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Making Plane Blades #1: Fabrication and Heat Treatment

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Blog entry by bobasaurus posted 07-06-2015 10:40 PM 2557 reads 10 times favorited 14 comments Add to Favorites Watch
no previous part Part 1 of Making Plane Blades series Part 2: Etching a Logo »

For the “Shop Made Tool Swap - 2015: Hand Planes and Spokeshaves”, I decided to make two planes (one to keep, one to swap) including the blades. I started with a 1/8” thick 2.5” wide precision ground flat bar of A2 tool steel that I happened to have on hand. After some requests for more information, I decided to start this tutorial blog for making them.

You can buy tool steel pretty cheap from MSC:

O1: http://www.mscdirect.com/browse/tn/Raw-Materials/Flat-Stock/Oil-Hardening-Flat-Stock?navid=12101782
A2: http://www.mscdirect.com/browse/tn/Raw-Materials/Flat-Stock/Air-Hardening-Flat-Stock?navid=12101777

Other sources of tool steel include Online Metals, Jantz, McMaster Carr, and Grainger. Be sure to buy the precision ground variety to avoid flattening hassles later. Blade thickness is up to you… most are between 3/32 and 1/4” for wood-bodied planes, I believe. Tool steel bars come in an “annealed” state, which means it’s soft enough to machine. We’ll heat treat it later.

These are the two most common steel types for plane blades. O1 is easier to sharpen and can hold a delicate steep (25 deg or less) edge without chipping, but the edge wears quickly so sharpening is needed more often. A2 will hold an edge a fair bit longer than O1, but is harder to sharpen and can have large carbides that tend to fracture on a steep edge… most sources recommend a 30 deg or higher bevel for A2.

I started by cutting the blades out of the A2 stock with an angle grinder and thin metal cut-off wheel:

I just winged it on the blade length… as long as they’re tall enough to protrude from the plane body and wedge, they’ll be adjustable with a mallet and work fine. I then sanded the rough edges square-ish on a stationary belt sander using a miter gauge.

To grind the bevel, I used my slow-speed 8” bench grinder and freehand ground to the squared edge I made earlier, leaving about a 1/32” edge thickness to prevent thin steel from burning/warping in the kiln:

I dipped the blades in water from time to time to cool them between grinding sessions. The angle is somewhere between 25 and 30 degrees, we’ll see if the A2 can hold up to this steeper angle… if not I’ll grind it back a little after heat treatment.

I then lapped both sides of the blade flat-ish, as this is much easier in the soft annealed state than after heat treating. I used this grinding jig on my stationary 9” grinding wheel for the bulk of it, then sandpaper on granite for the rest:

This step could probably be avoided if you were careful not to bend/warp the precision-ground stock when angle grinding.

Next up is heat treatment. Most tool steels shouldn’t be exposed to oxygen during the hardening, or the carbon atoms will react with it and burn away. This is called “decarburization” and can pit or even ruin tool steel in bad cases. To prevent this I first apply two coats of ATP-641 anti scale coating, a product used commonly for hobby knife making:

http://www.brownells.com/gunsmith-tools-supplies/metal-prep-coloring/heat-treating-accessories/anti-scale-coating-prod23076.aspx

I just paint it on with an acid/glue brush, wait a few hours, then re-coat. This is a thin clay product that prevents oxygen from contacting the steel while heating.

For hardening, I use my Father’s digital kiln that’s normally for jewelry work (precious metal clay, enamels, etc). It is possible to harden with a torch, but you can’t get the “soak” times at the hardening temperatures that the steel alloy’s elements need to go into full solution in the steel. A2 and O1 tend to benefit from this soak more than simpler steels like 1095, as they have many small quantities of rare earth elements that take time to distribute.

Here are the hardening steps for each steel type, which I came up with as an average of many separate internet resources (and review from knife form experts for the O1 steps):

O1 Tool Steel:

preheating: 1250 deg F, keep blade in kiln while warming, hold for 10 minutes
hardening: 1475 deg F, hold for 10-15 minutes
quenching: canola oil warmed to 125 deg F, agitate rapidly up and down (NOT side to side), keep going until bubbles stop and piece is hand warm
tempering: 375 deg F, hold for 2 hours minimum, cool with water, repeat for second temper at 375-400 deg F

A2 Tool Steel:

preheating: 1400 deg F, keep blade in kiln while warming, hold for 10 minutes
hardening: 1750 deg F, hold for 10-15 minutes
quenching: cool in still air to 150 F (able to hold in bare hand) before tempering
tempering: 400 deg F for RC 60, hold for 2 hours minimum

For A2, “quenching” just means taking it out of the kiln and leaving it in room-temperature air for a while. This is easier than dunking in preheated oil for O1. I actually do the tempering in a kitchen oven, as the kiln takes forever to cool down after hardening. I generally get lazy tempering and just do 400 deg F for an hour for both steels without bothering with a second cycle.

Here is the kiln and blades after removal:

Notice how the coating pops off during the quench:

After the kitchen oven tempering, you can see the yellow color of the steel below the remaining coating:

This color change indicates tempering hardness, hopefully I got it somewhere around RC60.

Now comes lapping the blades again to remove the coating and make sure they didn’t warp during heat treatment. This step took forever and I’m still not completely done:

I’m using a Veritas lapping plate here with grit powder, but it is extremely slow and labor intensive. Wish I had a big coarse diamond stone for this.

Next up I’ll work on etching a logo and actually make the wooden plane bodies.

