Why am I getting spiral grooves with gouge?

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Why am I getting spiral grooves with gouge?

Postby robgorrell » Mon Jan 07, 2013 1:51 am

Yesterday was the first day I really tried to use the bungie lathe that I built last year. I finally got it functional and set time to work on using it. I have used a power lathe just a little.

What I noticed right away is that when I am using the gouge to go down-slope I am seeing sort of a spiral or thread-like appearance on the work. It looks like the cutting edge is moving the downward area to fast?

I just wanted to ask before I try again in a couple of days. Lots to learn.

Rob
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Re: Why am I getting spiral grooves with gouge?

Postby jrccaim » Mon Jan 07, 2013 5:50 am

Bodger's strikes again. Remember to check the "log me in each visit" box or you will lose your post. Site administrator: can we make "log me in each visit" a default? Grrr.

OK, on to the question. Short answer is "you are traversing the tool at the wrong rate for the speed of the turning." Long answer, please bear with me, bit more complicated. Suppose you actually wanted to cut a screw. Suppose the pitch were some given, say 2mm . Well, then for each exact rotation of the piece, you would traverse the tool exactly 2mm. Furthermore the traverse must be steady, not jerky. Very hard, if not actually impossible to do, on a pole lathe since it is all done by hand. (We have to withdraw the tool when the lathe goes back but this is just incidental.) With a treadle or power lathe we can resort to gears and a feed screw to get the advance done; with a pole lathe we do not have this luxury. So I actually envy you. I have been trying to cut screws on a pole lathe for at least a year! What you are doing is traversing at a rate exactly multiple to spindle rotation. Oops, been corrupted by machinists. For spindle read workpiece. I wish I could traverse like that :) I do not by any means mean to insult your skills! Indeed I envy you.

If what you want to do is produce a cylinder, then a spiral is annoying, but easily cured. Just traverse the tool at some different rate. Myself I prefer slow, but faster is just as good. Just get it out of sync with the spindle, oops, workpiece, rotation.
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Re: Why am I getting spiral grooves with gouge?

Postby gavin » Mon Jan 07, 2013 10:48 am

robgorrell wrote:Yesterday was the first day I really tried to use the bungie lathe that I built last year. I finally got it functional and set time to work on using it. I have used a power lathe just a little.

What I noticed right away is that when I am using the gouge to go down-slope I am seeing sort of a spiral or thread-like appearance on the work. It looks like the cutting edge is moving the downward area to fast?

I just wanted to ask before I try again in a couple of days. Lots to learn.

Rob

JRC's post above analyses your problem elegantly.

Short answer:
If you wish to avoid the spriral - don't move the gouge left or right as you turn each time you depress the treadle.

Longer answer:
Assume you have a roughing gouge of 50 mm width. Move gouge 50 mm sideways between treadle depressions. You'll then have a series of valleys 50 mm apart. Then come back and remove the ridges between those valleys - which will also be 50 mm apart.
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Re: Why am I getting spiral grooves with gouge?

Postby robgorrell » Mon Jan 07, 2013 3:49 pm

Thanks guys. I think you overestimate my spiral cutting abilities :lol: . I know the answer is practice to cure the problem. The funny thing is that the "feel" in the tool was that it "wanted" to advance that way with each rotation. When I get at it again I will try to get a better grip on the tool and try to correct the advance to spindle speed rate. :D

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Re: Why am I getting spiral grooves with gouge?

Postby Bob_Fleet » Thu Jan 10, 2013 11:00 pm

One thing to help keep the tool still while the work moves is to have the tool rest as close to the work as you ca.
This means all of the benefit of the lever action is on your side.
Interesting how the horizontal speed and rotation come together like that.
All the best.
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Re: Why am I getting spiral grooves with gouge?

Postby Brian Williamson » Fri Jan 11, 2013 10:41 am

The guys are right. You can do a lot by adjusting your rate of traverse across the workpiece.

