LumberJocks

Marking Knife - it's all about the steel!

  • Advertise with us
Project by Mainiac Matt posted 06-21-2017 01:54 PM 1089 views 7 times favorited 24 comments Add to Favorites Watch

I’m setting up to hand saw dovetails and needed a spear point marking knife… but the last “name brand” one sided economy marking knife I bought off of amazon some years ago sharpened up very nicely, but then the edge bent over whenever any force was applied to it. The problem? Cheap steal!

I oversee a little proto-type machine shop at work, so I went rummaging through our scraps bin and found a nice little stub of O-1 tool steel and decided to spend a couple nights working late to make my own marking knife blade. I thought you might enjoy seeing some of the steps. I apologize in advance that I failed to take pics of my set ups, and these are really the key to success. But you can get an overview of the process.

Geometry
A quick internet search led me to a 2005 old Wood Magazine article by Chris Schwarz reviewing the top boutique marking knives out there, and he summarized the geometries in a table.

I chose geometry that was in the middle of the pack, 60° spear point, 25° bevel and 0.08” thickness

Machining
After shaping my blank on the milling machine. One advantage of O-1 tool steal is that it machines very nicely in the white (pre-heat treated). Forming the compound angle required either machining an angle fixture and loading it into a tool makers vise, and then mounting the vise on the grinders magnetic chuck at a precise angle, or making a quick set up to shape a precise 25° angle into the surface grinder wheel and then simply clamping the blank into the tool makers vise and positioning it at 30°

The key is to get the final form roughed in prior to heat treating, but leaving ~0.005” material for final grinding, as the rapid quench often distorts the geometry slightly.

Heat Treating
Time to fire up the oven… The process for O-1 (the O stands for oil cooled) is as follows:

> Put the blank into the oven and slowly raise the pre-heat temp to 1,300° F and let the temp saturate the part (doesn’t take long for a small marking knife blade). This step minimizes warping in the next step.

> Bump up the oven to of 1,500° F and hold for 30 min. This transforms the crystalline structure of the steel into the super hard Austenite phase.
> Pull the part out of the oven and immediately dunk it in the quench oil, where it rapidly goes back down to room temp.

> Now the part needs to have the hyper brittle crystal structures (Martensite phase) stress relieved. We do this in a little toaster oven as it takes several hours for the heat treating oven to cool down. 400° F for 30 min.

After all this drama, the part was ready for finish grinding and sharpening (I did scary sharp on a granite plate with 600, 1,000 and 2,000 grit wet dry paper).

I turned the handle on the metal working lathe from a hardwood dowel I had. I cut down and faced off a brass pen tube for the ferrule. I used a belt sander to create the flats so the knife wouldn’t roll off the bench and stab my toe.

Finish was dark Walnut gel stain and a quick spray of rattle can satin lacquer.

The knife is wicked sharp, and gives a nice deep score with just one pull. After many test scores in hardwood scraps, it hasn’t dulled at all. I can apply significant lateral force to the tip and it hasn’t bent or rolled a bit.

So that’s the story.

Of course, I have a couple days of labor into this, and I could have purchased a premium boutique marking knife for ~$45. But what’s the fun in that?

-- Pine is fine, but Oak's no joke!





24 comments so far

View helluvawreck's profile

helluvawreck

26816 posts in 2561 days


#1 posted 06-21-2017 02:02 PM

This is a beautiful shop-made tool.

helluvawreck aka Charles
http://woodworkingexpo.wordpress.com

-- If a man does not keep pace with his companions, perhaps it is because he hears a different drummer. Let him step to the music which he hears, however measured or far away. Henry David Thoreau

View Mainiac Matt 's profile

Mainiac Matt

6922 posts in 2022 days


#2 posted 06-21-2017 02:03 PM

For those who like to geek out. Here’s some info about heat treating steel.

Steel is carbon and iron alloyed together. It exist as a crystal structure. Tool steel has a higher carbon content, as well as other metals added to it.

O-1 tool steel is very common, as it machines well in it’s un-tempered form, and can be reliably heat treated and quenched in oil to get a hardness of 60 on the Rockwell ‘C’ scale. Here’s the chemical composition of typical O-1 tool is:
0.90% C, 1.0–1.4% Mn, 0.50% Cr, 0.50% Ni, 0.50% W, with the balance being Fe

A2 tool steel is air cooled. The super premium tool steels start with D & I really don’t know anything about them, except that they are expensive.

The heat treating process transforms the crystal structure from “body centered cubit (with 8 Fe atoms and 1 Carbon atom in each crystal) to Face centered cubit (with 8 Fe atoms and 6 Carbon atoms in each crystal).

This diagram shows the Fe atoms in red and the C atoms in blue. In reality, the Fe atom is 30x larger than the C atom.

This is done by carefully heating the steel to certain temps for specific times to create a phase change. Here’s a phase change diagram.

The goal is to create the Austenite crystal structure.

Note that these changes in crystal structure happen below the melting point of the steel, while it is still solid.

Then you rapidly cool the steel to lock in that crystal structure.

The rapid quenching in oil creates some hyper-brittle internal structures called Martensite. So you have to “draw down” or “temper” the steel by heating it back up and holding it at the tempering temp.

Clear as mud?

My favorite class back at Moo-U was the material science lab, where we would heat treat, quench and cold roll samples of different metals and then polish them up and view the crystal structure under a micro-scope. So I guess I’m re-living my miss spent youth :^)

-- Pine is fine, but Oak's no joke!

