Pull cut technique, and bouncing off a knot

[sploo]

I'm turning my first ever bowl (ok, second, if you count one under instruction). I glued together some sections of pine to make a blank, rather than waste "good" wood - though I've turned some practice spindles from the same stock and it's OK.

I've started on the base (top currently on a face plate) and was doing some push cuts from the centre/bottom out to the rim with a standard grind 3/8" Crown Bowl Gouge. I then switched to doing a pull cut, which is working well to remove lots of material. However, I've only previously seen pull cuts done with a more swept back (fingernail) grind.

I've got the tool rest relatively low, with the tool at about 45 degrees to the face, and swung about 45 degrees clockwise - so I'm sliding the tip along the toolrest towards me, with the handle on my waist. The image below shows approximately the area of the gouge that's doing the cut in this pulling movement:

It feels OK. I'm getting good ribbons of material and removing a lot of stock fairly quickly. Obviously this isn't riding the bevel and the cut is rough, but am I doing anything dangerous? I.e. given the grind of this gouge, is that an acceptable/correct way to present the tool?

I have a spare 3/8" bowl gouge and I was wondering about doing a more swept back grind, as I assume that'll be a little better for such pull cuts?

Also, the material does have a pretty hefty knotted section, and the gouge is "bouncing" off the harder material, then taking more off the softer opposing side. Is there a particular technique for dealing with this issue? I think I'll need to remove all of that knot for the shape I'm after, so if the answer is "just cut through it" then that's fine.

 

[CHJ]

Personally I would leave the Pull cuts and increased swept bevels for that matter until you have a lot more experience.
Unless doing a shear cut, also something for the more experienced then the bevel should always be providing the support to prevent the tool twisting and digging in (a catch) extremely likely with knots in the wood to the point I would say Will happen at your stage on the learning curve.

The fact that you are getting tool bounce tells me you have not yet mastered the correct holding priorities of the tool and is something you need to appreciate as to why it is happening.
We all experience it at times but knowing what it is and what you are not doing correctly is prime in sorting it out.


Your arms and fingers are flexible and soft.
When the tool encounters a tough bit of wood the wood pushes back, your hold being flexible lets the tool move back a fraction and then when it encounters softer wood dives back in deeper.
Next revolution this happens again and becomes more pronounced each time.

You must concentrate on adjusting your hold so that the tool/tool rest contact point stays the same and resists the forces that are trying to push the tool back at you, we are talking fractions of a millimetre initially but it rapidly compounds due to turning speed and less than optimum hold.

Think of a metal lathe where the tool is firmly anchored in a tool post, all things being equal it can't move and removes the same depth of cut regardless.

Pushing the tool too hard against the bevel will exacerbate the problem, take light cuts with the bevel rubbing but held firmly on the rest (fingers against rest).
Although we refer to a push cut this is really just the direction of travel, if you are having to PUSH to achieve material removal then the tool is not sharp enough and/or presented incorrectly.

To see the difference of bounce, when you experience this with a normal bowl gouge, use a parting tool or firm scraper held firmly on the tool rest (at or below centre on outside cuts) to approach the area where the bounce is occurring, It will knock as the wood passes and remove the high points but should not bounce back if you have tool/rest control correct.

If it does continue to bounce then you are pushing the tool into the work and following the wood surface rather than holding its position on the rest.


One other thing that can resemble tool bounce with uneven wood density is that even with best tool/rest control the work piece can move in relation to its holding point, most likely to occur if on a screw chuck, small spigot or socket, 'rouge wood' is best supported on a faceplate or with larger diameter jaws to reduce the chance of this work piece flexing. (The wood is doing what your fingers let the tool do.)

 

[sploo]

Thanks Chas.

The blank is held (with several screws) on a 6" faceplate - so hopefully pretty secure.

I take the point about pull cuts, though I've been careful to ensure there's a line directly from the cutting edge to the gouge's shaft on the toolrest (i.e. cutting with a supported part of the tool). Am I otherwise doing something that's OK (i.e. is the pull cut I'm attempting the "right" way to do it)?

In this blank I think there's a significant difference in density between the (fairly soft) pine and the (pretty dense) knot. I did try last night to do a really light cut to try to just catch the high spots, but it didn't seem to take much material out. Having said that, by that point I suspect the gouge may have been getting blunt, and I had to stop for the night.

I don't yet own a scraper, but would that be a better tool for getting my way through a hard knot?

On pushing with a push cut - yes, I'm not forcing anything; just letting the tool ride on the bevel, with small body movements to change the angle to "stay" on the cut and get the shape I want.

 

[Paul Hannaby]

Using a sharp gouge with a bevel rubbing cut would be the best way to deal with the knot.

 

[sploo]

Using a sharp gouge with a bevel rubbing cut would be the best way to deal with the knot.

