Sunday, May 29, 2016

Bevel grinding jig by Aaron Gough

When I started toying with the idea of knife making I was mostly put off by the fact that one needs a belt grinder (or so I thought at that time) to be able to grind the bevels and I had neither the workshop space, nor did I want to spend 1500€+ on one. This changed when I have accidentally came across the video from Aaron in which he describes in detail how to make a beveling jig using only very simple tools and materials - at that point I decided to give it a try - thank you Aaron! :)

I am not going to copy in words what Aaron describes very well in his video, I would only add my experience and some slight adjustments I did to the basic design. So before you read on, please watch the original video:


From my point of view the main properties of this jig are the following:

Advantages
  • Faster than a free hand filing. Takes about an hour per side on a 10 cm long outdoor knife blade
  • Fast learning curve
  • Easy to get very consistently ground bevels and plunge lines - much easier than when you start learning how to use a belt grinder or free-hand filing.
  • You can create flat or convex grinds
Disadvantages 
  • Once you get to kitchen knives you need to get more careful about the grinding angles (as these will be as low as 1°)
  • Also - with those low grinding angles (and large contact area while grinding) the file will have a tendency to skate on the blade (it is NOT dull, do not have yourself fooled)
  • Long thing blades of kitchen knife will tend to flex under pressure while grinding and thus changing the grinding angle.
  • Obviously slower than a belt grinder
  • Does not work on hardened blades/steel (the files usually have hardness around HRC 64 - 66 - that is too close to the hardness of the blade, so you would just dull the files without removing much material.

My changes to the original design from Aaron

Aaron pretty much nailed the design. I found that the steel rod that serves as a guide was producing rather loud and unpleasant noise as it was riding in the steel bolt, so I have replaced the bolt with a 10mm thick PVC board with a hole attached to a piece of wood on 2 threaded rods for height adjustment. Now this part of the jig is completely silent.


I have also added a second raw of mounting holes for working on smaller blades and to avoid grinding into the wood when getting closer to the tip.



A small update was adding a 4mm thick rubber ring to the bottom clamp to protect the blade from damage when accidentally bumping to it with the clamp. A simple, but effective solution.



How to grind kitchen knives with this jig

The main difference between kitchen and outdoor knives:

  • Longer AND thinner blade
  • Much more acute grind
  • A simple full flat grind will not make for a great  kitchen knife  - you want either a wide-bevel grind or a convex grind of some sorts
  • Kitchen knives have pronounced distal taper from the handle towards the tip.
The above means - you will be working on a large blade that in non hardened state is not all that stiff. To grind the convex grind will take some basic math to thin of in advance and calculate the angles (and thus the height setting of the bolt) to make.

Before you start grinding make sure you have the centre line (or two lines close to centre that mark how thick the blade should be once it is ground) scribed onto the cutting edge. It is also a good idea to scribe a centre line on the spine so that you can grind the distal taper symmetrically.

Once you have the design and in particular the grind (geometric cross section) on paper - start to work your way down and draw straight lines how you want to get there with the grinding jig. In general you want to start from the edge (with the steepest grinding angle) and work your way towards the spine of the knife in subsequent steps. After each step you want to remove the knife from the holder and measure the blade thickness at few positions (and different distances from the cutting edge) to see whether the grind follows what you have intended. Remember - you can always remove metal if needed, but not put it back on the blade. So proceed carefully. Important is - before you start with the first grind measure the thickness of the blade carefully and calculate how thick the edge should be once you are finished with the first grind.


Starting with first grind on 240 mm gyuto blade

Little tip: In the photo above note the brass screw used as a stop-pin (to keep the blade from moving). Kitchen knife blades are ground pretty thin towards the tip and thinner than any flat heat screw is. Using brass screw (softer than the steel) would mean that as your grind gets thin the screw will be also ground thinner and will not disturb the process. After that you have a nice flat stop-pin screw :)

In the knife shown below I have approximated the convex grind with 3 planes. You start with the one closest to the edge. Remember that you do not want to grind the edge to 0 thickness prior to HT - you want the edge to have some thickness (say 0.3mm) so that it does not crack or warp in the quench. The grind 2 and 3 are made under smaller angles, but you do not grind down to the edge, but stop a certain distance from the edge. I stopped 5 mm from the edge with the second grind and 10 mm from the edge with the 3rd grind. You can blend these multiple grinds afterward with cross-drawing a file (more on that below).



