Counterbores

I am after a set of counterbores and wondering if it would be better to go for a set of interchangeable pilot counterbores, I can see the benefit of these but are they a bit of a gimmick or are they fit for purpose.

I have the Arc set and they work fine. You do stand a better chance ( but no certainty) of getting a good fit for a cap screw.

Rod

Question: Do you do much counterboring? I have a set but have only used them once - they likely are either not the right bore or the best OD.

I generally use an end mill. Not the ideal (many might say) but does for what I need. I don’t do many, so lining up with the bore with a drill before changing to the end mill is not too time consuming, if not all done at the one setting. The DRO is also handy for re-setting, anyway.

Most counterbores available do not suit a lot of what I do, they all seem to have far to much clearance. OK in a general assembly of wide tolerances but cosmetically poor to my eyes. I have a set of suitable slot drills/endmills to give the results I desire. Having a cutter grinder gives me the luxury of making what is needed but counterboring is not something I do a lot of now.

There are standards for the centre hole size and counterbore size for the various thread and screw sizes. As Chris says the fit is "distinctly slack".

I recall taking Halls to task at an exhibition over not marking the centre hole sizes needed on either counterbore or box after the firm bought a fixed pilot set to use in the trials preparation workshop. The workers, not trained machinists, were forever moaning about not knowing what size hole to drill. Halls guy said its in the book so folk ought to know.

It is in the book, but try finding it.

Rep wasn't happy when I told him tha anythiny likely to be used in a basic workshop or fabrication floor whose data wasn't in a Zeus book should be marked up for the erks.

Clive

I too have a set from ARC and although they are to spec, I found them to have too much clearance for my needs.

Easy job to grind them down with a Dremel in the toolpost - sorted!

You are being very kind with your comments. No doubt there are very good technical reasons for the sizing but I have to agree, the counterbored holes are rather large and don’t look good.

Size of clearance hole and counterbore size in industry are based on cap screw under head radius clearance and positional tolerance. Individual companies set their own geometric tolerances and so create clearance hole and counterbore sizes to suit themselves. Their are standards these days but the places I worked with mostly had their own standards. When using soft materials the under used radius will form a corresponding form in the clearance hole if it is out of position or the hole is too small to clear the radius. Obviously if you take care and ensure the corresponding tapped and clearance holes are true to each other then hole and counterbore sizes can be reduced to as little clearance as you want.