honestly say that when it does happen it's invariably down to a lapse in concentration and/or rushed technique, and when it does happen, the fault is mine and not the screwdrivers.

 

Whether or not a hollow ground blade is less prone to slipping when compared to flat ground I wouldn't like to say as I've never tried one for comparison, however, I do think that the arguments used to back up such a claim need to be carefully examined.

 

It is often stated that because hollow ground blades are parallel where they interact with the screw head they better match the parallel sides of the slot thus increasing blade/slot contact and improving grip. This claim is however only true if the thickness of the parallel portion of the blade exactly matches the width of the slot. I wonder how often that situation pertains.

 

As I see it there are two scenarios that I believe would account for 99.9% of all likely hollow ground screw driver blade/screw slot interactions.

 

1) The parallel part of the blade is slightly thinner than the slot and the blade bottoms out in the slot, in which case the only torque transferring contact between the blade and the screw is a vertical line at each of the extreme edges of the blade that is only as long at each end as the depth of the slot. This results in almost the smallest amount of contact that it is possible to have and still turn the screw. A flat ground blade that bottoms out before filling the top of the slot has just two points of contact (not lines) at the top of the slot and so the hollow grind has a small advantage but either way I would consider this to be a poor fit.

 

2) The parallel part of the blade is slightly thinner than the slot but the blade does not bottom out in the slot, in which case the contact between the blade and the slot is two horizontal lines where the concave shoulder of the blade touches the very top of the slot. This is the same amount of contact that you get with a flat ground blade that does not bottom out and so has no advantage. 

 

To me it seems that in order to properly benefit from a hollow grind it would be necessary to have as many blade thickness's as there are slot widths in each of the sizes of screw driver that are to be used (have you seen the price of the hollow ground drivers?), and to assess each screw for slot width as well as length before selecting the screw driver for the job; for me this would be prohibitively expensive and painfully time consuming.

 

Then there is the complication and expense of maintaining the blades.

 

Also consider that a perfectly fitting blade (and remember it has to be a perfect fit or it might just as well be a flat ground) has to remain absolutely vertical in the slot as any deviation from upright will actually force the sides of the slot apart thus damaging (or at least weakening) the screw head.

 

So I use flat ground.

I maintain two sets of most sizes of screw driver; one in each size ground slightly thinner than the other in order to accommodate the different slot widths that I am likely to encounter.

For me the perfect fit of blade to slot is when the horizontal line of contact at the top of the slot happens just as the blade bottoms out in the slot. I reckon that this happens about as often as getting a perfect fit for a hollow grind as described above so in practise the blade should not bottom out as the horizontal contact is greater than the point contacts of a blade that hits the bottom without touching the sides. The right screwdriver to use is the one that extends furthest down to the bottom of the slot without actually touching.

 

I maintain my blades by dressing the flat faces with 600 grit wet and dry paper on a perfectly flat surface and I do not polish out the resulting lines (running up and down the face, not across) as these help to provide a relatively non-slip contact along that horizontal line.

And most importantly of all I use as light a touch as I can get away with whilst still doing the job, and I stay focussed on the task in hand. If I pull that off properly then the blade doesn't slip.

 

A couple of points to consider;

 

When a screw driver does slip it is the corners of the blade that damage the screw heads and the plates, and they don't care (or even know) if they are hollow or flat ground. The severity of the damage caused is proportional to the force exerted which is why a light touch is important. If you're having to push the blade into the screw slot with any kind of force then you might want to revise your technique as even a very tight screw will yield to a properly fitting blade irrespective of grind; remember it's torque that turns the screw and not downward pressure.

 

And just how tight do those pesky screws need to go any way? Remember, it's a watch, not an internal combustion engine!!

[Geo]

Marc, that is the most eloquent description I have ever read regarding screwdrivers.

I'm in whole hearted agreement, and like you I have two identical complete sets of flat ground screwdrivers sharpened to slightly different thicknesses, i.e. the thick set and the thin set keep them well maintained using a fine diamond slip.

Edited October 15, 2014 by Geo

[Charlie]

I will have to second that, it's good to have a second set. Also, it's not the installation that is troublesome, it's sometimes the removal of rusty or damaged screws that causes potential trouble. No watchmaker needs to be reminded what he is working on or how much torque to apply, it's common sense.

