Question about bowing of steel

[The_Iron_Joe]

So this might be a little out of left field, but when you guys face off your blades, ie, grind the flats before doing your profiling and grinds, do you find that your steel flexes and bows slightly in the middle? I've been lucky enough to work on a CNC and I've been playing around with some facing routines where I'm not removing much material (just enough to leave a nice finish), but the steel I've been working on tends to bow, so that I've had to flatten it out afterwards. I've been tempted to ignore facing with the CNC and go straight for milling in the blade 'grinds' and profiling the knife out. What do you all think?

 

[Stacy E. Apelt - Bladesmith]

Much blade stock was once on huge rolls. It is straightened for our use, but retains the stresses that made it be curved. With any machining, heating, or grinding, these stressed may be released and cause it to bow. One reason to get annealed steel is to avoid this.

Even with annealed steel, it may bow for another reason. The machining/grinding may work harden one area or side and cause bowing. Most folks do a little on each side and sneak up on the final grind to avoid this. If you do all the work on one side, it is very likely to bow.

 

[kuraki]

What Stacy said. I'd also make sure when milling that you're not inducing stress into the part with your setup. It should be in as close to an unconstrained state as possible, otherwise it will remove more material in one area than another and spring when you release it from the holding method.

When CNC machining knives I've found annealing them first, then making both sides parallel, then milling bevels and profiles, gave me the most repeatable results.

If you mill bevels before making the sides flat and parallel, you will have a difficult time making them even, especially in a machine. Where hand grinding allows you the flexibility to massage your angles and depths on the fly, the machine will only do what it's programmed to do straight and square, so it's easiest to start straight and square from the beginning.

 

[Kevin Powers]

I think that one thing also gets overlooked very often is localized heating causing distortion. When you heat metal in a localized area, the cooler surrounding metal acts as a restraint. The restraining effect ultimately leads to distortion as the metal cools. The first few minutes of this video explain it well. I show this video to my students every semester.
https://youtu.be/2vuGlcbDwKY

 

[The_Iron_Joe]

Thank you so much for the information, guys! Seriously. I hadn't found any real indication that bowing was an issue with annealed steel, as I had annealed / stress relieved the current pieces of steel that I'm working with. I hadn't considered that my tooling was generating too much heat, although this is very likely my problem.

Stacy; I think you hit the nail on the head; I've probably been removing too much material per side, and have not been flipping my work.

Kuraki; Annealing, making sides parallel and then milling bevels and profiles is exactly what I've done with other pieces out of different, thinner steel, and I've had no distortion issues with those since I didn't face off very much material to make things flat. I think that because I removed what I thought was 'not much steel', but was actually too much for the steel to handle without warping, I ran into problems. I'll continue trying to keep things as straight and square as possible.

Kevin; That video is awesome in many ways, and it clearly illustrated my problem, likely localized distortion caused by tool heat... Thanks for the share!


Looking forward, I'll be more careful about the amount of material I remove on each side. Wishing I had a better belt setup, so that I could get my sides closer to flat before approaching the CNC... ah well.

 

[P.Brewster]

To add onto what Stacey said - even if the steel is annealed, it may have been annealed as a giant coil. Then it is mechanically flattened (the industry term is "leveled"). So the annealing doesn't really fix the 'coil memory.'

Heat may not be a factor here. With waterjet cutting, it is very possible to put a flat piece of metal into the machine and end up with a warped cut piece - that is the coil memory.

 

[Kevin Powers]

To add onto what Stacey said - even if the steel is annealed, it may have been annealed as a giant coil. Then it is mechanically flattened (the industry term is "leveled"). So the annealing doesn't really fix the 'coil memory.'

Heat may not be a factor here. With waterjet cutting, it is very possible to put a flat piece of metal into the machine and end up with a warped cut piece - that is the coil memory.

It may not be, yes waterjet can have the piece "spring" up. Happens all the time.

 

[kuraki]

To add onto what Stacey said - even if the steel is annealed, it may have been annealed as a giant coil. Then it is mechanically flattened (the industry term is "leveled"). So the annealing doesn't really fix the 'coil memory.'

Heat may not be a factor here. With waterjet cutting, it is very possible to put a flat piece of metal into the machine and end up with a warped cut piece - that is the coil memory.

Does it knock there head off your jet like it does a laser? :grumpy:

 

[The_Iron_Joe]

To add onto what Stacey said - even if the steel is annealed, it may have been annealed as a giant coil. Then it is mechanically flattened (the industry term is "leveled"). So the annealing doesn't really fix the 'coil memory.'

Heat may not be a factor here. With waterjet cutting, it is very possible to put a flat piece of metal into the machine and end up with a warped cut piece - that is the coil memory.

I took care to anneal the steel myself, shouldn't that eliminate, or at least reduce the risk of the coil memory taking over, causing bowing as I mill down one of the faces?