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TTIT
3rd May 2007, 04:27 PM
OK - the last thing we need is another sharpening thread/argument/epic etc but I came across this statement and wondered if there was any truth in it or just this guy's (http://www.bigtreetools.com/articles/wishes.html) opinion.

8 FLAT GRIND – One day I discovered that chisels ground with a flat bevel work better than those that are hollow ground. I quickly re-ground all my cutting tools to the new flat grind, and I have never looked back. It’s hard to describe the feeling of that day. Without buying anything new or investing additional years of practice, I had suddenly made great progress in my ability, and I was seeing results that amazed me. If you attended the lecture by Michael Dunbar last September, you may remember he said the same thing about draw knives, scorps, etc. Any tool which is guided by riding the bevel should not be hollow ground.

I've often wondered about this myself but couldn't find anything in old threads about it. Has anyone here tried it and agree with him????????? I've got a crappy GMC belt sander gathering dust that could be put to use if there's any substance to this.

silentC
3rd May 2007, 04:46 PM
I believe he is talking specifically about turning chisels. Otherwise it's complete nonsense because with a bench chisel for example, you don't normally guide the tool by rubbing the bevel as you do with a gouge.

I have both flat ground and hollow ground my bench chisels and have never found a difference in performance, other than the time it takes to re-establish an edge.

TTIT
3rd May 2007, 04:51 PM
I believe he is talking specifically about turning chisels.
That he is Silent, and that's all I'm concerned with too!

silentC
3rd May 2007, 04:55 PM
OK well my non-turner opinion, or more of a question really, is that given the thing you are turning is round, how much of a flat bevel is in contact with it in comparison to a hollow ground edge? In other words, does it really make any difference?

soundman
3rd May 2007, 05:39 PM
Flat or hollow ground.... one thinh I do know is that you can manipulate the behaviour of turning chisels by changing the angle ait which they are sharpened.
I ahve played with angles on skews, gouges and scrapers and yes the angle makes a big difference, especialy for bevel rubbing applications.

Now I'm discounting longevity of the cutting edge here as that is another story.
Cutting performance is also another matter.
I'm just looking at tool dynamics.

with skews and gouges
where you often rub the bevel,
grinding a fine edge will allow a closer cuting angle and a finer cut to be made at shallower angles.
grinding the bevel at a steeper angle will not allow a finer cut but it will allow you to rub the bevel at more agressive cutting angles, thus allowing more control on a heavier cut.

with scrapers
grinding a more acute edge will allow more agressive cuts but will also allow bigger digs.
grinding a steeper almost square edge will result in a scraper that will tend to chip limit particularly inside things.

changing from a hollow grind to a flat grind at the same ground angle (measuerd with a line point to heel) will result in a tool that will rug its bevel as if it were ground steeper.

Its certainly worth a play with cutting angle rather than staying with the "one correct angle" dogma.

cheers

WillyInBris
3rd May 2007, 05:43 PM
Different grinds for different minds.

Toolin Around
3rd May 2007, 08:49 PM
Hmmmm maybe Derek Cohen is onto something. A few thoughts on it. There is no such thing as a flat grind. Not unless the tool is held in a very rigid device. Grinding by hand means you will always have a convex grind no matter what. How much of one though depends on the steadiness of your hands and the abrasive you use. A belt will round more then a rigid stone... To turn well in a pure cutting method it has always been assumed that support behind the leading edge was essential, that's where a concave bevel comes in. A catch will reinforce that idea for anyone. A convex bevel doesn't allow for that sort of support under a lot of situations. So most likely the tool is held somewhat differently to the wood so that it relies less on the support of the bevel. I would suspect that it may be a hybrid technique of scraping and cutting combined, which I see often in other turners when using a bowl gouge. I think with the introduction of HSS to the turning world the need to be a pure cutter has been relaxed a great deal. In the days of carbon steel you couldn't get away with scraping with such an grind with out dulling the tool real quick.

rat52
3rd May 2007, 09:13 PM
According to my bible (The Complete Guide to Sharpening by Leonard Lee) .While hollow grinding reduces the honing time it does so at the expense of a strong edge.

This becomes more of a problem with smaller dia wheels.

His suggestion to overcome this is to use micro bevels.

