Decreasing Drilling Time

Optimizing coolant pressure can make all the difference in holemaking applications

March 10, 2011

Increasing the tool penetration rate can have a dramatic impact on drilling time.

High-pressure coolant keeps the cutting zone cool and aids in chip evacuation.

High-speed drilling can have many definitions. It can simply refer to an increase in tool RPM, feed rate, or both, and is very subjective. Increasing the drill’s productivity by reducing the time it takes to create a hole depends on many factors, including tool speed and feed, coolant usage, and chip formation, and it is this time reduction that is a better indicator of proper drilling practice than just an increase in speeds and feeds.

Tool selection in a drilling application depends entirely on the type of hole being created. After all, there is a big difference between drilling a simple bolt hole and doing precision drilling work. In some applications, reaming also is necessary to guarantee hole quality. In these cases, high-speed, high-feed reaming can reduce cycle, as can the use of a single specialty tool that can mill, drill, and chamfer.

It all starts with the material, however.

The type of drilling tool used – typically indexable or solid carbide – also is based on the material being cut. Machinists need to ask: What type of tool do I need to use to get the chip I need?

For example, in low-carbon steel cutting, it can be very difficult to break off a chip and get it out of the hole because of the steel’s soft, gummy nature. A tool with the proper geometry and coating must be used.

New technology in the form of coatings and chipbreakers has done a lot for drilling recently. New insert geometry is enabling smaller chip formation, which becomes increasingly important as the hole’s depth-to-diameter ratio increases.

“As you move beyond 10xD, for example, it becomes very important that the chips are formed correctly and are able to move up the flutes and out of the hole,” said Iscar Tools Drilling Product Manager Dave Vetrecin.

Reducing Drilling Time

According to Vetrecin, increasing the tool’s penetration rate has the most positive impact on decreasing total drilling time; however, it comes with a price.

“The higher the SFM [surface feet per minute], the more heat will enter the carbide since the tool is always fully engaged in the material,” said Vetrecin.

When feed rate is increased, more cubic inches of metal are removed and more chips are created.

Vetrecin advises that spindle load should be monitored because it can be an identifier that something is wrong in the cutting process.

“When chips pack into the flutes of the drill, the spindle load will be erratic as the chips pack and are then released all at once,” said Vetrecin.

Gullet geometry plays a role in chip evacuation too. The larger the gullet, the faster chips can be brought out of the hole. Larger chips also can be evacuated quickly. In some of the tooling available from Iscar, the twisted flutes of the drills have coolant holes in them that are twisted.

“In most cases, if you have proper chip formation, your process will be fine,” said Vetrecin.

Machine tool settings, toolholding setup, and stability of the part also determine how hard you can push the drill. Maximum rigidity, especially on oddly shaped parts like a casting, is important.

If you don’t have stable fixturing, drill runout will affect hole finish and accuracy. This runout can be, at least partially, negated by proper toolholding.

“We suggest the use of hydraulic toolholders for drills with cylindrical shanks,” said Vetrecin.

Shrink-fit toolholding is another option.

The Coolant Conundrum

When feed rate is increased, more cubic inches of metal are removed and more chips are created.

High pressure or no high pressure, that is the question.

Different shops have different methods for using coolant, which eliminates the creation of long, stringy chips. According to Vetrecin, increasing coolant pressure to the maximum allows you to push the drill as hard as you can.

“High-pressure coolant is most preferable,” said Vetrecin. “Flood coolant is usable, but high-pressure coolant keeps the cutting zone cool and aids in chip evacuation. If you are doing a lot of hole drilling, you should be looking at this technology.”

Material also dictates coolant choice. For example, if you are cutting stainless steels or high-temperature alloys, the coolant concentration needs to be in the 12 to 15 percent range. In typical steel applications, a concentration of 5 percent is adequate.

“Higher concentrations create better lubrication, reduce heat at the cutting edge, and allow for better chip formation,” said Vetrecin. “Correct coolant concentration, especially in high-temperature alloys, is very important.”

Deep-hole Drilling

Coolant-through tooling becomes even more critical as holes get deeper. However, a very high-pressure coolant flow in deep-hole drilling often can be detrimental.

“The shape of a standard twist drill is basically that of a chip auger,” explained Walter USA Senior Project Manager Brian Baker. “That is the mechanism that removes the chip from the drilled hole.”

Essentially, the coolant is added to the process to cool the chips, workpiece, and cutting tool, while adding lubricity where it is needed.

New tooling can reduce vibration while improving straightness and roundness.

“The coolant is not there to force the chips out of the hole. By adding too much coolant pressure you can literally force chips out of that hole faster than the auger mechanism can remove them,” said Baker. “That will cause chips to pack in the flutes or even force small particulates past the tool’s margins.”

If, for example, you are using a 12xD drill, Walter USA recommends a coolant pressure of 1,000 PSI. However, as you approach depths of 30xD, the company recommends a reduction in pressure to approximately 300 PSI.

“Deep-hole drilling is starting to become very popular, and we are very happy to start showing results in this area,” said Baker. “It is becoming easier for drills to be able to go to depths of 30xD, and we can even create specialty tools that go down to 70xD. In 30xD applications we can drill six times faster than gundrilling will allow, and at 70xD, we are probably 10 times faster than gundrilling.”

Coolant pressure is not just of importance when using solid-carbide drills either.

“In indexable drilling we try to optimize coolant volume rather than pressure,” said Walter USA Product Manager Pat Nehls. “Pressure helps, but if it is too high when using a smaller-diameter drill, it can have a negative effect.”

If the coolant pressure is too high coming through the tool, he explained, the coolant’s energy impacting the workpiece material ahead of the drill has a tendency to deflect the tool.

“In these cases, we want to be able to have a little less coolant pressure than what would be considered typical,” said Nehls.

As mentioned, the concentration of the coolant can be changed to suit the material being cut. What also can be changed is the lubricity. Different additives can be placed in the coolant depending on the heat protection necessary.

Tool Design

New indexable drills can have wiper facets on the outer insert to give a better surface quality.

New tooling introduced recently was created with one goal in mind: Reduce production costs.

“The needs of production drilling are for the drills to run twice as fast as they normally would, or last longer, and there are several ways that this can be accomplished,” said Nehls.

And, according to Baker, the speeds and feeds of Walter’s new Xtreme® line of double-margin drills can run up to 30 percent faster than the company’s standard drilling line.

“These drills are very stable in the cut. Vibration is reduced, straightness and roundness are improved, and these tools also enter and exit uneven surfaces better,” said Baker.

Today’s manufacturers want to produce a higher-quality hole, and do it in less time, with less cost. Cost per part will come down as productivity improves.

This is true in both solid-carbide and indexable tools.

“We have a new indexable drill that has wiper facets on the outer insert to give a better surface quality,” explained Nehls. “These inserts also have precision-ground periphery to hold a better tolerance in the drill hole. The design of the insert provides and improved balance between the inner and outer inserts that enables us to run higher speeds and feeds.”