Everything you need to know to specify the right quill for your precision grinding application
Anyone who does CNC grinding knows what a grinding quill is, but few actually understand the important contribution the quill makes to optimized abrasive machining processes. In the rush to fine tune their grinding operations, abrasive machinists turn their attention to such things as feeds & speeds, abrasive wheel shapes and materials, dressing options and appropriate coolant choices.
Quills seem to be too mundane to merit much consideration. That is generally a mistake, because, like other grinding process variables, quills have an important role to play. Selecting the right quill for the job can make a tremendous difference in whether or not you realize your grinding process objectives.
The Quill’s Purpose
The purpose of the quill is to get the wheel into the part to reach the surfaces you need to grind. It is a highly customized and tightly toleranced tool that serves as an extension of the spindle itself.
It would not be economical for spindle manufacturers to produce thousands of different spindle designs to reach into every possible interior geometry. Instead, they produce a single design with a standard interface to which customers can design quills to suit their individual requirements. A well-designed quill has a long lifespan, allowing the user to mount grinding wheels to it over and over again.
Quill Features and Nomenclature
Quill Material Choices and Trade-Offs
Quills are generally constructed from one of three materials, each with its own pros and cons:
Tungsten Carbide Quills are the ideal choice for precision internal grinding applications because they are very stiff. Of the materials available, tungsten carbide is the most expensive, but is long lasting and ideally suited to precision grinding applications because of its rigidity, which results in the least amount of deflection under load. For the tightest possible part tolerances, carbide can’t be beat. Due to it’s brittle nature, however, care must be taken when handling carbide quills to prevent breakage.
Heavy Metal Quills are ideally suited for applications where vibration absorption is desired. Heavy metal has a cost and stiffness in-between that of tungsten carbide and steel, and it’s damping characteristics make it particularly desirable for certain applications. Heavy metal is relatively soft, so may not be the ideal choice for applications where the quill is subject to frequent abrasion, for example when mechanically removing glued-on wheels. Also, as the name suggests, the material is quite “heavy”, so care must be taken to ensure it is properly balanced.
Hardened Steel Quills are the least expensive and fastest-delivery choice but do not measure up to the other alternatives in terms of tensile strength and stiffness. However, they are an acceptable choice, particularly if a short, stubby quill geometry can provide sufficient stiffness to suit the requirements of the application.
On the Horizon….New Advanced Quill Materials. New materials utilizing Metal Matrix Composites (“MMCs”) are beginning to gain a foothold in the industry. Examples include quills constructed from a combination of carbon-fiber/steel and also from solid ceramics. Although generally more expensive these materials offer high stiffness at a fraction of the weight of traditional materials.
Quill Configurations: Mounting to Spindle
Quills are generally mounted to the grinding spindle via one of two methods. The chosen method depends on the spindle manufacturer’s design.
Threaded Spindle Method – This is the most common quill/spindle interface. It comprises of three key features; a) the precision thread, b) a tight-tolerance locating diameter to ensure the quill screws-in on center, and c) a ground face on the back of the quill shoulder to ensure a square and solid support once the desired mounting torque is achieved.
Quill Configurations: Wheel Mounting
There are several methods by which wheels may be mounted to the working end of the grinding quill. These choices are governed by customer preference along with the requirements of the application.
Glue-on by Customer: Users may select a style of quill that allows the wheel to be glued to it. This approach provides excellent stiffness at the lowest costs because less precision machining is required to manufacture the quill. However re-gluing replacement wheels to this type of quill requires removing the worn wheel and residual glue from the quill, gluing on a new wheel and allowing for the glue to set. Manufacturers who use this approach generally invest in additional quills so that they have pre-mounted wheels available as needed.
Glue-on by Meister: Customers may also elect to send used glued-on wheels and quills back to Meister where wheels are carefully removed, and the quills are cleaned of residual glue and inspected to make sure they are still in good condition. Then new wheels are mounted on the quills as part of a low-cost continuous process. Meister trues all of its wheels before they leave the factory. In this case they are trued while mounted on the quill; so that grinding surfaces are in proper alignment to begin grinding immediately after changeover. Many users find that this approach saves time, provides better control over wheel and quill inventories, avoids the re-use of damaged quills, and results in more efficient changeovers.
Screw-In Mounting: Screw-in wheel mountings allow for fast wheel changes on the fly without having to remove the quill from the spindle. The precision threaded mount makes alignment of the wheel very easy. This style is frequently used in high production applications that require frequent wheel changes. The quills are more complex due to the internal threads required, however fewer of them are needed in inventory because they can stay on the machine.
Collet Mounting: For shops that perform many short run-grinding operations using abrasive wheels of different sizes, it is possible to mount a variety of straight-shank mounted wheels to a single quill using a precision-machined collet system. This gives manufacturers the flexibility to change wheel sizes frequently without always having to swap out the quill. Carbide wheel shanks provide excellent rigidity and a given size quill can accommodate a wide range of wheel sizes.
Thru-coolant quills: There is one other quill configuration abrasive machinists should be aware of– quills that allow for the passage of high-pressure coolant from the spindle directly to to the grinding zone. This approach provides exceptional flushing of debris from grinding operations within deep holes and pockets. Not all machines have the ability to accommodate thru-coolant but when they do, this allows a precision grinding operation to make the best use of it. In some cases thru-coolant spindles are used in conjunction with specially designed wheels with holes allowing for passage of coolant thru the quill and the wheel directly onto the grinding area.
Rules of Thumb
Here are some rules of thumb to make sure the quill you are using or designing is adequate for your application:
Quill Length: The shortest possible quill that allows the wheel to reach the part and to be dressed for the full usable life of the wheel. For bore-grinding operations with oscillation, it is generally a good idea to design the quill long enough to be able to push at least ½ of the wheel thru the bottom of the bore you are grinding. Also, when moving to the deepest position, you need to consider how much room is required at the nose of the spindle for peripherals like coolant nozzles, gauge fingers, etc.
Quill Diameter: To insure good rigidity, optimal quills have the largest possible shaft diameter that will fit safely inside the feature being ground, while still allowing dressable life on the wheel and adequate flow of coolant in/out of the grind zone.
Stiffness: When designing your quill you should keep in mind that the effective stiffness is proportional to: (E)*(D⁴) / (L³) , where:
- E – Modulus of Elasticity of the chosen quill material
- D – Diameter
- L – Length
For example, when comparing possible design alternatives, an increase in diameter of 10% will increase stiffness by 46%. Reducing the length by 10% will increase stiffness by 37%. Changing from steel to carbide will increase stiffness by 257%.
The wheel of choice and the quill should be designed to work in harmony with each other.
Users who specify precision super-abrasive wheels for their grinding application and couple them with a less than adequate quill undermine the total performance of their abrasive system. This is why Meister routinely presents the most appropriate wheel specifications, wheel geometries, and quill configurations as part of a single superabrasive system. Each of these elements works in concert to give the user the best possible result for the application.