[Marc]

Thanks for the compliment Geo. I get call a whole load of different things during the average working day but rarely is "eloquent" one of them  :D

Fair comment Charlie, there is always going to be that one screw that is a "special" case, either due as you say to corrosion, or because someone who shouldn't have been fiddling has installed it with Loctite and tightened it with an impact wrench set to max. :pulling-hair-out: Often as not in those circumstances I will declare it FUBAR  and a more aggressive approach to removal is adopted on the grounds that the offending screw will be replaced, in which case anything goes so long as the the only thing that suffers is the screw.

[noirrac1j]

Hey just buy the inexpensive French screwdrivers and buy the Bergeon blades....they are very good quality, fit the French screwdriver holders exactly and none cost more than $6.00! You can buy a bunch of sizes for about $22.00.

 

 

JC

[noirrac1j]

:geek:

Edited October 16, 2014 by noirrac1j

[Waggy]

On 10/15/2014 at 1:20 PM, PadraicB said:

 

Time for physics ;). Torque is the product of force applied and the force’s distance from the axis of rotation.

 

τ = r x F

 

τ: torque

r: displacement (i.e. distance between torque point and force point)

F: force

 

An increase in either translates into an increase in torque. The same principle underpins how levers work, or why wrenches with a longer handle are easier to turn. For screws, this means that torque is the product of the force used when rotating the screwdriver and the blade size which determines the distance of the applied force from the axis of rotation (being half the blade length as force is applied on both sides of the blade). You can further modify this by reference to the angle between the force applied and the axis of rotation, e.g. a screwdriver not held perfectly vertical won’t be as efficient.

 

So, as far as I can tell, the shape of a screwdriver’s profile will have zero effect on torque unless it modifies either a) the force applied or B) the distance between that force and the axis. A hollow grind changes neither directly.

 

Personally, the benefit of a hollow grind would be that it better fits the screw and helps prevent slips and screw slot damage. A screw slot is cut parallel so a hollow grind blade will engage for the fill depth of the slot maximising friction on the sides (i.e. grip). A flat angled blade will only contact the top of the screw slot’s vertical sides so engagement is minimal, with reduced friction, making it more likely to slip. A hollow grind would also reduce the distance between the screw slot and blade at the bottom of the slot – which also reduces the chance of slippage or weird angled rotation axes which might cause damage to the screw slot or reduce torque (i.e. hollow grinds can indirectly increase torque if you were holding screwdrivers off vertical habitually – the gain would be minimal however).

 

The above also explains why using the right screwdriver size is important, the longer the blade, the greater the torque you create, for the same amount of force applied. Screw slot size is doubtlessly chosen in watches to take advantage of this.

 

I now return you to your regular programming…

As a fellow engineer I agree that you are correct, however, there is a contextual factor that is missing from the above. If the hollow grind gives the user more confidence to apply more force (i.e. less, real or perceived, risk of slipping- see below) then the resulting torque would increase. Hence, from this perspective someone using a hollow grind screwdriver could impart more torque than if they were using a straight grind version if the human factor is taken into account.

Just being extra picky to get my engineer geek on:

But I was also thinking that the hollow grinds appear to be shorter than the straight grind, which will make it more likely that the screwdriver will engage both flanks of the screw slot before bottoming out and centralise the screwdriver. The longer flanks of the straight grind must increase the risk of an asymmetric engagement with the screw slot (i.e. the screwdriver bottoms out before both flanks of the screw slot are engaged) which may lead to a eccentric application of torque, and as every action has an equal and opposite reaction the screwdriver will want to eject itself from the screw slot in the opposite direction to these point loads... ie a slip.

In addition to the above and we accept that the contact of the straight cut are more likely to be point load(s) on the screw flank(s), rather than a distributed load. A point load will concentrate the force (F= l/a) to a point on the screw slot where it exceeds the yield strength of the material and cause damage (plastic deformation) to the screw head. Unlike the distributed load of the hollow grind which dissipates this force over a greater surface area and would remain below the yield of the material for a given torque, allowing more torque to be applied before damage occurs to the screw head.

Did I just talk myself into needing a hollow grind set of screwdrivers and a hollow grinding machine?!?!?

[Klassiker]

1 hour ago, Waggy said:

Did I just talk myself into needing a hollow grind set of screwdrivers and a hollow grinding machine?!?!?

Read Marc's comment above and save your money.