How to drill a perfect hole

01 Coolant

Proper use of coolant is critical to good drilling performance, and it directly affects chip evacuation, tool life and hole quality during machining.

(1)    How to use coolant

a) Internal cooling design

Internal coolant is always the first choice to avoid chip jamming, especially when machining long chipping materials and drilling deeper holes (greater than 3 times the hole diameter). For horizontal drill bits, when the coolant flows out of the drill bit, there should be no undershoot of the cutting fluid over a length of at least 30 cm.

b)    External cooling design

Use of external coolant can be used when chip formation is good and the hole depth is shallow. For improved chip evacuation, at least one coolant nozzle (or two nozzles in the case of non-rotating applications) should be positioned close to the tool axis.

c)    Dry drilling technique without using coolant

• Can be used in applications with short chipping materials and hole depths up to 3 times the diameter

• Suitable for horizontal machine tools

• It is recommended to reduce the cutting speed

• Tool life will be reduced

Dry drilling is never recommended for:

• Stainless steel material (ISO M and S)

• Interchangeable drill bits

d)    High pressure cooling (HPC) (~70 bar)

The benefits of using high pressure coolant are:

• Longer tool life due to enhanced cooling effect

• Improved chip evacuation in long-chipping materials such as stainless steel and potentially longer tool life

• Better chip evacuation and therefore higher safety

• Provide sufficient flow for a given pressure and orifice size to maintain the coolant supply

(2) Skills of using coolant

Be sure to use a soluble cutting oil (emulsion) that contains EP (Extreme Pressure) additives. To ensure optimum tool life, the oil content in the oil-water mixture should be between 5-12% (10-15% when machining stainless steel and superalloys). When increasing the oil content of the cutting fluid, always check with the oil separator to ensure that the recommended oil content is not exceeded.

Where possible, internal coolant is always preferred over external coolant.

Absolute oils improve lubrication, which is beneficial when drilling stainless steel applications. Be sure to use with EP additives. Both solid carbide drills and indexable insert drills can use clean oil with good results.

Compressed air, cutting fluid mist or MQL (Minimum Quantity Lubrication) can be successful in steady conditions, especially when machining certain cast irons and aluminum alloys. Due to the increased temperature which may have a negative impact on tool life, it is recommended to reduce the cutting speed.

02 Chip Control Tips

Chip formation and chip evacuation are key issues in drilling and depend on workpiece material, choice of drill/insert geometry, coolant pressure/volume, cutting parameters.

Chip clogging can cause the drill to move radially, which can affect hole quality, bit life and reliability, or cause bit/insert breakage.

Chip formation is acceptable when the chips are able to exit the drill without any problems. The best way to identify it is to listen while drilling. A continuous sound indicates good chip evacuation, and an intermittent sound indicates chip clogging. Check feed force or power monitor. If there is an abnormality, the cause may be chip clogging. Check the shavings. If the chips are long and curved, but not curled, it indicates a chip clogging. View hole. After chip clogging, a rough surface will be visible.

(1) Tips for avoiding chip Clogging:

• Make sure you are using the correct cutting data and drill/nose geometry

• Check chip shape - adjust feed and speed

• Check cutting fluid flow and pressure

• Check the cutting edge. Cutting edge damage/chipping can lead to long chips when the entire chipbreaker is inactive

• Check if machinability has changed due to new workpiece batch - adjust cutting parameters

(2) Chips from indexable insert drills

The conical chip formed by the center insert is easily recognizable. Chip formation by peripheral inserts is similar to turning.

(3) Chips from solid carbide drills

A chip can form from the center to the periphery of the cutting edge. It is worth noting that the initial chip produced when first drilling into the workpiece is always very long, but this does not cause any problems.

03 Control of feed and cutting speed

(1)   Influence of cutting speed Vc (m/min)

In addition to material hardness, cutting speed is also a major factor affecting tool life and power consumption.

• Cutting speed is the most important factor determining tool life

• Cutting speed will affect power Pc (kW) and torque Mc (Nm)

• Higher cutting speeds will generate higher temperatures and increase flank wear, especially at the peripheral tip

• When processing certain soft long-chip materials (ie, low carbon steel), higher cutting speeds are conducive to chip formation

Cutting speed too high:

• The flank wears too fast

• Plastic deformation

• Poor hole quality and out-of-tolerance hole diameter

Cutting speed too low:

• Build-up edge

• Poor chip removal

• Longer cutting time

(2) Influence of feed fn (mm/r )

• Affect chip formation, surface quality and hole quality

• Affect power Pc (kW) and torque Mc (Nm)

• High feed will affect the feed force Ff (N), this factor should be considered when the working condition is unstable

• Affect mechanical stress and thermal stress

High feed rate:

• chip breaking is hard

• Short cutting time

• Less tool wear but increased risk of bit chipping

• Decreased hole quality

Low feed rate:

• Chips are longer and thinner

• Quality improvement

• Accelerated tool wear

• Longer cutting time

• Keep the feed rate low when drilling thin, poorly rigid parts

04 Tips for Getting High Quality Holes

(1)   Chip removal

Make sure the chip evacuation performance meets the requirements. Chip jamming affects hole quality, reliability and tool life. Drill/insert geometry and cutting data are critical.

(2)   Stability, tool clamping

Use the shortest bit possible. Use refined rigid holders with minimal runout. Make sure the machine tool spindle is in good condition and accurately aligned. Make sure the parts are fixed and stable. Apply correct feedrates for irregular surfaces, bevels and intersecting holes.

(3)   Tool life

Check the wear condition of the blade and preset the tool life management program. The most effective method is to monitor drilling with a feed force monitor.

(4)   Maintenance

Replace the blade hold down screw periodically. Clean the tool block before changing the blade, making sure to use a torque wrench. Do not exceed maximum wear before regrinding solid carbide drills.