FILENAMES: gundrill.jpg DESCRIPTION: This an image of diagrams which go with the text below on gun drilling. Posted by Paul Dickman . Paul \ provided the following description: ================================================================= This a short paper by Frank Zagar published in the 1962 ASTME collected Papers. It discusses accurate hole drilling and is a good primer on gundrills The drawing that goes along with it is gundrill.jpg The guy who wrote this is probably dead and the copyrights have probably faded away but just in case, show some consideration when making copies. Precision Holes in Metal Frank Zagar President Zagar, Inc. To qualify as a good machinist, is to be able to consistently make holes in exactly the right place --exactly to size, straight, and with good finish. To be able to make good holes of every possible size and descrip- tion requires a lifetime of experience. Each of us in the metal working business will always suffer through all the aches and pains of broken taps, oversize holes, undersize holes, egg shaped, out-of-round, bell mouth, broken drills, broken reamers, tight pushings, loose bushings, cutting oils, soluble oil, lard oil, broken center drills, holes off layouts, holes where they don't belong, and all the daily agonies of producing holes. After most of the usual miseries have been experienced, then there comes a time when the apprentice hole driller gets involved in what is known as "Precision Holes “. Here, the problems and the tech- niques never seem to be completely resolved. This discourse is an enu- meration of the why's and whereśore's of "Precision Holes" and the factors that have to be regarded to produce quality holes . Our prime function at Zagar, Inc. is to make holes of all types in great quantities. It is a natural pitfall that to us should very often be assigned the task of making many precision holes in parts for sewing machines, washing machines, carburetors, aircraft, etc. It is not unusual for us to see many hole dimensions specified to within .001 of true position and .0005 in size. In experience we have all kinds of trouble. The holes will be out of position, they will come oversize or undersize. The reamers will "pick-up” and make rough holes, they will even break off. We try different kinds of coolants and lubricants. Some reamers make the holes undersize, some oversize, some of them work for the first small batch of holes and then endless troubles seem to result. In order to clarify and think on somewhat of a logical basis, let's review the general techniques of making precision holes, and what we must consider regardless of how simple making holes might seem. In the first place, we must know what should be classified as a precision hole. In my opinion, any hole that is represented as having to be in a true location within a plus or minus .002 or less is by nature of its location a precision hole. Any hole that bears size tolerance of within ..002 or less is by nature of its size necessarily a precision hole. The most direct method of producing precision holes is on preci- sion boring or grinding equipment. I am sure that all mechanics will agree that boring machines with precision spindles using diamonds or carbide tools will produce excellent holes at the first try. The location of these holes will be as good as the slides of the boring machine. The hole sizes will be as good as the condition of the bearings of the boring spindles and the condition of the tool tips .In the long haul, boring ma - chines seem to be the most positive way to make precision holes for both location and size. Unfortunately, this most positive way is actual prac- tice, not always practical or possible. Most of us will first try reaming in order to produce good holes. Here is where the fun begins and it happens over and over again in every shop in the country. The position of a reamed hole is somewhat determin- ed by the position of the rough drill operation. If the rough drill operation off position, then the reamer will have a tendency towards bending with the stock in the hole. As a result, the reamed hole will be off-position in some degree, proportionate to the amount of the off -positioning of the orig- inal roughing operation. This is particularly true as the diameter of the hole in relation to its length becomes excessive. For example, a 3/4" hole 3" long is very difficult, if not almost impossible, to hold for straightness and position if the original cored hole is off position. Reamers do have a tendency to follow an original hole location. It is necessary to use bushings with reamers to assure the hole location. Bushings guide the reamers to locate the holes in correct posi- tion. Reamer bushing lengths should be at least three times the diameter, long bushings are preferable. The clearance between bushing and reamer should be held to a very minimum. The size of the bushing should be a good running fit inasmuch as it will determine the size or bell mouthed condition of the hole produced. It is advisable to float reamers. This can be done with conventional floating holders, or, it can be done to a modified extent by weakening the shank of the reamer to allow it to flex in conform- ance location to the position of the bushing. Due to the close fit between the reamer and the bushing, it is essen- tial to provide ample coolant to the assembly. If coolant is not provided, the reamer will seize in the bushing. If possible, provide coolant from the back side of the hole. It is also important that reamer bushings be reasonably close to the part being worked on, the closer the better. On the subject of hole size, reamers will "pick-up" stock on their "lands" and make rough holes. They will furthermore, make tight holes when they are dull and oversize holes when they are sharp. Reamers will cut under size if wall sections are thin and will cut larger holes on thick sections. They very often will cut large holes on the start of the hole and tight holes in the