Accurately measuring widest point of spine to edge

[SparkyLB]

Please excuse the crappy Microsoft Paint depiction. If this was the cross section of my SR, and I wanted to calculate my edge angle, it would be 2 x theta. I can handle the trig to find theta, (cotangent of O ÷ H). But I must have the exact length of line H.

I am a stickler for "hard" measurements. I detest eye-balling or "close enough," approximations. I can get a good, solid caliper measurement for 2xO, but measuring with certainty the hypotenuse is the issue. Since the spine is somewhat rounded, I need to eliminate that included chord/sector from the equation to get a good, hard measurement for H.

If my calipers must touch the top edge (which I suppose they will), I will use a piece of 3m Scotch 33 for protection, then subtract the tape's thickness.

So if you have any ideas, I'd sure appreciate it. How would you measure the hypotenuse of the triangle)?

As always, thank you gentlemen!

 

[Darth Scandalous]

Don't measure X, measure the hypotenuse from the top of the spine wear to the apex. Still takes a careful hand and a close eye but with calipers it will get you more than close enough.

 

[SparkyLB]

I gotcha. A true measurement isn't really possible, is it?

 

[Darth Scandalous]

I gotcha. A true measurement isn't really possible, is it?

You should be able to get a pretty precise measurement if you're careful. A lot more than you could trying to measure the height rather than the hypotenuse.

 

[SparkyLB]

Nice catch, BTW. Sine is O/H which is NOT line X, but H. I've remade the drawing and fixed the text. Sorry, @Darth Scandalous, I sort of messed up your post with my trig mistake. But if I had MY drawing correct, you're saying measure the hypotenuse, which is correct.

 

[SparkyLB]

You should be able to get a pretty precise measurement if you're careful. A lot more than you could trying to measure the height rather than the hypotenuse.

No argument there! I suppose a thou or two won't greatly affect the calculated angle. That's probably ballpark margin of error.

 

[Darth Scandalous]

No argument there! I suppose a thou or two won't greatly affect the calculated angle. That's probably ballpark margin of error.

In terms of precision, the spine thickness is a lot more critical anyway, and that one you can measure directly with a caliper.

 

[SparkyLB]

Agreed. The smaller the dimension, the more critical the accuracy. Hadn't thought of that.

Thank you, my friend!

 

[Darth Scandalous]

I usually lay the razor on something with a little give, like my thigh, to minimize that chance of dinging the edge with the caliper. Although if you take the measurement prehoning it's less of an issue.

 

[H Brad Boonshaft]

Here’s a good video by Matt on how to measure a bevel angle.
And use the excel calculator. There is some good info on what angles work best, watch the whole video.


DrMatt357(Razor Bevel Angle - How & Why)

 

[APBinNCA]

Jeez, he always has a good video explaining anything I can think up. I just can't handle 20-30 minute videos of something I can digest in less than 10. So I forget to look for his videos, my loss.

 

[Gamma]

For the most part, for most people getting into this level of investigation, a ball-park measurement more than 'works'. That said, I do use a magnetic platform to hold the blade, a loupe and a digital caliper. I would say My method is reliably precise, but maybe someone else would be more precise, dunno. How good someone is with their caliper might matter a whole lot.

When I am really being fastidious, I'll read in 6-7 place across the blade, esp when the blade is a smiler. And I'll use an average most of the time. Most, not all...
Accuracy is a lovely thing but with this type of exercise we approach the diminishing returns quickly. I prefer accuracy over inaccuracy but I also need to figure in 'tolerance' and what it affects.
For example, I can be off by a 1/4 of a mm on blade width and only see a difference of .2 degrees on the bevel angle. Practically speaking, 19.6˚ vs 19.8˚ = the same shave. I would say I would never be 'off' or 'over' by 1/4 mm.

A lot depends on the actual measurements being used, the above is from the last blade I worked on, 21.75 mm width. And if tape is in the picture, it starts to wear instantly on the hones, so the calculations 'dry' won't be the same as when 'wet'.

