Setting up an OD Traverse Task

Setting up an OD Cylindrical Traverse Grinding Process

Part 2: External Cylindrical Traverse Grinding, OP 3

Task:

External cylindrical traverse grinding of a shaft made of case-hardened steel, 16MnCr5, DIN 1.7131 (US: SAE5115), 58 to 62 HRc, surface finish Ra 0.35 μm according to the drawing in Figure 1:

Figure 1: Test piece

The easily machinable case-hardened steel, 16MnCr5 (DIN 1.7131), has established itself as an ideal material for highly stressed components in automotive and transmission construction and general mechanical engineering.

• Grinding allowance (stock allowance) 0.3 mm on diameter
• Operation 3: Traverse grinding of diameter 30 mm
• Grinding wheel dimensions 500 x 32 x 203.2 mm

Figure 2: Operation plan

Steps to be taken for OP 3, traverse grinding:

• Selection of grinding wheel specification
• Selection of the grinding wheel speed (the selected machine is designed for a peripheral speed vc of 50 m/s)
• Selection of the dressing tool and the dressing parameters vd
• Specifying the workpiece speed nw and calculating the infeed amount ae per grinding pass and traverse feedrate vfa in mm/min

Step 1:

Considering the good grindability of this case-hardened steel, a pink aluminum oxide wheel, grit size 80, medium hardness H, and medium open structure, e.g., H8 could be selected. So, 57A80H8V, for example.

Let's say we also want to grind harder materials such as tool steel. In this case, we could choose a microcrystalline ceramic grinding wheel, e.g., 30% Cubitron 321 or 3SG, again grit 80, medium open structure, e.g., H8, which would translate into a wheel specification of 93A80 H8V601W (using my old employer's (Winterthur/3M) specifications). With a grit size of 80, a surface finish of Ra 0.35 μm can easily be achieved.

Figure 3: Diagram for grit size and surface finish.

Step 2:

Select a wheel surface speed of 45 m/s. In this way, one leaves a margin of 5 m/s in both directions. It should be remembered that a change in speed of 5 (five) m/s represents a change in the effective hardness of the grinding wheel by one hardness grade. (If you slow the wheel down, it acts softer, or harder if you increase the surface speed.

Step 3:

The workpiece RPM nw can be determined using the speed ratio qs between the grinding wheel and the workpiece.?

Figure 4: Speed ratio qs 60 and 120

The speed ratio qs can be represented as illustrated above. For example, a speed ratio qs of 80 means that the peripheral speed vc of the grinding wheel is 80 times higher than that of the workpiece.

The speed ratio qs for rough and finish grinding is between 60 and 80, which means that the surface speed of the workpiece is 60 to 80 times slower. In the present case, we choose a speed ratio of 80, so the surface speed vw of the workpiece is 45 m/s divided by 80, corresponding to 0.56 m/s. To convert the circumferential speed of 0.56 m/s into the rotational speed of the workpiece, we need the following formula:?

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Step 4:

The traverse feed rate vfa in mm/min can be determined using the overlap ud. The overlap ud describes how many revolutions the workpiece must make until the grinding wheel has shifted laterally by its own width. For roughing, we select an overlap of 3 to 4; for fine finishing, we select an overlap of 6. In concrete terms, a 32 mm wide grinding wheel moves 5.33 mm per revolution for fine finishing. (32/6). If our workpiece rotates at 358 revolutions per minute, this results in a feedrate vfa of 358 x 5.33 = 1910 mm/min for fine finishing. With an overlap ratio of 3 for roughing, the feedrate vd is 3820 mm/min (32/3 x 358 = 3820).?

Figure 5: Overlap ratio ud in OD cylindrical grinding

Now that the feed rate has been determined, the infeed per pass ae (depth of cut per pass) can be calculated on the basis of the specific material removal rate Q'w, aka Q-prime. Let's target a Q-prime 3 mm3/mm/sec, which represents a reasonable material removal rate, however, even somewhat conservative for a such a rigid workpiece.

The definition of Q-prime (Q'w) is given below:?

Figure 6: Q-prime (Q'w)?

Figure 7: Q-prime for roughing - finishing - fine finishing

As stated, the reference basis for this calculation of the infeed ae is Q-prime (Q'w), which describes a material removal in mm3 per 1 mm grinding wheel width in 1 second (mm3/mm/sec).

The following guidelines represent a reasonable industry standard:

• 1.5 mm3/mm/s finishing
• 3.0 mm3/mm/s Good average
• 3.5 mm3/mm/s Target
• 5.0 mm3/mm/s Good performance Grinding
• 8.0 mm3/mm/s High performance grinding
• Above 8 mm3/mm/s: mostly CBN on rigid machines with pure grinding oil.

• ae = Zustellung pro überlauf in mm
• nw = Umdrehungen/Minute des Werkstückes
• dw = Durchmesser des Werkstückes in mm

Let's solve the formula for the depth of cut (infeed per pass) ae:

The depth of pass or the infeed ae varies between roughing - finishing - and fine finishing; thus, changeover points must be defined before reaching the final workpiece size. For this purpose, we must judge how stable the workpiece is and, based on this, how great the risk of deflection might be. The stiffer the workpiece, the closer the switchover points can be to set to the final workpiece size. As a guideline, for roughing, the switchover point can be set to a grinding allowance of ≈ 0.1 mm in ?; for finishing, the switchover point can be set to a grinding allowance of ≈ 0.03 mm in ?; and for fine finishing, the switchover point can be set to a grinding allowance of ≈ 0.005 mm in ?.

Figure 8: Switchover points

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Figure 9: Switchover points

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Step 5:

Selecting the dressing tool: Assume a fixed dressing tool has been selected, such as a CVD dresser with three inserts of 0.8 mm cross-section. ?

Figure 10: Calculating the dressing feedrate of a CVD dresser.

To calculate the diamond dressing feedrate vd in mm/min, one uses the overlap ratio ud again. Practical experience has shown that the following values for the overlap ratio ud are useful guidelines for dressing:

• Roughing:?????????????????????2 - 3
• Standard grinding:????????3 - 4
• Finishing:??????????????????????4 - 6
• Fine finishing:???????????????6 - 8

Example of calculation of dressing feedrate vd of a CVD dresser:

• Grinding wheel speed per minute ns: = 1720-1
• The effective width of the diamond bd: ?= 0.8 mm
• Selected overlap ratio ud:?????????????????????????= 5

Using an Excel spreadsheet

Figure 11: Using an Excel?spreadsheet to calculate dressing feedrates

The above explanations help you to understand how the author approaches a grinding task. He prefers using an Excel spreadsheet to make his task easier in the real world.

Figure 12: Excel spreadsheet data input for calculating grinding parameters

Figure 13: Extract from an Excel spreadsheet for calculating grinding parameters

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Figure 14: Extract from an Excel spreadsheet for calculating grinding parameters

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Grinding Book: "The Philosopher's Grindstone" by Walter Graf, 2023

If you would like to learn more about how we can look at grinding processes, you are welcome to purchase the author's book, "The Philosopher's Grindstone," which provides in-depth knowledge about grinding in about 370 pages. For the time being, the book is only available in English. To order or for more information, please e-mail the author at [email protected].?

Figure 15: Grinding Book: The Philosopher's Grindstone

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Walter Graf, Copyright?, June 2023, The Philosopher's Grindstone