How to reduce head diameter of countersunk wood screw?

[Clive FosterParticipant @clivefoster55965]

Been asked to source some small steel countersunk slot head wood screws for use in restoring a very old piano.

Fortunately the ancient screws are almost exactly the same size as No 4 x 1/2″ which can be obtained. At a price. Approaching £1 each for GKN new old stock. However the No 4 head is approximately 5 mm diameter whilst the old ones are only approximately 4 mm diameter, many need to go down holes that can’t be enlarged for the later screws. Real head diameters are of course fractions of an inch but mm is easier to follow here.

So I need to hold the screws to turn the heads down. Preferably without breaking too many screws in the process.

Plan A is to:-

grab some high quality doweling in a collet with minimum stick out

countersink the end to take the head

drill an appropriate size so the screw goes in true and firm

turn down both dowel and screw head to size

Rinse and repeat, cutting back the dowel to show a fresh end of each time. Simply deepening the hole for the next screw seems risky as the drill is likely to start running out of true.

I have a stash of piano hammer shanks which are a sensible size and high quality material.

Does the team think this is feasible method.

Suggestions for a better way to go about things gratefully received.

Clive

[Charles LamontParticipant @charleslamont71117]

I take it you don’t want to just face them off, as you would lose the slot. Do you want to reduce the diameter so there is a short cylindrical bit at the top of the head, or to cut the whole head cone down at 45°?

Either way, why can’t you just chuck them by the shank?

[Ian PParticipant @ianp]

Just machining of the top face means you will end up screw that will not sit or look correct the countersunk hole that the original screws did. I feel your pain regarding the cost of NOS GKN screws as I make metal part for a furniture designer/maker and have to adapt and modify screws and other fixings.

I would probably use a cheap drill chuck (keyless off a discarded battery drill) mounted in the lathe 3 jaw to grip the shank of the screw so that the head and a small portion of the shank is exposed. You can turn off the head OD and  extend the shank length slightly with a tool ground to the countersink angle.

I know the screw tank is tapered and the drill chuck is meant for parallel drill shanks, but if you dont tell anyone then no one will care!

Ian P

 

[duncan webster 1Participant @duncanwebster1]

Reading the original post, the problem us to reduce the head diameter so the screws will go down a counterbore which cannot be enlarged, so the OP is trying to reduce the diameter. Old fashioned brass screws are tapered, so chuck isn’t ideal. I’d be tempted to try a bit of plastic, UPVC or similar in the three jaw, drill a pilot hole and follow it with a drill just a tad bigger than the unthreaded part of the screw, drive in the screw and away you go. Should last for a fair few screws, then just repeat. Might need to experiment on drill sizes a bit, pilot drill a bit bigger than you’d use for wood.

[Michael GilliganParticipant @michaelgilligan61133]

Please forgive a slight digression [it may be helpful]
In memory of my Dad, who worked fot Nettlefolds when I was a kid, I am sharing the Patent which effectively started the business:

https://worldwide.espacenet.com/patent/search?q=pn%3DCA37168A

The proportions of a wood-screw are beautifully illustrated therein.

MichaelG.

[bernard towersParticipant @bernardtowers37738]

If you only machine the c/sunk portion you will still be left with the slot, possibly

[duncan webster 1]

On duncan webster 1 Said:

… I’d be tempted to try a bit of plastic, UPVC or similar in the three jaw, drill a pilot hole and follow it with a drill just a tad bigger than the unthreaded part of the screw, drive in the screw and away you go. Should last for a fair few screws, then just repeat. Might need to experiment on drill sizes a bit, pilot drill a bit bigger than you’d use for wood.

Aluminium might work for that too, and be a bit more durable? Soft enough for the woodscrew to bite into but stout enough to stand up to repeated use, if the stepped hole was made just right. You might even be able to put a lengthwise junior hacksaw slot along the aluminium bar so the “collet” can be nipped up a bit in the chuck once it has worn?

Or could you use some of those plastic wall plugs/rawlplugs used for screwing into a hole in brick and concrete etc? Screw the screw in then grip the plastic wall plug in the chuck. The plastic should take up the tapered shape of the screw on the inside and the parallel shape of the three jaws on the outside?

Bit of experimenting and you will get there.

[JasonBModerator @jasonb]

I’ve done a few so the heads don’t look as big on carts for Hit & Miss engine models. Made a split aluminium top hat bush to fit the shank and then a pointed tool to turn the back of the chamfer down. Does not matter if you cut into the bush a bit.

Other option is to face them off and then recut the slot which is easy enough using the same bush in a collet block and a slitting saw. At least the old ones are not hardened.

 

[Nigel Graham 2Participant @nigelgraham2]

Is there no metric equivalent to No.4 X 1/2″, that would be suitable and far cheaper? (It would be something-mm X 12mm, but I don’t know the equivalents.)

I’d go with Jason’s solution of a split aluminium bush, but not try turning the back of the chamfer. Since the idea is to reduce the head diameter, simply turn that. It will achieve the same result.

It make work combining this with Duncan’s suggestion: use an aluminium bush with a shank-diameter counter-drilling then the threaded part, so the shank is guided concentrically.

Using a plastic plug as Hopper suggests, risks the result being non-concentric because the plugs are soft, have a thin wall and will distort rather unpredictably under the chuck and cutting forces.

[Mark RandParticipant @markrand96270]

You don’t say which collet system you are using. Are soft collets available for it?

[Martin KyteParticipant @martinkyte99762]

I would at least consider fitting threaded inserts to the piano to take a parallel thread screw. Inserts and screws are easy enough to make to suit.

regards Martin

[Michael GilliganParticipant @michaelgilligan61133]

I think the easiest way to chuck them might be to use Shellac

Make a little stub-mandrel with a suitably close-fit plain hole to accept the screw … a small butane flame should suffice to soften the Shellac for each change, and for clean-up just use Methylated Spirits.

