7. Drill Speeds, Feed Rates and Lubrication

Speeds

A suitable spindle speed (revolutions per minute) should be selected when using the press drill. The speed at which the drill should rotate is decided by its size (diameter) and by the hardness of the metal being drilled.

A small diameter drill should be run at a faster speed than a large diameter drill when cutting the same metal.
Drills should be run faster when cutting soft metals, eg aluminium, than they are when cutting hard metals, eg mild steel.

As more materials of different hardness become involved in metalwork projects, there is a constant need to change the speed of the press drill spindle.

Attempting to drill at too fast a speed will cause the drill bits to overheat, become blunt and damage the cutting edges.
Drilling at too slow a speed will easily cause the breakage of a small diameter drill under pressure.

Where:

N = spindle speed
Vc = material cutting speed
d = diameter of drill bit

Calculating RPM

Machine RPM for drilling can be calculated in the same way as for turning on the metal lathe, except that the drill diameter is used in the calculation. The formula for approximating spindle speed (RPM) can also be used for drilling RPM.

Note: When using the chart and formula, the cutting speed must be multiplied by 1000 so all dimensions are in millimeters.

Using the spindle speed chart

If the cutting speed and drill size are known, RPM can be read from a spindle speed chart such as the one below. RPM is found by reading down from the nearest cutting speed value and across from the nearest diameter value (drill size).

The RPM value read from the chart could be adjusted proportionally depending on how close the nearest cutting speed and diameter values on the chart are to actual values. However, for most drilling operations rough approximations will usually do.

Example 1

A drilling machine with the following spindle speeds is to be used to drill mild steel bar with a 6mm high speed steel drill bit:

Available speeds are 100, 440, 860, 1280, 1620, 2040, 2580 and 2900 RPM.

Using the charts above, select the most suitable of the available spindle speeds.

Cutting speed for mild steel when drilling is 24m/min which is found on the cutting speed chart.
The closest cutting speed on the spindle chat is 25m/min.
Trace that down to the 6mm diameter drill bit row.
The speed on the chart is 1326 RPM.
The closest speed on the available drill press is 1280 RPM.

Using the cutting speed chart

The cutting speed chart can be used for both lathe operations and drilling operations. Note that there are two specific columns for each operation which correspond to different cutting speeds for materials depending on what process is being performed.

When completing calculations, ensure you are selecting the cutting speed for the relevant task e.g. Mild steel has a cutting speed of 30m/min when turning, but has a cutting speed of 24m/min when drilling.

Example 2

Calculate the lathe RPM for rough turning a mild steel bar of 42mm diameter with a high-speed steel cutting tool.

The basic formula for calculating revolutions per minute is:
RPM = Cutting Speed/Circumference.

When using a high-speed cutting tool, the cutting speed for rough-turning mild steel is 30 metres per minute. Circumference of the bar is calculated as follows:

Circumference of the bar = ℼ x diameter of the bar
= (22/7) x 42mm
= 132mm

RPM = Cutting Speed/Circumference
= (30m/min x 1000) / 132mm
= 230 approximately

(Note that the cutting speed 30m/min is multiplied by 1000 to convert to mm)

How to change the spindle speed

Safety! Switch the power off at the machine isolator switch.
Do not rely upon just the machine itself being switched off.
Remove the safety guard over the driving pulleys.
Move the V belt to the required position. Consult the table of speeds attached to the drill to find the correct position to suit the drill size and the metal. After moving, the belt must be horizontal and in the same relative position on the pulley at each end. If the belt is fitted on the slant, it may be stretched and badly worn.
Replace the safety guard over the pulleys.
Switch the power back on.

Feeds

Although small diameter drills are run at a faster speed, they must be fed slowly into the metal because they are easily broken if too much pressure is applied. Large diameter drills can be fed into the metal more quickly because they are stronger.

Whether large or small diameter, it is important that the drill is kept cutting and not allowed to just rub, or it will quickly overheat and become blunt.

When drilling, remove the drill from the hole periodically to clear the waste metal. If this is not done in a deep hole, the drill flutes will become choked and the drill will jam and break.

Lubricants

A suitable lubricant should be used to keep the drill cool, to preserve its cutting edges and to assist the swarf (waste metal) to escape up the flutes of the drill. If applying lubricant by brush, withdraw the drill regularly and fill the hole with the lubricant.

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