Thread: Sharpening Spring Loaded Centre Punch

I've been making up a jig for the table saw and needed to make a small 50x55 metal plate with four holes to locate things.

I cut the plate out of an old CMT saw blade using the angle grinder and filed the edges square without problems, but when I attempted to drill it I started to run into problems. I have a cheapie spring loaded centre punch that I assumed was a metal working tool and used it to mark the hole locations.. By the 4th hole the punch point was slightly mushroomed over with an annoying sharp catch. I touched it up with a file to get rid of the sharp bit but I think it needs some more work.

Q: What is the best way to sharpen punches and scribes? I don't think I have a steady enough hand to hold them up to a grinding wheel.

I also had trouble drilling the plate. I have some old reasonable quality HSS drill bits (mainly used drilling wood), but they weren't making much headway in the saw plate. Would it be particularly hard? It filed fairly easily. I have one of those cheap drill sharpener thingies that slip over the chuck of a hand drill. I eventually got through by resharpening the drill bits twice for each hole.

Sounds to me like it might be more the fault of it being made of cheaper, softer metal. Ive seen a lot of cheaper punches at work that are just case hardened stock and once youre through the case hardening, you may as well be using a punch made of mild steel.

Id just sharpen it as I would any punch or scribe, pointing into the wheel's rotation. What I would try is a more obtuse included angle on the punch. If its super pointy, like you would normally make a scribe, then it might not have the strength to put up with the abuse and thats causing it to round over. Im not sure how Id go about sharpening without using a grinding wheel, that a file sharpens it doesnt fill me with confidence in its hardness. Maybe file a more obtuse point on it and see if youre not wasting your time before you get too carried away.

My quick & dirty method to sharpen centre punches etc is to hold them in a drill chuck. The spinning helps to get a nice conical point.

Jordan

Originally Posted by Fuzzie

I've been making up a jig for the table saw and needed to make a small 50x55 metal plate with four holes to locate things.

I cut the plate out of an old CMT saw blade using the angle grinder and filed the edges square without problems, but when I attempted to drill it I started to run into problems. I have a cheapie spring loaded centre punch that I assumed was a metal working tool and used it to mark the hole locations.. By the 4th hole the punch point was slightly mushroomed over with an annoying sharp catch. I touched it up with a file to get rid of the sharp bit but I think it needs some more work.

Q: What is the best way to sharpen punches and scribes? I don't think I have a steady enough hand to hold them up to a grinding wheel.

I also had trouble drilling the plate. I have some old reasonable quality HSS drill bits (mainly used drilling wood), but they weren't making much headway in the saw plate. Would it be particularly hard? It filed fairly easily. .

What you are seeing is fairly normal.

Centre punches should be sharpenable by a good file, if not they would be too hard and the punches tips would risk shattering when the punch is struck.
Circular saw blades are usually hard and some are harder than a normal centre punch would be so the fact that it mushroomed does not surprise me.
You haven't mentioned hole size or RPMs use but if it is a large hole it helps to have a pilot hole.
When the material is approaching the hardness of a HSS bit reducing the RPMs can help reduce blunting the bit
For drilling into saw blades and the like I recommend using a Co bit or in cases where they are really hard a (new or sharpened) TC masonry bit works - high pressure, low RPMs, lots of lube/coolant.
I use TC bits this when drilling holes through the middle of chainsaw bar nose bearings where even Co drills don't cut it Even the masonry bits struggle and they usually only last for 2-3 holes.

Originally Posted by scottyd

Sounds to me like it might be more the fault of it being made of cheaper, softer metal. Ive seen a lot of cheaper punches at work that are just case hardened stock ...

Well it did come from Harbor Freight in the US, which doesn't actually inspire a lot of confidence. I think I'll just keep it for woodworking.

Look here what it says about centre punches
PB SWISS TOOLS: Centre punches and pin punches

Centre punches made of tool steel are only suitable for working on materials < 40° HRC. For harder materials, you should use centre punches with tungsten carbide tips (see illustration).

Your saw blade is likely to be around 50 or harder so that's your problem.

HSS has HRC of around 62-63, files will be 65+

Thanks Bob, The holes were only 3/16". I'll try low low speed and a bit of lube next time. I think the saw plate was much harder than expected, it was from an old nail cutting demolition blade.

Originally Posted by Fuzzie

Thanks Bob, The holes were only 3/16". I'll try low low speed and a bit of lube next time. I think the saw plate was much harder than expected, it was from an old nail cutting demolition blade.

I used some saw plate material to make a chainsaw bar nose adapter - I remember it was quite hard.
The chainsaw bar had a sprocket nose and I wanted to replace that with a roller nose which had a different attachment profile.
The adapter is the M shaped plate (it's actually a sandwich) between the red arrows between the bar and the nose.
It was cut roughly to shape with an angle grinder and then shaped using a carbide bit in a die grinder.
The Allen bolts are not a problem because this is for a chainsaw mill
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