Questions on sharpening the tip on a belt sander

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kmcdonou

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I have had a belt sander for the past couple of years (Kalamazoo 1x42) and really have struggled with consistency in getting sharp knives, particularly on the up-sweep of the blade and the tip. I looked at many videos and there seems to be two different techniques when approaching the blade tip. One is to slowly raise the handle of the knife straight up. The other pulls the handle back towards the sharpener. I tried the handle straight up tonight, but the tips of the three knives I sharpened are still not as sharp as the base, and not really very sharp.

I start with a Trizact A65 then go to an A30 and lately to an A20 and then a leather belt with green compound. I have gotten knives really sharp before, but I never really checked the tips as carefully as I do now and suspect even when I had a sharp knife the upsweep and tip were not as sharp.

Could really use some help here. Thanks.
 
Now I am no master by any means on the belt sander and I have had my share of screwed up tips. I would love to hear what the knife makers on this forum have say about this so I hope they chime in. At the end it does not matter since the angle stays the same. Whether you turn the knife so that the bevel is always more or less perpendicular to the belt or whether you pull the handle towards you. I think most more experienced makers do a little bit of both at the same time. For me I understand the principle of turning the knife better, so it follows its curve. However on a knife with a pronounced belly, it is very awkward and sometime impossible to do that all the way to the tip. That's when a hybrid approach come in I guess. In your case getting the tip not sharp, it may be original bevel grind or you do not keep the angle the same along the bevel. use marker, see where you grind!

Here is a video from Gavko knives, at 6:31 into the video u see him turning the knife quite a bit for the belly part and tip, where as he more or less pulls it off the grinder before. He also really only uses the very edge of the belt, you can see it when you pay attention to the sparks. That way I guess he can focus on one abrasive "spot" at a time. This is all for grinding the primary bevel though: https://www.youtube.com/watch?v=DrteNOp6ZvU&list=PLXHECohzv9aWBKC9IpAripl_xOTy-JFPW&index=4

Here at the beginning of this video (https://www.youtube.com/watch?v=jIilMg2TTgU&index=2&list=PLXHECohzv9aWBKC9IpAripl_xOTy-JFPW) he however puts the secondary bevel on after heat treatment and you can see he only slightly turns the knife near the tip. At 7:52 he puts the final edge on, he sharpens the knife at the slight slak area just above the platen.
 
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Its really a good idea to learn a passable edge freehand, will give you a very good foundation for problem solving.

Two issues come to mind right off the bat - either you are inclining the blade too high and grinding the belly and tip to a steeper angle that way, or maybe the decreased contact area has you using too much pressure and the edge is sinking into the slack on the belt, increasing the angle that way. Will also round off the tip.

You could get a sacrificial tool and play around with a Sharpie to see what's happening - make the tip and belly too thin on purpose to give you a better idea of how the geometry works with the belt. You could also take a hardwood shim, shape it to a knifelike profile, and practice putting an edge on it with different pitch etc.
 
I think you might be right about grinding the belly and tip to a steeper angle. When I look back from the knife edge it is not as ground as much on the belly and tip as on the straight part of the blade. I just can't figure out how to maintain a consistent angle for the whole length of the blade. It's frustrating because I have tried lots of knife sharpening systems and the belt sander is just so quick to get a really nice edge. If I only could figure out the process for the curved part of the blade I would be sooooo happy.
 
I figured this one out pretty good by backlighting the edge while I rested it on my stone, right at eye level. Note the relationship to the angle on the stone and slowly slide it across. As it reaches the belly and tip, you will find you need to elevate the handle to maintain the relationship.

How much (up to a point) is determined by the amount of belly. Straight edge do not need to be elevated at all. The key is to elevate the least amount possible, this will keep the edge nice and acute.
 
For *me* sharpie helped me figure out how to get the tip sharp. Sharpie will absolutely show you where you are grinding. If you're going past the tip, you'll see a wider area at the very tip. If you're not getting all the way to the tip, you'll see sharpie still there.

Watch Ken's video above and try to emulate that motion through the curve; it seems to be the closest to perfect I've seen.

In my WSKO video I show how to go through the belly, but it's not focused on that. Just an aside on how I was thinking about it at the time.

Good luck to you,

Brian.
 
One other thing I forgot:

To get a sharp tip, the biggest thing is to STOP right at the tip. Murray Carter (paraphrasing) describes this as sneaking up on the tip. If you stop right at it and go no further, you'll get very sharp tips. In the past few years I've gone from just normal blunt tips, to extremely sharp tips on just about every blade I sharpen. These two things (this and the one from my previous post) have changed everything for me.

