Angular plunge grinding- Walter Graf

Introduction

Introduction

When looking at angular plunge grinding, I was surprised that I did not find much information in the relevant technical literature. Over 30 years, I have purchased all the books published on grinding that I could get my hands on, both in English and German (and other languages). I found it disappointing that none of the authors I came across have spent much time on angular plunge grinding to offer practical information. In fairness, the purpose of academic literature is not to give practical advice. However, in my experience, practical advice is what people need and want. They have workpieces to grind to specific tolerances and quality demands. They set up a process, grind the workpiece and measure the results. Unfortunately, the results are often not what they have wanted to achieve. Hence, they must correct their process parameters to get the desired results, and for this, they like actionable information.

Furthermore, searching for helpful material that the machine tool builders may have published proved equally fruitless.

My aim in writing my articles on grinding is not to measure up academic standards but to give practical advice on how a process can be set up, what to watch out for, and how to implement corrections. Throughout my career in grinding, I have seen myself as a translator of academic literature into the language of the workshop.

Variants of Plunge Grinding

There are two forms of OD plunge grinding: Straight plunge grinding and angular plunge grinding.

Why Angular Plunge Grinding?

What are the significant advantages of angular approach grinding? First, the angular approach allows grinding both shoulder and diameter simultaneously. Furthermore, the angular position of the grinding wheel reduces the contact length on the shoulder, thereby minimizing or eliminating the risk of burning altogether.

Angular plunge grinding requires cylindrical grinding machines to swivel their wheelheads into a 60° position or feature a fixed wheelhead at a 60° position. The angular X-feedrate interpolates the vertical X-feedrate and the horizontal Z-feedrate. When calculating the material removal rate, Q-prime (Q'w), it is essential to note that the angular X-feedrate must be converted into a vertical feedrate, which is explained later in this article.

By setting the grinding wheel at an angle of 60 degrees, when grinding different diameters simultaneously, the diametrical difference between the different grinding diameters on the grinding wheel is reduced. Hence, we have less variation in the speed ratios across the different diameters of the wheel. As angular plunge grinding of multiple diameters generates high grinding forces, the use of steady rests may be advisable. In general, angular plunge grinding is used for large production lots. Straight plunge grinding also allows us to grind different diameters simultaneously. However, the grinding of shoulders with the side of the wheel is not an efficient process. This inefficiency is due to the large contact area and the extensive arc length of this process. However, sometimes a cross-hatched pattern is required, which can only be achieved by grinding with the side of the wheel. In a tool shop environment, straight plunge grinding combined with shoulder grinding can be applied. However, great care must be taken when grinding the shoulder as the risk of burning is high.

Risk of Burning

In angular plunge grinding, the risk of burning on the shoulder must also be considered. Although, this risk is much lower than for straight plunge grinding. Contributing factors may be insufficient coolant supply into the grinding gap at the shoulder, a closed wheel structure, or slow dressing feedrate at the side of the grinding wheel.

It is advisable to choose grinding wheels with induced porosity to reduce the risk of burning in angular plunge grinding. This selection is essential if the shoulder height is more than seven millimeters.


Further measures to reduce the risk of burning would be to apply different dressing feed rates at this side of the wheel and its diameter. For grinding the shoulder of the workpiece, the dressing feedrate at the side of the wheel should be much higher than the dressing feedrate on the wheel diameter. Increasing the dressing feedrate on the side makes the grinding wheel structure more open, and the grinding wheel acts more aggressively, thereby further reducing the risk of burning.
 

Angle plunge grinding can be combined with reciprocal grinding if the length of a diameter is much greater than the width of the grinding wheel. One can also distinguish between single angular plunge grinding but just one diameter and one shoulder ground on one individual workpiece. Multiple angular plunge grinding describes the same grinding wheel performing several individual plunge grinding operations. Lastly, multiple simultaneous plunge grinding describes the process whereby a grinding wheel has been dressed to simultaneously grind several diameters and shoulders.

Grinding Parameters

Regarding the surface speed of an angular plunge grinding process, the maximum wheel speed should be limited to 45 m/s. The higher the wheel speed is chosen, the higher the risk of burning at the shoulder section. The material removal rate, called Q-prime (Q'w), is a vital parameter for establishing the efficiency of a grinding process. In angular plunge grinding, the wheel approaches at an angle of 60 degrees. Q-
prime, however, is calculated based on vertical feedrate into the workpiece diameter. The following Illustration shows the formula for Q-prime in straight plunge grinding.


