Here are a few tips for dealing with some of the places where things commonly go wrong.
Dealing With Warpage
Sometimes the blade comes out of the quench crooked. It can be straightened hot, above roughly 500 degrees, immediately after the quench. Use welding gloves. Once it’s cool, it can be straightened during the temper cycles. Clamp the blade to a piece of steel that is flat. Use shims if necessary. Run through temper cycles as usual. Sometimes takes a bit of trial and error, but works almost every time. If you want a two-piece knife, go ahead and try and straighten one cold by putting it in a vise or three point jig. High risk of catastrophic failure with cold straightening.
Edge chipping is an indication that your edge is too brittle, either from poor normalization prior to quench, too high a temperature going into quench, or from being too hard. If the edge is chippy, try tempering 25 degrees higher. May not be able to fix if brittleness is from poor temperature control during the austentizing heat.
Edge rolling is an indication that your edge is too soft, either from failure to harden or from tempering too hot. There is no easy fix for this one. Re-heat treat.
Overheating during heat treatment often results in the blade not hardening correctly due to grain growth. The overheated blade can often be rescued by normalizing prior to quench. Overheated blades can be soft or brittle, with large grain.
Overheating after heat treatment is typically from grinding with too much heat. If the edge turns blue, you’ve over-heated. Overheating after heat treat makes the knife edge too soft. Re-heat treat, and dip the blade in water next time to keep it cool as you grind.
Tempering too long happens when you forget you have a knife in the oven. As long as the temperature remains steady, there are no ill effects.
Tempering too hot results in a soft knife. Most of the time two cycles are recommended. Do the first one 25-50 degrees below the second one. If you over-shoot, you have to re-heat treat.
Knife didn’t harden
-Not enough carbon in the steel. Is it a known steel, or something you found somewhere?
-Incorrect heat prior to austentizing. Did you normalize?
– Wrong austentizing temperature. Do you have temperature control? Try 25 degrees higher. If not, use a magnet or table salt to get closer.
– Wrong quench for the steel. Not every liquid will harden every steel. Your steel type must match what you quench in. Use a faster quench liquid.
-Layer of decarburized steel on the outside of your knife due to forging. Grind in a little further and test again.
Divot in the blade flats, usually about 2” from the plunge lines- This is caused by tipping the blade toward the point, usually because of trying to focus on the plunge. You’re going easy and careful on the plunge, and the other side of the belt digs in. You can’t polish it out. Go back to at least 120 grit and grind it out, keeping the blade moving to avoid any divots.
Handle material burns or discolors while shaping- Light materials like maple, or hard materials like oak or osage, or oily materials like desert ironwood are the worst. The common causes are a dull belt, or too fine a belt run at too fast a speed. Switch to a coarser belt, a fresh belt, or a slower speed, and use a lighter touch.
Gaps between the tang and handle are the result of surfaces not being flat. Either the tang isn’t flat, or the scales aren’t flat. Go to a flat platen, disk, or surface plate and smooth everything up.
Scratches still visible after polish are the result of a poor underlying grind. Go back up to at least 120 grit and polish again. Make sure and get all the previous grit scratches out before you progress to the next grit. If hand sanding, make sure you are switching directions with each grit change, and make sure you have good lighting.
A dark ring around handle pins is either the result of oversized or out-of-round holes, where the glue fills the gap and shows a ring, or is the result of grinding your pin stock down too quickly so that the heat burns a dark ring in your handle material.
Uneven plunge cuts are a sign of beginner’s level work, and even plunges are one of the telltale marks of a well made knife. Consider using a file guide. Make sure you scribe a center line on the blade before you grind. Consider grinding your final plunge cuts after heat treatment. Above all, practice! This is one of the most difficult parts to consistently get correct. Perfect plunges are the result of practice and attention to detail, and are never “easy”, even with practice.
Handle pins or bolsters sticking out further than the surrounding handle material are the result of hand sanding with no backing, or with a soft backing. The sandpaper cuts handle material faster than it cuts pin material. Try a hard metal backing to your sandpaper, and sand from metal to wood, not wood to metal.
Handles popping loose are typically the result of poor surface preparation. Oil on the tang, including fingerprints, makes the glue not stick. Use acetone or denatured alcohol to clean the tang, and rinse with clean water. Make sure your tang and scales are flat and clean. Don’t squeeze all the glue out of your joint, and use good glue.