The NMS (Non Medical Scalpel) project

[Merwe knives]

I've had a lot of interest from a lot of people in the small scalpel like knives I posted with the knives I made during the three months I've been busy with the knife making hobby.

To recap; The small knives came about by accident, I salvaged a hunter blade I messed up figuring out the grinding thing. The little knife I made from the hunter blade proved so handy that I made another.

I received input from BB guys and local guys interested so decided to re-design the knives putting all the best features together. Let's call this scalpel like knife the NMS (Non Medical Scalpel) has a nice ring to it :

-Blade angle in relation to handle allows for lots of control for precision cutting while maintaing compactness.
-Sharp blade point for removing metal shavings from fingers, cutting stencils and doing fine carving.
-Cutting edge runs off blade at choil so slicing through something bigger than the blade is long will not see it stuck in whatever is being cut. This type of choil also makes blade sharpening easier.
-A lot of guys indicated that the NMS will do kitchen duty so the cutting edge is 10mm longer.
-Stainless pins (also HairyDave's idea) and epoxied handle scales will leave nowhere for food to get stuck and the open simple lanyard hole will be easy to keep clean.
-Lanyard hole so the knife can be used as a neck knife.
-A hollow grind on 3.25mm steel will provide a cutting edge that can be sharpened like a razor.
-3.25mm steel will leave a strong spine above the hollow grind and provide a strong full tang for such a small knife.
-Well proven Bohler N690 hardened and tempered to 59 Rockwell will provide a tough durable stainless knife.

I have to fire up the heat treat kiln this week to harden the hogspear I'm busy with so decided to make a first prototype of the NMS by stock removal and share with all interested and all who provided valuable input on the project. Once done I'll transfer the design to CorelDraw to have a batch lasercut.

Tracing onto 3.25mm N690 and cutting from the bar:


Almost there; Removing metal using a fresh 60grit ceramic belt, dull belts only create excessive heat while not grinding much and great care must be taken not to overheat or burn the steel.

Flat grinding the blank to remove scale and marks while using a welding magnet to save skin from getting nicked by 60grit sandpaper:

With the profile done some handling of the knife to test the 'feel' before proceeding:

Test passed, feels good, handles well

The holes are drilled for the handle pins and more holes to remove metal and weight where it will not influence strength. All holes are chamfered to prevent stress risers that might cause microscopic cracks during heat treatment. Another advantage of the weight reduction holes are that they provide an epoxy bond through the handle between the scales.

Next up will be the rough grind then heat treat.
Thanks for looking, to be continued...

[ANDYLASER]

That looks like an excellent design for a scalpel.

[TallNHairyDave]

Oh yeah, I'm liking the look of that a lot

I think you've got a winner here!

Will be watching this with much interest!

[gorilla]

Oh yeah, I'm liking the look of that a lot

I think you've got a winner here!

Will be watching this with much interest!

absolutely! Looks like i'll be PMing you about one of those top ones

[TallNHairyDave]

Well, the Mrs has now seen the design, and she wants one too...

[Darkwoodecho]

will be nice to follow the progress of these blades all the way through, thanks for posting

[Larsanders77]

That is indeed a very lovely shape - I look forward to the next installment!

/Lars

[S.C.M.]

Great! looks like it'd be a good carving knife, and brilliant for kolrossing/barkrossing

another project idea for my to do list...cos I don't have the cash to buy one



if you'll let me copy/modify that design, that is
not that I'll get round to doing something like it any time soon

[Merwe knives]

I had lucky break today, a client cancelled a meeting and was rather suspicious about my joyous response. I had some knives to make but that would be difficult to explain to an art director so I just thanked her for cancelling and rushed out to the workshop
The 'prototype' multiplied and I managed to make 3 more by stock removal before starting up the heat treat kiln. 2 of the total of 4 already have owners and I'm hoping to get to keep one for myself.
I've decided to make these knives by stock removal instead of having them laser cut. That way they won't be 100% identical even though you'll have to put them side by side to spot the difference.

