Aim and Scope: This part aims to be a valuable resource on deep-hole machining. It is intended to familiarize the end users of deep-hole machining products with the basics of deep-hole machining explaining advantages and disadvantages of different deep-hole machining methods. Particular attention is going to be paid to drill geometry and design,  different aspects of cutting fluid (coolant) application and to the coherency of the deep-hole machining system. Although the emphasis is placed on gundrilling, specific points of STS (BTA) and Ejector drilling will also be considered.  

Be Aware That: Domestic deep-hole machine producers (except very few), particularly for the automotive industry, have very limited knowledge on gundrilling. To the best of my knowledge no one has a test bed to model and study gundrilling systems. As a result, their machines have the following “bright” features:

• “Inaccessible” starting bushings. It other words, if you like to check and/or change the starting bushing you have to be prepared to spend a lot of your time.
• There is no way to check and to correct “starting bushing-spindle” alignment. This alignment is one of the most important characteristics of any deep-hole machine affecting quality of the machined holes, tool life, and process efficiency.
• Most of the coolant supply systems have the wrong type of pumps, called variable-displacement pumps (Thank you, Mr. Wolfe for pointing this out). A variable volume pump is designed to maintain 'set' pressure. Unfortunately, if an obstruction is encountered by the coolant flow, the 'set' pressure (the pressure seen on the gauge by the operator) will be maintained but the flow rate supplied to the tool will actually decrease because coolant will be diverted through the pump's internal relief valve. As a result, the obstruction (in the case of a chip jam in the flute of the tool) can, in fact, be worsened and quickly lead to drill failure.
• The process control systems control and display irrelevant process parameters. For example, the inlet coolant pressure is controlled instead of the flow rate; the current of the driving motor is used to measure the load on the drill.

Although a relatively wide variety of gundrill designs are produced, they have the following ‘advance’ (as viewed from the 1950's) features:

• Obsolete point grinds including "popular" stepped and stepped-slash grinds. Most of the point grinds can be broadly divided into two categories: (a) CAM reliefs proposed by Eldorado 30+ years ago, and (b) facet grinds "borrowed" from German companies.  In any case, no one can provide a reasonable explanation or justification for a particular point geometry.
• Inherent instability. All gundrills having supporting continuums instead of two supporting pads are inherently unstable. Back to basics: there should be three circumferential points (for example, two supporting pads and the circular margin) for proper location of a cylindrical body (the drill tip).
• Many gundrills produced today are 'non-micable’. In other words, their diameter cannot be measured using standard micrometers or similar measuring tools. In my opinion, there is no ‘science’ or other logical reason behind this ‘genius’ design.
• Many gundrills are produced using obsolete carbide grades. There are two grades of carbide commonly used to produce gundrills in this country, namely C2 and C3. It is believed that C2 is ’forgiving’ (God forgives) carbide and thus can be used on any drilling machine. A ‘small’ price to pay includes tool life and the surface finish of the machined holes. The other is C3 proving to be much harder and thus more wear-resistant. Unfortunately, it is also more brittle and therefore cannot be used in gundrilling systems having excessive misalignment and runout. I should point out that these ideas are wrong.  The advancement of carbide materials has enhanced the tool's strength (you can look at the mechanical properties of carbides by PlanseeTizit and Duramet and compare them with say, HB).  The problem now is to select the proper carbide grade for a given work material.

Ask simple questions:
       To keep it simple, I would like to ask you a question. If you call a doctor and he just asks for your address to send you a bill and a prescription for some pills instead of asking where it hurts, what is your medical history, what medicine you are taking and so on, would you believe and trust your health to such a doctor? I hope your answer is no.  If so, my next question is: When you pick up a telephone and make a call to order a gundrill do you believe that you are going to have "the right stuff", i.e. the right gundrill for your application?  If the producer on the other end does not ask you about the work material, its hardness and structure, about what accuracy of the drilled hole (surface finish, diametral tolerance, runout, etc) you need, what productivity you are hoping for, what brand of deep-hole machine you have and how old it is, what coolant you are going to use and what range of the coolant pressure and flow rate are available at your disposal, would you still buy your gundrills from this guy!!!  I rest my case. 
      Just remember that the gundrill is a very application-specific tool and without knowing the answers to the above-mentioned questions it is next to impossible to give you the right product.  Why don't gundrill producers ask you, their customer, these questions?  Most, even if they do ask, simply do not know what to do with this information....  Therefore, if you really want to get the right gundrill for your job, you should know your gundrilling system and work with a gundrill supplier who understands your needs making sure that the gundrill you order is meant to work on your particular system for your particular job.