Is it the Concave bevel or is it Me ?

[myturn]

What are people's experience with concave bevels on fluted gouges as regards them possibly causing a rippling or ridged effect when running down the inside of a bowl?

I often find I can't get a smooth progressive curve down the inside, particularly on tighter curves and it seems to be worse when taking finer finishing cuts. A scraper cleans it up but I would rather get a clean cut in the first place.

I was reading an article on a.n.other forum and the symptoms sound similar. It suggests that the heel of the bevel on a concave grind, being relatively far from the cutting tip, is the cause of this.

Is there any benefit to be had by adding a secondary bevel (has anyone done this) or is it unnecessary and better tool control is what is required ?

 

[loz]

I tend to use a bowl gouge with quite a short bevel ( mainly due to being a small gouge ! ) for finishing cuts on the insides of bowls, adding a secondary bevel would provide similar shape.

How concave are your bevels though to prove such a nuisance ?

 

[jumps]

I would have thought that the 'benefit' of a shorter bevel (when traversing concave surfaces) is that you are going to get a shorter pitch and a less pronounced (possibly not even noticeable with the naked eye) effect for the same tool used at the same angle of rotation.

I would have thought that theoretically any bevel, less than a convex one matching the concavity of the cut, has the potential to create an impression on the surface behind the cut when presented at 90 degrees, whilst on a flat surface you would require a concave bevel.

So in practice the shorter bevel may give you the option of a presentation angle that avoids the issue over a wider range of angles and radius of cuts - but the fundamental issue remains. Same applies to the degree of concavity to the grind of the bevel.

 

[Mark Hancock]

Mick

Hollow grinding can produce a harsh keel on the grind which can cause a problem behind the cut of brusing or rippling the wood fibres. If you soften this keel ie put a second bevel on the grind you reduce this problem.

There are tools available (Eli's from Hamlet and Hannes' own tools) where the secondary bevel is more pronounced making the primary bevel behind the cutting edge very small, maybe 1mm. In effect you have a convex grind which doesn't have the issues you are experiencing.

Mark

 

[Leo]

Sorry if this is stating the obvious, I do not know your level of experience, I am quite new on here, so forgive me in advance if I am trying to teach you to suck eggs,

I think, without watching you, that you may get hundreds of differing replies/methods on this one, but for a final cut I usually, regrind, knock off the back edge of the bevel with a diamond file or whatever you've got handy, turn up the rpm, use the left hand edge of the gouge and go slowly and take a thin cut. If it is actual ridges that you are getting, it could be vibration ( keep your left hand on the outside of the bowl and thin cuts ) or movement by you ( try to keep it gentle and keep the handle along the forearm )

Like I say, without a vid., or seeing you, it is always difficult to help....................there is always my favourite 80 grit gouge (hammer) :mrgreen:

 

[myturn]

How concave are your bevels though to prove such a nuisance ?

They are only slightly concave but so the ripples are only slightly noticeable. It may just be my use of the gouge.

... whilst on a flat surface you would require a concave bevel.

I would have thought that a flat bevel would be best on a flat surface, as when using a skew to plane square across the end of a piece?

If you soften this keel ie put a second bevel on the grind you reduce this problem.

Do you actually use secondary bevels Mark?

If a secondary bevel is advantageous this brings me onto how to sharpen and maintain the cutting edge.

Presumably you create both bevels at the start and when sharpening you only sharpen the cutting edge leaving the secondary bevel untouched. After a while, and depending on how much you take off to sharpen the cutting edge, the secondary bevel will reduce in length such that you have to start again.
I'm wondering how often the need to regrind the secondary bevel will be (don't want to spend too much time at the grinder) but I guess that will depend on the gouge, the angle, how much I take off and how often I sharpen. In other words, try it!

....................there is always my favourite 80 grit gouge

I don't use my hand on the outside of the bowl as it's too busy guiding the gouge and I've had too many cuts trying to do two things at once with it, inattention is not a good thing!
Scrapers and paper can always be relied upon. :mrgreen:

 

[Leo]

Mick,

Sorry I see what you mean, but my thumb guides the gouge and forefingers on the outside, the turning bowl and the bevel rubbing hold the tool down, I have found that too much concentration can cause a jittery/ forces movement anyway, so I try to be cool, which is difficult for me !!!!!!!!!!!! hehehe.

 

[Mark Hancock]

... whilst on a flat surface you would require a concave bevel.

I would have thought that a flat bevel would be best on a flat surface, as when using a skew to plane square across the end of a piece?

With a skew I hone with a diamond card so you do end up with a flat bevel as the diamond card rests on the cutting edge and the heel.


I have secondary bevels on all my gouges. I sharpen the primary bevel first using the grinder tool rest or a jig and then simple move the tool up the grinding wheel and do the secondary bevel. I do this every time I sharpen gouges.

 

[Bodrighy]

Could you post some pictures of your grind Mark? I have seen so many variations on this 2nd bevel and have yet to successfully do it myself.

Pete

 

[jumps]

... whilst on a flat surface you would require a concave bevel.

I would have thought that a flat bevel would be best on a flat surface, as when using a skew to plane square across the end of a piece?

the omitted words before the statement quoted are rather significant to meaning here

whilst on a flat surface you would require a concave bevel.........to create an impression on the surface behind the cut

 

[steadyeddie]

Mick,
If you can get hold of Woodturning issue 204 (from Sept 2009) there's an article about this problem & how to solve it. If you like, send me a PM
with your address and I'll photocopy the article and post it to you (or would that be breaking copyright laws?)

Dave

 

[myturn]

Mick,
If you can get hold of Woodturning issue 204 (from Sept 2009) there's an article about this problem & how to solve it. If you like, send me a PM
with your address and I'll photocopy the article and post it to you (or would that be breaking copyright laws?)

Dave

Thanks for the offer Dave, I'll see if I can get a back issue and if not may take you up on your kind offer.

I added a secondary bevel to my favourite bowl gouge this evening, the Taylor KHS1 5/8" Superflute Kryo which has a swept back grind. After a few tests it does seem to traverse the curves more smoothly but I'll have to use it on more varied work before I can confirm that it's not a placebo effect.

 

[jumps]

there's reference in this sharpening guide to easing the back of the bevel - with guidance as to how etc

http://www.tormek.com/en/accessories/bg ... 100_en.pdf

 

[myturn]

That's an excellent guide, thanks.

I have what looks like the Irish grind and have now taken the heel off to see how it goes.

A lot of manufacturers would do well to follow that as an example of how to write a proper instruction manual. Some I've seen are barely legible photocopies of a very cursory and minimal description and virtually useless.

 

[Richard Findley]

Hi,

If it's any help I often compare this problem to using a bandsaw, which I know is of no use if you don't, but bear with me...

If you want to cut a 2" disc on a bansaw, you will struggle to do it with a 1/2" blade, the back of the blade binds in the cut. The answer is to use a narrower blade. It is the same when you look at the bevel of your gouge inside a bowl. The heel of the bevel interfers with the smooth cut you planned and causes bumps in the surface, not so big that you notice making them but big enough to see and certainly to feel. The answer is, as sugggested, either grind away a section of the heel, reducing your bevel by around half or grind a "micro bevel" on the front edge. Both work but I currently favour the first option as it leaves the angle of the bevel consistant.

HTH

Richard

 

[Mark Hancock]

Could you post some pictures of your grind Mark? I have seen so many variations on this 2nd bevel and have yet to successfully do it myself.

Pete

Pete

Sorry haven't had chance to photograph my grinds but the Tormek instruction linked to show the idea.

Richard, that analogy to a bandsaw is great.

Mark