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

I sometimes use square section spring washers under the heads of cap screws, not so much to prevent loosening* [Junker demonstrated that spring washers under screw/bolt heads do not prevent loosening when parts are subject to transverse loads], but

a). to allow the underside of the screw head to turn against the washer not the part, and

b). because there is sometimes a small radius where the shank of a cap screw meets the head, and this radius can prevent the underside of the cap from sitting absolutely flush with the part.

My question is why are there such things as spring washers that fit under the heads of cap screws without protruding laterally [enabling them to be used in counterbored holes] but there is no such thing on the market as a standard plain washer that can fit under the head of cap screws without protruding?

*Though spring washers do not prevent loosening in all conditions, I believe they more often prevent complete separation, because once a bolt loosens enough to release the spring tension on the washer, the splayed ends of the washer dig into both the underside of the screw head and the surface of the part, and in some circumstances this can act as a brake to prevent further loosening, at least temporarily.

Edited By Bill Phinn on 19/10/2022 03:27:41

Smaller OD flat washers, suitable for cap screws, etc are commercially available but you will have to go to a special nut and bolt supplier. They will not be available at the local hardware store.

Shnorr washers are made to fit under cap screws. They are the same diamiteras the head but have a sedated face and can be dished to help friction and are Usualy hardened like a spring.

David

Standard counterbore sizes for cap heads are quite generous so the washer can protrude beyond the head and still fit into a counterbored hole. eg for an M5 fixing the clearance hole is 5.3mm and the counerbore 10mm. A suitable washer for use under cap head sis 9mm so will fit down the hole but protrude a little beyond the 8mm OD of the head.

A quick google for cap head washer will bring up plenty of suppliers and size charts such as this

If you are plunging with an 8mm milling cutter to get your counterbore then all bets are off.

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I was avoiding the temptation to have my usual rant about how much better the Imperial cap-heads can be … but let me just reference this Unbrako page … which offers free downloads of some very informative documents:

**LINK** https://unbrako.com/catalogs-downloads

MichaelG.

could make them

If you cannot find them stack some on a mandrel/bolt and turn the required amount off. I know its a pain but quicker than sarching for and awaiting delivery.

regards

Thanks to everyone for your replies.

My post in this thread:

https://www.model-engineer.co.uk/forums/postings.asp?th=177296&p=2

will explain why the DIN433 cheese-head screw washers Jason and others are referring to will not fit my counterbores. The metric counterbores I use aren’t rare anomalies dimensionally speaking but are widely available in the marketplace. Their closer clearances (e.g. 13.4mm for the 13mm head of an M8 cap screw) give a much better appearance, I think, than the standard ones Jason refers to.

Michael’s mention of imperial cap screws led me to look into the relative dimensions of imperial cap screw heads and the counterbores that exist to accommodate them. Obviously what I’ve unearthed in a brief search isn’t the whole picture, but…

Mc-Master-Carr’s 5/16” cap screws have 15/32” (11.90625mm) diameter heads. Mc-Master-Carr’s standard 5/16” counterbores have a cutter diameter of 0.475” (12.065mm). Their 1/64” and 1/32” oversized counterbores, for “when the screw holes in sheets or layers of material are misaligned”, have cutter diameters of 0.491” (12.4714mm) and 0.506” (12.8524mm) respectively.

Hence, with their standard counterbore, a 5/16” cap screw head’s diameter is 98.7% of the diameter of the counterbore.
With the 1/64” oversized counterbore, the 5/16” cap screw head is 95.5% of the diameter of the counterbore.
With the 1/32” oversized counterbore, the 5/16” cap screw head is 92.6% of the diameter of the counterbore.

Now let’s compare DIN 912 M8 cap screws. The head is usually 13mm diameter. Standard M8 counterbores of the kind Jason has in mind typically have cutter diameters of 15mm. This means the head of an M8 cap screw is only 86.7% of the diameter of the counterbore. When measured by the standards of typical imperial counterbores, therefore, these standard metric counterbores produce massively oversized recesses for the heads of cap screws to sit in.

The question is why they are "standard" when they are so oversized?

In the case of the apparently non-standard M8 counterbore I have chosen to use, the head of the cap screw is 97.01% of the cutter’s diameter. This puts it dimensionally about midway between Mc-Master-Carr’s standard and 1/64” oversized counterbores. Isn’t this a sensible sizing decision, far more sensible at any rate than the unaccountably “standard” M8 counterbores with 15mm cutters?

Yes, I can make plain washers to fit, and have done, but the question remains why, when square section spring washers are practically the same diameter as the cap screw heads they are intended for and commonly available, isn’t there such a thing on the market as a plain washer with essentially the same dimensions?

