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Just about every engineer who has ever used a centre lathe has at some point had the misfortune of uncontrollable chatter.
There are many causes, and a good number of solutions, I will do my best to explain causes and solutions as thoroughly as possible.
The Causes of Chatter
Many causes of chatter are obvious, once you have experienced them, but identifying the actual root cause can be trial and error, more often than not chatter is caused by more than one of the items below working against the turner.
- Worn spindle bearings (obvious but generally overlooked), are usually very easy to diagnose as chatter will be difficult to control unless heavy cuts are being taken. In fact the lighter the cut the worse the chatter will be.
- Turning long shafts without support. This is another obvious one, but you may be surprised to learn how long is defined as long when it comes to unsupported turning.
- Incorrect tool height.
- poorly adjusted or worn slide-ways or machine bed.
- Loose tailstock or worn tailstock bearings.
- Spindle speed.
- Feed rate.
- Depth of cut.
- Tool condition.
- Thin walled sections.
- An accumilation of a number of the items above works against each other to cause chatter. This is the hardest type to cure.
Cures for Chatter
While I could simply list the solutions to all of the above, that would not be helpful and I am quite certain you could have worked out that the cure for worn spindle bearings is to adjust or replace them. Also curing one thing, may, and often does cause the chatter to get worse, so I am going to go into as much detail as possible, trying not to state the blindingly obvious to give you the best chance of curing the problem of chatter, once and for all.
First of all, don’t take anything for granted, if the machine is new, it does not mean the bearing must be in good order and can’t be the cause of chatter, likewise, poorly adjusted slideways and jib strips, or loose lead screw nuts on a brand new machine can be the cause of chatter. if you are suffering from chatter on just about every job, go back to basics, and take the time to check and properly adjust your machine according to the manufacturer’s instructions. Of course, it would be just as easy to blame wear and tear of the bearings and slide-ways for chatter on older machines, only to find the problem is related to a combination of tool wear, depth of cut, and the wrong spindle speed or feed rate.
Mechanical faults apart, the largest cause of chatter is probably a combination of rigidity of workpiece, tool condition, tool height, tool rigidity, spindle speed, and tool feed. Although either one of these could be the cause. Let’s take each one in turn…..
Work Piece Rigidity
As a general rule, if a workpiece is unsupported for a length of more than 3 X the diameter, you run the risk of chatter. However, it you have a really long slender shaft that is supported by a live centre you also run the risk of chatter unless you use a traveling steady. Common sense will normally tell you if your workpiece is sufficiently rigid, but sometime you may have to think a little outside of the box to find the solution.
By this, I mean that the tool is either too sharp or too blunt, again there are no easy answers. However as a rule sharper is better, or to put it another way the smaller tool tip radii the better chance you have of reducing chatter. This is fine for shallow cuts, but for deep cuts (more than 3 X tool tip radii) the larger the radii the better, but too big is worse.
You may start to notice a theme, there are no easy answers when it comes to chatter, it really is trial and error. But, Tool Height…… I personally found that running the tool a little above centre gives less chatter for smaller diameter, longer pieces while having the tool below centre always appeared to help with chatter on larger diameter components. However, colleagues past and present would either agree or disagree with these opinions at a ratio of about 1: 1, which goes to show, that whatever works for you at the time, the trick is to try everything!!!!
Again, really basic stuff, but if your tool is not rigid, either because it is too thin, or has too much overhang, it will cause chatter. This one is easy to cure, by making the tool as rigid as possible, however, if you have already done this and are still suffering from chatter, I would suggest changing tool height, tool geometry, or spindle speeds and tool feeds.
After rigidity, feeds and speeds are the biggest cause of chatter, but there may be no logic to the best method to cure it. Usually having the spindle running too fast when turning long slender shafts too will cause chatter, but as the diameter of the shaft incases chatter can be more evident at slower spindle speeds. As a general rule, long shafts under 20mmØ should be turned at relatively fast spindle speeds with a slow to medium feed rate and shafts over 20mmØ should be turned at a slower spindle speed with higher feed rates. I know this is a little vague, but the best way to cure speed and feed-related chatter are by trial and error, what works for one may not work for another. It may be machine orientated or just down to luck, to be honest, I have never quite worked it out.
If you are lucky enough to have a lathe with an infinitely variable speed you can sometimes get lucky and reduce chatter considerably by increasing or decreasing the spindle speed a little. if not, try making adjustments to feed rates, I generally advocate higher feed rates than others would normally use, and generally get good surface finishes with very little chatter, but of course, your mileage may vary.
How to get rid of chatter once you have it on a part
Assuming you get chatter during the roughing stage, tighten everything up and increase rigidity as much as possible and then take as large a cut as you dare. This SHOULD get the tool under the chatter you have, and remove it completely. If you can’t get under it, you will only make things worse. Again there are no real rules for this, sometimes a really quick feed rate works sometimes a slow one works, it really depends on the material and the underlying cause of the chatter.
However, if you are unlucky enough to get chatter on your finishing cut, I am afraid it is very little you can do apart from either live with it or make a new part.
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