Masonry Chisels with carbide tips

[TireguyfromMA]

One of my knife customers recently asked me if I could sharpen his masonry chisels with carbide tips. One chisel with 2" wide, the other was 1.5" wide, the handle area about 6" long, 1" wide hex diameter on the handle. The striking end of the chisel was rolled over quite a bit from being hit  with a heavy hammer.  I had to use my angle grinder with cut off disc to remove most of the roll over, then I used a bench grinder to clean up and dress the end with a 1/4" chamfer. Important to keep the striking end cool with a dunk in cool water after a few seconds of grinding. Moving on to the tip end I used to pieces of 1"aluminum angle stock clamped to the shaft with a kant-twist clamp to make a stop that would rest against the top of the USB. Set the angle up to 45 deg on each side.  At first I tried my SB-250 stone to grind with but this was going very slow. I then switched to a 200 grit CBN while using PB-Blaster on the wheel to prevent the wheel from loading up with carbide. That worked much better but later found out CBN should never be used to sharpen carbide...ugh.  The best TORMEK wheel to use for these chisels would have to be the DC-250 (360 grit). A coarser grit, closer to 200 would be better.  If your looking for another tool to add to your list of things to sharpen, this would be a good one.  Stone Masons spend about $140 for a 2" wide chisel w/ carbide tip. The masons I've talked to found these very difficult to sharpen and would often just replace than spend time trying to sharpen. 

[Ken S]

Wide, carbide tipped masonry chisels truly qualify as "chisels from hell" for sharpeners. The only similar experience I have had was sharpening two wood splitting wedges for a friend. These were quite old. No carbide, but decades old severe mushrooming on the backs. I used a 46 grit Norton 3X wheel in my ancient six inch dry grinder. Many hours later, I was able to complete the job. I did it as a friend; no reasonable fee would have been rational.

This would not have been such a problem if the tools had been resharpened frequently. However, how many thrifty craftsmen will return a tool for sharpening before it is well beyond "dull"?

Keep us posted.

ken

[TireguyfromMA]

I will definitely keep you posted.  I hope to get the chisel back for touch up sharpening in a couple days. I'm doing this more out of curiosity than anything else. I'm convinced that I can make the chisel like new again without going to "chisel hell"...lol.  Wish me luck

[tgbto]

I can't speak for chisels, but I experimented with a carbide knife. It was not real sharp out of the box, and it did not get much sharper after quite some work.

The way these seem to work is they have very hard bits of carbide bound together by some kind of cement. When you try to sharpen those with a diamond wheel, you grind the cement and tear the carbide bits off. That being said, it seems to me the overall geometry of the chisel striking end is of more consequence, function-wise, than the actual edge ? Or does a sharp edge have any hope of staying sharp past the few first strikes ?

[TireguyfromMA]

So I finally got to sharpen the carbide tipped masonry chisels and it went very well!  All the pads I used are 4" diameter diamond pads, they use velcro to attach to a backing pad that is attached to the end of a variable speed drill, set at a medium speed.  The end of the chisels had about a 1/4" or more radius on them, very rounded, almost flat. The goal is 45 dps, the final "edge" should have about a 1/16 radius to it. To make the edge durable and resist chipping or fractures, you don't want a sharp end, the left and right ends should also have a very slight rounded edge. I started with a 80 grit, then 120, finishing off with a 220.  Total time spent was about 15-20 minutes. I'm sure I'll be able to cut a few minutes off on future sharpenings. The customer was very happy with the results, said they look brand new.

[Ken S]

Well done, Tireguy! You have added and shared a new skill for our bag of tricks.

Ken

[Herman Trivilino]

I can't speak for chisels, but I experimented with a carbide knife. It was not real sharp out of the box, and it did not get much sharper after quite some work.

The way these seem to work is they have very hard bits of carbide bound together by some kind of cement. When you try to sharpen those with a diamond wheel, you grind the cement and tear the carbide bits off. That being said, it seems to me the overall geometry of the chisel striking end is of more consequence, function-wise, than the actual edge ? Or does a sharp edge have any hope of staying sharp past the few first strikes ?

Tungsten carbide steel is very hard, so it makes for a long-lasting and durable edge on a tool. But it is brittle. You cannot sharpen it to a fine edge because it so thin that it chips. You can notice this on new carbide tipped tools. Like the point on a carbide tipped circular saw blade has about a 70 degree bevel. Try to sharpen this to a more acute angle and chips will fall away from the tips.

For this reason, I've never seen or heard of a carbide steel knife. There are carbide tipped scribing tools that are used to scribe vinyl floor tiles, for example. When they get dull people usually just replace them with a new one.

[tgbto]

For this reason, I've never seen or heard of a carbide steel knife.

There it is. I'm not saying these are good knives, just they do seem to exist...

[TireguyfromMA]

Tungsten carbide steel is very hard, so it makes for a long-lasting and durable edge on a tool. But it is brittle. You cannot sharpen it to a fine edge because it so thin that it chips. You can notice this on new carbide tipped tools.

Yes, it's important to put a small radius on all the edges and finish with a 220  grit finish on these chisels. To coarse a grit on the finish could lead to small cracks.  I should also mention these chisels cost $130-40 each.  As you can imagine, when they get dull they slow down the work for the mason quite a bit. I checked back with the mason I did the work for a few days later and he was very happy with the "new" chisels and that he has a place to bring them for sharpening.