To answer David's suggestion of Schnorr washers, although it seems these will fit my counterbores, I wouldn’t want to use these washers very often, partly because of cost and partly because they may leave gouges on the surfaces of parts, just as spring washers do when you disassemble connections they’re used with.

Well you did mention standard, this site lists 6 different standards from around the world such as ISO, DIN, JIS, ANSI, etc for metric socket screw counterbores and all are 15mm for M8 so I can see why most washers fit. I think there are a couple of closer and looser standards about possibly in Machinerys handbook. Also worth looking at the ANSI imperial which gives a larger dia than your McMaster Carr ones at 1/16 over head dia.

What make are yours and are you using metric ones or a 5/16 one which is rather close to the Ansi chart?

Edited By JasonB on 19/10/2022 18:11:53

Edited By JasonB on 19/10/2022 18:14:14

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I am reasonably convinced that I sussed that one a long time ago, Bill

To paraphrase what I have said over the years:

• The proportions of Imperial cap-head screws were designed for toolmakers and aircraft equipment manufacturers
• The proportions of Metric cap-head screws were designed for IKEA
• … and the same goes for quality of materials and manufacture.

Yes, that’s a gross oversimplification, but I think you will get my drift.

The reason that the specified counterbore is so big, is that somebody calculated the maximum build-up of tolerances in the various Metric specs. and gasped.

It’s great to see that you have done the independent research and come to the same general conclusion.

Corroboration for much of this will be found in those UNBRAKO documents that I referenced.

MichaelG.

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

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

Edited By Michael Gilligan on 19/10/2022 18:17:23

Have you put M8 x 13 washer into Google, plenty come up there eg Amazon, Accu, e-bay so they are available to suit your tighter holes.

Edited By JasonB on 19/10/2022 18:41:01

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Probably overstock 5/16” items

MichaelG.

Too many on the net to be oversize metric and they tend to be called shim washers and come in a range of METRIC thicknesses eg 0.1. 0.2. 0.3. 0.5, 0.75, 1.0. That chap Ali who sends express delivery has a lot to offer 8x10, 11, 12, 13, 14, 16 which don't all tie in with imperial sizes

Just wondering if Bill's 13.4 counterbore is a 5/16 imperial one being sold/described as metric.

Edited By JasonB on 19/10/2022 18:57:40

..too much Botox causes that, I gather from the internet...

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That’s probably a good call, Jason

17/32” nominal according to UNBRAKO

MichaelG.

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

Edited By Michael Gilligan on 19/10/2022 20:01:56

Interesting that they give 14.25mm for M8 but don't say what standard that is to yet all those in my earlier link are a larger 15mm

Though could the fact they are US based mean they suggest a 9/16" hole so the "standard" is nearest inperial expressed in metric

.

It’s probably because they have a long tradition of doing things meticulously, and 15mm would be abhorrent.

MichaelG.

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Here’s a rabbit-hole quoting a wide range of standards: 

https://engineersbible.com/counterbore-socket-din/

… haven’t followed-up the links yet.

Edited By Michael Gilligan on 19/10/2022 20:40:53

I think it'd be easier to make 'em unless you need a serious quantity.

Many thanks for the further replies.

My counterbores are branded SWT, made by a big company in Xinan, China. They’re sold by many sellers on eBay, which is where I acquired my set of M3-M12. The dimensions chart I’ve posted clearly shows they’re sold as metric.

I had already seen the Engineer’s Bible pages showing the many different standards for counterbores. It confirmed my perception that standard metric counterbores are very oversized for the fasteners they’re intended for. I’d not read the Unbrako pages before today [and they do look interesting], but I was aware that the Unbrako name was acquired a few years ago by an Indian company called Deepak Fasteners Ltd.

I was surprised to read in my 17th edition of Machinery’s Handbook that “Counterbores for screw holes are generally made in sets. Each set consists of three counterbores: one with the body the size of the screw head and the pilot the size of the hole to admit the body of the screw; one with the body the size of the head of the screw and the pilot the size of the tap drill; and the third with the body the size of the body of the screw and the pilot the size of tap drill”.

I’m sure I’ve never seen counterbores sold that way.

I wasn’t aware of the existence of shim washers. Their dimensions fit the bill but a casual search suggests they're rather pricey.

I remain forlornly hopeful that manufacturers of metric counterbores and reduced diameter washers will see sense some time soon and start producing both with smaller diameters that more closely correspond with the real-world dimensions of metric cap screws.

Despite our negative impressions of some Indian engineering suppliers … it looks like the Unbrako name has found a good home:

**LINK**

https://deepakfasteners.com

MichaelG.