Brian.
 
I've found that when working on the tip with the belt sander, my best results were when I placed the tip on the beginning of the curvature of the belt sander (where it rolls underneath). I used a slide across technique then reversed the blade at same angle, and avoided the edges of the belt. Once you get near the edge of the belt, you may slice it and lose the belt and have to replace it. 400 grit first, then 1000 grit with a final touch of green or black rouge. This resulted in a mirror polished shaving edge-most of the time.
 
The edge has to be kept radial to the moving belt as well as maintaining a consistent angle.

I've found tips on many knives to be incorrect in the original grind. To fat, wrong angle. As a knife maker the tip is the most challenging part of the geometry to grind, so it makes since that its also the most difficult area to sharpen.
Finishing the tip on a stone or diamond plate after the belt can be useful. Because you have the body of the blade sharp, the edge is the focus. I use a degree wedge to maintain this angle.
There may be some information that is useful on this video we recorded last year.
[video]https://youtu.be/iFzVjH8DXYo[/video]
 
awestib said:
In your case getting the tip not sharp, it may be original bevel grind or you do not keep the angle the same along the bevel.
Click to expand...

You can't keep the primary edge bevel angle the same, because usually the entire geometry of the blade changes completely near the tip on any kind of significantly bellied blade: Unless the original blade secondary grind surface thins out in perfect proportion to the narrowing of the blade (a rare and difficult feat, with the consequence of tip fragility), if you keep the edge bevel angle the same on a thickening blade cross-section ratio, the edge bevel will then "broaden" tremendously as the blade thickens in proportion to its width: This is -fairly- easy to do, but tends to look not so great, which is why almost all "professionally finished" bellied blades will grow duller towards the tip... Unless they happen to have a deep hollow grind, that tapers smaller in hollow radius, deep into the point, and even then the edge bevel's side surface still tends to get wider if you keep the edge bevel angle the same.

Generally, bellied blades will grow duller towards the tip, the deeper the belly the duller, unless you accept an ugly gradual widening of the edge bevel, which doesn't look "professional"... Some deeply hollow ground blades may be able to minimize this, and even some flat ground blades can do it if they are very thin at the edge: This means however, since you can't get something for nothing, that the point on these tends to be fragile, if it is truly going to maintain the same blade geometry/sharpness...

Even so, sharpening the belly to match the rest of the edge is always a tricky proposition for an average user, or even a maker, and the deeper the belly the trickier...: The overwhelming majority of knives/makers go for a simpler solution: The knives open the edge bevel angle at the tip, and so grow duller as you go towards the tip, precisely the most useable part...

HeavyHanded said:
I figured this one out pretty good by backlighting the edge while I rested it on my stone, right at eye level. Note the relationship to the angle on the stone and slowly slide it across. As it reaches the belly and tip, you will find you need to elevate the handle to maintain the relationship.

How much (up to a point) is determined by the amount of belly. Straight edges do not need to be elevated at all. The key is to elevate the least amount possible, this will keep the edge nice and acute.
Click to expand...

"Straight edges do not need to be elevated at all": This is true, and even slightly curved edges, that are close to straight edges, can easily keep the same sharpness throughout, because the aggravating factor is the radius of the edge belly: The deeper the radius, the harder it is to keep track of a consistent edge bevel angle.

Ideally, knives should start out sharper at the tip than on the straighter rear edge portion: This is because, while it is easy to maintain the same edge bevel angle on the straight edge portion, the tendency towards the tip is to open, while sharpening, the angle further than what is originally provided...: This means that, to balance things out, the edge bevel should actually be sharper towards the tip: Fat chance though of ever seeing that: When it happens, it seems more of an accident than intended by design: Usually it never happens on any kind of thick stout blade or edge, but more on cheap crude machetes or the like...

Gaston
 
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Interesting post Gaston.

In discussing this with Ken Schwartz, he insisted that NO LIFTING was necessary, as long as you keep the edge facing the "normal line" (90 degrees to the tangent), which means rotating the blade as you go through the belly towards the tip. I argued that it was necessary to increase the edge angle near the tip because the edge will be cut from a thicker part of the blade (nearer to the spine it's thicker). All of this to maintain a sharp edge *AND* a consistent bevel width. Ken's counter argument was that most blades have "distal taper", which means that the stock gets thinner as you approach the tip, which allows the bevel width to stay the same *and* keep the edge angle the same.

