Home Shop Machining/Machining Operations/Turning Small Diameters

Turning Small Diameter Parts on a Lathe
If the work extends more than a few diameters from the chuck, then you will need to deal with flex. If the work flexes while cutting it will:
 * chatter,
 * end up tapered,
 * climb up the tool,
 * or just bend or break off.

There are several ways to mitigate this.

Using a Center
If drilling a center hole in the end of the part is acceptable (or if it's possible to leave a sacrificial section on the end that can be center drilled), then using a center to support will help a lot.

Using a Center Rest
If the work is too long to turn supported by a center at the end, it may be possible to use a center rest as near as possible to where you need to cut.

Using a Follow Rest
If the work is so thin that even supporting at both ends is not enough then it may be possible to use a follow rest directly behind the tool tip. If a ready-made follow rest is not available, it's possible to improvise by clamping a wood block with a slight V cut in it to support the work.

Using Sharp Tools
If there's no way to use some form of rest to support the work, very sharp tooling mounted exactly on center may help. HSS, sharpened with a negative rake and honed to fine edge, will require less force to cut.

Another option is using carbide inserts meant for aluminium. They are as sharp as HSS can be made and allow very fine depths of cut.

Using a Vertical Shear Tool
A vertical shear tool does not have the problem of needing to be exactly on center, the work doesn't have the tendency to climb over it, and very, very light cuts can be made. The problem with a vertical shear bit is that only very, very light cuts can be made.

The Single Pass Method
Instead, start with a larger rod, and a tool bit with a sharp end, pointed at the left when looking from above, and with all the cutting edge at the left side. Zero or near zero tip radius. A good rake on top surface sloping from left to right, 15 deg or even more.

Start cutting on the end of the rod, and adjust until you are finishing to final size. Now turn the entire small length to that size in one pass.

Because you are NOT exerting force on the side of the thin portion, that part is just not a factor. You do not press against it, you do not try to cut the thin part, it is the "leftover", which you arrange to be the size you want.

External Resources

 * YouTube: Extended Small Diameter Turning by Joe Pieczynski