Cutting speed and RPM (or spindle speed) – the difference

Cutting speed and RPM – what is the difference, and how is it important ?

Cutting speed and RPM are different but related. Here’s an explanation.

When you rub your palms together fast, you’ll notice that they get heated up (we in fact do this to get warm when we are feeling cold). This is because of friction, mechanical energy being converted to thermal energy. The faster you rub your palms, the hotter they get. Basically, the higher the relative linear velocity between your palms, the higher the heat generation.

Cutting speed and RPM - heat generation in cutting
Heat generation is proportional to the cutting speed

The same phenomenon occurs during metal cutting. Mechanical energy is used to cut metal by causing relative motion between the cutting tool and the workpiece. This is converted into thermal energy. Just like when we run our palms together, the thermal energy produced is proportional to the relative linear speed between the tool and workpiece. This relative linear speed is the Cutting speed. It is measured in meters per minute, different from the spindle speed, which is the number of revolutions per minute. The cutting speed is proportional to the spindle speed AND the diameter at which cutting is being done.

Tool wear rises as the temperature at the cutting edge rises. The tool material is designed to work at a certain temperature range. Below this range, you are under-utilizing the tool and the cycle time will be unnecessarily high. Above this range the tool wear will be too high, and you end up with high tools costs and high machine downtime to keep replacing the tool. High cutting speeds and temperature also result in poor surface finish and unwanted metallurgical changes in the workpiece.

With a constant RPM, as you perform various operations the cutting speed fluctuates, so sometimes you are cutting below optimum speeds and sometimes at speeds that result in high tool wear (and hence high tool cost) and poor workpiece quality.

This is why it is preferable to cut at constant cutting speed, also called constant surface speed (CSS).

Action point
Check if your CNC programmer knows about Constant cutting speed and is using it properly. A LARGE percentage of (I think most) programmers do not know what it is, and do not use it. Cadem CNC lathe programming software uses CSS and constant spindle speed automatically and appropriately.

Idea bulb

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Dhaba dinner in Ludhiana

Something I really look forward to when I travel to the North is the dhaba food. Like the simple but seriously mouth-watering dinner that I had in Ludhiana. Aloo paratha, dal, aloo sabzi, and salad of mooli (radish), onion and green chilly. Also a tall glass of lassi.

After dinner I went back to my rather unusual hotel, Gulmor, a 100 year old structure built to house the then Viceroy of India, Charles Hardinge. Hardinge is the guy who moved the capital from Calcutta to New Delhi in 1912. Till then Delhi was just Shahjahanabad (now Old Delhi), the walled city around the Red Fort built by Shah Jahan.

Gulmor hotel

Hardinge was also the target of an assassination attempt by Rashbehari Bose, founder of the Indian National Army. That was a (relatively) happy assassination attempt. Hardinge survived with minor injuries, and Bose escaped to Japan.