using spotting drills for countersinking

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.

Doug

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.

John

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,.

Mike

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.

JIm

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.

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.