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.

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.

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.

Chamfer Mills are one of the most common tools used by machinists, because nearly every machined component requires at least one chamfer operation. Many machining operations leave a burr or sharp edge on the workpiece, and a chamfer mill removes the sharp edges, leaving a smooth, angled surface or chamfer.

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.

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Titanium Aluminum Nitride (TiAlN) coating increases the resistance to wear, ductility, and heat transfer through the chips, making these cutters a good all-around choice for roughing and finishing steel, stainless steels, cast iron, and high-temperature alloys.

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.

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

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.

Titanium Carbon Nitride (TiCN) provides excellent abrasive resistance, and hardness to the cutting edge. It does not perform well at very high temperatures, so properly applied coolant is a must. This coating is perfect for aluminum and abrasive high-silicon aluminum alloys.

In addition to chamfering, these versatile chamfer mills can be used for several other machining operations, such as creating a full bevel along the edge of a part, breaking sharp edges, countersinking holes, or preparing material for welding operations.

Home › Forums › Beginners questions › Counter sinking for Cap Screws This topic has 13 replies, 10 voices, and was last updated 15 June 2015 at 15:11 by Neil Wyatt. Viewing 14 posts - 1 through 14 (of 14 total) Author Posts 15 June 2015 at 07:49 #193471 Alan RawlinsParticipant @alanrawlins60482 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? Advert 15 June 2015 at 07:49 #7670 Alan RawlinsParticipant @alanrawlins60482 15 June 2015 at 08:03 #193474 Michael GilliganParticipant @michaelgilligan61133 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 15 June 2015 at 08:26 #193476 JasonBModerator @jasonb 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 15 June 2015 at 09:44 #193482 VicParticipant @vic 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. 15 June 2015 at 10:09 #193486 KWILParticipant @kwil 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!! 15 June 2015 at 10:25 #193488 Paul LousickParticipant @paullousick59116 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 15 June 2015 at 11:47 #193492 VicParticipant @vic 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 15 June 2015 at 12:01 #193493 Gordon WParticipant @gordonw 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. 15 June 2015 at 13:27 #193499 Michael GilliganParticipant @michaelgilligan61133 Posted by Vic on 15/06/2015 11:47:25:

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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)

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!

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.

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.

HSAM1 series chamfer end mills are 2-flute cutters designed for high-speed chamfering primarily in aluminum and steel, with secondary applications in stainless steel and cast iron. They are solid carbide end mills, with a TiCN coating.

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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.

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

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.

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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?

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.

This price includes shipping cost, export and import duties, insurance, and any other expenses incurred during shipping to a location in France agreed with you as a buyer. No other mandatory costs can be added to the delivery of a Haas CNC Product.

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?

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?

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!!

HSPM1 series chamfer end mills are 4-flute cutters designed for high-speed performance chamfering in steel, stainless steel, and high-temperature alloys, with secondary applications in cast iron and aluminum. They are solid carbide end mills, with a TiAlN coating.