MichaelG.

[Clive FosterParticipant @clivefoster55965]

Thanks for the suggestions.

Regrettably holding by the shank isn’t possible because, assuming the 6 used ones I found at the bottom of one of my “keep after removal ‘cos they might be useful” box are typical, there is effectively no parallel shank. Tapered all the way up except for tiny land so the thread doesn’t hit the countersink.

Realistically they have to be screwed into something to ensure they stay concentric when being cut and to support the head. The overall diameter of a No 4 is, I believe, officially 7/64″ or near enough 2.4 mm in new money so the maximum core diameter is tiny. Probably somewhere between 5/64″ and 1/16″ say 1.9 to 1.6 mm in new money. Michael Gl would dig out the travelling microscope to look! If the head isn’t properly supported the screw will bend almost as soon as you look at it when working on the edge of the screw due to the considerable leverage.

Using aluminium rather than wood as a support material is tempting but I fear that anything soft enough to easily take the screw without damaging the slot will be a bear to turn. Certainly the only piece of relevant size I have is the horrible “2 seconds to a built up edge” stuff. There was a reason i was given 4 ft of it!

I like Jasons idea of shaving the countersink from the back. Have to be done by running down the taper rather than straight in. Probably in two or three steps so the head is supported. Straight in with an angled tool immediately destroys the support and the effective cut is large so odds are the head will twist off.

Face turning and deepening the slot with a slitting saw is an interesting idea. Major issue is going to be lining up the slot. It has to be dead nuts to look right. If I have an appropriate slitting saw I can set it up on the Bridgeport but it’s not really the right machine for the job. Much easier on a baby horizontal mill.

The right tool for reworking screw head slots is, of course, a slotting saw blade such as the long discontinued Starrett #249 set. Fit a short hacksaw frame. Square cut teeth with no set so you get a flat bottom and straight sides. High up on the “shoulda bought” tool list. But too young to understand when opportunity knocked.

On reflection I’ll try my Plan A first, Plan B will be cutting the back side of the countersink and Plan C will be trimming the face and recutting the slot.

Thanks very much for the ideas. So nice to have options.

Clive

[noel shelleyParticipant @noelshelley55608]

A small tapered dowel pin reamer might make a suitable hole in a soft collet to hold the shank ? Just a thought ! Noel.

[CirclipParticipant @circlip]

Reducing head DIAMETER by 1mm? How many thousand are you going to convert? Jason’s split collet answer is the obvious choice. Two bits of flat Aluminininium bar in chuck, turn to  suitable diameter and drill for plain shank diameter and then drill to clear threaded part of screw before parting off to generate two half collets.

Bet Sir John is laughing at this one.

Regards  Ian.

[Andrew CrowParticipant @andrewcrow91475]

Could you make a d bit as close to the taper as possible, use the d bit to make a bush, then cut a slot through  the bush which would hold the screw as if in a collet.

I do have some 5/8 x No.4 GKN screws that are at least 60 years old but the heads are nominally 7/32 dia. I could post some to you if they would be of any help.

Andy

[HopperParticipant @hopper]

If the screw heads are going to be down inside counterbored holes, and you don’t have too many to do, I would carefully grip each one in small vice grips and grind the head diameter down on the fine wheel on the bench grinder.

[MichaelRParticipant @michaelr]

I have a box of Nettlefolds 1/2″x3 the heads are 3.8 mm shank 2.4mm if you think they may be suitable I can send you some.

MichaelR

[Clive FosterParticipant @clivefoster55965]

Well what a waste of some perfectly good worrying!

Plan A worked just like that! And the slot on the sample one was deep enough that I could thin down the head by a useful amount.

Hafta see what Nigel, the Piano Shop Man, thinks when he comes to look at it.

Must confess to considerable surprise as to how easily things worked. My instincts just aren’t geared to this sort of tiny job where it’s both imperative to pay proper attention to deflection under cutting forces and there is no room for a steady.

MichaelR

Thanks for the splendid offer but No 3 will be too small. No 4 is already a thou or three under size in the thread but that shouldn’t be too much.

Thanks

Clive

[Nigel Graham 2Participant @nigelgraham2]

Well, Clive, you certainly caused a lot of cogitating!

I trust all these modified screws will help bring the instrument to Leeds International Piano Festival standard. 🙂

[Trevor Drabble 1Participant @trevordrabble1]

<p style=”text-align: left;”>Clive , Should you require any more then I have a box full of No4 X ⅝” as per pic .</p>

[Clive FosterParticipant @clivefoster55965]

Trevor

Thanks very much for the splendid offer.

But it looks like sourcing 1/2″ long screws is now sorted.

I’ve still got to take about 1/16″ off them so they don’t protrude though. The larger tip diameter after equivalent shortening of 5/8″ ones probably risks splitting the 200 year old wood of some rather delicate components.

Clive

[Trevor Drabble 1Participant @trevordrabble1]

Clive , You most welcome , and glad you sorted . FYI My local supplier has traditional slotted head counter sunk wood screws No 4 x ½” in solid brass at £1/10 off.

[File HandleParticipant @filehandle]

There are still a few hardware stores that sell them. Many of my traditional slotted screws have come from an antique / house clearance center. Typically a few hundred for £1 to £5.

[Mick BaileyParticipant @mickbailey28509]

I’m reading these prices of screws with astonishment and feel like I just woke up in a parallel universe. When did wood screws become £1 each?

[Author]

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