However, it is one of the immutable laws of nature that you will ALWAYS get spiral groves when traversing your cutting edge across your rotating workpiece.

A narrow, steep-sided gouge will give very pronounced grooves. A wide, flattish gouge much less pronounced grooves.

And when you move to your skew or other flat-bladed chisel to finish off, the spiral groves will still be there but they will have become almost imperceptible.

When hand, eye, tool and motion all come together in perfect harmony you may well need a magnifying glass to spot the spirals, but they ill still be there. We'll just neither notice nor care as we marvel at the quality of your workmanship.

Brian.

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Re: Why am I getting spiral grooves with gouge?

Postby Bob_Fleet » Fri Jan 11, 2013 11:58 am

Once you minimise the grooves, possibly by using planing cuts with a flat chisel as Brian suggests, let the workpiece dry.
Then you can enter into the contentious subject, see multiple topics, of whether to put it back on the lathe and sand it.
Much discussionabout whether the customer wants tool marks or not.
Basically it depends on your own taste.
There might even be a market for spiral grooved stools and chairs which remains untapped.
You're even approaching miniature barley twists and might set a trend..
Only question remaining would be; left or right hand thread?
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Re: Why am I getting spiral grooves with gouge?

Postby robgorrell » Fri Jan 11, 2013 5:06 pm

Hi gang. Interesting comments. So I am not to traverse during the downstroke. Well I was. In my head you needed to move during the downstroke to see where to go.. I will work on that. So it is more of an upstroke and traverse a little, downstroke cut, upstroke move a little more, repeat?

A question on the flat chisel. I tried to use a sharp 1 1/2 inch wide timber framing chisel. It sheared OK, but wanted to dig and chatter along the circumference of the work. Also, the left and right ends dug in bad and made big feathery messes of the work. Lots to learn there. I tried using a standard 3/4" skew tool I had in the box but it was dull and I think much to high of an angle.

I made a couple of modifications to the toolrest so that the cord would not drag against the base of the rest on the away side. That helped some also.

Rob
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Re: Why am I getting spiral grooves with gouge?

Postby Bob_Fleet » Fri Jan 11, 2013 6:04 pm

robgorrell wrote:I tried to use a sharp 1 1/2 inch wide timber framing chisel. It sheared OK, but wanted to dig and chatter along the circumference of the work. Also, the left and right ends dug in bad and made big feathery messes of the work.
The ends shouldn't be near the work.
Hold it at an angle, not a right angle to the work, and tip it so that the corner nearest the work is upwards. It should cut in the centre of the blade only. You'll get the feel for it and reduce the angle as you plane. Hope that explains it.
Bevel side down so it can also ride on the bevel will also stop digs.
I usually get folk to let the tool ride on the bevel then gradually lift the handle to find the cutting point.
That way the tool has 3 points holding it in place, your hand, the toolrest and the bevel.
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Re: Why am I getting spiral grooves with gouge?

Postby Brian Williamson » Sat Jan 12, 2013 9:35 am

robgorrell wrote:Hi gang. Interesting comments. So I am not to traverse during the downstroke. Well I was. Rob


And so you should be! When you're roughing out, you'll cut at right angles to the work piece on the down stroke and move the gouge along on the upstroke. That way you get a series of ragged, concentric, grooves. You'll then go back and take off the ridges between the grooves. How many times you do this depends a lot on how well you've done your drawknifing. You might get away with one pass; you might need two or three. So no traversing yet.

After this, it is likely that all your cuts will be made whilst traversing the tool. Pressure on, cutting and traversing on the down stroke, Pressure off but stationary on the up stroke. If your chisel remains in too close a contact on the upstroke it can ride up on the work piece and then come back down in the wrong place, with resultant dig-ins and other mishaps. It's quite a subtle movement, though,when you get used to it.

