A toolholder for parting with back rake
March 2010 - (last updated 05 April 2010)
Parting operations can be difficult, as the parting blade is the cutting tool that have a larger contact surface with the workpiece. For a good parting experience a well fitted saddle and cross slide is quite more important than the size of the lathe. If there is any play on any carriage component, it will be evident when parting.
Good practices for when parting include:
- parting as close as possible to the chuck;
- using the steady rest to hold long parts;
- setting the compound so the toolpost is entirely over the cross slide;
- locking the saddle;
- lots of coolant;
- a constant cross-slide feed, neither too slow or too fast. If feed is too fast the blade will dig-in, if feed is too slow it will cause chatter;
- the tool must be set exactly at the lathe center height. I have read that it can be positioned a bit lower than the center height but that's not a good idea. The parting tool flexes and if positioned lower than the center height it will be cutting with negative rake, making the parting experience harder;
- the parting tool must be set truly perpendicular to the workpiece;
- use half the recommended speed for turning the workpiece;
- grind the parting tool with small front clearance (I found 5 degrees to work fine and much better than the 7 degrees front clearance I was using before)
- avoid too thin blades as they flex more. I have been using a 1/16" blade that works fine with the standard motor ratio. After the speed reducer mod changed for the 5/64" blade, that is stiffer and flex less.
I was never happy with the parting toolholder for the mini-lathe that can be found on many stores. It holds the blade too far out and too far aside from the toolpost, making the blade to act as a lever pulling up the right side of the cross slide and saddle. As it holds the blade parallel to the cross slide, the blade will be cutting with no back rake.
For the Norman Patent QCTP, made this parting toolholder that holds the blade at 4.5 degrees angle. Decided for this angle to be able to part up to 35mm diameter stock while having lowered the compound 2.5mm. As the blade tip height varies with the amount of blade protruding from the toolholder, a choice must be made. It works very well and parting is much easier with it.
On pictures the parting toolholder is set on a plinth that replaces the compound. I use the compound on a vertical position for milling operations and, having now a leadscrew handwheel, I use to leave the plinth on the lathe while the compound is resting on the milling setup.
On front, the parting toolholder was made with a angle to make clearance when parting. This allow the blade to be supported by the toolholder always over the compound or plinth, while protruding as less as possible. On picture you can see that the unsupported blade amount on bottom is very short.
By the time this picture was taken I was using about 7 degrees front clearance angle. Later have tried a smaller 5 degrees front clearance angle and found it to be much better.