Buying steel in different forms

bodog

bodog

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Can someone explain some of the things to consider when buying steel? I'm referring to some of the way steel comes from the supplier. Belt ground, precision ground, decarbed, saw cut, shear cut, etc. Some of these are significantly more expensive than others.

Say I wanted shear cut and decarbed steel because the price is right. Is there a giant difference between that and belt ground, shear cut?

If something is cut with a saw, how much extra steel would there be rather than getting it shear cut and simply grinding/cutting off the small piece that's uneven because of the shear?

Precision ground would pretty obviously be best but holy crap, it's like an extra 50 bucks per bar. And is it really that hard to grind off the crap?

Thanks in advance.
 
It depends on what you want and how you make knives. If you forge it doesn't really matter. If you're buying steel for stock removal there are major differences. Decarbed steel has been sandblasted and has a very uneven surface. Blanchard ground steel is what I buy when its available. It is decarb free with a nice surface but it isn't dead nuts perfect from one end to the other like precision ground.
Saw cut usually has cleaner edges than shear cut. A lot of suppliers are starting to use waterjets to cut sheets into strips. I really like it better than either sheared or saw cut.
 
Darrin Sanders said:
It depends on what you want and how you make knives. If you forge it doesn't really matter. If you're buying steel for stock removal there are major differences. Decarbed steel has been sandblasted and has a very uneven surface. Blanchard ground steel is what I buy when its available. It is decarb free with a nice surface but it isn't dead nuts perfect from one end to the other like precision ground.
Saw cut usually has cleaner edges than shear cut. A lot of suppliers are starting to use waterjets to cut sheets into strips. I really like it better than either sheared or saw cut.
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If it's getting CNC'd would it matter? The sheared portion would be what gets clamped into the machine and the rest would be cut off, right?

Say I ordered .187 bar stock and wanted a .170 thick knife and it's going to be CNC'd then shear cut/saw cut and decarbed/not decarbed wouldn't matter, right? It wouldn't need to be precision ground, either, as the CNC machine would cut off any imperfections better than a grinder could, right?
 
The sides need to be straight, clean and parallel to hold in the CNC. So I stack them up and mow them down, 20-30 at a time. Then they're deburred.

The problem with shear cut isn't a small piece that's uneven because of the shear, it's the bar is bent. You can straighten it, but it often shows up again in HT unless it was straightened by stretching (which doesn't create a residual stress gradient across the section) but most places aren't set up for that.

Most "precision ground" isn't precision ground, it's run through a TimeSaver, which is basically a big belt sander. The time and cost to clean up a surface vs having clean material to start with is pretty significant. I usually try to get ground material when I can.
 
I don't understand the "let's say" part in your questions...
You don't buy a bloom of steel whitout shape, well the market offers you different options and you choose to pay for the work saved in your shop according to your workflow and needs.
 
If it's getting CNC'd I'd say it won't matter too much what you start with. As Nathan mentioned, I put a stack of blanks in the vise and planed the edges flat first. If you're sending the job out to a shop to be done, they'll figure out how they want to set the part up and all the steps they need to do to complete the job.
 
bodog said:
If it's getting CNC'd would it matter?
Click to expand...

If you're running the CNC machine, then you should know from prior experience. If not, buy some mild steel in various forms and make some chips.

If you're not the one running the CNC machine, then this should be a discussion to have with the machinist or the shop rep.

Milling (CNC or otherwise) is not a magic process where you put in a warped piece of steel, press a button, and get a perfectly flat/straight/square part - especially when you're dealing with parts that are long and thin, like knives. Material stress = warp. Milling can both induce a lot of stress in the metal, and release or reveal a lot of existing internal stress. Grinding is always going to induce less stress than milling.

Are there ways to workaround the problems associated with machining warped stock? Sure. Is the cost of those workarounds less than the cost of buying straight/square stock? Maybe, maybe not.

Also, it is important to understand that a sheared "bar" is almost always sheared to width and length. The width shearing is the real problem - it causes a twist down the length of the bar. The twist is not something that is simply milled or cut off.

As others have said, precision grinding is primarily about dimensional accuracy (size, perpendicularity, flatness, etc). Belt grinding (e.g. 'timesaving') is primarily about the quality of the surface finish.
 
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