What is your flatness/straightness tolerance?

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wizzer

Joined
Jun 23, 2016
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I am curious what each of you give yourselves for a tolerance related to flatness/straightness.

I would imagine the longer the blade the more you keep an eye on it as it will be more noticeable?

Is your straightening to prove you master over the craft or is it mostly performance based?

I would imagine those of you who make knives for a living could end up spending a lot of time trying the eek every blade back to perfect.
 
I would never buy a blade that isn't straight so I would never consider selling a blade that isn't straight. I consider straightening to be part of the heat treating process. I also surface grind after heat treating to ensure the blades are straight. I also use a disk grinder to make sure the bevels on my flat ground blades are perfectly flat.


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Straightening is part of the process. If you forge it's fairly easy, since the material is being heated and cooled throughout the process. It's amazing what can be learned in the process of trying to keep a blade straight.

Fred
 
" tolerance related to flatness/straightness."
what are you referring to specifically, the gap for handle fitting? or something else?

i think the human eye can easily distinguish a .003 gap
 
suppose you make a full tang hunting knife 8 inches long and lay it on a surface plate. how much of a belly or hump will you send out the door. I have several blanks started as shown here http://www.bladeforums.com/forums/showthread.php/1408115-S30V-machinability . If I lay them on a surface plate with the bow up, I can press about .0045 before it lays flat. As these are for hunting buddies I'm not overly worried about making it read .001 or better.

I'm a machinist by trade so its always interesting to discuss tolerance with folks in other trades. I know flat is a relative term meaning a different thing to a carpenter than It would to a grinder.

HSC /// said:
" tolerance related to flatness/straightness."
what are you referring to specifically, the gap for handle fitting? or something else?

i think the human eye can easily distinguish a .003 gap
Click to expand...
 
Straighter and flatter is a sign of quality. It has to look straight when sighting down the blade. For users for buddies, not as important, but if you put your mark on it, it's a sign of your quality and skillset when others look at your work. I use two marks, one for the economy series, for users who just want a quality heat treat, but aren't looking for a showpiece, and my full signature for the ones where people want to show off fit and finish.

When you taper the blade and tang, a granite plate doesn't work.


The abs doesn't measure straightness. It has to look straight.
 
wizzer said:
suppose you make a full tang hunting knife 8 inches long and lay it on a surface plate. how much of a belly or hump will you send out the door. I have several blanks started as shown here http://www.bladeforums.com/forums/showthread.php/1408115-S30V-machinability . If I lay them on a surface plate with the bow up, I can press about .0045 before it lays flat. As these are for hunting buddies I'm not overly worried about making it read .001 or better.

I'm a machinist by trade so its always interesting to discuss tolerance with folks in other trades. I know flat is a relative term meaning a different thing to a carpenter than It would to a grinder.
Click to expand...

ah ok, now I understand
I'm not a machinist but run a CNC aerospace machine shop, so I feel your desire for quantifying a measurement or known criteria,
But in this knife-making trade, I think it's more important to train your eye.
You know when it looks straight and you know when it doesn't

nice work on the machined bevels, not my interest but I can see it's well done. :-) :thumbup:
 
The flatter the better imo

An acquaintance of mine knew I was into knives and wanted to show me his "custom" he just bought to see what I thought of it.

It immediately made me realize what some people consider 500! Worth of knife when I thought it was about 100 lol

My first response was if he wanted the bend in his blade for some custom reason ;).

Edit but it was worth it to him , as long as it's got a buffed edge "that shaves hair" lol

I get it if your starting out and freehand grinding .

But I have to be frank as I don't know how else to be .
I have bought a couple knives from a maker on this forum .
They look great in pictures
In reality they are pretty horrible :( the grinds twisted ,plunge not square across, scales unevenly contoured epoxy staining . The list could probably go on if I look at it more.

Am I going to buy from the maker again ...no.. would I let my product walk out my door in that state .....no .

It did teach me a lesson , which is to ask to see more pics if I'm interested in. Custom work :thumbup:
It will also serve as a Guage for my own work. When I start nit picking myself I can always look to it for motivation ..... as I would NEVER want someone to look at my work and feel as if they got hosed.

Paul.
 
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I agree with HSC///, there is a responsibility to train your eye to see straightness. I check on granite, but it only ever confirms what I can already see. I sold a tomahawk blade on here a few days ago without making any money on it because of a bend that both my dad and brother decided wasn't there :rolleyes:
Bottom line, if there is a perceptible issue, it's an issue. It's my job to have the best perception I can. Willie makes a good point about catering to customers at the same time.
I also think straightness is probably more valuable than flatness, but a knife should be both.
 
My heat treat process causes more warping and more stubborn warps, so I've had to learn to deal with this. There are techniques for correcting warp that is still present after quench during the temper that are well known and what I do. But on a 16" blade I can have .020 warp as measured with feeler gauges on a surface plate that I let go. That's about the limit.

There is no such thing as a perfectly flat knife (or anything else for that matter), only a matter of how flat and ability to perceive and/or measure. I sight down every blade from both directions to try to see warp, and I check each blade on a surface plate. If I get it where it isn't visible and it's under about .020 on the surface plate it's good-to-go.




edit to add: obviously I wouldn't accept .020 on a shorter knife. .020 across 16" isn't a lot, but across 8" it would be pretty bad. You can't just give an absolute limit, the scale of the work piece is relevant. For example, I think a full 1/16" wouldn't represent a problem on a big sword.
 
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Park Swan said:
I agree with HSC///, there is a responsibility to train your eye to see straightness. I check on granite, but it only ever confirms what I can already see. I sold a tomahawk blade on here a few days ago without making any money on it because of a bend that both my dad and brother decided wasn't there :rolleyes:
Bottom line, if there is a perceptible issue, it's an issue. It's my job to have the best perception I can. Willie makes a good point about catering to customers at the same time.
I also think straightness is probably more valuable than flatness, but a knife should be both.
Click to expand...

If I may say I saw your ad before this thread .
YOU did it right and honest .
You had full disclosure .
You took the high honest road and I would have bought it myself :thumbup:
Nothing wrong with makers trying to recoup their costs , as long as they are honest which imo you were
 
wizzer said:
could end up spending a lot of time trying the eek every blade back to perfect.
Click to expand...

This statement makes me grit my teeth.
I'm never going "back to perfect". I'm always going 'toward' perfect.

If we're making Damascus billets or forging down from large stock, our blades are never perfect to begin with. We can't 'go back'.

I didn't just start making knives a couple years ago. It's been 20 now. I see a lot of people always professing how "normalize, normalize, normalize' is the solution to keeping blades straight during heat treating, and that is just a tiny, tiny bit of the equation.
I have many times found that even, equal symmetrical grinding, even heating and not over heating blades is, if not equal, but greater in importance than normalizing when keeping blades straight.

I'm a jig freak. I use my surface grinder and surface plates religiously. And even then especially when filing the handles to shape, I use a height gauge and jig that holds the knife equally from side to side to check straightness. They must be straight and in line with the blade or you've wasted your time on the blade.


I'll never make a perfectly straight knife, but I use every advantage I can to at least try.
 
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