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Speeds and Feeds for Turning and Parting with Indexable Tipped Tools


Speeds and Feeds for indexable tipped tools on big industrial CNC lathes tend to be very much higher than what are possible on the Emco Compact 5PC and 5CNC lathes. The speeds and feeds used by model engineers are more appropriate.

Speed
This is the surface speed of the rotating billet measured in metres per minute. Surface speeds vary with the material being turned. For example, 45m/min is suggested for free cutting stainless steel whereas 200m/min is suggested for free cutting aluminium.

The actual rotational speed of the billet varies with its diameter. A small diameter billet must turn much faster than a large diameter billet to achieve the same surface speed. Rotational speed, in rpm, is given by:

Spindle rpm = (surface speed x 1000)/(pi x billet diameter)


For lathes, like the Emco 5CNC and 5PC, which have several fixed and/or manually adjustable spindle speeds, the actual speed used for a job is based on the billet diameter and is the nearest available to the calculated value. The whole job is often turned at that rotational speed even though this means that the surface speed decreases as diameters get smaller as machining proceeds. In spite of this, the end product is usually satisfactory.

Modern lathes have a computer controlled variable spindle speed option. When this is switched on, the spindle automatically speeds up as the tool moves to smaller diameters thus keeping the surface speed constant.

Speed and Safety
If, instead of a cylindrical billet, the part to be machined is an irregular object and therefore out of balance, reduce the rotational speed to a safe value.

Speed and Surface Finish
If the machine has worn bearings or drive belts or other imperfections, it may be necessary to reduce spindle speed in order to get a good surface finish.

Suggested Surface Speeds
Stainless Steel 45m/min
Mild Steel 70m/min
Cast Iron 90m/min
Brass/Bronze 160m/min
Aluminium 200m/min

Feed
This is the distance the tool moves through the material in one revolution of the billet as it cuts. It is measured in mm/rev.

If possible, indexable inserts generally like to be worked at a minimum feed rate of 0.1mm/rev. However, some experimentation may be necessary to determine what works well on a particular lathe with a particular material. Start with the values below and adjust up or down in the light of experience:

Suggested Feed Rates
Roughing 0.08mm/rev
Finishing 0.06mm/rev
Parting 0.04mm/rev (at 1500rpm constant spindle speed for aluminium)

Minimum Feed Rates
On the Emco Compact 5PC and 5CNC lathes, do not drop the feed rate below 0.0139mm/rev. This is the distance the tool moves with one step of the X or Z stepper motors. A feed rate below these values will mean that the tool tip rubs the work for part of a revolution and does not cut it. This will decrease tool tip life.

Feed Rate in mm/rev or mm/min
WELturn will accept feed rates in mm/rev or mm/min. If using mm/min take care that, when the mm/min value is divided by the spindle rpm, the resulting feed rate in mm/rev does not fall below the minimum feed rate quoted above.

General Comments
Feeds should be set at the maximum possible per revolution, bearing in mind: