Thread: blade width vs, thickness question.

Originally Posted by Bruno

Or you could just keep above the 1 / 4 ratio and don't bother with formulae.
That is what vintage makers did and it seems to work well.

Of course

Again for a 7/8"

7 ÷ 8=0.875
0.875 ÷ 3.5 =0.25
0.25=1/4"

Or 4 instead of 3.5 if you want a slightly steeper bevel.

Of course a ratio describes an angle just as accurately as any other method. What peeves me a bit is that people use these same ratios, but measure differently or to different places on the razor, and act as though that doesn't effect the angle. One person using the 1/4 ratio always measured the full height of the razor regardless of how far up the actual grind line was relative to the top of the spine.

Also people argue that anything between 15-20 degrees will shave fine. That is true, but I feel a considerable difference in how those two angles shave and I much prefer the feel of an edge with a bevel angle closer to 15 degrees. This may be subjective on my part because when I make or hone a razor I always know the bevel angle before the test shave.YMMV

And of course, as you point out, the actual height of the BEVEL will determine the width can use a piece of stock, even one that might normally be a tad too "thin" if you measured all the way from the spine. That is a significant consideration say in the event that one of us could somehow get hold of the unobtanium 3/16/4.5-5mm thick AEB-L stock. Perfect stainless razor steel but typical not available in perfect or even close to suitable thickness for straights. unless you want to make a French style frame back.

Originally Posted by bluesman7

Of course a ratio describes an angle just as accurately as any other method. What peeves me a bit is that people use these same ratios, but measure differently or to different places on the razor, and act as though that doesn't effect the angle. One person using the 1/4 ratio always measured the full height of the razor regardless of how far up the actual grind line was relative to the top of the spine.

Also people argue that anything between 15-20 degrees will shave fine. That is true, but I feel a considerable difference in how those two angles shave and I much prefer the feel of an edge with a bevel angle closer to 15 degrees. This may be subjective on my part because when I make or hone a razor I always know the bevel angle before the test shave.YMMV

Originally Posted by bluesman7

Of course a ratio describes an angle just as accurately as any other method. What peeves me a bit is that people use these same ratios, but measure differently or to different places on the razor, and act as though that doesn't effect the angle. One person using the 1/4 ratio always measured the full height of the razor regardless of how far up the actual grind line was relative to the top of the spine.

In the link I posted earlier in this thread it clearly states in the Excel file "Measure width of blade from the tip of the edge to the hone-wear on the spine, including the width of that honewear" . That would obviously be for a used razor. Oth on a brand new razor the measurement would be to where the grind line starts as that would be the contact point with the hone, not the top of the spine. Hard to go wrong if you follow that.

Bob

Originally Posted by BobH

Oth on a brand new razor the measurement would be to where the grind line starts as that would be the contact point with the hone, not the top of the spine. Hard to go wrong if you follow that.

Bob

Agreed. I have spoken to many that don't seem to get that though.

Originally Posted by bluesman7

Agreed. I have spoken to many that don't seem to get that though.

In many cases the difference doesn't matter as there are other, larger factors about the razor that dominate.

For example, depending on the steel, a change of 0.1Rockwell (0.17%) in hardness can lead to a larger difference in how a razor shaves than a difference of 3 degrees (17%) in the bevel angle.

Measuring the width to the bottom of the hone line or the top of it is rarely more than 1/16" which on a 5/8" razor is 10%, so you'd overestimate the bevel angle by about 10% or less than 2 degrees.
Similarly the first terms are Taylor series of sin and tan are sin(x) = x(1 -x^2/6+ O(x^4)) and tan(x)=x(1+x^2/3+O(x^4))
Thus the error from using the wrong formula is of the order of x^2 (x^2/2 if you want to be more precise) or in numerical terms for angles on the order of 16 degrees (half angles of 8 degrees ~0.15 rad) the error is on the order of 0.15^2 or 1%.

At the end of the day it is important to be precise where it matters and pointless to be precise where it doesn't.
Incidentally this was the first lesson I was told to impart on students as a lowly TA; when they put in an answer straight from their calculator with a whole bunch of digits we were to penalize them. Unfortunately, them understanding what is the correct number of digits is equivalent on completely understanding the whole problem they are solving which is something to differentiate the top 1%-5% of students (most of the TAs were not that good).
The result was that the students discovered empirically that 2-3 digits is correct in 99% of the problems we give them and they didn't really learn all that much from that rule. Of course, the professors in charge understood that this doesn't really teach the students much but it was sort of a pet peeve for them and it didn't really matter.