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Parting-off Feed Rate and Spindle Speed


Parting-off from a billet can be a time of anxiety and disaster! But, it can be done successfully even with the wide blade of the Emco parting tool on free-machining stainless steel, provided it is very sharp.

The parting tool
The blade of the Emco parting tool is around 3mm wide. It must be really sharp. We advise against use of this tool. Very thin parting tools with replaceable tips are a good investment: see the Sandvik Coromant and Plansee Tizit catalogues and the Greenwood Tools web site www.greenwood-tools.co.uk.

Parting tools with push-in tips must only be used for radial cutting, never axial cutting such as cleaning the bottom of a groove. Axial cutting will force the tip out of the holder and the holder, work and tip will be damaged.

Be aware of the maximum diameter of billet from which a parting tool can part-off a workpiece.

Auto Tool Change Turret
Greenwood Tools excellent Quick Q-Cut parting tool cannot be used in the auto tool change turret: the tip holder has considerable depth and this will foul the lathe saddle when the turret rotates. Iscar do a suitable, narrow-tipped parting tool with a 12mm square holder.

Alignment
The blade of the parting tool must be perpendicular to the centre line of the lathe. If it is not, the side of the blade will bind on the billet or workpiece during the parting operation. It is worth taking the trouble to align the tool post, and therefore the parting tool, with a dial gauge.

Centre height
The cutting edge of the parting tool must be on centre height or minutely below, never above. If it is above, the contact point with the work will be slightly below the cutting edge. There will be rubbing of the work, not cutting - unless great force is used to get the edge into the material. The edge can be a minute amount below centre height. However, this can increase the risk of the billet riding up on top of the blade, especially if the billet is held in a 3-jaw chuck.

Position
Part-off as close to the chuck as is safe. The greater the distance from the chuck the great the chance of the billet being levered out of the jaws. However, keep a hand near the Stop button just in case "close" becomes "too close". A non-machining "dry run" is always a good idea before the real thing. To do this, prepare for machining in the normal way with a billet in the chuck. When the right-hand tool tip has been touched on the end and surface of the billet, remove the billet from the chuck and continue as normal.

Feed Rate and Spindle Speed
Keep the spindle speed high. Use a feed rate which moves the tool in at one stepper motor step per two spindle revolutions. For example, 2400rpm and a feed rate of 16mm/min (2400 rpm = 1200 steps per minute = 1200/72 mm/min = 16 mm/min. In the first revolution after a step, most material is cut away. But there is spring in the billet and the tool etc, so, the second revolution is a "spring cut" clearing away any residue. If this is not done, the residue will build up and, at least, stall the spindle motor. This method works well with the wide Emco parting tool. With a modern, narrow tipped parting tool the feed rate might be higher. Experiment: try 1500rpm and a feed rate of 0.03mm/rev in free cutting aluminium.

Lubrication: A spot of WD40, 3-in-1 oil or thread cutting compound, for example, Rocol RTD Cutting Liquid (available from Axminster Power Tool Centre and other suppliers).can make things run more sweetly.

Plunge and Parting Tool Widths
Speeds and Feeds for Turning and Parting with Indexable Tipped Tools
Spindle Speeds - Emco Compact 5 CNC
Spindle Speeds - Emco Compact 5 PC