My experience with blades is very small compared to someone like Ken and I haven't made a huge study of blade geometry. But I still think you need to "open the angle" some on a lot of blades as you say Gaston. When done on a belt grinder, that means leaning the spine towards you a bit as you get to the last bit of the blade. I talk about that a little in my video here:

https://www.youtube.com/watch?v=ApyDcJzKBgk?t=16m:20s

[video=youtube;ApyDcJzKBgk]https://www.youtube.com/watch?v=ApyDcJzKBgk?t=16m:20s[/video]

There are some other tips on getting sharp tips throughout the video.

Brian.
 
bgentry said:
Interesting post Gaston.

In discussing this with Ken Schwartz, he insisted that NO LIFTING was necessary, as long as you keep the edge facing the "normal line" (90 degrees to the tangent), which means rotating the blade as you go through the belly towards the tip. I argued that it was necessary to increase the edge angle near the tip because the edge will be cut from a thicker part of the blade (nearer to the spine it's thicker). All of this to maintain a sharp edge *AND* a consistent bevel width. Ken's counter argument was that most blades have "distal taper", which means that the stock gets thinner as you approach the tip, which allows the bevel width to stay the same *and* keep the edge angle the same.

My experience with blades is very small compared to someone like Ken and I haven't made a huge study of blade geometry. But I still think you need to "open the angle" some on a lot of blades as you say Gaston. When done on a belt grinder, that means leaning the spine towards you a bit as you get to the last bit of the blade. I talk about that a little in my video here:

https://www.youtube.com/watch?v=ApyDcJzKBgk?t=16m:20s

[video=youtube;ApyDcJzKBgk]https://www.youtube.com/watch?v=ApyDcJzKBgk?t=16m:20s[/video]

There are some other tips on getting sharp tips throughout the video.

Brian.
Click to expand...

I believe this only applies based on orientation. If you were to set the worksharp so the belt was running horizontal, the same mechanics would make it appear the handle were being elevated.

True, the line that is being referenced is the spine/centerline. As long as the angle is constant relative to this, the tip and belly will turn out fine. One could set the belt at the desired angle and keep the blade dead horizontal, the effect would be the same. The advice to "elevate" the handle is only valid when working against a horizontal abrasive surface.

In my experience, only very overbuilt edges will have any real difference in edge angle going into the belly and tip - it needn't be any more broad than any other part of the blade. On freehand and powered edges, it is very common to see added curvature in this area that could be eliminated - I agree with Ken on both parts.
 
The preferred angle of approach as the belly and tip are being sharpened relates directly to how this area was ground originally. Sharpening a blade with full distal taper, from plunge to tip, accompanied by bevels that are maintained at a specific angle from plunge to tip can be sharpened without any change in the angle of approach. It can be constant. Blades without distal taper or blades that have been ground where the taper starts just behind the belly.
You can tell a lot about a makers skill by looking at how this area of a blade is ground. Its the first place I look, if this is done correctly I will take a further look.
The plunge area of any knife is the easiest part to do, its the tip and the belly that takes a lot of skill to get the most out of the steel.
To ad, this is the area where convex bevels can cause real problems; they make the belly and tip area far to thick and in the process, make it necessary to use very creative sharpening techniques in order to get this area sharp. IMHO convex bevels are the mark of a rank amateur when it comes to grinding bevels.

Fred.

Fred
 
HeavyHanded said:
True, the line that is being referenced is the spine/centerline. As long as the angle is constant relative to this, the tip and belly will turn out fine. One could set the belt at the desired angle and keep the blade dead horizontal, the effect would be the same. The advice to "elevate" the handle is only valid when working against a horizontal abrasive surface.

In my experience, only very overbuilt edges will have any real difference in edge angle going into the belly and tip - it needn't be any more broad than any other part of the blade. On freehand and powered edges, it is very common to see added curvature in this area that could be eliminated - I agree with Ken on both parts.
Click to expand...

The geometry is fundamentally not the same towards the tip: The thickness to width ratio almost always goes way up near the tip, simply because the blade gets thinner slower than it gets less tall...

Blades that have little change in tip geometry vs main blade geometry usually have very slender tips.

Quote: "In my experience, only very overbuilt edges will have any real difference in edge angle going into the belly and tip"

: I am a bit astonished to read this, since besides the exception of thin convexed edges on flat ground blades (which are a separate issue entirely), virtually every knife I have ever seen in my whole life grows duller towards the tip, and my reprofiling them inevitably makes the bevel look broader (in profile) towards the tip, because the geometry of a point is usually completely different, and much fatter, to the main body of the blade...

