Grinding and plunge lines

U

unky_gumbi

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Aug 28, 2009
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So I recently got my grinder up and running and made my mini EDC with no problems. I guess I got a little over confident because of my initial success. On the first full sized knife I ground my plunge lines didn't come out right. One side is nice and crisp while the other side is rounded out. I did check the tracking to make sure that the belt was centered before putting steel to grit, so I can pretty much rule that out. I am thinking that the fault is with me and not the grinder or its set up. The plunges are equal it is just that the one side is rounded and not as crisp as I want. I think I can make them the same with a little file work so the blade isn't scrap yet.

Ant tips or tricks for getting sharp crisp grind lines and plunge lines would be appreciated.. If it makes any difference I was using zirc 60 grit Y weight belts to rough in the grinds and 120 grit X weight belts to refine them and get closer to my pre HT thickness. I was going to hand sand to my pre HT finish from there so I wouldn't chance screwing up with the grinder.

The rounded plunge is on what I would consider my strong side.
 
I already have a home made version of a file guide so that my plunges are at the same position on the blade. The problem with my plunge lines is not at the ricosso but at the inside where it transitions into the bevel, the radius is huge compared to the other side. Like I said I think I can file it back and make the two sides the same, but I think I should be able to get the closer right off the grinder. I would post a picture but I am down to my blackberry for internet access till after my move.
 
The problem with your radius is likely one of two things:

You wheel or platen are not evenly radius'd on each side
or
You are not letting the belt roll over as much on one side as the other.

If I'm reading your problem correctly in that the radius is not the same on each side.
 
You are reading the problem correct, but I am not letting any belt run off the sides of the platen, but I can see how if the edges of the platen are not the same it would push the blade over towards the tip even if I am not running the belt over the edge. Not sure how I would go about fixing the glass platen if that is the problem:confused:
 
Most makers radius the edge of the platen and let the belt track so that it overhangs the edge where you are grinding the plunge. The edge of the belt rolling over the radius on the platen creates a radius on the plune line instead of a square shoulder.
I lap mine on a granite plate and sandpaper after grinding. The edge of the granite is radiused in a similar manner as radiusing the platen, I use a file guide to run along the edge of the granite and keep the strokes straight and consitent and lap the plunges to a comparatively fine grit (relative to the final finish) before hand sanding the bevels.
 
There's a correlation between the pressure used to "keep" in the plunge grind, if you use a lot of pressure then the plunge will be more square, little less pressure it'll round out some. A coarser belt will square out more, when it wears it'll round out more.

Try grinding with your weak hand first then use your strong hand to match that grind. You may want to try slipping the belt over the edge a bit to round out the plunge somewhat. Also start at a higher grit until you get more practice. I used a worn out 80x or w/e Gator belt with a rounded off edge, it seemed to do a great job on plunges for me.
 
Is your steel Straight/Flat? I had a similar problem and fought one side constantly trying to adjust my grind on one side. I found that I had some Hills & Valleys on that side of the steel. Now I spend a bit of time making sure both sides are perfectly flat before starting my grinds. Just a shot in the dark.
 
Not sure if this has anything to do with your issue, but...on my Craftsman grinder, when I would apply pressure to the belt/platen, the belt would tend to run over to one side (can't recall which one at the moment), and this would contribute to my plunges looking different from one side to the other. I've noticed it happening occasionally to a much lesser degree on my KMG.
 
I am running the GIB kit and did have a bit of a tracking problem because my 3" contact wheel was a little loose, but I think I fixed that. I have another blank to grind tomorrow and I will check everything mentioned with a scrap piece of steel before I start.
 
Use a more flexible J weight belt for the plunge lines. The X weight belts are not very flexible and don't roll over the edge of the platen well and less controllable compared to J belts.
 
Sometimes the edges of the lets are not equal, either.
If you want things the same - you need to do things the same.
You may need to switch the belt around so that each plunge cut is defined by the same edge of the belt.
That was also a lot of good advice given above.
One of the benefits of experience is being able to see what the problem is at THAT time.
Keep grinding - you'll get it.
 
Flipping the belts is one I never would have thought of. I had just bought the cheaper belts to make sure all my wheels lined up and the grinder tracked right. I do have the intentions of buying better belts and a wider selection once I get moved and settled in. So J weights are in the future.
 
A problem may be how you are approaching the plunge. I find if I approach the plunge, line it up, apply pressure and immediately start my horizontal sway to the tip I get a nice radiused sweeping plunge. If I instead apply pressure and stick at the spot for a moment I get a nice crisp straight line. Clear as mud? Another thing I have found that affects them is making sure you are riding the flat before applying pressure (I think Nick Wheeler described this somewhere better than I can) and making sure I'm applying equal pressure with both hands so the blade doesn't "tip" on the platen. I'm relatively new to grinding so mistakes and cause/effect are still fresh in my mind.
 
If you can get a 80ish gator belt I'd really recommend trying that. I know that the j-flex is the tried and true method but give the gator an $8 shot. Round the edges of the belt equally or flip the gator. I also used it as a backing for my j-flex belts in fine grits to clean up the plunges. I'd drift a belt over the edge of the gator. Kind of like a poor redneck version of a rotating platen.

And what can't be overstated is practice practice practice. You're not really at the beginning but grab a handful of thick paint stirrers from a hardware store and grind a couple dozen with a cheap 120 ao belt.
 
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