Surface Grinding with Rotary Tables

Illustration 2: Surface grinding with rotary tables

This article focuses on rotary surface grinding. The aim is to give the user the necessary information to set the correct grinding parameters. All parameters are directly compared to standard surface grinding to understand this process better. Apart from the rotary movement of the table, rotary surface grinding is the same a standard surface grinding. The rotary movements are converted into linear movements to support this argument of sameness. This conversion allows for a better understanding of the rotary process.

Illustration 3: Surface Grinding Processes

?Grinding Parameters

Wheel Speed

For both processes, rotary and standard surface grinding, the wheel surface speed ranges between 25 and 45 m/s, with 35 m/s being a good starting point.

The four most important parameters of surface grinding processes are the speed ratio qs, the specific material rate Q'w (Q-prime), the theoretical chip thickness hm, and the overlap ratio ud.?

Illustration 4: The four most important parameters

?Speed Ratio qs

In the author's view, the speed ratio qs is of paramount importance in all forms of precision grinding, and surface grinding is no exception to this view. In simple terms, the speed ratio compares the surface speed of the grinding wheel and the workpiece, with the grinding wheel surface speed being at least 50 times faster than that of the workpiece.

Illustration 5: Surface grinding speed ratio qs

Imagine a grinding wheel with a single grit to visualize the speed ratio. Through the linear feedrate of the workpiece, this single grit cuts a wave pattern into the surface of the workpiece, as shown in Illustration 5. The faster the grinding wheel surface speed is in relation to a constant workpiece speed, the shallower and finer the wave patterns (Rt & ls') become. The changes in the wave patterns indicate how one can influence the surface finish. Experience has shown that the ideal speed ratio in surface grinding is 50 to 120. Below a speed ratio of 50, one may experience vibrations; above a speed ratio of 120, the risk of burning increases, as shown in Illustration 6. Incidentally, for creep-feed grinding, the recommended speed ratio should be higher than 1000.

Illustration 6: Speed ratio ranges

The Specific material removal rate Q'w (Q-prime)

The specific material removal rate (Q'w) refers to the material removal capacity of a grinding process, given in mm3 per mm wheel width per second (mm3/mm/s). Using the ranges that have been established in the industry and the simplicity of the Q'w formula, this is a convenient parameter to establish the grinding process:

ae = depth of cut per stroke in mm

vw = feedrate of the workpiece in mm/min

Illustration 7: Specific material removal rate (Q'w)

The target range of Q'w for surface grinding should be around 3 mm3/mm/s. However, values of around Q'w 5 mm3/mm/s can be achieved with ceramic abrasives. Values higher than 5 mm3/mm/s often require the use of Vit CBN.

Illustration 8: Range of the specific material removal rate Q'w

The relationship between the depth of cut (ae) and feedrate (vw, also sometimes called vfa) is very important as it establishes the material removal rate Q'w. Furthermore, Illustration 9 shows the range of depth of cut ae and its influence on the grinding temperatures. In surface grinding, the depth of cut should not be higher than 0.025 mm per stroke as higher values may lead to burning. To have parts free of grinding burns, the ae range of > 0.025 to 0.5 mm should be avoided. Hence, creep-feed grinding should start at a depth of cut of 0.5 mm. Processes such as Viper surface grinding often operate in the critical range of ae > 0.025 to 0.5 mm. Therefore, Viper processes need special precautions in high coolant delivery, both in terms of pressure and volume.

Illustration 9: Temperature progression and depth of cut ae?

Theoretical average chip thickness hm

While it is challenging, maybe even impossible, to determine the actual chip thickness, the theoretical chip thickness hm is extremely useful for evaluating grinding processes. It is called theoretical because it is based only on the geometrical kinematics of the grinding wheel and the workpiece. Actual chip formation, of course, may differ substantially. Nevertheless, using the theoretical average chip thickness gives control over the grinding process. Illustration 10 shows the geometrical basis and the formula for calculating the theoretical average chip thickness hm based on one individual grit.

