Question about edge geometry

[gumbo]

Quick question that I'm sure will engender a number of competing responses, but here goes.

Is it possible to get a high-quality shave-ready edge (and please don't look too far into this or include the caveat of subjectivity here...you all know what I mean) after removing a chip (~1mm) from an edge without also removing the comparable amount of spine? Or is the geometry altered enough that no decent bevel can be achieved going forward unless the spine is also worn down.

I know the angle calculation is possible and that the bevel will be marginally more acute, but so long as both the edge and the spine are still making contact, it should still come to a decent edge, correct?

 

[Heespharm]

Yes it's possible... just do it and see if you can get a good shave with it... If you read past posts about bread knifing you see an extended discussion about it.. But in the end your making your bevel more OBTUSE not acute.. when your getting bevel angle past 22-25 degrees I feel like you've gone too far

 

[Gamma]

Forgive me if I bungled the math here.

If you started off with a 19mm wide blade that had a spine of 4.8 mm, then the angle is roughly 14.5º
Take 1mm off the blade's width, and nothing off the spine - the angle is roughly 15.3º.

That all seems fine and the edge just might be more to the user's liking.

I suppose it all depends on the starting/ending numbers.

 

[Doc226]

Forgive me if I bungled the math here.

If you started off with a 19mm wide blade that had a spine of 4.8 mm, then the angle is roughly 14.5º
Take 1mm off the blade's width, and nothing off the spine - the angle is roughly 15.3º.

That all seems fine and the edge just might be more to the user's liking.

I suppose it all depends on the starting/ending numbers.

This

Keep doing it and eventually it will become a problem

 

[gumbo]

Great. Thanks for the replies. I have a nice wosti with beautiful ivory scales on the restore block and I wanted to make sure it wasn't a waste of time using the wosti - working with ivory always makes me nervous as I've found pinning can be a bit risky given their propensity for cracking - i only want to do it once.

 

[SliceOfLife]

Quick question that I'm sure will engender a number of competing responses, but here goes.

Is it possible to get a high-quality shave-ready edge (and please don't look too far into this or include the caveat of subjectivity here...you all know what I mean) after removing a chip (~1mm) from an edge without also removing the comparable amount of spine? Or is the geometry altered enough that no decent bevel can be achieved going forward unless the spine is also worn down.

I know the angle calculation is possible and that the bevel will be marginally more acute, but so long as both the edge and the spine are still making contact, it should still come to a decent edge, correct?

Too many variables to say for certain, but more than likely you'll be fine with 1mm depth removed. On a 3/8" sculptor edge or a blade that is already in the high teens angle by default, your chances will go down, but for a typical razor a change in geometry that minute shouldn't completely ruin the edge. It will change it. Again how significantly will vary. This time not only on the razor and the particular finish in question, but also the user and any variables his use of it would introduce. I can tell you that a 1mm change in blade depth on a number of blades has been not only noticeable but quite apparent. On other razors it has made little difference. There can't be an absolute answer. Just as a 1mm difference in blade depth may be the difference between an edge lasting 200 shaves between honings and one which fails during the first use in one direction, it can also be the difference between an excellent shave and one that is unacceptable in the other.

 

[Slash McCoy]

Quick question that I'm sure will engender a number of competing responses, but here goes.

Is it possible to get a high-quality shave-ready edge (and please don't look too far into this or include the caveat of subjectivity here...you all know what I mean) after removing a chip (~1mm) from an edge without also removing the comparable amount of spine? Or is the geometry altered enough that no decent bevel can be achieved going forward unless the spine is also worn down.

I know the angle calculation is possible and that the bevel will be marginally more acute, but so long as both the edge and the spine are still making contact, it should still come to a decent edge, correct?

It wont be more acute. It will be more obtuse.

I suggest you crunch the numbers and see what bevel angle you have before removing the chip, then recalculate using the new anticipated hypotenuse measurement and the original thickness. I dont think in mm I think in inches, but yes, a 1/16" edge removal with no spine reduction results in a significant change of bevel angle. Wirk the numbers for the blade you are talking about.

There is a large range of bevel angle that will shave. Increasing from 17 to 18 degrees might or might not be noticeable in the shave. Increasing from 17 to 20 degrees will definitely have a noticeable effect.

You know that you can calculate bevel angle and changes in bevel angle, so why not do it before you start removing steel? Dont guess. Make an informed decision and have a pretty good idea of the general outcome before you start.

If you simply cant be bothered with making a couple of quick measurements and hitting a few calculator keys, then I suggest that you simply lay the razor down on the hone and rub a dub dub until the chip is gone. Hone normally and naturally and the bevel angle wont change all that much.

