Using Counterbores

I am intending to counterbore two holes which will have M5 caphead bolts located in them. The holes are drilled M5 and the bolt fits nicely. I have a set of very good quality counterbores with the usual range of sizes and several individual counterbores.

The issue I have is that the locating spigot on all the counterbores are larger than I would have expected. i.e I would have expected the M5 counterbore to have a 5mm dia spigot but it is around 5.23mm. Is this 5.23mm diameter the correct sized clearance hole for M5.

Colin

Yes, they are intended for generous clearance holes, you can get some that use a smaller range but still more than nominal.

Think M5 standard is 5.3mm clearance.

If you want ‘toolmaking’ tolerances for capheads ...

avoid the ISO Metric system and use Imperial fasteners.

... I will leave you to find the comparative proportions 

MichaelG.

Edited By Michael Gilligan on 04/06/2020 20:37:47

Jason is correct & another point is you do want clearance around a screw as generally they are not used as a location component.

Tony

I've looked through the British Standard for metric threads and there is no mention of clearance holes. The tables in my Zeus books say 5.1mm is the clearance drill for M5.

Martin C

Most (all?) of my counterbores are also bigger around the head than I would like. One day if I can be bothered I will spin them down on the cutter grinder. Got to get energy levels back after a recent stroke first though.

Hi Guys,

One thing that that needs to be accounted for when counter boring for cap head screws is that the shoulder under the head is a fraction larger than the clearance diameter. So the counter bore pilot is bigger to allow for that.

Edited By John Baron on 04/06/2020 21:14:27

Cap heads have a radius under the head, the clearance hole diameter should clear this. Therefore the clearance hole is the screw diameter +twice the radius under the head + tolerance on position. Of course in the model world you just ignore all that, make the hole as tight as you want on the screw diameter and tighten the screw.

Colin,

It may be useful to look at this thread from 2017 : **LINK**

https://www.model-engineer.co.uk/forums/postings.asp?th=124417&p=1

MichaelG.

.

Edit: or this one, from 2015

https://www.model-engineer.co.uk/forums/postings.asp?th=107996

Edited By Michael Gilligan on 04/06/2020 23:15:58

Michael,

Thank you for that link. Very useful information.

Colin

Thanks Michael, I knew there was a standard

I did see some counterbores with 5mm pilot and 8.7mm head for those wanting a really close fit but they did have larger head sizes too. There is also the option to get counterbores with removable pilots so you can make and fit one to whatever size you want.

Colin,

If you desperately want a "size for size" fit you could still use the counterbore tool you have, providing the pilot length is shorter than the depth below the counterbore. If that makes any sense? Simply drill the pilot diameter depth to the length of the pilot + the depth of the counterbore. The rest of the hole diameter can then remain at 5mm.

Hope that helps.

From a purely aesthetic point of view standard metric counterbores don’t produce a very neat result. I’ve therefore used a variety of other tools including some home made ones to produce something that looks a bit better.

Does anybody know where I might buy Fractional Inch or BA Counterbores, please?

I have a set of metric counterbores and the Pilots are far too large for the nominal size holes.

I would like to investigate using Inch fractional or BA sizes.

Very many thanks for your help

I think you will find imperial just the same eg a #5-40UNC which is very close to 5BA would have a pilot of 0.140". have a look at MSCdirect.co.uk

As mentioned, the standard counterbore cutters are made for general industry which uses a bigger clearance hole for bolts. Bolts are not normally used for the precision location of parts and the tolerance on position of holes is not what we would expect in a tool room. Often just measured with a tape measure or rule and centre punched for position. Having larger clearance holes for the bolts allows mating parts with shightly different hole positions to still be assembled.

Easy to make your own counterbore cutter out of tool steel. Like a D-bit with a locating spigot.

Paul

The first six lines in the table on p27 cover counterbores for #4 and #5 caphead screws

**LINK**

https://www.bsaregalcuttingtools.co.uk/downloads/download.php?id=2

Very nice tools, but a little pricey for hobby use.

MichaelG.

Edited By Michael Gilligan on 21/07/2020 09:24:48

Home made counterbore made by AdrianR in a previous post

**LINK**

I have over the years found that the quality of commercially available cap screws has deteriorated, particularly if they are sourced from China. As I use them on parts for my motorbikes and on friends bikes, I make two mods. Firstly I mount them in a collet, and lightly remove the radius that has grown over the years where the head meets the main shank. Then I turn the outer diameter of the cap head to make it truly round and concentric with the main bolt shank. Over the years I have made and/or sourced counter bore tools to suit the modified head dimensions. This means a much tighter fit and a much more precise appearance to the finished work.

I don't think I would want to ride one of your bikes, the radius where head meets shank is to remove stress risers that could lead to fracture. This is actually one reason the clearance hole is larger than teh shank so that the head bears on the bottom of the counterbore and the radius does not foul a sharp edge.