This page describes the technology of centreless (centerless) grinding as well as centreless (centerless) grinding machines from Koyo Machine Industries. With centreless (centerless) grinders, a workpiece is supported by a work rest blade and set between a rubber regulating wheel that rotates the workpiece and a rotating grinding wheel. Koyo remains the world leader in centreless (centerless) grinding technology with products such as our 15-axis CNC control system.
Centreless (Centerless) Grinding Principle
Centreless (centerless) grinding is an OD (outer diameter) grinding process. In difference from other cylindrical processes, where the work piece is held in the grinding machine, while grinding between centers, the workpiece is not mechanically constrained during centreless (centerless) grinding. Therefore the parts to be ground on a centreless (centerless) grinder do not need center holes, drivers or workhead fixtures at the ends. Instead, the workpiece is supported in the grinding machine on its own outer diameter by a workblade and by the regulating wheel. The work piece is rotating between a high speed grinding wheel and a slower speed regulating wheel with a smaller diameter.
G: Grinding Wheel - R: Regulating Wheel - B: Blade - W: Work piece
The blade of the grinding machine is usually positioned in a way that the center of the work piece is higher than the virtual line between the centers of the regulating wheel and the grinding wheel. Also the blade is designed with an angle in order to ensure that the work piece is fixed between the blade and the regulating wheel. The regulating wheel consists of soft material like rubber and can contain some hard grain material to achieve good traction between work piece and regulating wheel.
Centreless (centerless) grinding can perform excellent roundness of the work piece. However, caused by the simultaneous suspending and machining of the work piece surface it is possible that process typical roundness errors are generated. Proper adjustment of the grinding machine and the grinding slot geometry is essential. When a high spot comes in contact with the regulating wheel, then on the other side of the work piece a low point will be ground. However this low point must not be exactly in the opposite side of the work piece. The grinding machine has to be set up in a way that a polygon form is ground with so many corners that it is almost round finally.
g: grinding wheel - r: regulating wheel - w: work piece - dg: diameter grinding wheel - dr: diameter regulating wheel - p: penetration depth - n: polygon order
Grinding Wheels and Dressing
Koyo is using on a large percentage of its grinding machines grinding wheels of cubic boron nitride cBN or diamond wheels. For the series 30 and 45 grinding machines a special dressing and truing system is available as an option: electrical discharge truing E-dt.
Automation
There are mainly two different types of centreless (centerless) grinding:In-feed grinding
The work piece can have different outer diameters over the length and either only part of the work piece is ground or the total work piece is ground using an adapted grinding wheel.Thru-feed grinding.
Cylindrical work pieces can be ground using this method. The work piece can be longer than the grinding wheel and will still be ground over the full length. It is also possible to grind small work pieces with this method. In this case, several work pieces are ground same time in the machine and high throughput can be achieved.
- By line wise support of the work piece it is possible to grind soft or brittle work pieces (low tension)
- It is not necessary to prepare the work piece for fixing in the grinding machine (faults cause by fixing are eliminated).
- Loading/Unloading of the work piece is simple and easy to automate.
- When using continues through feed grinding there is no time loss for changing the work piece.
- Long work pieces can be handled with rather small machines.
- Very high grinding wheel rotation speed can be achieved.
Main Applications for centreless (centerless) Grinding Machines
Mass Production
e.g. bolts, shafts, bearings, hubs, valves, needles, axles, pivot
Special Application
z.B. bars, tubes, cylinders, rotors, isolators, balls
Fork Regrinding
The process of fork regrinding is essential to achieve a consistent surface contour across the stanchion. Generally speaking, the whole stanchion is ground back to the level of the deepest area of corrosion and pitting. There is a limit to how far forks can be ground back and if corrosion is too deep then the forks are beyond repair. However, in some instances localised regrinding and plating can be undertaken to build up a localised area on the forks which saves time on grinding the forks completely and then building them back up.
Centreless Grinding: The principle of centreless grinding can be seen in the diagram below. Two wheels are used, the larger one operating at regular grinding speeds which does the actual grinding, and a smaller regulating wheel. The regulator wheel revolves at a much slower surface speed - usually around 50-200 ft/min, and controls the rotation and longitudinal motion of the work-piece. The work-piece is held against the work-rest blade by the cutting forces exerted by the grinding wheel and rotates at approximately the same surface speed as that of the regulating wheel.
Cutting: The cutting process is essentially quite simple. The fork is passed through the centreless grinding were approximately 25-50µm is removed. The wheels are reset and the forks passed through again to remove another 25-50µm. This process is repeated until the forks have been ground down to the deepest level of pitting and corrosion.
No comments:
Post a Comment