I'd guess they are some sort of counter-bore tool, like a D-bit.

Ed.

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Not wishing to take this too far off track … In some situations, it is good practice to counterbore as deep as is practical [it avoids bolt stretch]

MichaelG.

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To humour you … I just did:

What point are you making ?

MichaelG.

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Quote from **LINK**

https://en.m.wikipedia.org/wiki/Counterbore

A counterbore (symbol: ⌴) is a cylindrical flat-bottomed hole that enlarges another coaxial hole, or the tool used to create that feature. A counterbore hole is typically used when a fastener, such as a socket head cap screw, is required to sit flush with or below the level of a workpiece's surface.

< etc. >

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Edit: A typical 'use case' for deep counterbores would be items like this:

https://www.thermofisher.co.nz/Uploads/file/Environmental-Industrial/Environmental-Monitoring-Safety/Environmental-Simulation/Vibration-Test-Analysis-Systems/LDS-Test/pdf/Head-Expanders-Fixtures.pdf

… not one of mine, but I have designed a couple in my time.

Edited By Michael Gilligan on 02/03/2017 21:56:10

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Yes, I did wonder about the proportions …. maybe not for capheads after all.

Perhaps Alan could tell us the diameters

… I've had no luck searching for that Microtools reference.

MichaelG.

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Edit: another possibility is that the disproportionately large pilot diameter matches a flanged insert for use in a relatively soft component.

Edited By Michael Gilligan on 03/03/2017 08:10:10

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O.K. Ian … I should have said something like "[it reduces bolt stretch to manageable values]" … Note: I was using "stretch" in the dynamic sense; but I hadn't made that explicit.

On a 20,000 lb thrust shaker … I can assure you that short caphead screws, down deep counterbores, are much more effective than long ones in shallow counterbores. [see the 'head expander' that I linked.]

Feel free to continue the discussion without any further input from me.

MichaelG

Edited By Michael Gilligan on 03/03/2017 08:43:13

I think it's a deburing tool.

I agree, it is for deburring holes, and putting a radius on the edge. Ian S C

With regard to bolt stretch, the most efficient use of a fastener is to tighten it to JUST put the material into yield. We used to tighten W Range 1/2 UNF bolts so that they took on a permanent extension of about a couple of thou. By this time, the load in the bolt was circa 9 tons. And we found that the bolt could be retightened in this way 6 times or more, so we advised 4 times in the field, before replacement was necessary.

The machine monitored angular rotation vs torque for each spindle.  When the previous angle/torque relationship began to increase, it sensed yield and stopped rotating

To see the machine tighten 32 bolts from finger tight into yield, in barely 7 seconds , was quite impressive.

Lots of vehicle manufacturers now tighten the important fasteners into yield.

Howard

Edited By Howard Lewis on 03/03/2017 15:14:04

The meaning of the tooling word counterbore has been massively and probably irretrievably corrupted over the last few decades. The clue is in the word itself COUNTERbore. Think of counter-attack, counter-clockwise etc. In the reverse direction probably derived by distortion of the French 'contre' though I am not a linguist.

The original idea of the counterbore was boring from the far side eg when an item had been milled and drilled on the lathe saddle and the far side of the hole needed facing off or boring without taking the job off the saddle for accuracy. A rod with a suitable cutter is passed through from the far side to be gripped by the chuck but the cutting part is too wide to be passed through from the headstock side. It then cuts on the far side – counter direction to the original drilling.

If you make a wider hole from the same side as you drilled that is just plain boring not counterboriing.

Best picture I can find is on this thread: posting no 6 by Mcgyver.