First grind nearly finished


Before changing the grinding angle re-paint the already ground part of the blade and scribe (or paint) in the distance from the edge where you want stop with subsequent grinding.

3rd grinding step - here I am stopping cca 10mm from the edge

You want to switch sides after each step and ideally a few times during the first grind as this is the one step in which you remove most of the material. The reason is as you grind higher up the blade you start to loose support area (once you switch to the other side). While you can not avoid that, with regular swapping you can minimize the effect of possible asymmetry while grinding. 

Once you are done with the grinding jig you move to free-hand grinding. You get one or two steps finer file (the Cut #1 would leave crazy deep scratches) - I am using Cut #2.  Holding the file on both ends with you hands you get very good control. If you have you edge at the final, pre-HT thickness and do not want to grind it down, than mark the edge with a permanent marker.
Cross-draw 'jig'. Note the small clamp in front of the tip that serves as a tip protector.

My cross-drawing 'jig' is just a piece of wood slightly shaped as the blade so that the blade, so the wood will not get in the way. Important - make sure the cutting edge is not sticking outside the wooden support - you risk cutting yourself and/or damaging the cutting edge (which at this stage is weak and easy to dent). I also use a simple clamp some 5-10 mm from the tip that stops the file from sliding off the tip. The reason is - you work with quite some force and if you slip off the blade you will most probably not manage to stop the next stroke and will hit the blade tip with a file and badly bend it (beyond repair). My first blade lost about 5 mm on the tip for this very reason.

You can speed the this process by slightly tilting the file and using the edge of the file. This will make the file bite much more aggressively into the steel, but it will leave deep scratches. Removing them with files and sanding paper will yield some additional material removal and if you do not take that into account you will loose some blade height - that is what happened to me with this very knife.


Deep scratches after cross-drawing with edge of the file. These are deeper than they may appear.

The solution is - stop some 0.3 mm from the desired blade thickness (geometry) and proceed with using the whole contact surface of the file and once you have removed all the deep scratches then  proceed to about grit 60 wet sanding paper (I use WD-40 as lubricant). Clean the blade (remove scratches) up to about grit 240 (i.e. 60, 80, 120, 180, 240) before going for HT. It minimizes the chance of micro-fractures during the quench and will also make your life easier finishing the blade after HT (since the blade will be much harder then).

After HT the knife will be too hard to work with files. If you have left the knife too thick further up from the edge you will need the most aggressive material that allows you to remove metal - for me it was Atoma 140 diamond plate. If you did the pre-HT grinding right, than you only need to work on the first 5 mm behind the edge - that should be doable on coarse sharpening stones. You can also use a diamond plate, but I would not go with one as coarse as 140, because you may 'overshoot' if you do not account for removing the scratches in subsequent steps (which is also material removal). Atoma 400 would probably be a better idea. I plan to stick to water stones if everything goes according to plan and starting with either Bester 220 or JNS 300.

So, this is what I have to say to using this grinding jig including some subsequent steps. Remember - this in only one possibly way - do not hesitate to come up with your own approach. And of course - have fun :)

Last but not least - thank you Aaron for sharing this simple, but very effective grinding jig!

2 comments:

  1. Hi

    Can you please tell me what File does one use?
    Does one use a flat file that is Cut on all 4 sides or a Hand File that is only cut on 2?

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    Replies
    1. Hello,

      I have used a coarse double cut file (http://www.dick.de/en/files-and-rasps/products/precision-files) - the model 1112300, cut #1. This file cuts on 3 sides - on of the short sides is without teeth. Ideally both short sides would have no teeth, as otherwise you will be grinding into the screw that is used as stop-pin. For me it meant that swapping the sides of the knife I had to re-mount the guiding rod to the other side of the file. Should you have an access to a workshop equipped with some sort of belt grinder you could grind the teeth from the short sides away.

      Another possibility would be to get a large Dreadnought File. These are supposed to have less problems with clogging and will also have both short sides without teeth. I learned about these files when I had the whole jig ready and did not pursue it further as I eventually got a small 1x30" belt grinder.

      I hope this helps :)

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