need advice on grinding convex, problems with spine dips

D

DaQo'tah Forge

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I have trouble trying to grind a convex edge on my knives.

This is what I do, please read then advise:

I forge to rough shape.

I grind the profile lines (The outside edges all the way around) with 36 grit belts on contact wheel.

I grind flat on the platen starting with 36 grit, and go all the way up in belts until I don't have any higher.

I file the shoulders for when I place a guard on later.

I decide where the cutting edge will start. I mark that placement, then I begin with a 36 grit and "slack belt" grind from the marked position to the blade tip.


THE PROBLEM:
My convex edge never looks thin enough. To correct what I think is too blunt of convex edge as it dives down to the cutting edge, I push harder on the blade into the slack belt as I grind it.

However as I push harder on the knife into the sanding belt, (Blade is positioned always edge up so I can see what's going on) I have to make sure I don't dig too deeply into the cutting edge and thereby mess up the future cutting edge, so I tilt the blade's edge a bit away from the sanding belt in the hopes I can grind more off the sides first.

It is this tilting away of the cutting edge that always seems to bring the spine of the blade down into the slack belt and it starts to get sanded without my knowing about it.

All the sudden when I check how Im doing, I find that I have sanded a 'dip' in the spine.


Now I have to spend most of my time working on the other side of the spine to even out the spine taper so that one side will match the other.


Suggestions?
 
Hi DaQo'tah,

The way I do a full convex grind is to flat grind the blade on the platten and leave a little of the flat running down about 1/3 to 1/2 the blade at the top of the plung, next to the spine. The edge is a little thinner than a dime.

After heat treat I start with a 120 grit belt and slack grind it. I grind edge up real lightly and the extra flat at the top of the plung gets runded a little. Just a little distance from the plung I start pressing harder, but not real hard.

After finish grinding with 120 I still have a little flat on the spine leading away from the ricasso flat. Maybe a 1/4" of excess flat. After the 120 grit I go to 220 and make a couple light passes, then to 400 grit and just a couple of passes.

Done carefully like this the top of the spine will have a nice transition and look something like a flat grind. I then clean up with sandpaper and hard wood sticks that are flat and and work my way up in grits by hand till I have the finish I'm after.

I lot of times I just put a convex edge on a blade, it's flat ground witht the last 1/8" to 3/16" convexed on the lower tight slack portion of my grinder, between contact wheel and platen, maybe 2" section.

Hope this helps,

Will
 
I put a convex bevel on 95% of the knives I do (only take it off on request). I do full convex grinds on at least 5-10% of those.

However, I have never ground a convex bevel "edge-up"...:eek: yikes!

I always grind edge-trailing on the slack part of a belt.

Start off with a full flat grind and then lay into the spine real good to smooth it over. As long as you are patient and check it every few passes, it turn out just fine. Plus it practically sharpens itself as you take it up the various belt grits.

Tip: the final pass on the flat grind (before I start convexing) is made with the blade vertical so that when I start convex grinding, I can tell where I need more grinding - by looking for lateral grinding marks. Make sense?

Dan
 
Both of you have pegged it pretty well.
I would suggest this:
Clean up your shoulders after you have cleaned up the ricasso and before you grind the bevels.
Do a flat grind at least 1/2 way up leave it a little thick at the edge.
I use the same spot to grind my convex between the bottom of the Platen and top of the contact wheel about 2" Keep your belt as tight as you feel safe with.
I have a Coote 2 wheel grinder
Here is what will give you more control
USE a Fresh belt or the best one you have and make even controlled passes after your rough initial grind.
I grind edge up to start but finish edge down as I get close. That allows me to keep the spine out of the belt...99% of the time anyway.
I use a 50grit 3M 967 much easier to control and is designed to be used with a lighter touch!
one mistake with a 36grit and your Toast
This is key!
 
Will 52100...(and all)

While you guys are linked to this topic (perhaps you still are anyway) I thought I would toss a little story of what I have been doing and some of my ideas out you guy's way.

TOPIC; HEAT TREATING...

I took note of that Darn, big, over-sized, DEBATE we all had this year about the use of 3 different heat/quenchings on our blades.

I decided to set up my own little test.

I made two blades the same,,,(well, kinda close to being the same. they were from the same forged Load shaft and were about the same size)

Blade "1" got one heat/quench
Blade "2" got 3 heatings and quenchings. (with 3 tempering,,all of this took 6 days to finish)

TEST RESULTS > ...beats me if I can tell the difference...I sharpened them, they cut about the same, I bent the blade that was heated just one time, it seemed to bend okay, I sold the other blade.(hey I needed the money to get more MICARTA)

NEXT EXPERIMENT:

I decided to test different ways to heat the steel for the quench. Now up to now I have heated the steel for the quench just like Ed Fowler does, with a O/A torch. heating only the lower section .

Well,,,I ran a little low on gas one night, so I fired up my Wisper Low Boy forge and heated a blade with it for the 3-time quenchings. (I let the blade cool in the oil for 1/2 hour each time)

It seemed to work, I wrote to Ed Fowler and he seemed to think no real harm was done.
So on this weeks 2 blades I have set aside my O/A torch and just used the Forge for the heat treatments.

I still do 3 heatings and quenchings, (but due to some science-backed information that I have seen posted from METE, I dont do 3 H/T based out of any idea that such a set of H/T is better, I just do them for no real reason other than they are fun.)

Yes, I heat the full blade with the forge and this means that there is some fire during the quenching. I still just quench the edge of the blade. I dip the tip, waite for about 3 seconds, then lower the edge into the oil for about 12 seconds, then rock the tip, then down again...the last change I have added to this experiment is to no longer lower the blade into the oil to cool down after the edge has quenched enough.

In the Fowler video (my hero and only teacher) Mr Fowler lowers the blade into the oil to cool down for 24 hours...I have given up doing this right now..( "Why?"...easy, It's -40 below Zero in my unheated shop, I dont want to have to come and get it in the morning)I hold the blade edge in the oil until the redness of the blade spine is dark, then re-heat right away for the 2nd quenching, then the 3rd,,then I hold the edge in the oil until the blade is cool enough to hold with gloves and take it in to temper.



RESULTS > interesting...I end up with a darn cool looking set of temper lines in the raw blade . the temper lines are very deep in the blade as i take it out of the oil .

I dont know for sure but perhaps the forge-quench-no dunking the whole blade, might be something I should keep experimenting on...hey it might be my "way"

I have not etched a blade done this way yet due to the guys around here hate etched blades...But I will have to etch one soon to back-up what i think could be a very deep dark quenching line


I Just thought I would let you know what I have been doing with all the advice you guys have sent to me...I have been haveing fun, and trying to learn by doing...
 
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