Why is the pilot diameter of a counterbore so large?

"which is more likely to be cast than formed with precision" Afraid not, It is most likely to be forged on a cap head and is going to be reasonably accurate in a decent quality item.

Mark

Simples.

Because they are made for industry where they want a bit off leeway and it's industry that drive sales.

If Model engineers could guarantee sales they they would make them to what you wanted.

Having said this I was involved with the selection of pilots and cutters that make the ARC set up bearing in mind some need tighter tolerances, so you do have a choice. It up to you though where you spend your money.

For the purists...

Typed on a non IMB keyboard at 29 degrees C from a Staples armless office chair

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Brad,

Sorry if I was a little blunt in my response, but; as you now appreciate 5.1mm as 'clearance' is quite irrelevant to the size of pilot on a standard counterbore.

MichaelG.

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P.S. forgot to mention ... the reason I asked initially about Imperial or Metric is that the Imperial ones have much neater proportions [because the shape and tolerance of the cap-head screws is different] : Go Compare !!

Edited By Michael Gilligan on 30/01/2017 21:56:45

I make my own out of silver steel so they can be any size I want

Neil

It would appear that the counterbore size is related to the size of the cap screw heads and clearance due to the hole the screw goes through. An example is the hole drilled say M4 clearance which will give 0.5 clearance then the counter bore is 4.8 dia giving 0.8 clearance for the head of the screw used. It is after all a system.

Clive

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Well, obviously you do ... you're the Editor of Model Engineers' Workshop.

AS Neil says, if it's so important then make your own.

A selection made from broken and chipped end mills and slot drill with just a simple rotary jig and a bench grinder.

Choose what sizes you want.

L to R countersink tool. Counterbore. Tool for trepasnning the broken bit of a centre drill out. Another counterbore and a single flute boring tool.

In machine shop practice the amount clearance provided depends on the precision of the hole pattern. In the "old" days for fine work clearance was 1/64" oversize. For ordinary work - 1/32" oversize. For structural work - 1/16" oversize. "Old" is before geometric tolerancing.

And a homemade one for our Aussie members

No fancy grinding machines need to be built, teeth were filed and seem to do the job.

Ian, a quick look and the counterbores with interchangable ends go down to 3/16" head and 3/32" pilot, did not look long enough for metric. The reason for suggesting them to teh OP wa sthat he could make his own pilot to suit his prefered clearance hole which would be easier than grinding an all in one counterbore. They are known as "Aircaft counterbores" and that is also what the OP hinted at when he said he had heard that closer fitting ones are used in aeronautics.

I think I understand what you mean, but if the shank is uptight against one side of the bolt hole the head cannot then get any closer to the counterbore wall so it does not need extra clearance (other than to allow for head eccentricity).

My interest in the counterbore diameter subject is in regards to small fixings where it seems one would have to make or buy a custom cutter.

Out of interest, I just examined an A2 Stainless M4 caphead. There IS a radius where the shank meets the head but that radius starts at a diameter smaller the M4 thread OD so would not encroach on the bolting face, even if it was an M4 bolt in an 4.00mm hole.

I don't have a problem regarding oversize counterbore pilots or cutters (which some posters seem to infer), I was purely interested 'why' they were so large.

Ian P

**LINK**

Rod

The M4 counterbore OD in that set is even larger (at 8.5mm) than the other solid counterbore that ARC sell. There is no way I would put an M4 caphead in a 8.5mm counterbore, apart from the crude appearance the counterbore would break out of the edge of the part (say a flange) where the fixings are close to the edge.

The choice is not to purchase but to carry on using the ones I have made, my query was 'why' they were so large.

Ian P

From a 'Holo-Krome Ltd Metric Series Socket Screw Selector' , the following sizes are listed --

M1,4 - clearance hole 1,6 dia, counterbore for head 2,8 dia.

M1,6 - clearance hole 1,8 dia, counterbore for head 3,3 dia

M2 - clearance hole 2,4 dia, counterbore for head 4,3 dia

M2,5 - clearance hole 2,9 dia, counterbore for head 5 dia.

M3 - clearance hole 3,4 dia, counterbore for head 6 dia

M4 - clearance hole 4,5 dia, counterbore for head 8 dia

M5 - clearance hole 5,5 dia, counterbore for head 10 dia

M6 - clearance hole 6,6 dia, counterbore for head 11 dia

M8 - clearance hole 9 dia, counterbore for head 15 dia

As these sizes are the manufacturer's recommendations, can it be assumed that they allow for any under-head fillet radii? In general, both in model engineering and 'full-size - on steelworks and mining machinery' I have found that metric fasteners both socket head and hexagon to be over-sized (across-flat in particular) and clumsy compared to their near-equivalent imperial sizes. Please, why??

Ian

Here's an alternative way of dealing with a broken off centre drill.

I actually took the bit I broke, ground the notch with a Dremel and was able to recover the part I was making without wasting more than 10 minutes.

Ian P

Something seems to happen (in the progression of OD sizes) between M3 and M4, unless there is an error in the table. 0.60 clearance on M3 and 1.12mm on M4 (by my quick measurements of two hand screws).

Maybe the suggested diameters use a range of 'preferred' sizes or maybe Holo Chrome just looked in a counterbore makers catalogue and used those.

Ian P

Looks like they just settled on nearest whole mm once the size gets to M4 and above, same a hex sizes they are not a direct proportion to the shank

The original Whitworth spec had huge heads but they finally made them all one size smaller, metric have a few standards and ISO does seem a bit clumsy wit 13 on an 8mm and 17 on 10mm but the Japanese have 12 on an 8mm and 14 on 10mm which to me are much neater especially on a motorbike. You would think that the head size would have had calculations done to arrive at the optimum size in which case why do the standards differ? Unfortunately just because it is metric does not mean it is standard, European car makers often use a smaller head like VW who use 16 and 18mm. If you are making a metric fastener use whatever size you fancy it probably meets someone's standard.

Mike