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Grinding Jig Instability


The jigs used for sharpening gouges are very popular.  Almost every turner has one of one type or another, and they are used routinely and with great success.  I learned to sharpen free hand before I even knew that a jig existed, so I don’t have one.    


This article describes a hidden danger in using a jig. In certain circumstances, the grinding wheel may “grab” the tool and jerk it downward rather violently. This can result in damage to the jig or wheel, and possibly an injury to the user.  The scary part is that if it happens, it will happen quickly and with little if any warning.


What causes the tendency to grab?


The root cause is that the grinding wheel rotates toward the tool.  If the wheel rotated in the other direction, the tool would be “in trail” relative to the wheel and there would be no possibility for it to grab.


Beyond this, there is an interaction between two forces that always gives an instability in the simple fixture.  One is a force applied by the user; the other is the force exerted on the tool by the wheel.


The tool must be pressed against the wheel.  It may happen that the weight of the tool and jig alone provides enough force, or additional pressure may be supplied by the user to make the grinding more aggressive.


The wheel exerts a force (FW) on the tool because of friction and the cutting action of the abrasive.  The direction of the force is tangent to the wheel at the point of contact.


With the simple fixture, the force exerted on the tool by the wheel adds to the pressure forcing the tool against the wheel. The wheel will seem to pull the tool in.  The actual force  FT  pressing the tool against the wheel can be far greater than the weight of the tool or the additional force applied by the user.  


This is where the instability comes in.  The force  FW  causes  FT  to increase.  An increase in  FT causes  FW  to increase.  This circle of cause and effect escalates until something either breaks or gives way. It’s at this point that the tool will be “pulled through the wheel” with great force.


Fortunately, this seldom happens.  Even so, it’s important to know that it can happen and how to avoid it.  


Beware of steep bevels.  A problem is more likely to occur when the bevel on the tool is short and steep.  To match the bevel to the surface of the wheel, the axis of the tool must be positioned at a comparatively small angle (around 30º or so) to the horizontal.  The drawing at right illustrates this and shows a very dangerous situation.


A spindle roughing gouge placed in a simple fixture is positioned to grind the bevel at an angle of 45º, which is typical.  A large force will be in effect that tends to pull the tool down against the wheel.  A greatly magnified component of this force will be directed to the left and exerted against the part of the jig where the end of the tool handle seats.  For this reason, . . .


It is extremely important for the horizontal arm of the jig to be clamped securely.  If it slips, the tool will then be able to move downward.  The situation will then escalate and the tool will be thrown from the jig.


A much safer way to sharpen a spindle roughing gouge is to position the tool rest of the grinder at the proper angle, and then place the tool flat on the rest.  Simply rotating the bevel against the wheel is all that’s required.  Or, you can grind it free hand.  


Bowl gouges may also be ground with a steep bevel.  I have one ground at 55º that I use for the finishing cuts across the bottom of bowls and platters.  You must not attempt to sharpen one of these with the simple fixture.  It is too risky.  


A better jig is one in which the tool is clamped into a holder with the tool at an angle.  Such a jig is much more stable than the simple fixture that only provides a seat for the back end of the tool handle. With it, you can safely grind bevels that are much steeper.  The instability can still occur, but the tendency for it to get out of control is far less.


Stable or unstable?  There is an easy way to tell if the configuration of the angled jig is stable or unstable.  Imagine a line drawn tangent to the grinding wheel where the bevel of the tool makes contact.  If this line passes above the pivot point of the jig, the situation will be stable.  If it passes below the pivot, it will be unstable.  


If it’s stable, the force exerted on the tool by the wheel will tend to lift the tool from the wheel.  If it’s unstable, the force will tend to pull the tool in.  Having the wheel “grab” is then a possibility.


Because other factors come into play, such as the grit of the wheel and the hardness of the steel, you can grind in the unstable regions of the wheel uneventfully.  However, the possibility is always there that a grab will occur, and it is more likely when grinding a tool with a steep bevel.  


Always make sure that all adjustment screws, bolts, clamps, and so forth are tight before using the jig.  


A note: One of the worst turning-related injuries that I have seen first hand (after the fact) happened when the tool slipped off the side of the grinding wheel.  It resulted in a deep cut across the knuckles and back of one hand.  Be careful, and have respect for the grinder. Sometimes things will sneak up on you.


Another note:  While sharpening a tiny, shop-made scraper (hand held) that was only 1/8” in diameter and about 2” long, it caught on the wheel and was pulled through.  No harm was done, but it took a small divot out of the 120-grit wheel and got my attention in a hurry. The small shaft flexed enough to make it unstable.


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