Illustration 10: Theoretical average chip thickness

Many grits work simultaneously and thus overlap, so we cannot know the actual chip thickness produced by an individual grit. Chips are formed during three stages of interaction between the grit and the material to be ground, as shown in Illustration 11:

Illustration 11: Chip formation

In stage 1, when a grit first gets into contact with the material, it generates an elastic deformation, which progressively turns into a combination of elastic and plastic deformation in stage 2. The actual chip formation occurs in stage 3, as shown in Illustration 11. Nevertheless, as mentioned previously, the theoretical chip thickness has proven an excellent grinding performance indicator.

Illustration 12: Calculation of the theoretical average chip thickness

The theoretical average chip thickness is a function of the depth of cut ae and the speed ratio qs. The target value of the chip thickness hm should be around 0.25 μm, with an upper limit of around 0.5 μm. At values higher than 0.5 μm, conventional grinding wheels tend to break down. The above statement is, of course, equally valid for reciprocating and rotary surface grinding.

Overlap Ratio ud

When looking at conventional surface grinding and its linear movements, the lateral feed movement per stroke ap is a fraction of the grinding wheel width bs. In other words, the overlap ud is calculated as follows: ud = bs/ap = factor 1.5 to 2 for rough grinding, and factor > 2 to 8 for fine grinding.

Illustration 13: Overlap ratio for reciprocating surface grinding

Illustration 14: Surface reciprocal overlap ratio ud

When calculating the overlap ratio for rotary grinding, one looks at the relationship of the feedrate vw per one table revolution, as shown in Illustration 15:

Illustration 15: Overlap ratio ud for rotary surface grinding

Assuming the grinding wheel width bs is 25 mm, the feedrate vw is 70 mm/min, and the table rotation is 17 RPM, the overlap ud would be a factor of 6 as shown in the spreadsheet of Illustration 16:

Illustration 16: Spreadsheet for the calculation of grinding parameters

To generate a spreadsheet from the instructions given in this article is relatively simple. The speed ratio is not constant as the table RPM of the rotary grinders known to the author have constant feedrates and constant table RPM. Hence, one must assume a "middle" or average diameter, as shown in Illustration 16. In general, the grinding would not take place in the center of the rotary table anyway. As the wheel position moves towards the center of the rotating table, the surface speed of the table decreases. However, it would be easy to make a CNC program that takes the decreasing surface speed into account and compensates by proportionally increasing the table RPM as shown in Illustration 17:

Illustration 17: Table diameter and RPM variation at constant speed ratio qs

Assuming a table diameter variation from 500 mm to 100 mm, the RPM ranges from 17 to 84 RPM is one were to compensate and achieve a constant speed ratio of 72 as in this example.

Going back to reciprocating surface grinding, the calculation of the parameters is similar. However, as the table moves linearly, all the parameters are constant. Again, making a spreadsheet is not complicated.

Illustration 18: Surface reciprocating grinding with continuous stroke (diagonal stroke)

The above illustration shows a surface grinding cross-stroke, which is diagonal. Some grinders have intermittent cross-strokes at the end of the workpiece. For practical purposes, it does not matter as the overlap parameters are the same, as shown in Illustration 19:

Illustration 19: Surface grinding cross-stroke (intermittent)

?Illustration 20: Spreadsheet for surface reciprocating grinding

Using Ceramic Abrasives

It may be worthwhile to mention the use of ceramic abrasives for surface grinding. Micro-crystalline ceramic abrasives such as SG, TG, Altos, Cubitron, etc., are now very common in workshops. The author has observed that, as a rule, these abrasives are not used to their full potential. Many users do not push these abrasives sufficiently, and therefore, may even experience burning issues. Ceramic abrasives have a high threshold level before forming a chip and maintaining self-sharpening. Self-sharpening only takes place when the grit is sufficiently pushed to perform at higher Q'w rates, as shown in Illustration 21:

Illustration 21: Using ceramic abrasives

?Walter Graf, Copyright January 2022, The Philosopher's Grindstone

Uploaded on January 8th, 2022

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 way beyond grinding and offers a whole plethora of engineering courses. Please check them out on their Webseite below (www.thors.com). For us engineers, curiosity is a constant and so is our hunger for continuous learning!

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