by Gary Sneesby

PARTING OFF using a rear tool post is one way to dramatically reduce the risk of a dig in, but very few modern lathes have the mounting holes to take a rear tool post. The BTF Parting Tool Holder gets around this by working on the normal tool post, but instead is mounted upside down with the lathe running in reverse. There are several reasons why this method of parting off works effectively to reduce the chances of a jam.

Using a conventional parting tool, if you get a dig in, the cutting edge will tend to get pushed down and towards the revolving workpiece. As a consequence the tip will get pushed even deeper into the cut resulting in huge loads on the tool and lathe, often breaking the parting blade in the process and damaging the workpiece and occasionally the lathe itself.
If you use a parting tool mounted upside down and the lathe running in reverse, or a rear mounted parting tool upside down and running in the normal direction, the cutting edge will tend to be pushed up and away from the cut if the tool starts to dig in, thereby lessening the pressure on the cutting edge and stopping the blade digging in further.
While this is the main advantage of a BTF or rear mounted parting tool, there are more advantages.

Firstly the swarf that is created is more likely to drop out of the cut with the blade upside down, while with a normal setup the swarf will pile up on the top of the blade increasing friction and the chance of a jam up between the sides of the blade in the slot.
Coolant should always be used when parting off and because there will be hardly any swarf on the top side of the workpiece, the coolant will be able to get down into the slot where it is required, rather than seeping through the swarf that is piled up on top of the blade with a conventional setup.
Because the cross slide will tend to pull up tight into the dovetail when cutting, this can lessen any slop between the ways. Clearly this feature will be much more effective on a smaller, older lathe than a large, brand new one.

The BTF holder takes the common 1/2" x 3/32" tapered type parting blade, these also have a bevel on the top and bottom so that they lock securely into a holder. The BTF Parting Tool Holder does not include a blade in the purchase, but these are widely available from several manufacturers and many suppliers worldwide. The blade is easily sharpened on any bench grinder and instructions for setting up, sharpening, and tips for best results, are included with the holder.

More details from Eccentric Engineering