The material removal rate, called Q-prime (Q'w), is a vital parameter for establishing the efficiency of a grinding process. In angular plunge grinding, the wheel approaches at an angle of 60 degrees. Q- prime, however, is calculated based on vertical feedrate into the workpiece diameter. The following Illustration shows the formula for Q-prime in straight plunge grinding.

As mentioned before, for angular plunge grinding, we must convert the angular feedrate into the vertical feed rate. This conversion is shown in
Illustration 10: Q-Prime for straight angle plunging

. This conversion is shown in Illustrations 11 & 12.

Illustration 11: Q-Prime for angular plunge grinding

Illustration 12: Feedrate approach at 60°

Given an approach angle of 60 degrees, we multiply the X-feedrate by a factor of 0.86 (cosine of 30°), the complementary angle to 60°. This factor converts an angular Z-feedrate of 1 mm/min to 0.86 mm/min in vertical mode. Modern CNC machine tools offer the option to change the approach angle to 45 degrees while leaving the wheel positioned at 60 degrees. This method is applied when much material is left on the shoulder, which requires a more aggressive approach. It must be borne in mind that the grit penetration on the shoulder is much smaller than on the diameter. Hence, grinding on the shoulder produces finer chips than grinding on the diameter. Knowing that the chips remove heat from the grinding area, applying the 45° approach makes the grinding more aggressive.

Illustration 13: Feedrate approach at 45°

When using the feed rate approach at 45 degrees, we must divide the feed rate in the X-direction (mm/min) by the square root of 2, i.e., by 1.41, to get the vertical feedrate needed for the calculation of Q-prime, as shown in Illustration 13. If the feedrate in X is 1 mm/min, the vertical feedrate is 1/1.414, therefore 0.7 mm/min.

When grinding multiple diameters simultaneously, the resulting Q prime rates vary considerably. This fact is easy to visualize, given that each diameter rotates at the same RPM. Therefore, a large diameter has more material removed than the small one in one revolution, as shown in Illustration 14.

The Q-prime values vary proportionally in the above case, with a diameter range from 100 to 50 to 25 mm.

The variation also applies to the speed ratio of each diameter on the grinding wheel. Angular plunge grinding has the advantage that variation between the different diameters on the grinding wheel is much smaller than in the case of straight plunge grinding. Illustration 15 shows a straight and an angular approach wheel, both 400 millimeters in diameter, and demonstrates that the angular plunge wheel has only about a third of the variation of the diameters. This smaller variation in wheel diameters, in turn, reduces the speed ratio variation that may occur in the case of straight plunge grinding wheels.

The grinding literature tells us that the speed ratio should start at 60 for roughing, at 80 for standard grinding, and not exceed 120 for finish grinding. This rule has proven to be an excellent guideline for OD grinding. If we go much below a speed ratio of 60, we risk entering the machine bed's harmonics, creating chapter marks. If we exceed 120, the risk of burning goes up. The following Illustration shows that we plunge grinding of multiple diameters, we do have a significant variation of speed ratios. These speed ratios are way off the recommended guidelines, particularly for straight plunge grinding. Lower speed ratios on some of the diameters can lead to surface finishes that are is too rough. Inversely, it can also lead to burning on the diameters and shoulders where the speed ratio is way above 120. For these reasons, we can see that setting "correct" grinding parameters is a compromise. This compromise also applies to dressing when using rotary diamond dressers. For rotary dressers, the recommended speed ratio between the dresser and the grinding wheel is 0.8. However, since several different grinding wheel diameters engage with an equal number of different dressing diameters, setting a speed ratio is again a compromise.

Regarding the speed ratio, angular plunge grinding is superior to straight plunge grinding as it has a lower variation in speed ratios. This is an essential advantage as it allows the operator to control surface finishes and burning more efficiently.

Specifying the Grinding Wheel Dimensions

The following Illustration contains all the information that is required to order an angular plunge grinding wheel correctly

Walter Graf, Copyright March 2022, The Philosopher's Grindstone

A final note: The author, from time to time, consults on grinding courses for the Thors Academy's eLearning Solutions, assisting in the writing of courses on precision grinding. However, Thors goes beyond grinding and offers a whole plethora of engineering courses. Please check them out on their website below (www.thors.com). For us engineers, curiosity is a constant, and so is our hunger for continuous learning!

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