Why is the pilot diameter of a counterbore so large?

I have bought a number of counterbores over the years but have always had to grind them down so that the thinner part will fit in a clearing-size hole. If you enlarge the hole instead, you end up with a component, such as a lid, having a sloppy fit. Why is this? I have heard that in aeronautical engineering smaller diameters are used.

Brad,

Are you buying Imperial or Metric counterbores ?

The commonly preferred clearance holes for Metric fasteners are proportionally larger ... so the pilot sizes follow suit.

MichaelG.

Dear Michael,

My workshop is all metric. The clearance drill size for M5 screws is 5.1mm, but the pilot of my M5 counterbore is 5.43 mm in diameter, which seems unnecessarily larger than the specified clearance size.

Brad Amos

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Believe that if you will, Brad

Other versions of reality are also available: **LINK**

https://www.trfastenings.com/Products/knowledgebase/Tables-Standards-Terminology/Tapping-Sizes-and-Clearance-Holes

MichaelG.

As Michael says the industry standard metric clearance holes are quite large even though it is quite easy to put a M5 bolt of screw down a 5.1mm hole, infact more often that not I will just use 5.0mm for my own use.

One other reason for the counterbores using these large clearance holes is that a socket head screw would generally be used in the counterbore and having the large clearance ensures that the tightening force is applied by the underside of the screw/bolt head and the radius between shank and head does not make contact. If you look closly under the head of a cap head you will see they have a radius to reduce the risk of stress risers.

If you really have an issue with this then you can buy counterbores with removable pilots and a range of pilots is available for each counterbore dia so you can fine tune to the fit you want.

J

Edited By JasonB on 30/01/2017 18:19:47

The clearance hole for a single screw can be anything that will clear the screw. If you have a hole pattern, clearance holes need to be somewhat larger to allow for hole-to-hole tolerances in the pattern on both the part and whatever mates with it. Recommended sizes have an allowance for this.

I would agree with metric standards being on the generous side. I recently made up some brackets for a friend who is building a telescope. His drawing called for M10 clearance. Standards say 10.5mm for "close fit" and 11mm for "standard fit". So I purchased a 10.5mm drill for the purpose. Even with a close fit an M10 bolt rattles around like a wotsit in shirt sleeve. Friend was still happy with the job though.

Unless you are buying known make metric caphead screws most likely you will find 5mm drill will provide an acceptable size hole, always lightly countersink to allow for the radius under the head.

If you are using known makes of caphead bolts then you may need to go 5.1mm for clearance on the shank.

Emgee

Looks like I and Jason are on the same planet for this one !!!!!!!

Edited By Emgee on 30/01/2017 18:38:04

Screws are meant to secure an object not locate it, therefore decent clearance is advised.

Tony

Nicely put.....

I'll remember that for the future...

A 5mm screw will go through a 5mm hole, provided it is in the right place. Using a larger clearance hole gives some wriggle room, bearing in mind the 2 mating components may not have been made in the same place at the same time. Standard counter bores are made for standard cap screws with this in mind.

Chris Gunn

Cap screw have a form under the head which according to standards lies within a given diameter. This form may include a radius and taper. Where clamping forces are critical and calculated to prove an application the clearance hole must clear the form diameter to ensure that the applied force is directly on the underside of the head. So that is read why clearance holes are so much larger than the thread diameter.

In model engineering we don't care so much about screw holding forces (until something breaks), we care more of how a thing looks. Make up your own standards for what works for you, or if you want to buy tooling stick with what you can get and work with that. I have worked for many firms and they all tweak the clearances to a degree. Look at Unbrako or other manufacturers for information, well worth a read to understand better.

To qualify what has been mentioned about standard sizes; Taking the M5 (to suit a cap head in the form BS EN ISO 4762 or DIN 912, which covers most of the screw/bolts you will get hold of in the uk - far eastern will probably not conform) the standard is three classes of fit, loose, normal and close.

In numbers they are all 10mm diameter counter bores taken to 5.4mm deep but the clearance diameter for the screw can be 5.3 close, 5.5 normal and 5.8 loose. When detailing them on a drawing you can add further information to tie down the small under head and counter bore chamfer (or radius if you so wish).

Mark

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... To allow for the permitted eccentricity on the heads of 'commercial quality' fasteners.

MichaelG.

I don't see why 10mm is big - it is only 1.5 mm bigger (0.75mm a side) than the head (8.5mm). They look "right" to me but perhaps I am not very demanding?

Mark

And you could reduce that 0.75mm clearance by 0.4mm if the screw were up tight against one side of the 5.8mm pilot hole so you could be looking at 0.25mm which is not a lot.

J

Ian, if there half a mm clearance for the shank, you will need as much plus a bit for the head.

At the end of the day a lot will depend how accurate the mating holes can be drilled. Commercially some tolerances will be applied to hole centres, taking into account the hole sizes and so on. You may well be able to work more accurately and adjust the sizes accordingly as close as you can work, but this could be more expensive to achieve commercially.

Chris Gunn

Oh no, geometric tolerance..... run away!

Mark

Thanks, Jason B, for explaining about the radius between shank and head of a socket cap screw. I have just measured the inner diameter of the flat on the underside of the head on a good-quality M5 stainless screw and it is 5.36mm, which explains everything.

My ' version of reality', as regards recommended clearance sizes, comes from the widely-used 'Zeus' precision tables. I now understand that this clearance size for the screw is less than the hole size needed to accommodate the radius on the base of the screw head, which is more likely to be cast than formed with precision.

Thanks to all the advisors who weighed in on this!