Middle overgriding problem

[wootz]

Comrades, I need help of the collective genius, as I've run out of ideas.
Sharpening a knife on a rotating wheel concaves its middle, and I cannot avoid this no matter what I try.

In the photo below is a brand new knife.
Before sharpening its edge was straight and touched all the way down on a  flat surface.
After sharpening on Tormek, it got a concave in the middle, seen in the photo as a white line of light under the edge.



Having realised that this concaving happened before to other knives as well, I did a controlled sharpening.
Made sure the next knife edge is straight by leveling on a DMT plate.



Trued the stones.
Locked a collar on the knife jig to ensure the edge moves strictly perpendicular to the stone during sharpening (as described in the "Knife jig aid" thread).
Sharpened it on Tormek with our standard sequence of #220 - #1000 grinding wheels, making sure I slide it across the stone continuously, not spending too much time anywhere along the edge. And it got a concave in the middle, seen in the photo as a white line of light shining under the edge.



At least on my kitchen knives I see that repeated sharpening on Tormek deepened the concave.

Unwillingly, I arrive at a conclusion that middle concaving is intrinsic to sharpening knives on a wide rotating wheel  due to the middle part of the knife spending extra time on the wheel.
When we sharpen on bechstones, we minimise that by placing the knife diagonally rather than across the stone.
If the Tormek grindstone width was, let say, half of what it is now, this effect wouldn't be noticeable (but narrow wheel would be worse for chisels).

Can this be helped?

[Jan]

Wootz,

I see this tendency on my knives also. 

You yourself have formulated the reason for it: you spent too much time in the middle part of the knife. So the correction may be easy, spend less time in the middle of the knife and/or reduce here the pressure on the jig.

I was wondering why my overgrinding is not as pronounced as yours. 
It may be given by the fact that I, as a hobby sharpener, spend more time sharpening the tip (point) and also the heel while you, as a professional sharpener, do not need so much time for the blade extremities. 

Good luck!
Jan

[Herman Trivilino]

Unwillingly, I arrive at a conclusion that middle concaving is intrinsic to sharpening knives on a wide rotating wheel  due to the middle part of the knife spending extra time on the wheel.

Not intrinsic. It's due to the operator spending too much time in the center.  If you simply pass the knife at a steady speed across the 50 mm wide grindstone, less time will be spent on the 50 mm sections on each end of the knife. And it's not uniform. The closer to either end of that 50 mm wide piece to the end, the less time will be spent on it. In other words, a small section, say the last 5 mm near each end, will spend very little time in contact with the grindstone. You are right in that the narrower the grindstone the smaller the magnitude of the problem. On the other hand, if the blade were only 50 mm long you would completely eliminate the problem.

The operator must intentionally spend more time, or exert more force, near the ends.For a kitchen knife you can usually err on the side of making the knife concave rather than the convex shape you show in your photographs. For other knives it may not matter if it's convex or concave.

The best way to teach yourself how to handle this problem is to continuously check your progress so that as soon as you see too much concavity you can adjust for it.

If the Tormek grindstone width was, let say, half of what it is now, this effect wouldn't be noticeable (but narrow wheel would be worse for chisels).

Or if it were wider and equal to, say, the length of the knife! 

[wootz]

Jan, the middle overgrinding is more pronounced with shorter knives, in the 3-5 inch range; and the longer the knife, the less.

Herman, I wish it were an operator error that could be corrected.
When you say, "The operator must intentionally spend more time, or exert more force, near the ends", this works for long knives.
But with shorter knives the knife bolster & handle won't allow to spend equal time on each part.
As soon as you move to the knife heel, the bolster/handle is in the way, and you always grind heel + middle.

Let's for simplicity take a 4.5 inch knife with 1.5-inch heel, middle and tip, and sharpen it on a 2-inch Tormek stone from heel to tip:
heel + part of middle;
middle;
part of middle + tip;
the tip.
The middle part gets more grinding.
It is somewhat possible to equalize grinding of the tip and the middle, but what to do with the heel?

[Herman Trivilino]

As soon as you move to the knife heel, the bolster/handle is in the way, and you always grind heel + middle.

You have to exert more force on the bolster end of the knife. Plus, spend more time there.

[wootz]

Appreciate your advice, Herman.

