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A sharpening request for advice....

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Kalimna

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Greetings folks,
For about three years Ive had a lathe and not progressed really beyond some basic bowls (though I am at last generally happy with the shapes I am achieving). And this I don't have a problem with as I only turn rather infrequently due to other commitments (mostly a 1 1/2 year old boy at the moment).
However, and this is where I would like some advice, following a break for a smidgen over a year (due initially to my right hand out of action for a month following a purely accidental bench chisel injury) I fired up my Record CL1 and attempted to turn a couple of bowls over the weekend.
I was havign great difficulty indeed achieving a finish straight from the bowl gouge that didnt require very coarse grit sandpaper to rectify prior to running up through the grits. Two things seemed to be happening (on wood varying from yew, hornbeam and sycamore), firstly I was getting significant tearout and secondly a lot of chatter, resulting in spiral-like ridges running down the bowl. The chattering seemed to happen even with the lightest of cuts.
The way I sharpen my bowl gouge is freehand, rotating through a 'U' shape on the grinder.
So I wonder if anyone could offer any advice as to what may be happening - is the gouge not sharp/correct profle/correct angle? Tool presentation not correct? etc,

Any help appreciated, thanks,
Adam
 

John. B

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Adam,

At a guess, I would say you are correct on both counts. That is to say, they are not sharp and possibly a wrong profile.

these are bowl gouge profiles.



Also as yourself pointed out, you are relatively inexperienced in turning so I would suggest you invest in a sharpening jig.

It would be wrong of me to suggest what jig, as the cost of these range from reasonable to ridiculous. I would however suggest joining a woodturning

club. Failing that attend a woodturning show to see for yourself how they are used.

I am really sorry to be negative, but I've always used a jig, namely a Tormek for my gouges, one of my club members uses a wolverine and

occasionally does it by hand as you do. But! he always says it isn't as good as using the jig.

John. B
 

nev

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i know what i want to say but dont know if i can do it without a drawing (which means changing computers)
one day i will get a sharpening jig, till then like you i am freehand ,and newish to the sport so please dont take this as FACT, but may be worth an investigation.
i had a similar problem and it turned out that i had a very slight concave profile on my gouge so hardly any cutting edge was reaching the wood #-o and what was was 'grooving', and not in a good 70's way :)
a slight flick of the wrist at the top of the 'U' whilst sharpening was all that was required to get back the convex profile and a decent cutting edge.

hth
 

Paul Hannaby

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Hi Adam,
I also sharpen my bowl gouges without a jig and if you are sticking to a basic straight grind, I don't see a problem with that. The shape I use for my bowl gouges can be seen here - http://www.hannaby.com/bowl-shaping.html.
The torn grain might be due to a blunt tool? Try resharpening and repeat the cut to see if there is any improvement.
The chatter might be due to a number of things - losing bevel contact because your gouge has too shallow a bevel in a deeper bowl (again, see the link above for an explanation). A bevel of around 50° would be a good place to start. It might also be due to incorrect tool presentation including not swinging the handle back far enough as you progress round the internal curve so bevel contact is being lost. It could also be due to the gouge handle being held too high, leading to an incorrect angle between the cutting edge/bevel and the wood surface. Remember to follow an arc from the edge of the bowl down to the centre.
The chatter might also be reduced by using a secondary bevel (again, see the above link for details). This allows the gouge to better follow the internal curve.
Another cause of chatter is thin wall thickness in the bowl but this would only be evident at the final stages of turning. The answer to that one is all of the above or to reduce wall thickness in sections to maintain support.
 

chipmunk

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Hi Adam,
I feel I should try to save you some hard earned money!

Grinding a bowl gouge straight-across as you are doing is absolutely fine for basic bowl work and can be done by anyone freehand. Maybe buy a jig later when you know what you want it for and how much money you have. I do agree with John about joining a woodturning club.

Have you tried applying felt tip pen to the bevel on the gouge to see where you are grinding it? That's a good tip.

The aim should be to lightly touch the bevel on the grinder and rotate from side to side while sitting flat on the toolrest of the grinder. All of the bevel should be ground with one facet and all of the felt pen removed if you're doing it right. Don't worry about wearing your gouge away - provided you are only using light pressure it'll last long enough... and although you're using up HSS, gouges are a lot cheaper than jigs.

This is your basic grind. It can sometimes help to grind the tops of the wings a bit more to make it look more like John's profile B rather than A but A is fine to begin with.

HTH
Jon
 

Kalimna

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Many thanks indeed for the advice so far.
From what has been said above, I think that the way I have been sharpening has been the main source of problems, which have perhaps led onto the chattering (I had wondered if a resonant frequency type phenomena was happening which just exacerbated the problem) as well. The chattering was happening on the outside of the bowl also...
The profile I have been grinding is probably closest to figure 'e' in the above diagram, but one side wing tends to end up higher than the other, and frequently with a sharp angle rather than the flowing edge.
I shall try grinding some more (there's plenty of tool left!), at the angles suggested. Unfotunately, there are no nearby turning clubs that I am aware of (I live in Alloa, near Stirling), so that I dont think is an option.
The reason I was going down the freehand route was that it would (in theory) take me much less time to 'touch up' an edge rather than fiddling around setting up a jig each time. Also the chap Ive had a couple of (very good) lessons from made it look much easier than my brain remembers!

Again, many thanks for all input so far!
Adam
 

Spindle

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Hi

I may be wrong but the spiral ridges you describe sound a lot like 'barley twists' experienced by me when spindle turning. They are usually indicative of a blunt tool or flexing of the workpiece. If flexing is the case then reversing the direction of cut can remove the ridges.

Regards Mick
 
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