I would like to type out the page he devotes to this with all the data but 1/ It would be violating copyright:no: and 2/ at my typing speed of 8wpm I would not get any sleep tonight:zzsoft:

soundman
3rd May 2007, 09:38 PM
As with several discussions in the past we must remember we are talking about turning chisels here.
The whole micro bevel concept is then pretty redundant.

size of wheel though is a valid issue.

how hollow is hollow and does it matter?

cheers

Toolin Around
3rd May 2007, 09:53 PM
According to my bible (The Complete Guide to Sharpening by Leonard Lee) .While hollow grinding reduces the honing time it does so at the expense of a strong edge.

This becomes more of a problem with smaller dia wheels.

His suggestion to overcome this is to use micro bevels.

I would like to type out the page he devotes to this with all the data but 1/ It would be violating copyright:no: and 2/ at my typing speed of 8wpm I would not get any sleep tonight:zzsoft:

This is one area where I completely disagree, unless you're talking about chipping the edge that is. And then all I can say is you're using the chisel wrong. When a tool dulls it's because you're removing molecules of the alloy steel and or you're getting microsopic chipping. A hollow grind has absolutley no bearing on that what's so ever. When you hone an edge you have at least a mm of edge that is at about 30 degrees. There's a lot of molecules packed into that one mm. And microscopic chipping wouldn't exceed the honed area on the bevel either. If you plan to pry with the tool you have sharpend then yes you can expect that the edge may fail. That's why a pig sticker (mortice chisel) has a steeper angle to reduce the chance of chipping and or breaking. All my carving chisels have an angle that I would suspect is in the 10 degree range and they stay sharp for a fairly long time. But I would never dream of trying to "pry" with them (I did once when I was first starting out many years ago). I would suspect that the average bench chisel ground on a regular grinding wheel would have a steeper angle than my carving chisels and would more than likely (unless you bought crap chisels) would last a lot longer than my carving chisels.

powderpost
3rd May 2007, 10:11 PM
Most of my turning tools are ground generally at about 30 degrees. I don't want to get into an argument about "correct" grinding angles, that angle suits me. If I am turning hardwoods that angle is reduced. If working soft woods theat angle is increased. The difference is for hardwoods the edge is a bit thicker, therefore stronger and for the softer woods the chisel has a finer and sharper edge. I disagree that the ground bevel can be flat, simply because with consecutive honings the bevel will become rounded, causing difficulties in getting the chisel to cut. This all applies predominately to the skew.
Jim

Skew ChiDAMN!!
3rd May 2007, 10:30 PM
As has been said, the angle is more important than anything and it varies from wood to wood and turner to turner. (I like 60-80° scrapers, 40-50° gouges and around 25-30° skews. Early on I had a table listing "correct" angles for tool types and had nothing but grief until I threw it in the bin and discovered for myself what worked for me.)

Saying that a hollow ground bevel has less support than a flat grind is misleading. It's true if you're measuring from high-point to high-point, but although people say the "angle of the bevel" what they should be saying is the "angle at the cutting edge."

That's what's important... and is totally independant of the size of the grinding wheel or radius of the grind. :rolleyes: In reality, a hollow-ground tool with the same cutting angle as a flat-ground tool actually has more support!

If someone is finding that a flat bevel is cutting better, then they should try cutting their hollow grinds at a slightly steeper angle than what they're used to... and I'll be they find a matching improvement. :p

Salty
3rd May 2007, 10:46 PM
For my money the benefit of a hollow grind is that you get the chance to hone a sharp edge back on the tool without having to remove as much steel as with a flat bevel.
I am not in the class of turner which can detect the difference between a hollow grind or a flat bevel when actually turning:no: How about anyone else?

Cliff Rogers
3rd May 2007, 10:58 PM
Vern..... :ohcrap:

Frank&Earnest
4th May 2007, 02:17 AM
For my money the benefit of a hollow grind is that you get the chance to hone a sharp edge back on the tool without having to remove as much steel as with a flat bevel.
I am not in the class of turner which can detect the difference between a hollow grind or a flat bevel when actually turning:no: How about anyone else?

I agree. My practical experience is very limited but I have been reading books about sharpening for 40 years. All consider a concave or convex grind "wrong" because in cabinet making and carving it is important that the whole bevel touch the wood, but this is not important in turning, as Skew said, provided the actual angle remains within the desired range (that's why using a small grinding wheel is more problematic). Because turning requires more frequent sharpening, also, the balance between purity and practicality tilts very much towards practicality. I had this discussion with the instructor of the TAFE course I have been taking, who has been turning wood for as long as I have read books about it, and he is of the same opinion.

