Blued Steel Defect on Freshly Ground Shaper Knives

Bluing of the steel in a new or reground knife is a defect that indicates damage to the temper of the steel. December 31, 2013

Question
Amana Profile Pro head with 40 mm blank knives to match custom profiles. Is it normal to have this much burning on the knives? I had a shop custom grind some knives for me and the edges are blued and they are telling me it is normal. I am concerned they will dull extra fast. Does anyone know who else custom grinds this type of shaper knives?


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Forum Responses
(Solid Wood Machining Forum)
From contributor J:
I can't say for sure, but no, I do not think that is normal. They don't come from the manufacturer blued, nor do custom ground corrugated knives come with that bluing/burning. So why should they get it from being ground properly elsewhere?

I believe FS Tool still sells these knives, possibly others as well. There's also a fair supply on e-bay for them. Unfortunately I don't know who grinds them - maybe give FS Tool and/or Connecticut Saw a call. If you're feeling adventurous, I started grinding my own profiles and it's not too bad. As the steel is somewhat thinner than the corrugated I use, I've had good luck free handing simple profiles on my shop grinder. It takes a bit of time and patience, but it's doable.



From contributor M:
Not enough coolant from under while grinding to keep the knives cool, resulting in burning, which in turn takes the temper out of the steel. You can hone it off, but the fact remains about the temper of the steel - it is now prone to chipping in the burned areas. Burning on the edges of knives where the actual cut was made is somewhat normal from the use of chop saws, but it should not be in the profile area.


From contributor O:

Can't tell in the picture if the cutting edge has degraded, but bluing is simply the steel's reaction to heat. When tool steel is heated up and cools down slowly, it tempers not hardens. Your cutters are now more susceptible to dulling and not chipping.


From contributor A:
I definitely would not pay for those knives. Any of the blue areas have lost their heat treatment. He ground them too fast or did not use enough coolant (or did not use coolant).


From contributor C:
These 40 and 60mm universal knives are pretty common. They are okay for one off and short custom runs. The problem is the steel is very low quality, dulls fast and is not the best quality cut. They are less expensive than corrugated in some cases. I have heard a better knife is in the works but have not seen it. Amana gave me names of a few grinding shops that custom profile these. Give them a call. That does look like a bad grinding job.


From the original questioner:
The shop that did these was the only shop that was highly recommended by Amana. Am I wrong to expect knives that are not burnt?


From contributor S:
You can have that same knife made in a better grade of steel.


From contributor U:
Those knives look very poorly done. Discoloration means the steel has lost temper. Maybe it's a byproduct of low quality steel in the first place or over aggressive grinding, but regardless, it's not right.


From contributor B:
While I am new to woodworking, I am a retired tool and die maker with over 43 years experience.

I can tell you for sure this is a bad job grinding. Whenever you turn the steel blue, you lose carbon in the process. Basically you have changed the structure makeup of the steel. You either have to grind the part of the steel off that has turned color, or heat treat the steel again, depending on what type of tool steel it is. I have seen a lot of cutters that use A2 tool steel, and it's a good steel that heat treats very well. If it is another type of steel, like S7, you actually create tiny fractures of extremely hard and brittle pieces on the edge, and it will flake right off and dull very fast. O1 steel will just dull fast after submitted to this type of abuse. During the tempering process, the steel turns brown, not blue.

This is the result of someone trying to do the job too fast, and taking off more stock than they should have. Itís sad that any shop that has pride in their work would let this job out the door. It's just sloppy work and it may be normal for a bad shop, but not a good one.

The only other time I have seen work like this is if the original tool was made and not enough grind stock was left on it. Some steel warps worse than others in heat treat. You have to leave the tool oversized and grind it to size after heat treat. If not enough stock is left, you get the blue areas from it not cleaning up in the grinding process, but that only happens on new tooling.



From the original questioner:
I ended up sending the knives back to be reworked. I received them back to me over a month later with rust spots on them. I was overcharged on the bill from what I was quoted on the phone. After numerous calls and emails to the owner, I had to call my credit card company to get the refund. They insisted that the burning was very normal for all their knife grinding. I have used these knives and they do seem to get dull fast. Also, the first grinding the knives were the same width and lined up perfectly. When they sent the reworked knives back to me, they were different widths so that only one knife is finish cutting. Very poor service! Wish I had the means to grind my own knives.