I resharpened the gouge and started taking very light cuts (whilst holding the tool down hard on the rest) to minimise the "kicking" and that worked ok.

However, I realised I was going to end up removing all the knot for the shape I wanted, so in the end I just cut it off with a saw, then continued turning.

I got the basic shape I was after, but there was still a lot of tearing:

I quickly resharpened again and took a few push cuts across the face (I just about managed to get a few uninterrupted passes across the whole face) and that made a huge difference - clearly this timber seems to be pretty aggressive on the tool (there are still some knotted sections, so I guess that's understandable).

A fair bit of sanding and some (not very evenly applied) sanding sealer and it's not looking that bad (despite it turning out to be pretty grotty wood, that was lazily slapped together with some glue):

 

[duncanh]

The problem with glue-ups like this is that you need to be careful to get the grain orientation consistent so that it reacts like a normal bowl blank would do. If you get one or more of the pieces backwards then some sections will get tear out whilst others won't.

Also, the glue is likely to be blunting the chisel quite quickly.

Pretty good for a first bowl though.

If you're planning on holding the foot in a chuck I'd have put in a flat shoulder to rest on the jaws. I'm sure Chas will be along soon with a useful diagram...

 

[g7g7g7g7]

If you have an extremely soft wood and an extremely hard knot (like spruce whitewood) would it be advisable to drill the knot out if you are going to be removing it completely?

 

[sploo]

The problem with glue-ups like this is that you need to be careful to get the grain orientation consistent so that it reacts like a normal bowl blank would do. If you get one or more of the pieces backwards then some sections will get tear out whilst others won't.

Also, the glue is likely to be blunting the chisel quite quickly.

Pretty good for a first bowl though.

If you're planning on holding the foot in a chuck I'd have put in a flat shoulder to rest on the jaws. I'm sure Chas will be along soon with a useful diagram...

You're spot on with the grain orientation - it was obvious that the tearing was in stripes (on the strips of wood that had the grain in the "wrong" direction). Lesson learned in that regard.

Good point about the glue too - I was using Titebond II, which does dry pretty hard.

I did have a small flat shoulder by the tenon - mostly because I don't own a tool that would get into the "corner" yet!

I put the blank on the chuck, drilled a centre hole as a depth marker, then started making push cuts to hollow out the bowl. That went really quickly and cut nicely, though I did encounter one issue; if I make a deep pass then I found I'd be getting quite a lot of material being removed by the right "wing" of the gouge (image below), and I was worried that might roll the tool over. It occurs to me that a fingernail grind would keep the wings out of the way, but I'm heeding the advice to stay away from that until I get more experience.

Anyway, I finished the hollowing, and it went pretty well. I certainly see why a wide curved scraper might be nice to smooth off the curve on the inside (don't have one of those yet), but the basic shape of a mild ogee outside, a dome on the inside, and a slightly curved top went basically as planned:

I've bought some wood jaw plates, and my intention was to make a wood "chuck" to hold the bowl so I could remove the tenon on the bottom. I deliberately cut the hollow fairly shallow, but on looking at the finished shape I think I want to remove a bit more material from the bottom (to reduce the height of the bowl). I think I've left myself enough thickness at the base so I'll be OK.

It may be a week or so until I get time to try that, but for a first ever bowl, made from some strips of cr@ppy pine, it's not bad.

The cellulose sanding sealer went on a bit thick, so isn't the best finish. I think I have some cellulose thinners from my paint spraying days, so might cut it down a bit next time.

 

[sploo]

If you have an extremely soft wood and an extremely hard knot (like spruce whitewood) would it be advisable to drill the knot out if you are going to be removing it completely?

See my post from today (at 13:37): I cut it out.

 

[CHJ]

Just bring the wings of gouge back a little more each time you sharpen and as you gain experience, but take care that you do not end up with a 'pointy' front end, it's a case of watching the amount of 'dwell' time on any one segment of the tool profile as you swing it on the wheel.

In the mean time the prominent wings you currently have are making you aware of the catch potential and the need to watch the tool approach angle to the work piece.

 

[sploo]

Just bring the wings of gouge back a little more each time you sharpen and as you gain experience, but take care that you do not end up with a 'pointy' front end, it's a case of watching the amount of 'dwell' time on any one segment of the tool profile as you swing it on the wheel.

In the mean time the prominent wings you currently have are making you aware of the catch potential and the need to watch the tool approach angle to the work piece.

I was very aware earlier on :wink:

I guess I'm bound to have a proper catch at some point. I had a couple of minor ones when I started with spindles, but I'm generally trying to be a bit cautious - and doing a "practice cut" with the lathe turned off whenever I'm doing something new. I suppose that's "air turning", instead of air guitar. No gurning though :mrgreen: