How hard?

[paulthefish2009]

Morning all,bit of advice needed, currently making a knife for myself using an old file,at the moment after softening the blank it is measuring 30-31 rc (302-310 hv) what sort of values should i be looking for when finished?

[dave budd]

depends what you intend the knife for and how you like to sharpen it ;-) if it's a throwing knife then go mid to high 40sRC, if its a craft knife/scalple/razor leave nearer the 63RC, for most general purpose knives somehwere between 55 and 61 is best (obviously the softer end will need sharpening more often but will be more durable than the higher end. Geometry of edge makes a difference

horses for courses as they say! But most people would opt somewhere between 57 and 60

[Bernie]

How do you measure rc/hv?

[paulthefish2009]

luckly i work for a engineering company and we have a pretty sophsticated machine in our inspection dept! Basicly it presses a diamond shaped carbide stylus into the surface with a known force, then you measure the indent left in the steel and the machine outputs the hardness in either the rockwell scale or vickers.

[Whistle]

luckly i work for a engineering company and we have a pretty sophsticated machine in our inspection dept! Basicly it presses a diamond shaped carbide stylus into the surface with a known force, then you measure the indent left in the steel and the machine outputs the hardness in either the rockwell scale or vickers.


Thanks for that !!! I was wondering too !!! Cheers Whistle

[paulthefish2009]

Thanks for that Dave,just got to add to those who think about great big dents in there blades, the indent made is only about .1mm across, you need a microscope to measure it. If anyone wants something tested i will gladly do it, the test only takes a few seconds so its no problem

[CanadianMike]

Saw this just after getting up with barely a sip of coffee into me, hovered over the new topic after seeing the words "How Hard?" then the first word I see after is "Morning", and had a funny thought flash across my brain...... ya, tent pole/kickstand kinda thing. :D

Anyways............ I have a portable electronic RC tester at work that I've used on a couple of my knives, is difficult to get a trustworthy reading off of. Pain in the ass really, a file is much easier. Lol

[dave budd]

all hardness testers have their limitations and proper uses.

The portable ones are OK for big stable objects like anvils and machine beds, but small things like knives just flex/move too much.

I used a modern micro-hardness tester at uni for a while and it was great for measuring the hardness of materials in a very specific location, but you couldn't use it on something hetrogenious like wrought iron or damascus for example coz it would read each layer differently (and each layer flexes and compresses indipendently as they are slightly different hardnesses, so the reading is all over the place).

I have a solid old fashioned Rockwell tester from the cast iron and brass era rather than the white paint and somputer read out style (it's just as reliable but more rodust in a non-lab environment! and is a bit more simple on fuctions). Simple to operate, accurate enough to read and they come up second hand quite frequently

A nice flash modern one from a lab such as Paul most likely has access to will read in a range of scales (like kg and lbs, the hardness of metals is read in several scales including Vickers, Rockwell, Knoop and Brinnell, then the rockwell has 3 scales for different materials). Then there are different shaped indenters and loads applied to them for different materials, etc. Alot of the modern machines will read several scales with quick change indentors and the ability to caluculate all the right things as load is increased/decreased by pressing buttons ( on mine you hang more or less weights on the back!), they have built in microscopes with reticules so you can actually see what you are testing as well as the hardness read out, etc, etc. Lots of whizzy features that make life in the lab easier and more accurate for all :).

then there are hardness files. They are little files that are heat treated to a known hardness (with a chart on the inside of the box and colour coded handles). You simply try to file your object and if the file cuts then it is harder than the metal, so you try the next softer one. repeat until the files don't cut and you know that the metal is between the two files. So for example the 60RC file cuts easily, but the 55RC skates. The chart in the box says that the actual hardness of the files is 61.5RC and 57.2RC, that narrows it down so that the metal must be between 57 and 61, then you can narrow it a little from experience; so if the 60 only just bites then it is nearer the 60 mark and if it cuts easily then it is closer to the 57. They are a little subjective but good for ballpark figure and are less prone to the problems of poor sample preparation that proper machines need, also new files will cut harder things than slightly work in files and they work when a dull file says the metal is soft! But the sets are about £40 instead of £2000! These are a good buy for bobby knifemakers IMO

You can get a simimlar subjective view of hardness with normal files, especially if you have a brand new one of a brnad that you are very familiar with. It will be of a certain hardness and you can judge VERY approximate hardness by how easily it bits. very subjective though!

The proper machines are always the most reliable , but they do need the samples to be prepared well. Like I said above, mine (and the micro hardness that I used before) requires the sample to be flat and have paralel surfaces, the surface finish makes a huge difference too. If I take a 1/4" slab of steel and finish it to 200grit, 600grit and a 1000 grit paper I get three diferent readings with a range of 10 points on the RC scale! When you get down to very fine tolerences (ie not knives) then humidtiy and temperature make a difference too. The samples I tested at uni were small bits of metal mounted in a cold cast resin block and the metal ground and polished in a metalographic lab, so everything was flat, hard and smooth with only the metal itself for variation (the resin is very strong under compression, and at 1" thick nearly wasn't going to flex!)

All very interesting to know the hardness of your blade, but its very relative in the real world I think :D