I've got a collection of blunt centre punches and its some time since I had
a go at resharpening one. I've never been told the "proper" way to do it.
Could someone please tell me how I should go about it?
You'll need a bench grinder, with built-in tool rest.
I've no idea whether this is the recommended "text book" method - but it
will work! Hold the punch on the rest in such a way that it is horizonal,
but at the correct angle to create a cone at the sharp end. As you grind,
slowly rotate the punch about its own axis. You should then get a sharp
point, which is exactly on the centreline of the punch.
According to some machining books, the center punch should be held
so that the grinding marks run from the sharp point toward the back of
the centerpunch. In line with the axis of the centerpunch. Reasoning
behind this is that grinding "around" the point creates the
possibility of the centerpunch "mushrooming" slightly behind the sharp
point during use whereas grinding in line with the axis transfers the
force of use in line with the axis of the centerpunch. But what do I
know??? I typically grind mine in line with the axis and have no
problem. (you really don't need a "long" point on them - look at the
angle before you grind, and try to maintain the relationship).
i've been taught the same thing,
grind from the tip to the back so all
the grind marks are radial.
i use a belt sander, its just as easy.
be careful the point doesnt catch.. or
point it the other way. :)
i'm guessing the mushrooming would only be a problem
if you're using your punchmarks for delicate layouts.. where,
for example, a steel rule could get hung up when sliding it around.
in the end, as long as its 'pointy' it should be ok.
unless we're talking about transfer punches.
i sharpen those in a lathe.
Good centre punches should last a loooooonnngg time without sharpening
too. Do you start with a prickpunch and work up to a heavier size.
Prick punches a real "sharp" and use only a light tap to mark, but
centre punches are pretty dull becasue of the flatish angle they are
ground to. A 3/8" taper tap makes a good punch, and are "chuckable".
Good point, Brian. I find that prickpunches work even better if twirled
between the fingers with very light downward pressure. Then work up to the
more blunt centre punch. All this easier to do if one is using a headband
magnifier and layout blue.
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