Why aren't there plain washers on the market specifically for use under the heads of cap screws?

Not seen the last of the three but YG-1 list fine and medium screw hole as well as tapping size, all with the same 15mm for M8 heads

I suppose it makes sense as if the head hole was tight there would be little point in having the screw hole larger as the head would not be able to wobble about if there was no room around it. If you were using aircraft type counterbores then one 15mm would be needed and just swap pilots as needed to suite the clearance hole.

Don't know what standard those ones in the chart are made to but at least your washers are made and available at 12 or 13mm dia for M8.

The reason such washers are not commonly available is they are not commonly used, because there is commonly no point to them.

Cap screws work just fine in counterbored holes without washers. The heads don't have the corners that a hex-head bolt has, so no need for a flat washer to facilitate turning when tightening. And the radius where the bolt shank joins the cap head is small enough that if you use the correct sized clearance hole, the hole will clear that radius.

When I worked in toolmaking, press tools were exclusively held together with cap screws in counterbored holes, without washers, ranging from small dies used in presses running at 50 strokes a minute through to monsters run in a one-thousand ton press. Never saw a cap screw come loose even under those conditions. Of course, they were well torqued up, being a high tensile screw into tool steel and tightened with a suitable short length of pipe over the allen key.

The only place I remember seeing spring washers under cap screw heads is where they are used to hold the two halves of a brake caliper together on motorcycles and safety is critical.

Never ever seen flat washers used with cap screws in counterbored holes anywhere.

So use the correct sized clearance hole (1/32" or at least half a mm bigger than the shank diameter) and do them up tight and all should be well, sans washers.

Delighted to see Hopper's comment, because it fits my amateur thoughts on the subject.

I started by asking what washers are for? A quick look-see revealed three main reasons for using washers:

1. A washer is needed if the material being fastened is weaker than the bolt. Then washers provide support to the top surface as the bolt is tightened, which otherwise tends to break up around the entry point. The effect is most obvious on soft materials like wood, but it happens to metals like Aluminium, Copper, and Mild-steel as well. I imagine Cast-iron being weak in tension makes it likely to need washers too. The 'weak material' needing protection can be a layer of paint rather than the underlying metal.
2. They're designed to absorb vibration, split springs and so forth, which is important in anything that moves because alternating stresses are remarkably good at undoing threads. Not good when a wheel falls off!
3. To seal the connection against liquids, either to prevent leaks or corrosion.

As far as I understand washers from my armchair, none of the reasons for using them require the washer to be a particular size. So why do Imperial counter-bores come in different sizes? I suspect the reason is historic. Imperial tooling dates to a time before materials were made to a consistent specification, therefore the size of the washer needed depended more critically on what was being made. An unreliable material needs bigger washers than reliable materials, but drilling large counterbores is more expensive than drilling small ones. In this world it's a good thing that designers and production engineers had a choice.

Metric fasteners are a latter development, becoming popular after metals were made to a consistent specification. In this world, I think it's cheaper to drill one adequate size of counterbore hole sufficient for most materials. The metric system is deliberately simplified compared with predecessors, usually for the better.

However, although modern engineering strongly prefers using standard sizes it's always possible for the designer to decide otherwise, for example by specifying a larger or smaller counterbore. But they'd need a strong reason because it involves buying non-standard tooling in a brutally competitive world where extra cost is dangerous.

Dave

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Posted by Michael Gilligan on 19/10/2022 19:59:04:

Pages 82 and 83 of the ‘Engineering Guide’ give a nice summary

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Sorry to be a swot, Dave ^^^

https://unbrako.com/images/downloads/engguide.pdf

MichaelG.

Edited By Michael Gilligan on 20/10/2022 11:40:27

No problem Michael and the Unbrako link is suggestive because it mentions washers in the text only once and the use-case pictures don't show washers at all.

Washers are mentioned only in this torque measuring table, which adds a washer reason I missed load-indicating:

The accuracy values quoted are quite interesting too! That humans judge torque poorly isn't surprising, but I thought torque wrenches were considerably better.

