Useful math, or How to tell what angle you need for a FFG.

R

Rapt_up

Joined
May 4, 2012
Messages
968
Its possible with measurements and the help of a calculator to determine what angle will give you the FFG directly from the dimensions of the steel. (Yes it involves trigonometry, but don't run away, the calculator does the hard part. :thumbup:)

Example:
blade blank is 1.5" high and .220" thick. Lets say you want .010" thick edge before sharpening (secondary bevel) this means that for each side of the blade you need to remove half of .220-.010, of half of .210"... This is .105" on your FFG on each side. The angle is then the ATAN (or inverse TAN) of 0.105"/1.500" which is 4.004 degrees, which is plenty close enough to just call it 4 degrees. :)

So the formula is: Angle= ATAN(Depth of grind/Height of blade) (In the windows calculator accessories application its INV TAN instead of ATAN, depends on your calculator.)

You can change this if you want a specific height of grind, then just substitute that number where the formula has Height of blade.

Hope this helps when you're doing a design, or planning your work.
 
That's cool rapt. I usually make a scale drawing, then mark for grind height and edge thickness then extend the bevel lines up out of the scale drawing then use my protractor.
 
I just grind the mother until it looks right.:D
 
My eyes are calibrated for a pretty keen line, it's my hands that need a little work getting up to spec. :D
 
:) I know that experienced folks and people with an eye can do it that way and get exceptional results, but that usually takes time to get to that place.

But I thought it might be useful for those that want to know the "numbers", as it were. Particularly if you have rests or jigs that you want to set, as a way to help reduce some of the "time" taken in getting the experience. Not that time in is necessarily a problem.
 
Rapt_up said:
:) I know that experienced folks and people with an eye can do it that way and get exceptional results, but that usually takes time to get to that place.

But I thought it might be useful for those that want to know the "numbers", as it were. Particularly if you have rests or jigs that you want to set, as a way to help reduce some of the "time" taken in getting the experience. Not that time in is necessarily a problem.
Click to expand...

I agree. Glad you took the time to post it up. Good info.
 
Rapt_up said:
:) I know that experienced folks and people with an eye can do it that way and get exceptional results, but that usually takes time to get to that place.

But I thought it might be useful for those that want to know the "numbers", as it were. Particularly if you have rests or jigs that you want to set, as a way to help reduce some of the "time" taken in getting the experience. Not that time in is necessarily a problem.
Click to expand...

No, you are right. There is a place in the shop for math, and machines.

I have already had special 4-flute end mills with long reaches for hollowing out micarta hidden tangs made for me. I have been looking into tilting tables and angled parallel bars for a mill vice too. Having parts like these made for you is not cost prohibitive, many CNC shops can build you custom tooling and cutters if you are not ready to make your own.

-Ron
 
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But seriously, ladies and germs, I may have to start looking into such things as the accuracy of the Mod 4 Calibrated Eyeball does degrade with age.;)
 
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