When you're taking your finishing cuts, the tool will almost always be traversing, with it's bevel riding on the freshly cut wood, just behind the edge. You will almost always be holding the tool at an angle to the work piece, so that the cut has a shearing action. The angle on the skew chisel does a lot of this for you, but whether you're using a shallow gouge, a flat chisel or a skew, the cutting edge needs to be at an angle to the wood.

Brian.
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Re: Why am I getting spiral grooves with gouge?

Postby Robin Fawcett » Sat Jan 12, 2013 12:11 pm

robgorrell wrote:Hi gang. Interesting comments. So I am not to traverse during the downstroke. Well I was. In my head you needed to move during the downstroke to see where to go.. I will work on that.

When I'm roughing out I definitely traverse during the downstroke. It's more of a stroking action and those spirals will disappear after the finishing cuts.
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Re: Why am I getting spiral grooves with gouge?

Postby gavin » Sat Jan 12, 2013 12:57 pm

Robin Fawcett wrote:
robgorrell wrote:Hi gang. Interesting comments. So I am not to traverse during the downstroke. Well I was. In my head you needed to move during the downstroke to see where to go.. I will work on that.

When I'm roughing out I definitely traverse during the downstroke. It's more of a stroking action and those spirals will disappear after the finishing cuts.

Robin,
When you are teaching do you ask the beginner to traverse or to stay in the one spot?
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Re: Why am I getting spiral grooves with gouge?

Postby Robin Fawcett » Sun Jan 13, 2013 11:22 am

I demonstrate how I do it and then encourage them to have a play and try different things.
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Re: Why am I getting spiral grooves with gouge?

Postby robgorrell » Sun Jan 13, 2013 10:57 pm

Thanks guys. It may be a few days before I get a chance to use the lathe again, but will give the comments a try when I do.

By the way, are any of you anywhere near Bognor Regis England? I will be there in May.
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Re: Why am I getting spiral grooves with gouge?

Postby jrccaim » Wed Jan 16, 2013 6:35 am

Great thread, great comments. I would not have thought of my comment if I hadn't been intentionally trying to cut threads! Roughing to cylinder will either get you threads, if you traverse, or scallops of you do not. On the whole I prefer scallops. I then get rid of the ridges with a skew. Thus I have a cylinder. But you can do it any way you want. Nice thing about pole lathes. No hard and fast rules. So what works for you is fine.

What works for me is this. I do as much as I can off the lathe, with axe, knife, drawknife, ... because any of these tools removes much more wood per calorie of muscle energy than the pole lathe. Ideally you would shave to octagon. Practically get it as round as you can. Then put it in the lathe. Take the biggest gouge you have. Again ideally you would have a 35mm (or even bigger) roughing gouge. I have no such animal; I use a 25mm Japanese carpenter's gouge. I go right to left. It matters not at all if you go L to R. Push the gouge in until you have turned off a mini-cylinder bit. Do not traverse and do not worry about leftovers, come back again later. Move the gouge about 12 mm over (in my case left) and do it again. Repeat. You have at this point a very scalloped figure. Again using gouge, remove the high spots. Do not worry about scallops yet. Use the gouge as long as you can. Again: a gouge removes more wood per calorie of leg power than any other lathe tool. Our power lathe brethren use electric motors. We do not have this luxury. It is a matter of judgement as to when you have gone far enough with the gouge. There are a whole lot of YouTube videos on this; but in the end it is experience. Off another forum I found the following Leonardo da Vinci quote:
L'esperienza è il nome che si dà ai propri sbagli.. "Experience is the name we give to our own errors." Oh yes, how true. So now you pick up the skew. It takes some experience to use the skew, again the You Tube videos are informative but one must make one's own errors with the skew. Object is to shave down the high spots and wind up with a cylinder. Once you have a cylinder the real turning begins. Can't help you there; too many possibilities!

Lots of people on this forum. For some, I realize that I am trying to tell my grandmother how to suck eggs, as the saying goes, but for begginers the above might be helpful.
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