-- Allen, Colorado



14 comments so far

View WayneC's profile

WayneC

12642 posts in 3558 days


#1 posted 07-06-2015 11:07 PM

Very cool. Nice to have access to the kiln.

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

View bobasaurus's profile

bobasaurus

2658 posts in 2645 days


#2 posted 07-06-2015 11:08 PM

Thanks, it is nice to use the kiln. It is possible to build your own, too. It’s just insulated firebrick, a heating element coil, and a PID temperature controller. All three things can be bought online pretty cheap. I might make a big one someday for treating kitchen knives.

-- Allen, Colorado

View ColonelTravis's profile

ColonelTravis

1189 posts in 1355 days


#3 posted 07-07-2015 12:08 AM

Excellent stuff. I’m making my own molding planes, including blades, and in a how-to video I bought you can see the bluing of the steel.

He just grinded away and never took the steel off until it was done. This is before any heat treating. If the steel had been hardened, that end would have been ruined, so I’m guessing it doesn’t matter if you blue the steel like this before hardening?

View bobasaurus's profile

bobasaurus

2658 posts in 2645 days


#4 posted 07-07-2015 12:16 AM

It generally shouldn’t matter if you blue it before heat treating. However, If you get it hot enough to start decarbing the cutting edge, even the heat treatment might not repair it. At least I think so, I don’t have all that much metalworking experience, I’ve just done a lot of googling about knife making.

-- Allen, Colorado

View ColonelTravis's profile

ColonelTravis

1189 posts in 1355 days


#5 posted 07-07-2015 01:13 AM

I think you’re right and the DVD seems to confirm this but it’s something I’ve never read anything about, although my googling has been less than yours.

View Redoak49's profile

Redoak49

1938 posts in 1449 days


#6 posted 07-07-2015 01:15 AM

Do you have any concerns about blade or edge brittleness due the fact that you did not do any cyrogenic treatment. Without it, you will have significant retained austenite with a loss of hardness and increased brittleness.

IMO you might be better with O1.

View Ripthorn's profile

Ripthorn

1406 posts in 2446 days


#7 posted 07-07-2015 01:36 AM

When it comes to heat treatment, prior to hardening, bluing is not an issue, as you have not reach austenite stage yet, so you aren’t ruining anything. The problem with bluing a hardened blade is that you basically have tempered it so soft that it won’t hold a good edge. However, if you do that, you can anneal the steel, re-harden and re-temper, but it is a labor intensive process. Both bluing and decarbing the edge can be abraded past, it’s just a waste of steel. Also, if you don’t heat treat the entire piece (sometimes a piece is too large to be practical for heat treating the whole thing), you could go into unhardened steel.

-- Brian T. - Exact science is not an exact science

View bobasaurus's profile

bobasaurus

2658 posts in 2645 days


#8 posted 07-07-2015 02:04 AM

Redoak, I do have access to cryogen at work, I might just have to try a cryo treatment. I was under the impression that this was not a huge improvement over straight hardening and tempering, though. I might try the edge first as-is then go the cryo route if it might improve things.

-- Allen, Colorado

View kaerlighedsbamsen's profile

kaerlighedsbamsen

1177 posts in 1174 days


#9 posted 07-07-2015 08:54 AM

Great writeup!
With a background in toolmaking i allways felt that any woodworker should have at least basic understandig of metals. The protective coating you use is new to me. Perhaps since most critical hardening on a larger scale is done here in a bath of molten salt that does not allow for any oxygen to enter.

Keep up the good work. Looking forward to read more!

-- "Do or Do not. There is no try." - Yoda

View Redoak49's profile

Redoak49

1938 posts in 1449 days


#10 posted 07-07-2015 12:06 PM

I think it might be a good idea to consider the cryogenic treatment. With A2, you could have up to 20% of it still as retained austenite which has not transformed to martensite. This will limit the hardness that you achieve and could cause brittleness problems.

There are many people who heat treat A2 like you have done and are happy with the results. But it does mean that you do not get the most out of it.

View terryR's profile

terryR

6314 posts in 1769 days


#11 posted 07-07-2015 01:20 PM

Allen, excellent source of info!
You’re a lucky guy to have a kiln…solves many problems.

I’ve been interested in the cryo treatment, anyone read about an average woodworker doing it at home?

-- tr ...see one, do one, teach one...

View upchuck's profile

upchuck

540 posts in 1126 days


#12 posted 07-07-2015 04:03 PM

Allen-
Thank you for writing out about your blade making. I am grateful for the time and effort you took to explain your process in the step-by-step detail. I’ve benefited from your description from sourcing the materials to the final lapping. I will save this blog for future reference. Thank you.
chuck

View bobasaurus's profile

bobasaurus

2658 posts in 2645 days


#13 posted 07-10-2015 10:06 PM

After reading up on cryo treatment, I found it it needs to happen before tempering to have any effect. Oh well, maybe next time. I’ll test these blades after sharpening and see how well they hold an edge.

Edit: Actually I found a source (Crucible’s website) that indicates it may be worth doing a cryo treatment after the first temper then doing another tempering cycle:

“Technically, cryogenic treatments are most effective as an integral part of the original quench, but due to the high risk of cracking, as discussed in the “Quenching” section above, we recommend tempering material normally at least once before performing any cryogenic treatments.”

-- Allen, Colorado

View terryR's profile

terryR

6314 posts in 1769 days


#14 posted 07-11-2015 12:57 PM

Thanks for the research, Allen!
Seems as though when I watch a pro, they normalize and temper frequently.

-- tr ...see one, do one, teach one...

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