Stu in Tokyo
4th May 2007, 02:55 AM
I use an 8" slow speed grinder, I have the good Oneway wheels on it and the precision balancing kit.

I have started to freehand grind, I find the grind is so much better than with a jig (after some practice) and it is dead quick too!

Dunno about the hollow grind, as I do not get a hollow grind from freehand sharpening.:2tsup:

Cheers!

TTIT
4th May 2007, 09:05 AM
For my money the benefit of a hollow grind is that you get the chance to hone a sharp edge back on the tool without having to remove as much steel as with a flat bevel.
Damned good point Salty - without thinking about it, I use the same principle on my skew daily - starting to wish I'd never found that website now - the word 'sharp' really opens up a can of worms here eh!:doh:


........ and discovered for myself what worked for me.)

............ what they should be saying is the "angle at the cutting edge."

.......................................... If someone is finding that a flat bevel is cutting better, then they should try cutting their hollow grinds at a slightly steeper angle than what they're used to... and I'll be they find a matching improvement. :pSounds fair enough to me - think I'll stick to the 8" wheel and stop reading other turners websites!:;

Cliff Rogers
4th May 2007, 09:16 AM
...- think I'll stick to the 8" wheel and stop reading other turners websites!:;
Good move.... I mean, it is not as if you are a crook turner yourself is it. :D

rat52
4th May 2007, 11:11 AM
As my old grandpa said "There may be a 100 ways to do a job but it is the end result that counts"

PAH1
4th May 2007, 01:09 PM
TTIT

I can actually see the guys original arguement. It has nothing to do with sharpening times, andgles or the strength of the edge. If you have a curved bevel "hollow ground" as you make an entry cut you need to have the tool at a given angle to the wood, as you go in further you now bring into contact an area further back, because the two points of contact are on a curve you have to change the angle of the handle to maintain contact. Once you go in the full length of the bevel then you are in constant contact and no longe have to change the angle. Another way of achieving much the same thing is to grind away the heel of the bevel, I have recently changed to this and the difference is unbeleivable.

Frank&Earnest
4th May 2007, 01:37 PM
TTIT

I can actually see the guys original arguement. It has nothing to do with sharpening times, andgles or the strength of the edge. If you have a curved bevel "hollow ground" as you make an entry cut you need to have the tool at a given angle to the wood, as you go in further you now bring into contact an area further back, because the two points of contact are on a curve you have to change the angle of the handle to maintain contact.

No argument here. Is this such a big deal for 99% of people, though?


Once you go in the full length of the bevel then you are in constant contact and no longe have to change the angle.

Theoretically no, practically yes. When the difference is micrometers, who cares?




Another way of achieving much the same thing is to grind away the heel of the bevel, I have recently changed to this and the difference is unbeleivable.


Then you practically use the rounded heel as a very shallow mini-gouge. Could work very well, I don't know, but is it a chisel any more?

PAH1
4th May 2007, 02:14 PM
"Theoretically no, practically yes. When the difference is micrometers, who cares? "
Actually it is a bit more than that, as the angle difference is magnified by the length of the handle. I think that many people actually push cut the first bit and do not actually rub the bevel untill the heel rubs anyway.Then you practically use the rounded heel as a very shallow mini-gouge. "Could work very well, I don't know, but is it a chisel any more?"I can tell you the difference in tool control is rather large, you do not realise how much pressure you place on the heel of the tool. If it is not there you can not force it, it reduces chatter to an amazing extent when doing bowls. For spindle work I am not as sure because you should not be rubbing the bevel that far back and with back hollowing you do not want a prominant heel.

hughie
4th May 2007, 03:52 PM
Vern,

Just another variance in the life a wood turner. I reminded of Richard Raffans commments on sharpening.

quote:
"There is only one rule about sharpening, there is no rule."


horses for courses :U

Bruce101
4th May 2007, 04:36 PM
Rightly or wrongly, I have found that the longer I stay away from a grinding wheel, the better results I have with turning. I use predominantly a hone stone, when the chisels blunt, the course side, then turning to the fine grade for a few (very few) finishing strokes. A regular touch up with a small fine lapping stone also helps on hard timbers.

Angle would be about 25-30 deg. and using this method I have had more success than constantly turning the the grinder (and it's kinder to the chisels).

rsser
4th May 2007, 07:29 PM
I use an 8" slow speed grinder, I have the good Oneway wheels on it and the precision balancing kit.

