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using spotting drills for countersinking

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Douglas Johnston22/11/2020 11:50:24
760 forum posts
34 photos

Having spent many a long year getting frustrated by countersink tools that never seem to work well for me, I recently discovered something that I should have realized a long time ago. It started when I needed to make a number of countersunk holes that went deep enough to sink the screw heads about 1mm below the surface.

I had recently bought a set of 90 degree spotting drills from the place I cannot mention lest I be struck down, and realized that they make ideal countersinks. I was using M6 screws which had a maximum head diameter of just under 12mm, so by using a 12mm spotting drill I produced very nice countersunk holes to the correct depth. The cutting was very smooth indeed and much nicer than a conventional countersink bit.

I am probably the last person to realize this use for a spotting drill, but in case not, I thought I would give it a mention.


John Hinkley22/11/2020 12:05:28
1149 forum posts
384 photos

I'm with you on this one, Doug. For my current project, I had to drill and lightly countersink 36 holes using the mill. I always use a collet chuck to hold drills, so to reduce drill changes, I used a spot drill, then a 4mm drill and back to spot drill to take the edge off the drilled hole. On to the next position and the spot drill was ready to go, saving a change of drill. Obviously, countersinking takes more pressure, but by using a spot drill, I found it gives a more controllable countersink and a better finish. Maybe my dedicated countersinks are poor quality, more likely it's the way I use them.


Mike Poole22/11/2020 13:07:22
3015 forum posts
71 photos

I find that some countersunk screws have quite a thick edge to them and thus to get them flush or below the surface it looks much nicer to me to use a countersink that matches the head to effectively make a counterbored countersink,.


Jim Nic22/11/2020 14:36:56
347 forum posts
200 photos

Good tip Doug, thanks I also have difficulty with countersinks and have several of varius designs and sizes. I also have a bunch of spotting drills up to 12mm which are lightly used in the larger sizes so I will give them a go in future.


old mart22/11/2020 15:45:08
3201 forum posts
201 photos

Its always to put tools to good use, the 90 degree spotting drills are ideal for small countersinking, just be sure to leave enough Z height to be able to change tools without moving the X or Y axes. With a lathe it is usually not so important. Spotting drills are also available in different nose angles, I have 60 and 120 degree ones. The 120 degree one is not so good for basic spotting as it is difficult to see the end.

Chris Evans 622/11/2020 19:57:40
1938 forum posts

I've been using the 90 and 120 degree spotting drills for 25 or 30 years with good results. Somewhere in my workshop I have a few that I altered to suit UNC/UNF with the different (85 degree ?angle) never needed them since. I have to agree the appearance of a more controlled diameter is so much better.

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