The "best' angles for any particular razor/blade are very dependent on things like the steel's grain structure, HT&T, hardness, etc. Maybe even the types of hones used. Can't really put that into list format.

 

[SparkyLB]

For the most part, for most people getting into this level of investigation, a ball-park measurement more than 'works'. That said, I do use a magnetic platform to hold the blade, a loupe and a digital caliper. I would say My method is reliably precise, but maybe someone else would be more precise, dunno. How good someone is with their caliper might matter a whole lot.

When I am really being fastidious, I'll read in 6-7 place across the blade, esp when the blade is a smiler. And I'll use an average most of the time. Most, not all...
Accuracy is a lovely thing but with this type of exercise we approach the diminishing returns quickly. I prefer accuracy over inaccuracy but I also need to figure in 'tolerance' and what it affects.
For example, I can be off by a 1/4 of a mm on blade width and only see a difference of .2 degrees on the bevel angle. Practically speaking, 19.6˚ vs 19.8˚ = the same shave. I would say I would never be 'off' or 'over' by 1/4 mm.

A lot depends on the actual measurements being used, the above is from the last blade I worked on, 21.75 mm width. And if tape is in the picture, it starts to wear instantly on the hones, so the calculations 'dry' won't be the same as when 'wet'.

The "best' angles for any particular razor/blade are very dependent on things like the steel's grain structure, HT&T, hardness, etc. Maybe even the types of hones used. Can't really put that into list format.

Whether it’s my thread or someone else’s, I always appreciate your input. Thank you, Sir.

 

[rbscebu]

On a ⅝ blade you need to measure blade width to within about ±0.1mm to get a bevel angle measurement accuracy of about ±0.1°. To achieve the ±0.1mm measurement, you can use a vernier caliper. A wider blade measured to within ±0.1mm will give you a slightly more accurate bevel angle result.

A vernier caliper can measure to within ±0.005mm if you know how to read a vernier scale.

If the vernier caliper touches the blade's edge, it can damage the edge. The blade's width should therefore be measured before any serious honing.

If measuring after honing, you must measure without the vernier caliper touching the edge. This can be done by eye but will give you a less accurate measurement, possibly about ±0.2mm to ±0.3mm and this will give you a corresponding bevel angle accuracy of about ±0.2° to ±0.3°.

When measuring a happy (smiling) blade, most and I will just state the bevel angle at the blade's mid-length.

 

[rbscebu]

I have found that shaving with a SR having a bevel angle of anywhere between about 18.5° down to under 16° does not affect the shave result or even the shave comfort. The larger bevel angle does however require a little more pressure to be applied tangential to the skin against the whiskers to cut them. No biggy for me.

 

[Titleist]

Wedgek Angle Calculator - Calculate Knife Grind and Bevel Angles

Grind and Bevel Angle Calculator - by Wedgek

www.wedgek.com

 

[Frank Shaves]

Once you review the math it is pretty simple. Here it is using Microsoft Excel:

=2*DEGREES(ASIN((Spine/2)/Hypotenuse))

where Spine is the thickness of the spine.

Now let's apply the example from Filarmonica Bevel Angle - https://www.badgerandblade.com/forum/threads/filarmonica-bevel-angle.577422/post-10477703. If Spine is 7.14 and Hypotenuse is 23.5, =2*DEGREES(ASIN((Spine/2)/Hypotenuse)) should return 17.48 degrees.

I normally wrap one layer of blue painter's tape around the portions of the spine and the edge I am measuring, and subtract the thickness of two layers (about 0.25 mm) from the spine and one layer (about 0.125 mm) from the hypotenuse.

The hard part is measuring the hypotenuse. For this, I just hold the razor under good light, try to follow spine under the tape and measure, and measure multiple times using your caliper. What's important here is to be consistent. Right or wrong is less important than consistency, in my opinion.

 

[SparkyLB]

Thank you all.