'Turn of the Nut' is a new one on me. Can anyone explain how it works?

Dave

Only guessing, but could it mean something like those plastic pointers you see on lorry wheels?

Rob

To address Hopper’s point, the main problem, as I see it, with omitting a washer under a cap screw comes when the surface in direct contact with the screw head is a lot softer than the screw, e.g. aluminium. It’s chiefly for use on aluminium that I wanted plain washers that fit my counterbores.

I can’t really see why using cap screws in counterbored holes should do away with the need for one of the common functions of washers, i.e. to prevent chewing up of the surface of the thing that is in direct contact with the underside of the fastener head - a surely important consideration, particularly if frequent disassembly and reassembly takes place. It’s not primarily aesthetics I‘m thinking of here.

It seems I’m not the only person to have felt the need for plain washers under cap screw heads:

https://www.practicalmachinist.com/forum/threads/is-there-such-a-thing-as-a-washer-for-a-shcs.316893/

An additional reason why I’m sceptical whether it’s right to say there's commonly no point to using washers [spring or plain or otherwise] under cap screws in counterbored holes is the content I’ve managed to find [from a fuzzy image] of DIN 974 part 1:

“DIN 974 , part 1. Diameters of Counterbores

1 - …
2 - Counterbore Diameters

This standard specifies six series of counterbore diameters as a function of screw type, of whether used with or without washer, and, if used with washer, of the washer type…

Where there is the risk of the edge of the hole being damaged by the screw underhead fillet, the edge shall be chamfered. This, however, will increase the pressure on the head bearing face.”

What I can’t find mention of is quite what diameter counterbores DIN 974 recommends in these six cases and what these six cases are. It may well be that counterbores on the wide side are advised for use on softer materials, so that washers such as DIN433 [or even wider] can be used to spread the load.

If this is so, my wish to use narrower and perhaps non-standard counterbores with aluminium and have the narrower than standard washers to go with them may ultimately be misguided, though perhaps not as misguided, in this case at least, as using no washer at all.

Hi Dave, in recent times on cars, many nuts are torqued up in two or three stages, and then finally turned by a set number of degrees once or even twice. Don't know if this is what is meant by the term 'turn of the nut' but that's all I can think of. I have had to use that method in industry as well.

Regards Nick.

In the vibration-testhouse … The only time we used washers under cap-heads in counterbores was when using an an Aluminium/Magnesiun alloy ‘Head Expander’ and doing high vibration levels.… and those were always flat [hardened and ground]

Incidentally: Our biggest shaker was 20,000 pounds thrust

MichaelG.

Spark plugs into alli cylinder heads - instructions might read something like "...turn to finger-tight washer contact, then tighten 1/6th turn only."

Plus I'm with Hopper on this - in the tool design office I used to work in, capscrews were specified in counterbores without washers, leastways that was how every drawing I ever saw or did was done. And some of the press tools did big batches. Toolmakers had a bit of licence though, so it's possible they sometimes added them off their own bat - but if they did I never saw 'em.

Edited By Mick B1 on 20/10/2022 21:08:37

Ah well then, it seems you have "special needs" so to speak. Perhaps look to the aircraft industry where lots of ally is used?

I reckon in the home shop I would try to get away with using common spring washers that would fit in the counterbored hole rather than source flat "specials". They would still protect the ally.

As an aside,  you can make your counterbores whatever size you like if you drill them with a normal drill bit then finish off with a normal drill bit ground flat on the end to provide the square shoulder. That's all we ever used in the toolroom. Special counterbore tools had not been invented yet, I guess!

Edited By Hopper on 21/10/2022 05:07:47

As I said earlier there is really no need to go looking at specialist suppliers or aircraft industry. real problem is in the title of the thread as you don't want "Plain Washers"

Here is one of the suppliers I mentioned earlier, Hole for M8, range of diameters and range of thicknesses, upto 1mm depending on seller

Edited By JasonB on 21/10/2022 07:38:13

The clamp load applied for a given applied torque will differ greatly, depending up the pitch of the threads and the materials used in the threads, and lubrication, or lack of it.

