Counter sinking for Cap Screws

i recently wanted to use some of the black cap screws on a component I was making that needed the head of the cap screws to be below the surface of the plate I was fastening to something else.

The hole for the M8 cap screw was drilled initially to M8.5 .I sent to RDG for a suitable tool to counter sink the plate for M8 cap screws. When I used it to cut the counter sink, the cutter enlarged the M8.5 hole to M8.93 which isn't a problem.

The countersink though, cut a hole to 15mm which is 2mm (1mm all round) larger than the head of the M8 cap screw and it looks awful

The c/sk cut on parts of my lathe for an M8 screw were13.3mm which gave a much better looking clearance of 1mm (0.5mm all round)

Is it now normal to have such a large clearance for the head of cap screws? is there not any standards that should be used by the manufacturers of cap screw c/sks?

Alan,

... am I right in thinking that you mean counterbore rather than countersink?

I don't have the numbers to hand, but I do know that the standard clearances on Metric fasteners are 'generous to a fault' ... more suited to IKEA than the toolroom.

MichaelG.

.

Edit: Here is a useful chart ... compare the dimensions specified for 5/16" and M8

Edited By Michael Gilligan on 15/06/2015 08:32:28

Yes the standard clearance is quite large. You can use an end cutting milling cutter, grind a drill to about a 170deg point or ideally make a D bit to get a flat bottomed hole.

For stressed items you need to make sure that the slight radius between screw shank and underside of the head has clearance which is covered by the large almost 9mm hole or by easing it with a csk or spotting drill if using a smaller clearance size.

Edited By JasonB on 15/06/2015 08:30:00

I've used a variety of methods to counterbore holes including Bullet drills and hand ground drill bits with a flat bottom.

As you've found, the recommended counterbores for metric cap screws is rather large and looks ugly.

I am not surprised by the "large" counterbore sizes, some sources of cap head screws seem to have lost the ability to make the heads concentric with the threads!!

I have found that some counterbore cutters (especially the cheaper ones) vary in size for a particular size cap screw. Some were undersize and produced a hole which was too small. I now check the hole size that they cut before buying.

Have not found a proper standard for counterbores but this seems to be what is recommended.

Paul.

Edited By Paul Lousick on 15/06/2015 10:26:22

In many cases the cap head diameter size in that chart must be the maximum recommended size as in practice they are somewhat smaller. I often counterbore to the "head" size in that chart!

Am I right in thinking the counterbores for imperial cap screws are a bit tighter and therefore look better? Certainly the imperial cap screws I've seen have a smaller head in comparison to the thread diameter so that may have a bearing?

Edited By Vic on 15/06/2015 11:48:18

When cap screws were fairly new often the heads had axial grooves. The c/bores were quite tight, the heads were locked by deforming the material into the grooves. A special tool was available for doing this. May explain the variations in bore sizes.

.

Vic,

Have a look at my earlier post

The link includes information on imperial sizes

MichaelG.

If in fact you are using counter sunk screws, the bit should be a small clearance diameter over the head diameter, and a fraction of a mm of parallel hole to take the head flush.

Ian S C

Hi Alan Rawlins

In the past if I have made counter bores from old drills by grinding the tip away on an old drill leaving a centre parallel pilot section the same size as the hole, in your case to fit an M8 clearance hole. once this is done I just grind in to cutting edges at 90 degrees to cut the bottom of the counter bore hole. Maybe you have one. The shorter the drill is the better as the web will be thicker and you will get a more complete pilot. I guess 14mm is an odd size but worth a try if you have one. I keep a box of broken cutters and worn out or broken drills "Just in case" for this sort of job. Do tests on a piece of scrap until you get it cutting nicely.

Regards
John

Thank you everyone for your replies, I think I will try to grind an old drill up to do the job "properly"

Proper counterbored holes are not a close fit on either the shank of the screw or the cap - alignment should be by a dowel or some other form of register as screws a can be made to wide tolerances and only the underside of the head should bear on the part being fixed. Hence the generous clearances.

Also, spot facers often have greater clearance than counterbores (being intended for nuts) but the line between a counterbore and a spot facer is a vague and indistinct one.

Neil