I found more threads where this problem was stated.

http://forum.tormek.com/index.php?topic=2078.0

...
After my wife berated me for ruining her knifes I went to the forum and discovered that what I'd done appears to be a relatively common problem. Despite my best efforts I had successfully added a concave shape into the blade of the knives (2 in particular). While I'm pretty good at maintaining a consistent angle along the length of the blade I'm useless at keeping the original "straight' shape of the blade. I've had the Tormek a little over a year...

... the concave portion appears in the portion of the blade where the holder is attached (basically the middle).

"... It is a common problem. it does not reflect an shortcoming with the Tormek. It can certainly be corrected by a skilled Tormek operator. The trick is to be aware of the situation and deal with it."

The solution offered was, firstly, in skills - "Use less time and pressure in the middle of the blade. Requires some practise...".
"... It sounds like you're trying to keep your pressure even and consistent. That doesn't always work."
Or as Herman said above, "The operator must intentionally spend more time, or exert more force, near the ends... exert more force on the bolster end of the knife".

And secondly, in sharpening knives on a finer stone to give yourself time to apply those skills as "the coarse will eat away a thin knife rather quickly..."

I am happy to hear this can be corrected by skills.
I was definitely applying pressure evenly and consistently, and primarily onto the jig.
Off to my shop to retrain my hands, with 4 fingers spread out on the blade to apply pressure differentially.
Frankly, pessimistic about the outcome.

[Ken S]

I wonder if the practice of bidirectional grinding (bolster to tip and then tip to bolster) might be a cause of the problem.

Ken

[wootz]

I wonder if it is overkill to use metronome for pacing sharpening?

I remember when Ken first suggested to use laser to mark the LOC (line of contact), I snorted.

But then came Jan's setup


And now I use laser regularly.

But then I saw grepper's


In this line of evolution, adding metronome doesn't seem overkill any more.

[Jan]

I am happy to hear this can be corrected by skills.
I was definitely applying pressure evenly and consistently, and primarily onto the jig.
Off to my shop to retrain my hands, with 4 fingers spread out on the blade to apply pressure differentially.
Frankly, pessimistic about the outcome.

Wootz,

When I observe pronounced overgrinding, and the knife is not too hard or valuable, I usually file it away by hand and start to form a new bevel from scratch. It is much quicker than correction of an already overgrinded knife. 

This drastic procedure activates in my mind a continuous checking procedure described by Herman in reply #2. 

Application of different pressure along some parts of the blade to prevent overgrinding is for me only an auxiliary possibility, because I am afraid that it can easy result in uneven bevel width which is also an annoying bevel geometry. 

Jan

[wootz]

I wonder if the practice of bidirectional grinding (bolster to tip and then tip to bolster) might be a cause of the problem.

Ken

I slept over Ken's idea, and you know, in the morning it became concept changing to me.
I've always been tending to sharpen knives in both directions, and we see this in training videos.
But of all parts of the blade, the heel gets the least grinding time because of the bolster/handle, whereas it is the thickest part.
Sharpening unidirectionally from heel to tip compensates for this.
As does exerting more force on the bolster end of the knife.

Bidirectional sharpening of knives is a methodological error.
As is even pressure along the edge.
As is pressure onto the jig.

In all training and demonstration videos people press onto the knife jig.
This is contributing to the middle overgriding as the SVM-45 jig itself weighs 300 g, and should be avoided with straight end knives.

[SharpenADullWitt]

Yet another reason to find some thrift store knives to sharpen/learn on, before doing the good ones.
One reason why I want some of the inexpensive restaurant ones, is I can practice and get feedback from the users at my friends restaurant.

[wootz]

Straight edge looks deceptively easy to sharpen, but it is unforgiving to inaccuracies.
Knives with straight edge under 3" are sharpened on Tormek 2" wheel perfectly without any special precautions.
Knives where straight edge extends over 3" before it curves upwards, tend to concave in the segment under the jig.

After days of feeling around for a solution, a dozen of varied attempts and 3 ground down junk knives, I started getting satisfactory results. Satisfactory, but still not perfect.

Now to the rules I drew for myself to prevent concaving in knives with straight edge over 3":

POSITION
Right


Wrong. Positioning you hand over the jig deepens the concave.


