Using Counterbores

[Cornish JackParticipant @cornishjack]

Possibly OT but a repeat of a query I posted some time back without a real answer. The photo shows a pair of tools which appear to have a counterbore function but the construction seems at odds with normal counterbore – they appear to operate as 'pull' tools rather than 'push'. The shanks are 2MT without drawbars and the 'bottom end' has a hole but not suitable for centres. These features would seem to indicate that the cutters (only mountable to face the shank) cannot apply pressure for the cut. The cutters are special to type, quality material, and come in a range of sizes.

Does anybody recognise what they are, please?

rgds

Bill

[Jeff DaymanParticipant @jeffdayman43397]

It's a reverse side spotface tool I think. We had a similar version for finishing some spotfaces from opposite side, on valve castings in a firm I worked at years ago. We did them from the opposite side because the cating was sitting on a datum for other ops, and we wanted the spotfaces to be sized relative to that datum also. Picking up the datum after breaking the first setup would have been difficult.

[JasonBModerator @jasonb]

Yes they look like reverse counterbores, one in action here which was positioned with teh DRO but could not have been done with an endmill

[Douglas JohnstonParticipant @douglasjohnston98463]

I had the same problem with a couple of my counterbores in that I did not have the correct size drill before using the counterbore. I simply solved the problem by holding the counterbore in an accurate collet chuck and used a sharp polished carbide cutter insert to reduce the diameter of the front part of the counterbore. Worked a treat on the hard HSS and left a mirror finish. It never fails to amaze me what you can do with this type of carbide insert.

Doug

[KW56Participant @kw56]

Counterbores for bolt and screw heads can be made with a suitably sized drill and then another drill of the same size ground to produce a flat bottom in the hole. Grind the end flat then back off on the bottom edges to create the cutting edges.

Keep the back off angle as small as possible to help prevent chatter. For most work ( ie. gear covers etc. ) the standard drilled hole is perfectly good enough.

[Cornish JackParticipant @cornishjack]

Thank you for the replies. I quite see that they would appear to be reverse counterbores, but how do they operate without a drawbar fitting or provision for a centre to apply pressure ? Additionally, there is no adjustment for length. The design appears to be specific to one particular operation but what it was, I have no idea!

rgds

Bill

[Mike PooleParticipant @mikepoole82104]

if you have the facilities to grind this the you can make a counterbore to suit you own preference of pilot and counterbore size, many of our toolmakers used these as the commercial items were not preferred.

Mike

Edited By Mike Poole on 23/07/2020 15:33:50

[Tony Pratt 1Participant @tonypratt1]

if you have the facilities to grind this the you can make a counterbore to suit you own preference of pilot and counterbore size, many of our toolmakers used these as the commercial items were not preferred.

Mike

Edited By Mike Poole on 23/07/2020 15:33:50

Made loads of those 'back in the day' when I was a Toolmaker & still have them some where, as you said the commercial ones were 'non preferred'

Tony

[VicParticipant @vic]

I often work with aluminium alloy so making your own from Silver steel is not difficult. I never seem to have the size I need though.

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