back. Reamers are many times excellent and countless numbers of them are in use, but they are not the cure -all and many times are impossible to use. We have experienced excellent results with carbide tipped reamers. It seems that carbide over and above its ability to withstand considerable wear, does have the faculty of resisting the "pick-up" on the "lands” of the reamer flutes. To summarize the approach on reaming of holes we can conclude that the best approach is to use carbide reamers, and a close fit of carbide bushings, either the ring type or solid, and plenty of coolant. It seems with the combination of carbide bushings and carbide all the reamers and coolant, good holes are almost assured on the initial try. Gun Drilling ---There is another accepted way to make a pre- cision hole by the method that is commonly reśerred to as "gun drilling”, "gun boring" or “gun reaming". There have been some rather spectacu- lar successes registered with gun drilling here of recent years and it is on the subject of gun drilling that I hope this discourse takes its greatest emphasis. It seems to me that much can still be accomplished if we can properly evaluate and understand exactly the way a gun drill works in its cutting action of producing holes . A recent atomic research project required that there be holes pro- duced approximately 1/8” in diameter. .0002 of size. 18” long, and a rel- ative position from one end to another within .005 of true position. As impossible as this requirement may seem, it has been successfully done on a production basis. The story goes that a large producer spent a good many thousands of dollars in building a machine that would do this job. The machine never did materialize any good pieces. Somewhere along the line. a "jack of all trades" came into the picture and in his garage managed to produce these impossible specifica- tions with somewhat dilapidated equipment. The technique used was a modified type of gun drilling. In gun drilling. the start of the hole is the most important issue. Picture , for example. a 1/2" diameter rod 20” long with a tool bit on the end. It's obvious then that the end of this drill must have some sort of a starting or guiding device. It is this start that is all important. all other considerations except lubricant and chip removal being of minor impor - tance. After the cutting edge or the head in the gun drill has passed in the work past the depth of the bushing, then the drill produces its own guide bushing and it is continued indefinitely. Here of recent years. it has been possible to produce much better holes because of the use of carbide tipped gun drills. They maintain their cutting edges and their wear edges for long periods. It is not un- usual to make holes consistently good in lengths of anywhere from 6" to 30", holding size within .0005 and holding position within. 002. Most precision hole requirements in every day work are of a very comparatively short nature of less than 2 " long and it seems logical that what can be produced in 20" lengths could also be successfully produced in lengths under 2 " .Various drill manufacturers are now introducing short gun drills in what they refer to as "Bore Drills" or "Gun Reamers". All of these can lead to considerable difficulty unless a thorough under - standing of how these tools cut and operate is established. Gun drilling is basically a "one sided single lip boring tool". We once had a very interesting experience in attempting to multi- ple drill holes 3/8" in diameter and about 3" long in aluminum. The holes and a iadto be straight and slightly large. In order to do the job in the best possible manner. we sharpened all the drills on a precision drill grinder. Our night man tried drilling pieces. He didn't get very far and he assumed that all the drills were sharpened wrong, so he resharpened them by hand. We then found about fifty percent of the holes acceptable. After sharpening the drills again on the drill grinder. we found none of the holes were acceptable. Investiga- tion proved that hand sharpening the drills created an off -center condition, so the drills were actually acting as gun drills and cutting with only a lip. We sharpened all of the drills to sil'lgle flute cutting. the job passed with flying colors. Gun Drills -Gun Reamers ---There exists a degree of confusion -- between what should be considered as a "gun drill"and what should be con- sidered as a “gun reamer “. I believe that both tools act and cut basically the same. A gun drill should probably be referred to as 'one that is working into solid material where it is necessary to use a coolant throughthedriJ: ~ uI o ~!.!J ~. under pressure. A gun reamer should be considered as working on a hole that has been previously drilled or rough cored and coolant is supplied by the usual external system or by the nature of the coolant supply, the back side of the hole. The use of gun reamers are for limited length of holes. We would also use gun reamers where center distances are limited, or where it is desirable to do a great quantity.of holes at one time and the positive assurance of coolant to the tool tips is difficult if not practically impossible. The basic action of either a gun drill or a gun reamer is the same and it is described with the attached sketch. Sketch No. 1 indicates the gun drill in an oversize bushing. This has been done to illustrate the general reaction of the gun drill in some- what adverse conditions. In practice it is advisable to hold the size of the bushing as reasonably close to the size of the gun drill as is practical. Some wear of the bushings will not necessarily deter from the quality of the hole provided the general technique or principle is thoroughly under- stood and adhered to. There is still much to be said about the position of the pads on the gun drill or gun reamer, the oil pressure required, the volume of oil for various sizes of gun drills. the speeds and the feeds, and the possible sizes. paper number 402