I also tried clamping the jig at the heel just to check if this helps - nope.

DIRECTION
Unilateral from heel to tip.

Bilateral (heel to tip and then tip to heel) furthers concaving.

PRESSURE
Maintain light pressure on both the heel and tip segments.
Note, if there is room for 2 fingers at the heel, main pressure must be applied by the finger near the bolster.

With no or minimal pressure, the concave still forms under the jig due to its weight (300 g).
Extra force on the heel causes a concave between the jig and the heel.
I also tried applying extra force on the heel as I pulled the blade across, dropping it to almost none when the blade was about to reach the middle part - and still got a concave between the jig and the heel.
Extra force on both the heel and the tip deepens the concave in the jig segment.

TIMING
For the sake of experiment, I used a metronome at 60 bpm.
I tried extra 3 seconds at the heel, 2 seconds, and 1 second - in all cases this caused concaving between the jig and the heel.
Worked well only when I start pulling across the stone as soon as I firmly place the heel segment on the stone.

Pulling across at a steady speed of 1 cm per second (about 2 seconds per inch) works well for both the coarse and fine stones.

GRIT
Whenever possible, avoid the coarse grit.
Starting on #800, I can establish the bevel within reasonable time on mainstream knives.
However, it is not always possible. With seriously dull knives, hard and powder tools steels #800 will take ridiculously long to bevel.
When you have to start on the coarse grit to establish the bevel in harder steels, or when speed is the main factor like in bulk sharpening, alternate sides with every pass.
I tried alternating sides after every 2 passes on the coarse stone, and this caused concaving.

On #800-1000 grit alternating sides after every 2-4 passes works well, but as soon the burr starts forming, alternate sides every 1-2 passes.

BURR FORMING
Burr first appears in the middle of the blade. Middle part gets more grinding, and there is no clever way to get the burr all along the edge outright.
Everyone develops his own technique to get a continuous burr not overgrinding the middle segment of the blade. Mine is as follows.
As soon as the burr in the middle becomes tangible, I stop pulling across the whole edge, and start working on the heel and tip segments separately.
At the heel segment I spend a few seconds on each side doing short slides on the stone, with pressure applied to the heel only, alternating sides till I get an apparent burr.
Then I do the tip segment, with pressure applied to the tip only.

Having got the burr all along the edge, I finish with a hill to tip run along the whole edge on one side, then another, twice.

***
Note that said above is to prevent a concave, not to repair one.

REPAIR A CONCAVE
First, I flatten the concave on a low-speed linisher (thanking Steve Bottorff for advice), or side of the Tormek wheel where knife geometry wouldn't allow to use the linisher, and finishing on Extra Extra Coarse DMT plate.
If you look straight at the edge after flattening, it is thinner in the middle, and thicker at the heel and tip segments.
Set grinding angle on the coarse stone, and start thinning out the thicker segments of the edge.
At the thicker segments I spend a few seconds on each side doing short slides on the stone, with pressure applied to that part of the blade only, alternating sides, and frequently checking the edge.

Having got somewhat even edge, I start pulling all along the edge, alternating sides, to get a uniform bevel.

***
Thank you everyone.
Criticism and flogging are heartily welcome.

[Hatchcanyon]

Makes a lot of sense to me even if I had not much knives sharpened/modified? until now.

The only thing I did more or less intuitively was to work always from heel to tip.

Rolf

[Stickan]

Hi,
I use a very light pressure on the jig when I am on the middle of the blade and higher at the tip and bolster.
I realise that we would within short make a short new instruction video which shows different knifes models.

My advice is to start with cheap knifes and get to learn a technique where you will use little pressure in the middle of the blade and keep an eye on the curve of the blade that the manufacturer made from the start.

After 10-20  knifes you will get an understanding how it works and will get a hang of it.
If you don't sharpen much knifes keep in mind about using more pressure on the jig on tip end end, less in middle.

Best,
Stig

[wootz]

Thank you, Stig.
This forum has your explanations on the topic in 2013 and 2014, and I should have read your and Herman's and others' posts before I concaved a few good knives.

The main lesson I learned, the main thing I had to change in my sharpening was NOT to keep passing the blade back-forth across the wheel till the burr forms all along the edge, but work on segments of the edge that fall behind, checking frequently.