Chattering problem

[Karl Best]

I've been encountering some chattering while doing a finishing push cut, about halfway up the bowl from base to top, tailstock to chuck. I'll go back and try a second push cut, and the same thing happens. I can go back afterwards with a fat scraper to clean up the marks, but I'd really rather have the nice smooth finish cut.

I measured the runout on the 3520 lathe spindle and Vicmarc chucks; nothing there (spindle is perfect, chuck < .001"). I'm using an Ellsworth grind on multiple gouges (⅝" and ½"), and sharpening doesn't seem to help. This seems to have started just the last few weeks, but I don't recall that I've changed my technique, and I haven't changed any of my equipment.

What causes this chattering?

 

[Dennis J Gooding]

I have encountered this effect in the earlier stages of turning the outside of a bowl. On each revolution, the gouge encounters a high spot (or possibly a hard spot) and ski jumps a bit before resuming cutting. The best solution I have found is to position the gouge so that it cuts air most of the way around and only nips the high spot. You might consider using a pull cut or a shear scrape with your Elsworth gound gouges.

By the way, is the phenomenon occurring at the juncture of sap wood and heart wood in the bowl?

 

[hockenbery]

Asuuming the wall is not really thin which will cause this.

You may be putting too much pressure on the bevel
You may be gripping the tool to tightly

The bevel needs to float over the cut surface
A thumb and for finger are more than enough grip by the forward hand

be sure to keep the tool handle against your side and follow the curve by shifting you weight from back foot to forward foot.

tool rest could be tool low.

 

[Kevin Weir]

What type of wood are you turning Karl? And how thick is the wall?

 

[Karl Best]

Kevin: This is while turning the outside, so it's still solid (no wall thickness). And I'm turning ash (mostly dry) as well as dripping wet walnet that I just cut down.

Al: You might be right; I probably do grip the gouge too tightly and push too hard. Let me lighten up a bit and see if that helps.

 

[john lucas]

Also try rotating the flute very slightly CW and CCw as you go through the chatter area. Sometimes it only takes a slight repositioning of the cut to stop the chatter.

 

[hockenbery]

Kevin: This is while turning the outside, so it's still solid (no wall thickness). And I'm turning ash (mostly dry) as well as dripping wet walnet that I just cut down.

Al: You might be right; I probably do grip the gouge too tightly and push too hard. Let me lighten up a bit and see if that helps.

hope it solves the problem
a trick that may help too is once you get the chatter or bounce stop cutting, back up a to where it feels smooth, and move forward picking up the cut just before the chatter/bounce area.

In general Dry wood requires lighter cuts and a slower feed rate than wet wood.
ash has the hard and soft parts in the growth rings and often a slower cut will produce a better surface.

 

[odie]

(The following comments has nothing to do with the OP, but thought I'd interject my personal opinion here...only in an attempt to add to the discussion.)

Every bowl has a variance in resistance to the cut across long grain and end grain.....some more, some less, but it's always there. Species and grain pattern can contribute to the level of the resistance. As others have pointed out, wet wood generally has less resistance to the cut, than seasoned bowls and kiln dried stock. There are so many variables that can contribute to difficulty in a clean cut without tearout (as others have suggested), but the very most contributing factor is "sharpness" of the cutting edge. Sharpness decreases the resistance.

Personally, I believe there are many conflicting conceptions as to what is "sharp"...considering there is a narrow range of what sharp really is from a reality perspective. Anyway, I have my own beliefs about that, and I'm pretty sure there are many turners who are satisfied with sharpness that I would consider "needs sharpening".

For hundreds of years, wood carvers and master craftsmen knew what sharp was, and it's generally much sharper than modern woodturners believe it to be. We turners have the advantage of high speed cuts, but if you're going to get perfect cuts at slow speeds (like carving tools, and old fashioned bench planes), then it needs to cut cleanly at very slow speeds, regardless of the potential resistance to the cut.

Getting to the point, here......as stated, there is a plethora of contributing factors, but sharpness can be a major cause of chatter. This, when considering high speed cuts on alternating extremes of inherent resistance to the cut.

-----odie-----

 

[Lamar Wright]

Very good point Odie made about sharpness. I would say that 75 to 80% of the wood I turn is dry. I found out early on about wood chatter. I thought well maybe my gouge is not sharp enough.....well, found out each time I sharpened the gouge there was no chatter at all. Point is..... I just needed to make more trips to sharpen my gouge!....... Another thing tools that are not sharp, too much forward feed pressure and too much pressure on the bevel are frequent causes. Sharpen the tool, relax and slow down to let the tool cut at the rate it wants and try not to push on the bevel. I like to think of it as gliding the bevel rather than riding the bevel.

 

[odie]

I just needed to make more trips to sharpen my gouge!.......

Mornin', Lamar.

Ha,ha!............That right there, ought to be one of the "ten commandments" of woodturning"!

-----odie-----

 

[Leo Van Der Loo]

Lamar and Odie, in the same realm of resistance to the tool cutting, I have found that if there is this shatter or bounce, that using a smaller tool does often help, a lower speed and new smaller sharp tool is often the answer.

 

[robo hippy]

Well, one reason for there being some bounce or chatter is the grain orientation. The tools cut differently when going through end grain, and side grain, as well as when cutting against the grain and with the grain, which happens twice each revolution. With all turning tools, the hand on the handle is what does ALL the work. The other hand rests on the shaft of the tool, and nothing more. If you have experimented, you will notice that the more you clamp both hands down on the tool, the more chatter/bounce you get. This is a critical skill to learn to improve your surfaces so you don't have to sand as much. The one handed push cut is an excellent drill to practice. It does make many nervous, but try it with the hand that is on the shaft of the tool just resting on the shaft, no grip at all. Besides that, move with your body and sharp tools.

robo hippy

 

[John Torchick]

Many good thoughts for consideration. Seems like it is somewhat a process of elimination to find and cure chatter.
If I get chatter, I turn up the heater.

 

[odie]

Many good thoughts for consideration. Seems like it is somewhat a process of elimination to find and cure chatter.
If I get chatter, I turn up the heater.

Heh,heh,heh.......yep, that's right....chatter happens when you're outside and unprepared for the cold, too!

As for differing opinions, you know the saying......everybody's got one!...including me!

-----odie-----

 

[John Torchick]

No further comments as admin might red flag them as off topic.