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How to gauge depth of countersink

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mick H28/04/2020 11:12:01
723 forum posts
21 photos

This is probably another one of those questions to which, when I get the answer will leave me feeling stupid, but here goes anyway.

When countersinking, for countersunk screws, how do you gauge or calculate the depth of countersink. Until now I have adopted the trial and error approach but on occasion have come unstuck when I go too deep. How can I obtain consistency ?


mechman4828/04/2020 11:15:56
2726 forum posts
422 photos

Measure the OD of the countersink screw head, counter drill hole to match the OD, should lie flush if its correct depth.


Andrew Johnston28/04/2020 11:23:19
5635 forum posts
652 photos

Trial and (hopefully not too much) error for me. I normally countersink manually with the work on the mill table. I have a fairly good idea of the depth needed so I set a bit less on the knee, countersink and try a screw. I then raise the knee in 0.1mm or so increments until the screwhead is flush or slightly recessed. Then, with the knee and quill stop set, the rest of the countersinks should be consistent. For paid for jobs it isn't that important as long as the heads are slightly recessed. I don't use many countersink screws on my model engines, and those that are there are hidden. The last countersinking I did was for M12 on this steel plate, used to fix the broken bench that the flypress sits on:



Ian P28/04/2020 22:35:57
2412 forum posts
101 photos

Its trial and error on a bit of scrap material for me if I need to end up with a cosmetically good result.

Mostly though I uses an assortment of countersink bits that I have ground the OD to match the diameter of the screw head, that way the resulting countersunk hole is also counterbored slightly. Depth is then nowhere near as critical.

I find small differences between every batch of fixings I buy, supposedly they are to DIN standards but there still seem to be relatively big variations in the radius at the edge of the head and ones with a larger radius look awful by the time the countersink is deep enough to let the head sit flush, a counterbored/countersunk recess solves the problem.

Ian P

mick H29/04/2020 08:33:48
723 forum posts
21 photos

Thanks for the comments and advice gents.

For what it is worth, my method is similar to Andrew's and uses my small mill drill which has a quill digital readout. Having achieved a decent countersink by trial and error I then note the dro reading for subsequent countersinks. Unfortunately, this rarely proves to be consistent enough and to get a really good result have to resort to drilling a smidgen shallow and finishing with a hand held countersink. Perhaps this is because of variations in the fixings that Ian describes. If the required countersinks are in separate workpieces, however carefully I try to set it up, the above method does not work. To be fair, it is not very often that I need extreme accuracy but I have to do countersinks in six separate items that do require a very decent finish. Wish me luck.


ega29/04/2020 08:42:40
1797 forum posts
153 photos

Ian P:

I admire your perfectionist approach.

Woodworkers using slot head csk screws sometimes aim to get the slots in line and the heads nicely flush but there seems no easy way to do this. One factor would be whether the threads and slots have the same orientation to each other within the batch.

Hopper29/04/2020 08:53:40
4790 forum posts
105 photos

I'm another who does it Ian P's way. Grind a drill of suitable diameter to 90 degrees and then counterbore the OD that little bit so the screw head sits down in it, the edge out of sight.

If doing multiple holes, I use the depth stop on the drill press to get uniformity.


Edited By Hopper on 29/04/2020 08:58:55

John Haine29/04/2020 09:20:29
3278 forum posts
176 photos

I can feel a spreadsheet coming on...

not done it yet29/04/2020 09:39:43
4909 forum posts
20 photos

One factor would be whether the threads and slots have the same orientation to each other within the batch.

Might be alright for woodworkers but the actual thread start in the wood would be more important. Less than half a turn is likely less noticeable in timber.

In metal it might depend on where the tap started to bite when making the thread in your piece? Use posidrives, or similar, and lining up screw slots will only involve a 1/4 turn? Torx - even less.🙂

David George 129/04/2020 09:55:20
1305 forum posts
447 photos

In making special purpose machinery I had a selection of countersinks to suit the screw head as some were close together or an edge etc. We measured the diamiter if the head and got a drill a few thou bigger than the head and reground the angle of the head to suit the angle of the screw. UNC is different 82 deg. Then the cutting angle is flattened to almost zero to prevent chatter and grab. When you use one of these they all look the same even if you go a little deeper and looks professional. There is a slight flat on the edge of the head on most screws in engineering so make the hole to suit.

The fourth from the left is one I still have it was for 10 UNF



John Haine29/04/2020 10:03:47
3278 forum posts
176 photos

Actually I think it's quite easy at least for the normal 90* countersink. If you feed the bit down so the edge just touches the end of the initial hole, then for every extra mm you downfeed the countersunk recess expands by 2 mm (1 mm on each edge). So measure the diameter of the screw head, subtract the diameter of the hole, divide by 2, that gives the required downfeed. I think. Simples?

If you want to make sure the top of the head is recessed below the material surface, add an extra allowance for the amount required.

Edited By John Haine on 29/04/2020 10:05:06

JasonB29/04/2020 10:10:06
18675 forum posts
2049 photos
1 articles

Most metric countersinks have the right dia head for each size CSK socket head screw so no need for making specials.

Two hole the far one done with the correct dia CSK and the one in the foreground with a larger one that leaves a more visible larger hole.

photo 107.jpg

photo 106.jpg

AdrianR29/04/2020 10:24:12
488 forum posts
25 photos

This is something I have been meaning to look up, especially after my most recent attempts.To small, too small, HUGE! I also found that the two M4 CSK screws I had, have different dimensions.

I have looked it up, for metric this seems to be the closest data to the original ISO i can find without paying.


