Volume 35, Number 2, February 2000
DISPELLING THE MYTHS
by Rick Haynes
The Loch Ness Monster, Bigfoot, The Lizard Man, flying saucers, and diamond tools that last forever. What do all of these things have in common? They do not exist! Sure you have heard stories about them, but have you ever actually seem them? The funny thing is that the more experts investigate the facts about each of these phenomena, the more they agree that they do not exist. But, when it comes to diamond wheels for flat glass fabrication, many people involved with the sale or usage of diamond tools simply do not know the facts.
This article will attempt to dispel some false information and will specifically focus on resin bonded cup wheels. It will also attempt to answer a variety of questions. Why is a resin matrix used in the manufacturing of glass grinding wheels? How many different types of resins are used? What differentiates the wheels from each other? How can you be sure you are getting the best tool for your application? I will explore each of these questions and perhaps dispel some myths about diamond tools. We will leave the other myths for someone else to explain.
Resin as a Bond Matrix
The reason resin is used as a bond matrix is simple: manufacturers need a compound
that will hold small diamond crystals and release these as they become dull. When metal is
used as a matrix, ultra fine diamond crystals cannot be used. Additionally, when a metal
matrix and small diamond crystals are used together the diamonds are not released when
dulled. As a result, the metal will glaze or smear the metal
matrix over the face of the wheel. The closed face of the wheel damages the glass surface
making it impossible to polish the surface to an acceptable finish. The closed face also
creates heat, which will eventually destroy the wheel as heat is a destroyer of diamond.
The type of resin used for cup wheel manufacturing is a simple catalyst driven, plant
derived substance that is activated (set) by heat. Melamine and phenolic resins, as they
are called, are formed into shape by applying a combination of pressures and temperatures
over a predetermined time. The resin matrix, in powder form, is mixed with the required
diamond crystal size (grit or mesh), poured into a mold, and set using heat and pressure.
The resin powder and diamond crystals change to a liquid because of the heat. The pressure
then forms the powder into the shape of the mold, and a cooling period completes the
process. The wheel then is removed from the mold and precision machined to fit the
required glass grinding machinery. The problem with this type of resin is after it is
transformed from solid-liquid-solid, the compounds new form continually evaporates.
This result gives resin products a finite shelf life. The amount of shelf life is the
basis for argument, but this continual drying process will affect the
performance of this type of tool. The amount of time the tool can be inventoried and not
begin to have an adverse affect on the performance is approximately three
Super Secret Resin?
Is there a super secret resin? No. All resin wheels are manufactured basically the same and have the same basic characteristics. There is no super resin used and, in fact, there is no need for one: the diamond crystals determine the life of the tool. Super resins could be used, but if the diamond crystals are held too long there is no advantage gained. Diamond grains held too long by the resins or prematurely dulled by the grinding operation will cause a glazing on the wheel face. The resin face will actually melt preventing new diamonds from being exposed. Resin wheels sharpen themselves by continually releasing old dull diamond crystals and new sharp diamonds are exposed. When resin wheels must be dressed manually, use a very fine aluminum oxide stick (approximately 325 grit) or a pumice stone. Normally resin wheels will self dress as explained above, but it is not uncommon for them to need manual dressing. If you are dressing your resin wheels continually, contact your supplier because something is wrong.
Fillers and Porosity
Fillers are added to the resin compound to aid in lubricity. Graphite and Cerium Oxide are the two main types of fillers, but additional fillers are added in an effort to strengthen the resin. Resins have the natural physical property of being very brittle, thus fillers are added to give the resin a certain resiliency and to avoid breakage. These fillers aid in holding the diamond grain by enabling the resin to absorb the vibrations created during the grinding process and not fracture. Unlike when grinding metals, the grinding process of glass is not a cutting action, but a chipping action. The glass does not slice, the removal of glass is a controlled breaking process.
Porosity is added to resin bonded diamond wheels in an effort to reduce the surface contact of the wheels. Porosity is created by adding salt to the resin bond compound prior to the actual pressing process. During the pressing process the salt evaporates and leaves the holes (or porosity) in the bond. Slotted or turbo style resins accomplish the same results. Slotted or turbo wheels are manufactured by replacing the normally smooth mold tops with a mold top containing teeth. The teeth are pressed into the resin compound leaving the slots in the bond. Problems sometimes found with these methods of bond softening are the pores or slots can become clogged with the grinding slurry and actually scratch the glass. Properly-engineered bonds do not need the added porosity or slots to grind efficiently.
Diamonds used in resin wheels are all man-made. While there are many manufacturers of diamond grit, there are only two major manufacturers of man-made diamondsDeBeers and General Electric. These two manufacturers use state-of-the-art processes to manufacture, then separate grit and grain sizes.
The perfect diamond for glass fabrication has a slightly higher degree of roughness and is not too smooth and blocky. The diamond crystals must be friable (able to fracture and stay sharp) but not fracture prematurely. Several diamond wheel manufacturers have experimented with different styles of diamond grits, including those coated with various compounds, such as nickel, copper, and titanium, in an effort to increase the life of diamond grain. But, in the end the cost prohibits most of these processes and success of these programs has been, at best, questionable. This leaves most wheel manufacturers using basically the same diamond powders from these two vendors.
Super Resin Wheels
So, where do the resin wheels capable of running three to five million inches come from? Clearly they do not exist. The average life of all resin wheels being sold today is between 400,000 inches up to a maximum of 1,500,000 inches. This is not to say some users cant get more inches, but it is highly unlikely that in a production atmosphere these higher numbers could be maintained. Strangely enough, the actual expense of the resin wheels when compared to the amount of inches of glass produced is also much lower than most people realize. For example, a resin wheel that costs $145 and produces one million inches of beveled glass costs .000145 cents to the customer. A resin wheel that costs $145 and produces three million inches of beveled glass costs .000048 cents to the customer. Clearly, this is not the area where customers will find huge savings. The ease of polish, scrap-rate, grindabilty, and reputation of your vendor are much more important factors. Does your supplier work with your operators? Train your personnel on how to get the most from your tools and machines? When purchasing diamond tools, be careful not to exclude these areas as they can cause large losses or profits to your company.
So, how should an end user decide which resin wheel is best? How quickly a wheel can be set on the machine and begin to produce quality glass should be a major factor. The only time your machine is not making you money is when it is not running! The amount of scrapped metal produced by the wheels is also lost revenue and should be considered. Quality of sales representation should also be a consideration. Do you trust your sales staff to keep you updated on new products and proper usage of current products? Now, if we could only train Bigfoot and the Lizard Man to run good glass.
Deciding Which Resin Wheel to Buy
Choose wheels easy to set-up and use.
Choose wheels that produce a minimum of scrap.
Buy from a supplier you trust.
Use wheels consistent in quality.
Choose wheels first and negotiate price second.
Choose a supplier large enough to provide support if problems do occur.
Follow manufacturers instructions when testing new products and test in full sets when possible.
Rick Haynes serves as diamond tools sales manager for Salem Distributing Company located in Clemmons, NC.
© Copyright 2000 Key Communications, Inc. All rights reserved. No reproduction of any type without expressed written permission.