"Turn of the nut" methods are often used , in the field, when tightening to yield.

This maximises the clamp load applied , and makes it consistent, in the fastener.

The fastener is tightened to a "snug" toque, and then tightened a further increment to put the fastener into yield, The figures will depend on the grade of fastener used.

As an instance, a 1/2 UNF in W range might be snugged to 90 lb fit and then tightened another two flats to put into yield.

Torque wrenches vary in their accuracy, and how the are used also affects accuracy of the applied load..

One dial wrench supplied by very well revered tool supplier / manufacturer, was found to be grossly inaccurate when checked, no matter how it was operated..

"Break back" wrenches are VERY susceptible to how the torque is applied. A suddenly applied load can apply a vastly exaggerated load.

Torque wrenches are best used by applying the torque gently until the wrench either indicates to required torque, or clicks, or "breaks"

Strangely, some of the most accurate wrenches, in terms of applied torque vs reading are the "bendy bar" type where a pointer moves over a fixed scale as the bar bends under the applied torque.

A torque wrench can be checked for accuracy by using it to apply torque to a pulley of known diameter, around which is a wire carrying known weights, to lift the weight. Using different weights allows the wrench to be calibrated, so that the scale reading can be compared with the actual torque being applied..

(Then, one has to be confident of the accuracy of the pulley diameter, and the weights, to be sure of the torque calibration! ) .

Howard

Our factory had a torque tools calibration department so I thought I would get my Norbar torque wrenches checked. The lads who checked them were able to get consistent results but I discovered that I was the biggest variable in the process. The tools are accurate but using them consistently certainly requires some practice. Hoppers real experience is exactly what I have observed in a motor industry tool room but only from the position of an interested sparky. One of the most brutal demonstrations of the power of hydraulics was demonstrated when the sensor failed that checked the safety chocks were retracted. The chocks would take the weight of the press slide on a 1000 ton press. The slide probably weighed 50tons but the press powered straight through snapping about a dozen 30mm diameter cap screws. An awesome demonstration of hydraulic power. Just wondering if aluminium would have a tendency to yield under the reduced area that cap screws apply their clamping force over.

Mike

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That’s why we used washers in the counterbores of the head-expanders, Mike

They were sized to just fit the counterbore [i.e. slightly larger than the cap-head diameter] so as to optimise the surface area of contact.

Counterbores were also very deep, so as to minimise cyclic stretching of the screws.

It’s a hard life on top of a 20,000 pounds thrust shaker !

MichaelG.

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Just for info. __ this would be the current version of our big shaker:

https://www.bksv.com/en/instruments/vibration-testing-equipment/lds-shakers/high-force-shakers/lds-v964

Edited By Michael Gilligan on 21/10/2022 13:11:35

A ride with that shaker is going to be quite a tough time. One of the tests that a car was put through was a 500mile pavé test, the body shell was measured the measured again after being thrashed for 500miles on a pavé test track. The Princess prototype developed a droop and was reengineered to survive the test.
Mike

Thanks for the further replies.

If I can get confirmation from a couple of sellers of the shim washers what the actual rather than the nominal ID is of the 8x13x1 ones, I might give them a go.

Given the modest quantities I'm talking about, though, and the immodest price of these shim washers, my need for off-the-peg washers with those dimensions is correspondingly not very pressing.

Following other forum members' advice, I've just made some plain washers of the dimensions I want for M6, M8 and M10 cap screws.

M6 were made from Form A M5 washers drilled out to 6.4mm. No reduction of the OD was necessary as it is 9.95mm.

M8 were made from Form A M7 washers. ID was drilled out to 8.4mm, OD reduced by 0.4mm.

M10 were made from Form A M8 washers. ID was drilled to 10.5mm. Existing OD is only 0.1mm bigger than the cap screw head, so again no reduction was needed.

The task was a very quick one and these washers all now fit my counterbores.

I see that Drill Service Horley do sell at least 3 different sizes of counterbores targeted presumably at M8 cap screws: 13.5x8.4, 15.0x8.4, and 18x8.5. Their 14.0x9.0 would be another candidate.