I also found this site that has simplified data for a whole range of screws, BA, Metric Whitworth etc. **LINK**

The problem is we now know how far to push the cutter in, but we don't know what the starting position is. If we had a hole with the nominal diameter, we could lower the cutter till it's cone just touched the hole lip. Then we just have to cut to the depth of k. e.g. M5 hole diameter 5mm k=2.5mm.

The problem is we rarely have a hole nominal diameter, ie it is tapped or clearance.

With a pointed cutter and a bit of trigonometry we can calculate the depth of cut.

The depth for a pointed cutter to reach the nominal size Ns (d) with a countersink angle A

d = (Ns/2) / tan(A/2)

So the total depth to cut is d + k e.g. for M4 with a 90 degree cutter, Ns=4 A=45, k=2.2

d = 2 / tan(45) = 2 ( I know tan(45) = 1, I included this for other angles)

depth of cut = d + k = 4.4mm

So we should be able to zero with the cutter just touching the work, then cutting to 4.4mm should give a perfect countersink.

If like me you have a counter sink which is a truncated cone, then you have to work out what the offset would be due to the missing part.

I think it is probably easier to counter sink a hole in a piece of scrap that is too deep, but note the depth. Drop in a screw and measure how too deep it is. Subtract that from the depth you cut and that is the depth you should use.

If you are using a mill you could zero the cutter, cut a too shallow countersink, drop in the screw and touch the tool to find how much the screw is proud. That is how much more you need to cut.


Samsaranda29/04/2020 10:58:12
957 forum posts
5 photos

When working on aircraft and countersinking rivet holes we used a countersinking tool with the windy drill which was basically an adjustable depth stop that had very fine adjustment which accommodated different sizes of countersinking bits . The best way to control the size of the countersink is to use some type of depth stop.
Dave W

John Paton 129/04/2020 10:58:54
280 forum posts
17 photos

vanmar csink enlge.jpgIf you need to do a number of similar countersinks to a precise depth there are special tools to assist like this (brand names Vanmar and Zephyr). In use the countersink plunges down through the shroud onto the work using the pilot tip to centre it in the hole.

I believe they are used in the aircraft industry for countersinking rivet holes in aluminium skinning.

By adjusting the 'shroud' position relative to the countersink in the 'plunged' position you get exactly the depth you require and the shroud ensures that the countersink sits exactly perpendicular to the surface.

You could easily make something similar for occasional use but omitting the plunge feature for simplicity. A 40tpi thread would give fine adjustment to the shroud extension.

Please note that the countersink is not fully screwed home in this picture - I left it protruding to show its typical position when the countersink is 'plunged' during use (the spindle is spring loaded within the collar and the shroud is threaded onto the collar for adjustment)

vanmar csk set.jpg


Ps DW just beat me to it by seconds!!

Edited By John Paton 1 on 29/04/2020 11:02:23

Edited By John Paton 1 on 29/04/2020 11:03:44

John Haine29/04/2020 11:28:15
3278 forum posts
176 photos

Adrian, surely you know what size hole you have just drilled, and you know the head diameter you want, so just lower the cutter into the hole until it just touches the edges of the hole and feed down half the difference between the hole size and the head size. Doesn't matter if the tip is flat or pointed then.

Of course it's different for different angles but most csk heads are 90*.

Edited By John Haine on 29/04/2020 11:28:51

Speedy Builder529/04/2020 11:51:21
2085 forum posts
145 photos

All of the above, but wood screws don't have a sharp countersunk head (especially cross point heads). If you countersink to get the head flush, then the diameter of the countersunk hole will be MUCH larger than the screw head. You would need a countersink tool the diameter of the screw head which is then sunk below the surface of the workpiece. Also I have found that 'snail' countersinks wander off centre if not done on a pilar drill etc and the workpiece held in a vice or clamped.

Ian P29/04/2020 12:06:24
2412 forum posts
101 photos

Spreadsheets, trigonometry and other techniques might help in a manufacturing or production environment but are pointless for ME or home workshop purposes.

As Jason showed countersink bits are available with diameters to suit standard sizes but I found that I could only use the 10.3 and 8.3 in the set I have (Jasons has 10.4). I have acquired or made 10.0, 10.2, and 10.4 as well as an assortment of smaller sizes for M3, M2.5 and M2 fixings.

A quick check of my stock of M3 countersink screws shows head diameters between 5.04 and 5.83 (mixture of Philips, Pozi and Allen recesses). I assume they are all 90 degree types.

Trial and error in a bit of scrap (with the actual screws I am going to use) to determine the best settings still works for me. Dont forget that there are many more countersink angles in common use other than 90 degrees. (82, 100, 120 etc) so a spreadsheet would be a minefield.

Ian P

Andrew Johnston29/04/2020 12:20:04
5635 forum posts
652 photos
Posted by JasonB on 29/04/2020 10:10:06:

Most metric countersinks have the right dia head for each size CSK socket head screw so no need for making specials.

The diameters listed don't seem to tie up with either minimum head diameters or the maximum theoretical sharp head diameter, which would be needed if the head is to be flush?


Eccentric Engineer29/04/2020 12:32:22
23 forum posts
3 photos

My preference when it comes to countersinking metric socket head screws is to use a 90 degree spotting drill and do the csk before drilling the hole. The screw heads are a nominal diameter so you can slightly counterbore the hole below the surface and get a much tidier looking flush finish and not have the ring around the screw head like in JasonB's picture a few posts back. It also eliminates chatter doing it this way.

M3 = 6mm
M4 = 8mm
M5 = 10mm
M6 = 12mm
M8 = 16mm

With regards getting the right depth, that's best trialed on a piece of scrap first as the heads can differ between batches. Never had any issues with differing head diameters though.


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