Parameters That Affects Drilling On The ‘Whole’

Drilling a hole is not a great achievement today as compared to the period of Constructing pyramids. Evidence indicates that our ancestors used drills very similar to the present day counterparts. Those drills were mainly made of bones or stones shaped for the purpose . The activity is so common today, that it has become a household terminology, not to mention about the industry.

However it is still relevant and important to review this subject, being one of the critical activity in any industry. How often we really bother about the quality of a drilled hole? Is it the only responsibility of the workmen at grass root level who does the job?

Let us have an insight in to the activity as a “whole” rather than a “hole”. What are the parameters really influencing the drilling?

Job Material
Every work material has got its’ own behavior. These behaviors are related more to the characteristics projected by various parameters. Although hardness is considered as to be a major factor influencing drilling and machinability, more focus needed to consider the parameters other than hardness itself. Characteristics like ductility, work-hardening, powdery, sticky, abrasive and whatever way your reliable operator explains it to you. The process, through which the job undergone also plays an important role. Whether it has gone through a process of casting, rolling, forging or drawing, all these imparts a particular behavior.Heat treatment is another influencing process. Often you can find same steel of same hardness behaves different in a machining activity if those undergone two different kinds of processing. For example same steel undergone speroidised annealing and normal annealing may impart same hardness, but very different micro-structure. In case of spheroidised annealing, the material be too sticky resulting in edge buildup and subsequent failure of the tool; whereas plain normalizing or annealing results are rather crispy.This is dominant in machining processes like broaching.

Then comes the mind-set related to certain materials. For example, when we mention aluminium we are tempted to reach conclusion that component is ‘soft’. But many new generation materials with higher alloying elements like 12% Si behaves much different than expectations. Same is the case other soft materials like copper and many non-ferrous materials. One misunderstood modern material is Titanium, which is a very bad conductor. This explains how important is it to understand the job material

Machine And Fixtures
It is only a matter of common sense, that better the stability of the machine, better the results of tool. In other words if the tool (here drill) has to bear all the brunt of aged, helpless, shaky machines and loose toothless fixtures, then it is difficult to the tool to survive the ragging. Appropriate maintenance of equipment has long lasting influence.

Coolant
In a recent interaction with a team with us, one of the queries was which is the best coolant is for graphitic type cast Iron.(Gray iron /SG Iron).Most of the people have not even thought of a different coolant other than usual white emulsion. It was a new thing for many, that these type of cast iron machining best done without any conventional coolant, but dry air. Normal coolant forms an abrasive mixture with powdery chips and actually supports to blunt the drill by abrasion. Fact is that very graphite particles are one of the best coolants.

Right coolant selection is equally important to improve the efficiency of the tool. Various material demands its’ own preferrance. Use kerosene or paraffin while drilling aluminium. Best coolant for thermoplastic may be soap solution.

Tool
Many time the poor drill bit has to bear the blame for all the mistakes done by operator and application engineer. In fact we expect the tool taken out of pack to handle the entire needs. Well, a tool will always performs the expected way in justified conditions of its’ capacity. It can perform well in standard conditions of material characteristics. These std conditions are related to hardness and behavior pattern. For example a standard drill can perform very comfortably on a material having 30 HRC hardness. Behavior can generally may be treated as a medium range .say the behavior of mild steel in annealed condition.

However when the application becomes challenging we have to encounter these conditions with appropriate modifications because we are planning to exceed the capacity of a normal tool. Modifications can be of Geometrical or Metallurgical nature. Some alternations In point ,web,flute and helix geometry can do a lot. Metallurgy involves enhancement by way of external and internal strengthening. Coatings, special treatments, choosing special grades of HSS are beneficial .

Another angle of approach is modification in operations. Selecting appropriate speed,feed,coolant and fixtures makes a remarkable improvement.

Geometrical
Point modifications involves different point angles. Standard point angle of 118° can do a lot of normal jobs. Acute angles improve the thrust and perform better on softer jobs. Obtuse angle is good for tougher material, providing sturdy supports to cutting edge. Web thinning improves the thrust. A split point is ideal for deep hole drilling, especially harder jobs (viz.crank shaft).Web modifications often goes well along with suitable point angles. Many kinds of web modifications are in practice. Best suitable web modification sometime demands a bit of experimentation from the technical personnel.

Helix angle and flute modifications are usually possible at the manufacturing level only.

A slow and quick helix got its’ own advantages over standard helix. Similarly flute modifications from heel rounding to parabolic profile can do wonders in certain kind of applications

Metallurgical
Metallurgy being an inherent property, is relevant when the life of a tool is important. When frequent regrinding of a tool interfere the productivity, it is the time one can think of metallurgical enhancements. Often this situation is associated with an adamant job material. Tough and work hardenings, nickel bearing steels are, one for example.

Coating of drills come as an immediate rescue. New generation coatings combine titanium nitride, aluminium nitride with numerous combinations solve a lot of problems related to the wear resistance issues. These coatings are at least three times harder than HSS itself.Hence capable of enduring normal conditions.

Plain surface treatments like bluing (steam oxidation) can solve a lot of routine problems related to chip adherence and swarf flow. Induced treatments are helpful if dimensions are more important. Nitriding is a process where a thin hard nitride layer possible within the surface without any buildup coating.

Temperature treatments are also gaining popularity. Remarkable results are often reported by subzero treatments (cryogenic treatments) done at very low temperatures as low as -190°C using liquid nitrogen. These treatments help the internal refinement almost at molecular level, enhancing the tool properties and life by at least 30%.

Choice of tools made of special cobalt grade HSS raw material often comes last as these grades have better red-hardness properties, but at a higher cost. HSS grades like M35 and M42 are more popular. Talking about material, powder metallurgy products also reported to be highly enduring than many grades due to uniformity and inherent strength related to the manufacturing process of these steels.

Art of drilling is more to the skill, fixtures and approaches. Fixtures shall be sturdy enough to hold the jobs in position. Clamps must be capable of keeping the work piece in position . A machine vice, clamps or dog may be appropriately used for the purpose.

Use of wood blocks often used with clamps found to have more than one benefits; from Protecting the job and equipments and prevent distortion of holes in thin sections.

Good tool holders and and fixtures play a very important role in machining and shall be understood properly by the application engineers.

Techniques
While drilling deep holes, it is prudent to withdraw the drill at frequent intervals in order to remove the swarf out of hole and prevent choking. This method is popularly known as wood-pecking.The method allows proper transportation of coolant to save the tool life.

Another way to handle deep holes is by using multi drilling. In this method the longer hole is completed using three or more progressively longer drills. This will help to maintain the size of the hole more accurate by taking care of the excessive wear of the drill ,especially the land on body.

It is a common problem in sheet metal drilling, the holes becoming triangular and with burr at the bottom. One way to solve this problem is by grinding the drill point with an obtuse angle of 140°.Ensure that the full diameter of the drill enters before the point of the drill breaks through the hole.

Alternatively a drill with no body clearance also may work effectively on thin sections.

There are more techniques developed by intelligent artisans, to handle all kind of situations, difficult to explain in one stroke. Influence of speed and feed : These mechanical factors are very tricky affair although

We try to make our own thumb rules, but not always because of a number of parameters influencing.

A Word On SAFETY
Never use gloves while drilling. Use safety goggles. Beware of loose fixtures. We have come across many avoidable accidents for silly reasons.

Courtesy: Indian Tools (division of Birla Precision)