How far can you hone a straight?

[Haines1997]

Hi all - bought a set of old Straight razors to have a play with on restoring/honing etc.

Found a couple where a fair chunk of the razor had clearly been honed down or ones that weren't a "straight" edge, so would need to be down a bit.

Question is, how far can it go down before it's not usable any more?

Many thanks!

 

[Herrenberg]

Pretty darned far. I have some 3/8 razors that shave just fine.

However, people have pointed out here that the steel may get softer as you go, apparently because of the more rapid cooling of the thinner sections during tempering.

Another thing to watch out for is keeping the edge angle reasonable, not just taking off metal at the edge.

 

[Haines1997]

So, let's say the blade (if levelled out) is close to, or lower than than the shank - is that an issue?

 

[rbscebu]

So, let's say the blade (if levelled out) is close to, or lower than than the shank - is that an issue?

No, of you know how to properly hone a SR.

 

[Haines1997]

Brilliant, thanks all - that's great news. Would you have to take metal off the shank to level it out? Or could you effectively have a lower shank than blade edge?

 

[thp001]

You need enough thickness or width on the shank for ergonomics. I wouldn't grind any of the shank or you might make it too small and unstable in the fingers.

 

[bluesman 7]

Not sure of your terminology. Are you saying that the width of the blade is narrower than the tang? Pictures would be helpful.

 

[Haines1997]

Will be back tomorrow with them, but this photo should explain the shank meaning.

 

[Haines1997]

.

 

[RayClem]

When a straight razor is made, the steel is tempered. That process makes the edge of the blade very hard. However, since the interior of the blade does not cool as quickly as the edge, it will not be as hard as the edge. Thus, as a blade is honed, you will come to a point where the hardness of the steel will no longer be sufficient to hold a sharp edge. Where that point occurs for a specific razor depends on the type of steel, the exact tempering process and how the edge was ground in the first place (1/4 hollow, 1/2 hollow, full hollow). Thus, no one can predict when you will reach the point of no return.

 

[rbscebu]

When a straight razor is made, the steel is tempered. That process makes the edge of the blade very hard. However, since the interior of the blade does not cool as quickly as the edge, it will not be as hard as the edge. Thus, as a blade is honed, you will come to a point where the hardness of the steel will no longer be sufficient to hold a sharp edge. Where that point occurs for a specific razor depends on the type of steel, the exact tempering process and how the edge was ground in the first place (1/4 hollow, 1/2 hollow, full hollow). Thus, no one can predict when you will reach the point of no return.

Very true, however the hardness should remain fairly constant from the edge up to at least where the blade starts to thicken in the grind. This, of course will depend on if the blade was ground before or after tempering.

 

[SliceOfLife]

I've gotten some out of barbers sets that could only be described as "a spine with an edge". We're talking <1/4" deep. Generally the angle is WAAAAY out of whack at that point (I'd guess they were 20*+), but clearly they were pretty usable right up until that point (maybe for neck trimming, not face shaving though? Who knows).


Also I'm no smith, but my understanding would be that it's actually the difference in surface vs interior cooling during the shocking (submerging in oil/water immediately after removing from a very high state of heat) that would render the interior softer... and tempering actually completely corrects that by taking the entirety of the blade to a softer/less brittle state than any part of it was after shocking.

 

[bluesman 7]

How far toward the spine the blank hardens has to do with the quenchant, the thickness, and the hardenability of the steel. The hardenability refers to how quickly the steel has to be brought below the nose of the IT curve and does not correlate to the actual hardness that the steel can reach. Most older razors are pretty simple high carbon steels and have low hardenability and are likely not hardened all the way to the spine. While O1, 52100, 5160 have higher hardenability and are more likely to be hardened all the way to the spine.

The above picture is from Verhoeven and shows a test piece that was low enough hardenability (1086) to not through harden. Especially interesting is the expected result and the actual.

 

[SliceOfLife]

Very Casual physics that difference (which is interesting) suggests to me that the quenching material can't keep up with the steel's own internal thermal conductivity's ability to maintain temp long enough to prevent retaining/forming the martensite once a certain thickness is reached. Are there more modern quenching materials that would resolve that or is, like you suggest, simply a matter of changing the makeup of the steel to weight it more towards martensite formation? I would assume if you move too much in the colder/more conductive realm for quenching the steel would shatter or crack during quenching?

 

[bluesman 7]

Often differential hardening is a desirable feature (hamon) or irrelevant to the tool being produced. If through hardening is desired its easier to pick a steel with a slower TTT curve.

 

[thp001]

Very Casual physics that difference (which is interesting) suggests to me that the quenching material can't keep up with the steel's own internal thermal conductivity's ability to maintain temp long enough to prevent retaining/forming the martensite once a certain thickness is reached. Are there more modern quenching materials that would resolve that or is, like you suggest, simply a matter of changing the makeup of the steel to weight it more towards martensite formation? I would assume if you move too much in the colder/more conductive realm for quenching the steel would shatter or crack during quenching?

I don't thing there are any quenchants faster than water/brine, certainly no engineered quenchants. As Bluesman said above though, you'd just pick a different steel. I've done knives in A2 where you can just take the blade out of a 1750F HT and hang it somewhere in ambient temp until it's cool and reach almost full hardness (you get about +1 rockwell point with cryo).

The thing with razors is you want a certain type of very fine grain structure which plain carbon steels/low alloy steels excel at.