View cmmyakman's profile

cmmyakman

104 posts in 2350 days


#3 posted 06-21-2017 02:03 PM

Awesome! You should be on the “Forged in Fire” TV show.

-- You can't fail if you don't give up.

View Mainiac Matt 's profile

Mainiac Matt

6922 posts in 2022 days


#4 posted 06-21-2017 02:16 PM

I don’t know about that…. but I can definitely see how knife making is very addictive.

For all the time putzing around getting set up, I should have made a half a dozen of the blades.

-- Pine is fine, but Oak's no joke!

View FoundSheep's profile

FoundSheep

119 posts in 150 days


#5 posted 06-21-2017 02:30 PM

Awesome Matt, thanks for going into the technical details, it’s nice to get a good understanding of everything.

-- -Will, FoundSheep Designs

View Monte Pittman's profile

Monte Pittman

25018 posts in 2032 days


#6 posted 06-21-2017 02:30 PM

The more I get into the business of this obsession, the more that quality tools matter. Getting by just to be cheap is usually reflected in the finished product. Good job sir.

-- Mother Nature created it, I just assemble it.

View splintergroup's profile

splintergroup

1287 posts in 916 days


#7 posted 06-21-2017 02:31 PM

Great info Matt! I like geeking out 8^)

I’ve always had an interest in metal working and have often made items and tooling for special purposes. A typical approach would be to get an appropriately sized piece of leaf spring steel, make my shape, then do some seat-of-the-pants torch-tempering and hardening with Kasenit.

I now have access to several kilns with programable temperature cycles and a nice little chart above to stare at 8^)

View buckbuster31's profile (online now)

buckbuster31

131 posts in 209 days


#8 posted 06-21-2017 02:33 PM

If you ever want to make another and sell it, give me first shot!

View WayneC's profile

WayneC

13202 posts in 3791 days


#9 posted 06-21-2017 02:34 PM

And here I am sweating making a simple 1/2” straight cutter for a plow plane from O-1. lol

-- We must guard our enthusiasm as we would our life - James Krenov

View Mainiac Matt 's profile

Mainiac Matt

6922 posts in 2022 days


#10 posted 06-21-2017 02:40 PM

We used to outsource all of our custom machine builds to a bonifide “rocket scientist” (retired NASA mechanical engineer) who ran a small hole in the wall machine shop. This guy was really brilliant, though I had the privilege of interacting with him just a few times. Sadly, he died a couple years ago and though his son took up the torch and continued running the business, he was more of a welder-fabricator than a true machinist and machine designer, so our business relationship waned.

The son’s health failed last year and he liquidated the business. He called me up because he had a couple of boxes labeled with our company name that he didn’t know what to do with. By the time I retrieved the box, all of the premium assets (a CNC Bridgeport, a couple of massive lathes, etc..) were already spoken for, but nobody had yet purchased the heat treatment oven, so we jumped on the opportunity.

Not having to send parts out for heat treating is a huge plus and has freed us up to make replacement parts, design changes, etc… for the custom built machines we run.

I insulated the two car garage attached to our little office building and set up shop with a mill, lathe, surface grinder and oven. It’s not suited for production work, but it is pretty adequate for a proto-type shop and I’m extremely happy that the owners supported the project.

-- Pine is fine, but Oak's no joke!

View Mainiac Matt 's profile

Mainiac Matt

6922 posts in 2022 days


#11 posted 06-21-2017 02:45 PM

seat-of-the-pants torch-tempering and hardening with Kasenit.
- splintergroup

I’ve never done any case hardening… though it looks interesting.

Heck … it’s ALL interesting to me (just ask my wife)

-- Pine is fine, but Oak's no joke!

View Gene Howe's profile

Gene Howe

9151 posts in 3122 days


#12 posted 06-21-2017 02:47 PM

That’s one damned fine knife, Matt.

-- Gene 'The true soldier fights not because he hates what is in front of him, but because he loves what is behind him.' G. K. Chesterton

View Mainiac Matt 's profile

Mainiac Matt

6922 posts in 2022 days


#13 posted 06-21-2017 02:51 PM

I forgot to say… the handle is the very first thing I ever turned out of wood.

I need to get some hand turning tools and set up a tool rest though.

Turning the handles with cutter mounted in the tool holder is OK for straight lines, but not really any good for fairing a curve. Sand paper to the rescue.

-- Pine is fine, but Oak's no joke!

View Festus56's profile

Festus56

1791 posts in 922 days


#14 posted 06-21-2017 03:04 PM

That is nice Matt. Build the best and will last a long time and look good doing it !!

-- Mark -- Really we're not crazy, just "Limited Editions" -- http://www.jmh-limited-editions.com/home.html --

View John's profile

John

738 posts in 964 days


#15 posted 06-21-2017 04:04 PM

That’s a great looking knife Matt, nothing more satisfying than making your own tools. Good point about the flats on the handle, maybe I’ll try that next time I make one, my gouges are always trying to roll away on me. Are you in touch with Bobasaurus at all? I’m really impressed with his tools as well!

-- John, Sunshine Coast, BC, Canada.

showing 1 through 15 of 24 comments

Have your say...

You must be signed in to post the comments.

DISCLAIMER: Any posts on LJ are posted by individuals acting in their own right and do not necessarily reflect the views of LJ. LJ will not be held liable for the actions of any user.

Latest Projects | Latest Blog Entries | Latest Forum Topics

HomeRefurbers.com