Stationary dressing tools, Part 2: Application

In the first part of this motion blog, we discussed the importance of dressing and the different types and applications of stationary dressing tools.

In this second part, you learn which parameters influence the dressing process and how to adjust them to achieve the best results.

First of all, we would like to give you an overview of the most important parameters that influence the dressing process and are relevant to the rest of the blog:

a_d           Depth of infeed of the diamond per dressing path (mm)
b_d         Effective width of the diamond dresser (mm)
n_s           RPM of the grinding wheel per minute (rpm)
R_ts          Surface roughness of the grinding wheel in µm
s_d         Feedrate of the dressing tool per grinding wheel revolution (mm/rev.)
u_d        Overlap ratio (number)
v_c           Surface speed of the grinding wheel (m/s)
v_d         Feedrate of the diamond dressing tool (mm/min.)

Objective: Self-sharpening 

As explained in the first part of this motion blog, the dressing process generally pursues two objectives:

1. geometric shaping of the grinding wheel
2. influencing the surface roughness (R_ts)

Concerning surface roughness, the dressing parameters should be set so that the grinding wheel adapts to the specified material removal rates at the start of the grinding cycle. After a certain amount of material (v'_w) has been removed, each grinding wheel adjusts to its "own" surface roughness, regardless of the roughness initially dressed. Figure 1 shows how the wheel adjusts from different surface roughnesses of approximately 4 to 13 micrometers (µm) at a constant metal removal rate of Q'_w 1 mm³/mm/sec, to a roughness of 6 micrometers, regardless of its initial dressed roughness. It is fascinating that this phenomenon usually occurs at a v'_w of about 500 mm³ of material removed per 1 mm width of the grinding wheel.
(Source: Dissertation by K. Weinert, 1976, Technical University of Braunschweig).

Figure 2 shows the influence of the removed material volume v'_w on the original surface roughness R_ts of 6 micrometers at different material removal rates Q'_w and varying dressing feedrate v_d per wheel revolution s_d. The roughness of the grinding wheels adapts to different roughness values R_ts (4 to 13 µm) at different stock removal rates (Q'_w 0.2 to 3). In all three processes, the dressing was performed at constant dressing feedrate v_d of 0.2  mm/revolution of the grinding wheel. The following conclusions can be drawn:

• The results show that the dressing roughness should initially correspond to the chosen removal rate, in this case, 6 micrometers.
• It can also be seen that fine surface roughness becomes rougher with higher material removal.
• The two diagrams also show that the reverse occurs when a low stock removal rate is applied: The surface roughness becomes finer than that originally dressed into the grinding wheel by dressing.

This adaptation of the grinding wheel to the material removal rates is called "self-sharpening." Under perfect process conditions, grinding wheels should only be dressed due to loss of shape or profile, but not because the grinding wheel has become clogged or blunt.

Influence of the dressing parameters infeed and dressing feedrate

To achieve an excellent surface finish and high stock removal rates, it is extremely important to work with small infeeds a_d (cutting depth), i.e., 0.002 to a maximum of 0.03 mm per dressing pass.

To increase the surface roughness of the wheel, the dressing feed v_d is increased and not the depth of the dressing infeed a_d.

A higher dressing feedrate v_d leads to a higher surface roughness of the wheel and, thus, a more aggressive grinding wheel that cuts more freely.

A low dressing feedrate v_d leads to a finer grinding wheel topography and a better surface finish on the workpiece but to a lower stock removal rate during grinding. It should also be noted that grinding wheels dressed too finely can cause grinding burns.

Important: Always use grinding fluid (coolant) during dressing to avoid thermal damage to the diamond; see Figure 3. In addition, the grinding wheel should never be dressed without infeed (with a_d = 0 mm).

Effective width b_d of dressing tools

This dressing parameter specifies a diamond tool's effective width b_d at a specific cutting depth a_d.The ideal effective width b_d is listed below  for the dressing tools shown in Figure 4:

• Single-point diamonds: 0.5 to 1 mm
• Multi-point diamonds: 1.5 to 12 mm*
• Standard dressing blades: 0.7 to 1.0 (1.2) mm
• MCD or CVD dressing blades: 0.4 to 1.2 mm

Caution*: For multi-point diamond dressers (2) with an effective width b_d of more than 3 mm, only 35 % of the actual measured width should be used, as the total width of all protruding diamond points is approximately 1/3 of the measured width of the tool. If the actual effective width is used, there is a risk that the grinding wheels might be dressed too finely.

The effective width b_d for single-point dressers with natural diamonds depends on the wear caused by frequent use, as shown in Figure 5. The effective width b_d should never be more than 1 mm. When an effective width of 1 mm is reached, the diamond should be repositioned in the holder if the diamond has several usable points for dressing. Otherwise, the diamond should be replaced.

To determine the effective width b_d of a single-point diamond, see Figure 6; a ruler and a magnifying glass can be used. This method provides a good reference point for calculating the correct feedrate v_d.

Overlap ratio u_d

Figure 7 shows an overlap ratio u_d of 4. The overlap ratio indicates how often a point on the grinding wheel circumference is covered by the effective width b_d of the dressing tool for a certain number of wheel revolutions. The more often this happens, the higher the overlap ratio and the finer the grinding wheel surface. An overlap ratio u_d of 4, as shown in Figure 7, therefore means that the dressing tool has moved axially across the wheel's width by its effective width b_d in four grinding wheel revolutions.

Standard overlap ratio values:

Roughing:                  2 - 3
Normal loops:            3 - 4
Finishing:                    4 - 6
Fine finishing:             6 - 8

Calculation of the overlap ratio u_d and speed of dressing feed v_d

b_d = effective width in mm
n_s = grinding wheel revolutions per minute
u_d = overlap ratio (number)
v_d = speed of dressing feed in mm/min

The formula for calculating the overlap ratio u_d

u_d = Effective with of the diamond b_d / Dressing feed s_d per grinding wheel revolution = b_d / s_d = (b_d x n_s) / v_d

Suppose the overlap ratio u_d has been determined by calculation or other selection criteria. In that case, the dressing feedrate v_d of the diamond tool can be easily calculated. For dressing tools with defined radii, the dressing infeed depth a_d must be used first to determine the effective width b_d, see Figure 8.

v_d = (n_s × b_d) / u_d

Calculation of the dressing feedrate v_d for dressers with a defined radius

The formula shown in Figure 8 also applies to all dressing tools with a defined radius.

Dressing depth a_d

As mentioned, the depth infeed a_d per dressing pass should not exceed the range of 0.002 to 0.03 mm for all stationary dressers.

Dressing feed v_d versus depth infeed a_d

Is it better to increase the dressing feedrate v_d or the infeed depth a_d to obtain a more aggressive grinding wheel? Figure 9 shows the effect of doubling the feedrate v_d and the infeed depth a_d on the surface roughness R_ts of the grinding wheel. The graph shows that increasing the dressing feed v_d (exponential increase) has a greater effect on the surface roughness R_ts than increasing the infeed (linear increase) a_d.

Positioning of dressers in relation to the center line of the grinding wheel

All stationary dressing tools should have their center point on the center line of the grinding wheel to avoid possible distortions during CNC profile dressing, as shown in Figures 10 and 11.

Application guidelines for blade dressers 

Depending on the grinding wheel profile, blade dressers can be used in different angular positions (straight wheels, wheels with shoulders, angle plunge grinding wheels). In all cases, however, the basic rule of central positioning in relation to the wheel axis should be observed.

Please get in touch with us if you would like to know more about setting dressing parameters. We are happy to advise you.