Theoretical question on chisel life

[Jacob]

.... However there's no need to rough grind all of the secondary bevel off, as the cutting edge will hopefully still be pretty sharp. ....

Not if it's blunt, which is presumably why you are sharpening it in the first place. In which case just grind it off - only just as far as necessary, and be careful of overheating.
It may be necessary to go well past the original bevel if the face is also worn.
Honing a nicely ground new edge is the work of seconds. It gets progressively more difficult as the honed area increases, whether as a rounded or a flat bevel.

 

[RossJarvis]

.... However there's no need to rough grind all of the secondary bevel off, as the cutting edge will hopefully still be pretty sharp. ....

Not if it's blunt, which is presumably why you are sharpening it in the first place. In which case just grind it off - only just as far as necessary, and be careful of overheating.

I don't think the original point was about grinding a blunt tool, it was about when to regrind the primary (25 degree bevel) when the secondary (30 degree bevel) was getting very large, maybe too large to easily sharpen/hone. By regrinding the primary, the secondary, sharp bit, would be a lot narrower and therefore easier and quicker to sharpen, and wear the sharpening medium less. If you leave a tiny bit of the previous edge, you won't remove any more metal than you have to. This was to answer the best way of extending the life of the chisel. If the whole of the front is rough ground too often, you'll have a short chisel too quickly;



I think if the edge was pretty grim, we'd all agree you would have to regrind/rough-grind the whole face off.

 

[David C]

The goalposts have been shifted as usual, from tool life to Jakov's inability to understand techniques other than his own.

There are several good reasons not to grind to or past the edge.
1. Overheating.
2. Loss of existing shape, i.e. squareness for a chisel and camber for a plane blade.
3. Unnecessary shortening of the blade.
4. Bench grinders remove metal very fast, judging when to stop can be difficult. Refer to point 3.

After regrinding I will frequently get a good wire edge with 2 or three strokes on an 800grit waterstone. These cut incredibly quickly compared with oilstones and even diamond stones.

I suggest that grinding past the edge is bad practice, it will shorten the life of your tools, and create extra work. Point 2.

David Charlesworth

 

[Jacob]

Agree bench grinders not good. Belt sander better. You just have to do it carefully to avoid the problems above. You only do it because honing the bevel is too slow so you might as well start a new bevel. New bevels hone sharp in seconds. If you have bits of blunt old bevel left you have given yourself an unnecessary task.
It seems blatantly obvious to me, though it did take me some time to get sharpening well under control as there is so much contradictory information around - so many people saying how difficult, nay impossible, so many things are. :lol: :lol:
Shortening tool life - I've one or two shortish chisels, one or two plane blades replaced. Pennies per year. Many hours saved!

 

[RossJarvis]

It all reminds me of an argument I had with a mate once.

"The First Service station on the M1 was Blue Boar Services at Watford Gap" says I.

"No it's Toddington"

"No it's Watford Gap"

"No it isn't" "Yes it was" "No it isn't" "Yes....." This got more and more heated till I said;

"Okay then, when did they build the f*****g Toddington Services then?"

"How the b****y hell would I know" says he !?!?!?!?!?

 

[Jacob]

8) It was the Blue Boar.

 

[Jacob]

.......
I don't think the original point was about grinding a blunt tool, it was about when to regrind the primary (25 degree bevel) when the secondary (30 degree bevel) was getting very large, maybe too large to easily sharpen/hone. ......

Yes but surely you wouldn't bother until the tool is blunt and needs re-sharpening. So it is about blunt tools.
With rounded bevels you don't get this secondary bevel prob - the tool only needs regrinding when it is very blunt or chipped.

 

[bugbear]

... it did take me some time to get sharpening well under control as there is so much contradictory information around - so many people saying how difficult, nay impossible, so many things are.

Many people say double bevel sharpening with the cheap and simple Eclipse jig is quick and effective, as so clearly explained by RJ above.

BugBear

 

[bugbear]

.... However there's no need to rough grind all of the secondary bevel off, as the cutting edge will hopefully still be pretty sharp. ....

Not if it's blunt, which is presumably why you are sharpening it in the first place. In which case just grind it off - only just as far as necessary, and be careful of overheating.

I don't think the original point was about grinding a blunt tool, it was about when to regrind the primary (25 degree bevel) when the secondary (30 degree bevel) was getting very large, maybe too large to easily sharpen/hone. By regrinding the primary, the secondary, sharp bit, would be a lot narrower and therefore easier and quicker to sharpen, and wear the sharpening medium less. If you leave a tiny bit of the previous edge, you won't remove any more metal than you have to. This was to answer the best way of extending the life of the chisel. If the whole of the front is rough ground too often, you'll have a short chisel too quickly;



I think if the edge was pretty grim, we'd all agree you would have to regrind/rough-grind the whole face off.

I'll repeat the technique I learnt from a full time plane (and hence blade) maker on OLDTOOLS.

When grinding away damage from a blade, first grind the edge back to remove the damage, which leaves the front edge perfectly square and blunt (this feels very wrong when you do it). Then (and only then) grind the bevel.

This avoids performing grinding with a thin edge, which is always the place most prone to blueing.

BugBear

 

[Jacob]

Yes thats how it's done. It's actually difficult to shape an edge and form a bevel at the same time. Shape first 90º, bevel second 25º, then hone 30º.
PS and this is the answer to Dave's problem (point 2 see above) - grinding is the way to shape an edge, to recover it, not to lose it.