Particularly egregious examples of this are the Chris Reeves one-piece range, where you can clearly see the edge angle go from 13° at the rear, on a fairly thin 1 mm bevel base, to 30° easily at the tip... Again, this is a cosmetic issue when the blade cross-section thickness is not maintained thin enough to keep pace with the rapidly narrowing blade side-view towards the tip...

Even thin 0.5 mm Randalls grow much duller at the tip, but this time mostly by the fault of the so-so factory sharpening (and the geometry on 18 style blades), which squanders the tip's potential despite the thin-edged hollow grind: It is fixable, and this is where paying the extra price on Randalls comes through, I guess, as most other blades are much too thick at the edge to have any hope to get the tip's edge down to 10° per side without a grinder (and even if you do, it might look terrible)...

One thing that makes these considerations worse for me is that I consider 10° per side (20° inclusive) the bare bones minimum, even for big chopping knives, so that make thick-tip knives like the Chris Reeves one-piece range almost untameable without power tools...

Gaston
 
Very interesting thoughts here and once again I am amazed how much new insight I get here on BF even after so many years of reading and following threads.

I am not certain that I agree to some of the above though, but I am probably wrong. I try to imaging a few different blade types here, starting with the easiest, a scandi grind. The stock is the same all along, the single bevel is the same angle all along to the tip, same amount of metal "behind the apex" all along. So, no adjustment of any kind (as long as the bevel was ground properly).

2nd, a big bowie with a full distal taper ending in a "needle like" tip. So, the height of the blade and the thickness of the blade near the plunge gets smaller towards the tip. Depending on the amount of those reductions of material, we may have the same ratio of height and thickness so the main bevel angle (primary grind angle) would stay about the same. But this can vary, so the primary grind angle may have to get very acute if the taper is dramatic and the height remains large. Vise versa, the primary grind angle stays more obtuse or even has to be obtuser if the distal taper is not as much and the height is being reduced rather drastically.

Is that correct so far? Either way I believe at the end of grinding the blade, the remaining edge thickness before sharpening should be the same all along the blade. So the basis for the edge is the same and as long as final edge angle is more obtuse than the most obtuse primary grind, we should have the same edge bevel all along the blade?
 
awestib said:
Very interesting thoughts here and once again I am amazed how much new insight I get here on BF even after so many years of reading and following threads.

I am not certain that I agree to some of the above though, but I am probably wrong. I try to imaging a few different blade types here, starting with the easiest, a scandi grind. The stock is the same all along, the single bevel is the same angle all along to the tip, same amount of metal "behind the apex" all along. So, no adjustment of any kind (as long as the bevel was ground properly).

2nd, a big bowie with a full distal taper ending in a "needle like" tip. So, the height of the blade and the thickness of the blade near the plunge gets smaller towards the tip. Depending on the amount of those reductions of material, we may have the same ratio of height and thickness so the main bevel angle (primary grind angle) would stay about the same. But this can vary, so the primary grind angle may have to get very acute if the taper is dramatic and the height remains large. Vise versa, the primary grind angle stays more obtuse or even has to be obtuser if the distal taper is not as much and the height is being reduced rather drastically.

Is that correct so far? Either way I believe at the end of grinding the blade, the remaining edge thickness before sharpening should be the same all along the blade. So the basis for the edge is the same and as long as final edge angle is more obtuse than the most obtuse primary grind, we should have the same edge bevel all along the blade?
Click to expand...

You're hitting the important aspects on the nose.

I recently did a regrind on an old sabre grind that I had converted to a full convex. I converted it back to a sabre grind, but with far less meat on the primary grind and a very aggressive distal taper into the tip. Maintaining the same angle, the bevel gets smaller at the tip, as the tip has more aggressive geometry - more shallow primary angle - than the main body of the blade from the belly to the heel and so less metal to remove. If the cross section of the spine at the tip roughly matches a cross section of the primary elsewhere, the bevels will be similar. If the tip is overbuilt the bevels would have to be much wider at the tip to maintain the same angle.

After this thread I went back and did the backlight edge on a flat surface. Most of my knives that still have the factory primary grind intact have reasonably uniform edge bevel width and show the same angle from heel to tip. Most of my knives that I have thinned tend to have somewhat smaller bevels at the tip as I have ground the distal taper and primary above the belly a bit more aggressively.
 
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