counterbores, reamers, boring tool

How to counterbores compare to a reamer or boring tool if you want to enlarge a hole with a sharp step at he bottom to the smaller diameter through hole?

You can't do this with a reamer because of the lead-in taper.

Counterbores work fairly well, but you need a good pilot and may have to touch up the thru hole. You will also have to go fairly slow, both speed and feed. Bigger steps take a fair bit of power. (If this is a CNC job, I've never used one there.) When the corners go, you will have to sharpen the whole cutter. Depending on how sharp the corners really have to be, you might want multiple cutters for multiple pieces.

Boring is the way to go, aolthough it will take multiple passes. If the corner radius isn't too tight, you can use in insert bar and change the insert when needed. For really sharp corners, use a hand ground HSS bit and keep sharpening as needed.

I have seen this done with a reamer, grinding of the the tapered end and regrinding the end to cut. But when you do that, I guess you can no longer call it a reamer, but a counter bore.

Tom

It really depends on the machine you're using and what tolerance and finish requirements you have. On a manual machine, a counterbore is the most simple method of making a square shoulder hole. On a CNC mill, I would prefer to circle mill the hole. On a CNC lathe, I would bore the hole.
Both boring and circle milling can done while holding tight tolerances and fine finishes. A counterbore, not so much.

Brian

Depends on what machines you have to do the work, but if it's just a routine task size- and tolerance-wise, my (extremely) frequent choice is just an end mill (in the mill, of course). You end up with a very slight conical profile across the hole diameter at the bottom, but it isn't much (a few thou...) and the corners are sharp.

entoffice said:

How to counterbores compare to a reamer or boring tool if you want to enlarge a hole with a sharp step at he bottom to the smaller diameter through hole?

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Compare as to what?

Quality of counterbore?

Cost of tooling?

Speed of process?

Ease of resharpening?

Practicality?

Availability of sizes?

In general, a piloted tool will allow larger metal removal rates at the sacrifice of positional and surface quality. It can be done on machines of limited rigidity.

Make your question more specific and you will get more specific answers.