How was this kitchen knife made?

September 14, 2008 7:43 AM

[ManufacturingFilter] I have a kitchen knife that looks like solid metal, but it's far too light. How was it made?

The knife is 210mm (8.5") end-to-end, stainless steel, and reasonably inexpensive. I cannot see any marks indicating that the blade and handle are anything but a single solid bit of metal - but were that the case, the handle would be very heavy, which it is not.

I've heard that hollow silver knife handles used to be made by producing two halves, soldering them together, then polishing until the mark was invisible. I can't see any sign of that here, although I'm no expert on what to look for.

posted by Mike1024 to technology (9 comments total)

Care to share a picture or two?

It is possible that the handle is spun and so one piece and hollow. This would require it to be symmetrical about the axis of the knife, however. It theoretically could also have been cast, and the blade inserted with some filler material into one end. Without pictures it is impossible to say.
posted by Brockles at 8:13 AM on September 14, 2008

Crap, I missed the link, sorry. Morning brain hitting me.

There are significant polishing marks on the knife that would easily hide any welds. I suspect the handle is hollow and the blade is welded in. There is nothing about it that suggests to me that it couldn't be of several parts. A proper (especially machine-) weld can be ground down to effectively invisible if done properly. Any mark of the weld (ie a line) would be a sign of poor welding, rather than 'normal evidence of process".
posted by Brockles at 8:17 AM on September 14, 2008

Theoretically you could use super-plastic forming (a bit like molding a glass bottle, but for metals), which injects air or inert gas (usually nitrogen) into the centre of a blob of molten material and makes it expand to fill a mold, leaving a void. The hole could then be capped and the whole thing polished to hide it.

That said, SPF would be total overkill for this process - it's usually used in high-performance engineering like Aerospace and Power Generation because it's often lighter and stronger than welding.
posted by Nice Guy Mike at 8:46 AM on September 14, 2008

Besides spinning it, which would require symmetry, there is hydroforming, in which a fluid can be used to force the walls of a tube out against the walls of a mold to form asymmetrical parts. I know about this only because I used to do some prototyping work for a major faucet manufacturer that uses this process to form faucet spouts. There would have to be an orifice somewhere, but it could be small and easily repairable.
posted by jon1270 at 8:46 AM on September 14, 2008

I have a similar knife, and agree with Brockles. Manufactured in three pieces- two handle pieces and the blade. Weld them together and grind/polish away.

Note that welding is different from soldering- soldering is joining two pieces by melting a third metal that fills the gap between the two. The workpiece does not melt.

Welding melts the workpieces, along with a filler metal that's identical or nearly identical to the original metal, to form them into one contiguous piece. If you do a good job of welding and grind down the seam, it would indeed be identical to having had one single piece of metal.
posted by gjc at 8:50 AM on September 14, 2008 [1 favorite]

It could be made from only two pieces. One half handle and blade, the other half handle only.
posted by weapons-grade pandemonium at 8:59 AM on September 14, 2008

Outline of forging process for single piece steel knives.
posted by parmanparman at 9:27 AM on September 14, 2008

None of those have the hollow handle, parmanparman.
posted by weapons-grade pandemonium at 11:06 AM on September 14, 2008

Here's an article about one brand of all-metal, hollow-handled knives. It talks about the why, the how, and the difficulties of manufacture. Summary: the two halves of the handle are welded together, then welded to the blade.
posted by exphysicist345 at 12:53 PM on September 14, 2008