 

[Slash McCoy]

Forgive me if I bungled the math here.

If you started off with a 19mm wide blade that had a spine of 4.8 mm, then the angle is roughly 14.5º
Take 1mm off the blade's width, and nothing off the spine - the angle is roughly 15.3º.

That all seems fine and the edge just might be more to the user's liking.

I suppose it all depends on the starting/ending numbers.

You didnt bungle the numbers but you did use a wrong one. Dont use blade width. Use the distance from the top edge of the spine's bevel strip to the razor's edge. Thats the hypotenuse. Blade width is immaterial.

 

[buca3152]

Fast question. Spine up, edge facing down. Measure from top of spine bevel or bottom of spine bevel? I always measured from bottom. Thanks.

 

[gumbo]

OK so the blade is now 16.5mm from the widest part of the spine to the edge and the spine is 5mm thick. This is after chip removal.
My calculations give me a bevel angle of 17.24 degrees.
17 degrees actually puts me right in that sweet spot between 15-19, correct? I don't have calipers so it's possible that I'm off by as much as .5 mm on the hypotenuse, but I think the spine is just about dead-on 5mm thick.
Even if I were off as much as .5mm, that would give me an angle deviation of 1.02 degrees, and a range of 16.74 - 17.76 degrees - I think I'm fine either way.

Thanks again for the help - I haven't done trig in about 15 years! If this appears to be incorrect please let me know.

 

[global_dev]

Fast question. Spine up, edge facing down. Measure from top of spine bevel or bottom of spine bevel? I always measured from bottom. Thanks.

Top of honewear closest to back of spine

 

[Gamma]

You didnt bungle the numbers but you did use a wrong one. Dont use blade width. Use the distance from the top edge of the spine's bevel strip to the razor's edge. Thats the hypotenuse. Blade width is immaterial.

Thats exactly the dimension I was referring to.
I could be wrong, but I seem to remember reading a thread somewhere that this measurement is, technically, the blade's width.

 

[Slash McCoy]

Fast question. Spine up, edge facing down. Measure from top of spine bevel or bottom of spine bevel? I always measured from bottom. Thanks.

Top edge of the spine bebel, because that is more or less the thickest part of the spine. Remember you are solving a triangle and to measure the sides you have to measure from the corners of the triangle.

 

[Slash McCoy]

Thats exactly the dimension I was referring to.
I could be wrong, but I seem to remember reading a thread somewhere that this measurement is, technically, the blade's width.

I dont know... what I call the blade width is the width of the blade.

 

[buca3152]

Thanks gents.

 

[Slash McCoy]

OK so the blade is now 16.5mm from the widest part of the spine to the edge and the spine is 5mm thick. This is after chip removal.
My calculations give me a bevel angle of 17.24 degrees.
17 degrees actually puts me right in that sweet spot between 15-19, correct? I don't have calipers so it's possible that I'm off by as much as .5 mm on the hypotenuse, but I think the spine is just about dead-on 5mm thick.
Even if I were off as much as .5mm, that would give me an angle deviation of 1.02 degrees, and a range of 16.74 - 17.76 degrees - I think I'm fine either way.

Thanks again for the help - I haven't done trig in about 15 years! If this appears to be incorrect please let me know.

There ya go! I actually think thecsweet spot is at 16.5, but thats close enough to shave well. Obviiusly a slightly bigger angle will be milder, or at least have the potential to be a milder shaver, and a more acute angle, as long as the steel will support it, will shave more aggressively, with good honing. So there is some room for preference, and plenty of wiggle room between extremes. Still, it is good to know what you are doing to the blade geometry and not have to guess or assume anything.

 

[SliceOfLife]

Fast question. Spine up, edge facing down. Measure from top of spine bevel or bottom of spine bevel? I always measured from bottom. Thanks.

If you're using the depth of the spine, then measure from the top of the spine bevel. If you measure from the bottom, then you'd need to measure the depth of the spine at that location too (which can be done if your caliper is deep enough to fully insert the blade into edge first).

Sweet spots vary. The best coticule shave I ever had came off a 13 degree angle blade. I've also got a few blades around 16 that shave very well. For me, razors over 17 degree's have yet to impress me with the shave, but given the variance in blades and finishes out there, as well as peoples preferences for edges, there are certainly going to be examples out there where steeper angled blades than what I've preferred are well-regarded. My personal preference is to take my best guess to what the original grind angle is through research, estimations based on wear and condition, etc; and to put that angle into any razors I repair the geometry of. Almost all unworn and unrestored razors I've measured fell between ~14* and ~16.5*. This obviously won't be the best option if you're looking to keep from putting a lot of spine-wear on a razor that had a chip removed.