I had the hogspear I'm busy with and two ordered knives that also needed heat treatment so it was worth firing the kiln.

The knives are wrapped in special stainless foil and a cigarette butt included in the parcel. When the parcel is inserted into the kiln's 1080degC atmosphere the cigarette butt ignites and uses up the oxygen in the parcel. The oxidation of the blade is kept to a minimum this way, at those temperatures even stainless steel actually oxidises leaving scale and stains that need to be removed after heat treat.

The kiln was a major capital outlay but I view the precision of the heat treat process as vital to the final product quality so prefer to have full control. A micro processor controls the kiln temperature by 'learning' the kiln's characteristics. This way it slows down the heating just before the set temperature is reached to ensure it does not 'overshoot' the critical temperature.
The 'parcel' and the kiln:


Our NMS's are N690, 3mm thick so each had a turn to spend 6mins in the kiln after the kiln was up to temperature at 1080degC. This 'soak' time depends on the thickness of the blade, general rule is one hour for every inch of thickness. Once the 6 mins elapsed the envelope was removed and the blade quenched in special quenching oil to reach 600degC within at least 2 minutes. Once cooled enough to handle with gloves the blade is checked for warping by putting it flat on a marble slab. If it warped it is bent straight immediately. This can be done by hand, the steel is still malleable. It only stays in this state for about 15mins before it reaches 60+rockwell, hard enough to shatter if dropped.

The temper must now be 'drawn' to make it tougher and less brittle. To get our NMS blades to 59rockwell they need to be tempered at 175degC in two one hour sessions with a cool down to room temperature in between. Since I have a total of 7 blades to temper I'm doing it in the kitchen oven Pot roast is not as temperature sensitive as my blades so I'm using my multimeter with temperature probe to monitor the temperature of the kitchen oven, ( I know I'm a control freak) but the oven's thermostat is almost 12degC out of truth


The times and temperatures might sound confusing but the beauty of working with quality steel is that the manufacturers supply all the metallurgical data so there's no guesswork involved to obtain optimum results.

I'm sitting out the second one hour temper session now as I type here, will have an update soon!

[Merwe knives]

Babysitting the blades in the oven is done. 4x NMS's heat treated and tempered, can work on them over the weekend

Off to bed for now

[Darkwoodecho]

Excellent, good WIP very informative, keep it coming

[tomancik]

Excellent, good WIP very informative, keep it coming

+1. Really very interesting to see the details + to read your thoughts.

[Manofkent]

Wow, Great stuff! Quick work to!
Cant wait to get my hands on one of these. Might be worth doing a group buy.

This is going to make a lovely addition to my kitchen. I love small knives for rabbit and chicken processing.

[Merwe knives]

Thanks for all the interest guys!

We are a step further with the NMS's, I've cleaned up the blades after heat treat and started the final grind on one I planned to put titanium scales on and sandblast finish.

These blades are going to be scaringly sharp, a 15mm grind height on 3mm steel with a hollow grind from a 300mm wheel... They are indeed going to be over sized scalpels. We need to keep this in mind during use since chopping with or abusing the blade might damage it because of the thin grind. They are going to be very sharp cutting tools for precise cutting work, not machetes.


I have this piece of titanium I bought with other knife making supplies three months ago when I started out. I have a titanium bracelet and find it an interesting 'modern' material. So I optimistically set out to 'quicly' rivet titanium scales to a NMS. Needless to say, the time it took to shape and drill the titanium I could have fitted 6 wooden scales. The stuff is tough and weird to work with! It feels as light as aluminium but breaks drill bits and makes bright white sparks with 'popping' sounds when ground

Called it a day for now, will sandblast it tomorrow. It looks nice as is but too 'bling' to my liking, the sandblasting will dull it and give it a monotone 'metal on metal' look.

Will post pics of this one when finished tomorrow night.

[jonofdoom]

Nice... I like these. Look very useful.