Sunday, January 5, 2014

Drawknife Rehab - Part 4

Yesterday my day began a little before sunrise with a trip to my good friend Ralph's farm in the flat lands of eastern Indiana to pick up a little hard maple.  It was a beautiful drive with the fresh snow on everything and as the sun came up revealing a crystal blue sky.  After I got a close look at the pieces I think I may have some figured maple on my hands.  These are not ideal for leg blanks for my windsor chairs but I think I will be able to use them for another project.  That of course will be another post.

After an equally beautiful trip home I parked the truck and headed into the shop.  It was time to get back to rehabbing my old drawknife.  This next step, grinding the blade to remove the chip/nick, was the one that was going to determine if this was the end of the trail for this project or if I was going to be able to continue and make this a usable drawknife.

I had several concerns moving forward.  The one concern that loomed greatly in the back of my mind was a matter I had never considered or heard of, until Paul Frederick brought it to my attention in a comment. Hydrogen Embrittlement.  My understanding of hydrogen embrittlement is that atoms of hydrogen are absorbed into the steels atomic lattice like structure which causes the steel to become brittle.  Hydrogen embrittlement may have occurred to some extent because I removed the rust using electrolysis.

Michael Olsen is an electrical engineer that read the post and left this comment:  "I would be skeptical of embrittlement considering the relatively low energy levels used.  Most such brittleness occurs in either high voltage electrolysis of thicker material or combined low voltage and thin sheeting".  Michael gave a very interesting and more in depth explanation in his comment that helped put my mind at ease.

Further research revealed that the passage of time, letting the piece sit for an undetermined amount of time, or baking the piece at 200 degrees Celsius (392 degrees F) for four hours, would cause the hydrogen to be dislodged from the steel.  Since the knife sat over the Holidays it was off to the grinder.

In the picture above is the jig I use for grinding drawknives.  This jig is one that came from the wonderful mind of Pete Galbert.  If you are interested in building this wonderful jig and learn how to use it check it out here.  There is no need for me to try to explain what Pete has already done so well.  The first thing I had to do was to prep the spine of the drawknife.  I filed, sanded, and waxed the spine to ensure that it would slide easily over the hardened steel pins of the jig.  As a side note, since I do not have a designated spot in my small shop to work on metal, I take great care to shop vac, often, the metal filings so that I do not discover them in a future woodworking project.

After preparing the spine I adjusted the jig so that the wheel would be grinding in the middle of the existing bevel.  I measured this to be roughly 25 degrees.  I used no real forward pressure on the knife just enough to keep it in contact with the stone without jumping, and moved the knife back and forth across the spinning stone.  I do use a slow speed grinder.  While I was making progress it was taking quite a long time to get down to the bottom of the chip/nick.  So I decided to employ a method I have used on nicked plane irons in the past.

I used a set of dividers to gauge the distance from the spine to just at the bottom of the nick and scribed a line onto the newly ground bevel.  I then marked the metal that was to be removed with a red Sharpie.  Here is where the process goes against everything that seems correct.  I laid the flat back of the knife onto a grinder table and ground the edge of the knife to the scribe mark.  Zip-A-Dee-Do-Dah and Bob's your uncle, no more chip/nick.  Now what remained was a relatively large flat where a sharp edge is supposed to be.  I highlighted it in red so it would be easier to see.  Now it was back to grinding using the grinding jig.

With a little more time I was able to grind a nice even bevel across the knife and it is now ready for honing.  I was so excited to be able to reach this point of the restoration without the drawknife cracking due to possible hydrogen embrittlement and to discover, at least at this point, it appears the pitting that remains is not deep enough to prevent me from obtaining a good edge.  However, this will only be fully realized when I finish honing.  I was so pumped up over the results I broke out all of my old drawknives that needed to have the bevels reground and went to work.  In a matter of thirty minutes I was done.

The next thing that needed to be done was to make the angle of the handle tangs as close as possible to the same in a position that was comfortable to me.  It turned out to be just about 83 degrees.  I was able to accomplish this very easily without heating the steel.  I clamped the tang up to the bend in my leg vise and ever so gently moved the blade to obtain the desired angle.  I then ensured that the tangs were in line with the back of the blade.

What determines if a drawknife is bevel up or bevel down?  If the handles are in line with the back of the spine it is a bevel down knife.  When in use with the bevel down the hands and wrists will be comfortably in a downward position.  It is not very comfortable, nor is it recommended, to use a drawknife with the wrists cocked upward in an unnatural position.  By laying the knife on its back and then on its bevel, it readily becomes evident as to which way the drawknife is to be used.

In the next exciting episode of Drawknife Rehab, I will be posting on honing the edge, starring the amazing Galbert Drawsharp. If all goes as planned, I will be including the turning and installation of the handles.  So stay tuned for the next exciting episode of Drawknife Rehab.  Yeah I know, my bubble is a bit off.

~ Ray Schwanenberger

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