MACHINE TOOL SELECTION. Introduction

In 1938, in the first edition of Grinding Wheels and Their Uses, Heywood wrote: “Internal grinding machines are being improved so rapidly that a machine only a few years old is practically certain to make the internal grinding operation cost more than it would with a later model.” The most striking fact about this statement is that in the intervening 65 years it is as true as ever!

When selecting a grinder, the end user must be clear on the part dimensions and tolerances, required production rate, and required grinding operations. Some applications require high-volume, dedicated machines while others may have a range of part sizes or require the grinder to do multiple grinding operation not limited to just a single internal grind operation or even just grinding. The following are some examples of common machine types and applications.

18.5.2 Fuel Injection

Grinding fuel injectors are characterized by very high production requirements, small wheel diam­eters (2 to 6 mm), limited coolant access, and extremely tight finishes and tolerances. Bore roundness is especially stringent and must be held to <0.5 pm. Taking the requirement for CpK capability into account, the requirement is nearer 0.25 pm. There is usually a bore grind and an angled seat. For diesel injectors, there are often also faces and combination grinds.

Machine tool builders focused on this industry such as UVA and Bahmuller build modular grinders where the slides and spindle can be easily interchanged or modified thus maintaining flexibility and economics of scale in manufacture. Wheelheads run up to as high as 240,000 rpm [UVA n. d.] but speeds in the range of 140,000 to 180,000 rpm are more standard [Bahmuller 1997, Micron 2000, Okuma 2000, Studer 2001]. Spindle power is <3 kW at these speeds. CBN wheels dominate at around 270#-500# (B76-B36) grit size; and quills are carbide.

There are numerous ways of mounting the wheels. Gluing direct to the carbide by the end user is problematic. Wheels are, therefore, often supplied on wheel screws. However, producing quills in carbide with high accuracy electrodischarge machine-cut internal threads for these to screw into is expensive. Only a few companies such as Hamex Hardmetallverktyg, Linkoping, Sweden can produce to the necessary tolerances. Wheels are, therefore, also sometimes supplied on straight carbide shanks and held in a collet wheel mount.

Getting coolant into the grind zone is difficult. Fortunately, even at the wheel rotational speeds mentioned above, the wheel velocity is still barely 20 to 30 m/s, which limits the risk of thermal damage. Also low viscosity oil, chilled and filtered to <5 pm, is used almost exclusively. If spray holes exist in the injector tip, it may be fed with coolant from behind the wheel. Many wheel spindles can also provide coolant through the center of the quill, some as high as 120,000 rpm. Specialist wheel companies can produce precision wheels on hollow wheel screws with fine perforations in the wheel face for seat or end grinding to deliver coolant evenly right at the grind point. For new applications, especially grinding at the end of long bores, FEA analysis of the dynamic stiffness and resonant frequencies of the quill is required.

The work drive is the most critical component, for its running truth governs the running truth of the finished part. Aerostatic and angular contact ball bearing drives have been used but the most common are hydrostatic or hydrodynamic. Industry standard for precision is currently 0.1 pm; speed of rotation is up to 4,000 rpm. Since the workhead is also one of the stiffest parts of the grinder, some OEMs have taken advantage of this and mounted the dressing diamond as a ring around the chuck.

Diaphragm, collet, and jaw chucks are all used for part holding. The primary concern is to avoid distortion of the part and to maintain accurate running truth. The jaws should be regularly ground; the same wheel used as for the bore grinding can grind high-speed steel jaws. Otherwise, dressable vitrified diamond wheels are available for grinding carbide jaws in situ.

Most slideways are hydrostatic with ballscrew/ac servo drives accurate to 0.1 pm. The high oscillation/short stroke length required has recently led numerous OEMs to provide linear magnetic motors as options. All state-of-the-art machines have sophisticated CNC controls with a variety of the AE dressing and gap elimination options described above.

Load/unload is carried out with a pick and place arm or gantry as the parts are too small or awkward in shape to load by a gravity-fed method.