by Paul Lalewicz
In todays growing glass industry, glass fabricators encounter a wide variety of applications for drilling holes in glass. A days work could involve anything from cutting holes in glass ceilings to accommodate sprinkler heads, to drilling repetitious holes in auto glass, to cutting electric outlet openings in mirrors. Drilling through thick glass is necessary when fabricating glass furniture or drilling a variety of holes to partial depths in a piece of artwork.
These diverse demands have created a need for an even greater variety of glass drill types and the associated machinery to operate them. To provide the best results, glaziers need to properly define their applications and use the correct types of drills and machinery for each particular application. These include single-layer and multi-layer diamond core drills.
The word drilling brings many things to mind, most not applicable to the process of creating a round hole in glass. When we drill glass, we actually grind away material in a circular fashion to create a round hole. The drill bit in this case is primarily an abrasive holder. This holder must present the abrasive (which is usually diamond crystal) to the glass in an optimal way.
In order to achieve this, there are several important factors to consider. These include the type of diamond, how it is attached or exposed to the "holder" or drill bit, the speed of travel of the abrasive, the downward pressure (known as "feed rate") and the coolant applied to the contact area. These key factors exist regardless of the type of drills, drilling machines or particular application used.
In most drilling applications, a core drill is used, which means that the abrasive material will contact the glass in a circular path around the outside perimeter of the hole to be produced. As we explore drilling variations in this article, keep in mind the importance of this abrasive path and the optimal factors.
While there are many types of diamond core drills for glass, they can be divided into two basic groupssingle layer and multi layer. Single-layer drills usually are known as plated drills. They are durable, quick-cutting and not subject to a great amount of operator error. Plated drills also perform well with small portable machines, which are often imprecise.
The drawback to plated drills is the single layer of abrasive which has a short life-span. Because of this, plated drills are used primarily for smaller drilling volumes and portable applications that require quick trouble-free performance.
The second type of drill, the multi-layer drill, offers a great life expectancy. It is produced by mixing the abrasive with a bonding material and molding this bond to a specified depth. Multi-layer drills usually are referred to as metal bond or electroformed drills.
When dealing with multi-layer drills, the challenge is to work with the abrasive exposed on the cutting edge without too much interference from the bond. The bond must hold the crystal while still leaving enough crystal exposed to perform the grinding action. This relationship of bond/crystal exposure changes as the drill is used and must be maintained by proper use of the correct sharpening stone.
Since this bonding material must be soft enough to allow wear as needed, this type of drill is far more fragile than single-layer types. Careless handling or use in imprecise machinery will result in breaking, bending or cracking the bond and will prevent future use.
In addition to using the proper machines and good drilling practices, the most important thing for operators to understand is proper sharpening practices and when to apply them.
When sharpening drills, the first important consideration is to choose the correct stone. Proper porosity, hardness and grit are very important. Sharpening stones used for diamond wheels are usually a fine grade and should not be used. Rather, use stones that are specifically designed for drills. The correct stone should be placed in a tray of water before use. These stones are quite porous and need to be saturated thoroughly with water. This will let the abrasive stone remove the excess bond contact while exposing fresh diamond without generating heat.
For safety, the stone must be larger than the diameter of the drill being sharpened, or it can become a flying projectile. The stone should be held securely or fixtured below the drill. When drilling into the stone, a mud paste is created. After drilling, the operator should move the stone over slightly and drill down again. This abrasive action is all that is required to present fresh abrasive on the drill cutting surface.
All multi-layer drills will need sharpening when the average drilling time starts to increase. Most plated type drills can be sharpened lightly once during their lifetime when the remaining crystal becomes hidden.
The speed (RPM) and pressure (feed rate) are also very important for optimum drilling. As the diameter of the drill increases, the RPM is reduced to provide the same surface travel speed. Always start with manufacturers recommended RPM for the size you are using and fine tune as necessary. Multi-layer drills will always be run faster than plated-type drills.
As the diameter of the drill increases, the feed rate needs to increase to compensate for the larger contact area. When drilling submerged in a water/coolant mixture, it is best to cycle, that is, use an occasional up and down motion to allow coolant to be present at the contact area continually. This is also important when drilling deep holes in thick glass. In all cases, when single side drilling, the pressure should be greatly reduced, just prior to breaking-through, to reduce chipping on the exit side of the hole.
Using a good grade diamond coolant is another part of successful drilling especially when using plated-style drills. The coolant lowers the temperature in the drilling zone by reducing friction. This reduces diamond wear and helps the crystal stay firmly fixed in the bond, holding it to the drill. Turpentine or antifreeze are not good choices and may affect backings and coatings present on mirrors or high-performance glass.
A wide variety of machines are available for glass drilling today. Choices start at portable cup or tripod machines, progress into portable machines with center water feed and finally move into large, single or double drilling production machinery.
A double drilling machine is usually reserved for large fabricators doing high volume production work. These machines use two separate drill bits, top and bottom to complete the finished hole in the center of the drilled glass. This eliminates the exit chipping common in single-sided drilling operations. The large initial cost of these machines requires a high level of production to become cost effective.
For smaller fabricators, a portable machine, or a drill press modified to accept a water coolant chuck, may be the ticket. The user should review the intended volume of drilling, sizes required and desired finish requirements before deciding on what equipment to buy.
The old standby, the tripod type portable drill, has been used in the glass industry for more than 50 years. Originally designed for use with brass or steel tube drills, this machine still remains valid today for diamond drilling of low volume, assorted size work.
Cup-type portables are similar, but physically lock down to the glass surface with a large rubber cup or cups. Because tripod drilling machines are difficult to position near the edge of the glass, some users prefer the cup type portables for drilling close to the edge, as well as for specialty applications such as aquarium drilling. Locking the machines securely to the glass surface also increases stability and allows more precise drilling.
New entries include small portable machines, featuring a built-in water coolant chuck and electronic, variable speed controls. Multi-layer type drills are used with these machines. Because of this and the fact that they provide optimal cooling, they are an excellent choice for many applications in the shop and at the job site.
In conclusion, glass drilling does not require magical abilities. Best results are achieved by forging a relationship with a quality supplier who can support you with the right drills, machines and accessories for your applications and proper technical support when needed.
Paul Lalewicz has more than 25 years of experience in the diamond tool and glass industries. The former owner and president of Amazing Glazing diamond tool company, he now heads up the new C.R. Laurence Research & Development facility in Howell, MI.
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