I have started to freehand grind, I find the grind is so much better than with a jig (after some practice) and it is dead quick too!

Dunno about the hollow grind, as I do not get a hollow grind from freehand sharpening.:2tsup:

Cheers!

Well, may I say, in the nicest possible way ( ... yep, here it comes!) , what the hell does that have to do with anything??

......


Seem to recall Darlow advocating a belt to grind turning tools. Serious call as far as I'm concerned.

So how about some serious-minded posters doing comparative tests?

By way of a model, check out Lyn Mangiamelli's test of hand plane blade angles. http://www.woodcentral.com/bparticles/haspc.shtml

Stu in Tokyo
4th May 2007, 08:24 PM
Well, may I say, in the nicest possible way ( ... yep, here it comes!) , what the hell does that have to do with anything??

Thanks for being as nice as you can possibly be:2tsup:

What it has to do with, from where I'm sitting reading this thread, is that when you use a jig, the tool is held rigid, and while is kind of rolls, it really does not, IMHO.

When you freehand sharpen, the tool is rolled and turned, thus there is little, if any hollow grinding going on.

I'll go back and read the whole thread again, I could have taken it wrong, not like that has not happened before :-

Cheers!

Stu in Tokyo
4th May 2007, 09:13 PM
Well, I just read the whole thread again, and I'll stand by what I said.

I am talking about gouges, I don't see how the conversation could relate to skews, and in fact, the OP quoted article has this pic with it.......

45400
...... which, last time I checked, is a picture of a gouge...........no?

When you have a gouge held in a jig, like the vari grind or such, the gouge is rolled against the wheel, it arcs in only one plane, which simulates a good hand grinding, but does not duplicate it, 100%. Because the gouge is being ground by a round wheel, the arc, or hollowing of the grind will exist. Of course this hollowing would be much more pronounced on a 4" wheel, compared to a 6" wheel, compared to an 8" wheel.

When I freehand sharpen, I roll the gouge as well as swing it, and the results are, not hollowed in any way, that I can see, in fact, I'd say they are, if any thing somewhat convex in shape, but only very slightly.

So, I'd say that what I said in my first post to this thread has something to do with the subject manner, but then again, maybe I was not being clear enough.

My bad.

...........being as nice as needed

PS, Ern, do you freehand sharpen gouges?

hughie
5th May 2007, 04:31 PM
Hi Guys,



It seems there is a generally held view 'out there' for hollow grinds and this then is a good a place as anywhere to start for for all of us.

For those who have the eye and the dexterity, hand grinding is the way to go for them. :U

The rest of us have and most likely will continue to need assistance... a jig. :C :roll: or a friend

It seems as our experience grows in turning, so does our view point on whats the right way to go. This is not only based on experience but ability,equipment, application and attitude etc.

We are all on different parts of the learning curve and this is indicated by the wide responses to this thread :2tsup: and thats real healthy and it keeps the forum alive.

For me I have got some thing out of nearly every response. :2tsup:

Thanks guys!

rsser
5th May 2007, 04:57 PM
Stu, I take the hollow grind profile to be a function of the radius of the grind wheel. On an 8" wheel its observable and on a 6" job I imagine it would be significant.

It may be that with freehand grinding you are covering the same part of the bevel in subsequent passes differently presented to the wheel and so grinding out the hollow; I'm not sure.

As has been said, it would seem to be the case that a hollow grind would increase the distance between the edge and the part of the bevel that's rubbing. This may increase tool chatter and the quality of the finish by noticeable amounts.

Yes, I use a jig for all turning tools on the grinder but freshen the edges with freehand honing. After a while I can see the honing start to flatten out the bevel.

Stu in Tokyo
5th May 2007, 06:29 PM
Stu, I take the hollow grind profile to be a function of the radius of the grind wheel. On an 8" wheel its observable and on a 6" job I imagine it would be significant.

It may be that with freehand grinding you are covering the same part of the bevel in subsequent passes differently presented to the wheel and so grinding out the hollow; I'm not sure.

As has been said, it would seem to be the case that a hollow grind would increase the distance between the edge and the part of the bevel that's rubbing. This may increase tool chatter and the quality of the finish by noticeable amounts.

Yes, I use a jig for all turning tools on the grinder but freshen the edges with freehand honing. After a while I can see the honing start to flatten out the bevel.

So it seems my post earlier did have something to do with something :D