Rotary dressing tools Part 1: CNC form rollers

Nowadays, rotating diamond dressing tools are frequently used for high-volume grinding. Compared to stationary dressers, dressing rolls offer higher consistency in dressing results due to their large number of diamond grits. In this Motion Blog, we explore the different types of dressing rollers and the impact of various setting parameters on dressing outcomes.

A distinction is made between profile and form rollers. The naming of form and profile rollers can be confusing and may lead to misunderstandings. The difference can be remembered as follows: profile rollers already have the finished profile on their circumference. In contrast, form rollers must first create the profile through CNC path-controlled movement. The CNC form dressing roll is the most flexible solution for cylindrical and surface grinding, as numerous shapes can be produced with the same dressing tool. In contrast, using a profile dressing roll is more limited because it only has a single profile. Therefore, profile dressing rolls are best suited for high-volume production in the automotive, ball-bearing, and aerospace industries.

CNC form dressing rolls feature diamond inserts around their circumference, including natural, PCD, MCD, or CVD diamonds. Nowadays, CVD diamonds are primarily used for form rollers because they offer a good price-performance ratio and have a more precisely defined geometric shape than natural diamonds. These diamonds feature a precisely lapped radius, which is entered into the CNC control system to create the correct profile on the grinding wheel subsequently.

CNC form rollers generate the desired profile by synchronizing two CNC axes. This makes them much more flexible than profile rollers, though they are slower when dressing or re-profiling the grinding wheel's profile. CNC form rollers can have different diamond setting designs depending on the profile to be dressed.

Some of the setting parameters are discussed below, with the most important being the direction of rotation of the grinding wheel and the dressing roller in relation to each other. It is advisable to start with synchronized dressing, resulting in a higher effective roughness depth (R_ts) on the grinding wheel and increased cutting ability.

Compared to asynchronous dressing, synchronous dressing produces coarser surface roughness (R_ts) on the grinding wheel. This characteristic is consistent for both CNC form dressing rolls and profile dressing rolls, regardless of the infeed depth (a_d), which is the penetration depth of the dressing roll into the grinding wheel during dressing. The diagram also illustrates how the speed ratio (q_d) of the diamond dressing roll to the grinding wheel affects the grinding wheel's surface roughness (Rts).

The closer the speed ratio (q_d) approaches 1, the rougher the grinding wheel's surface roughness (Rts) becomes. However, a speed ratio of q_d = 1 would damage the diamond dressing roll due to the crushing mechanism between the diamonds and the grinding wheel. Therefore, a speed ratio (q_d) of no higher than 0.8 is recommended for synchronized dressing.

In asynchronous mode, there is only a slight variation in the surface roughness R_ts of the grinding wheel. This mode produces a significantly finer surface roughness of the wheel compared to the synchronous mode. When grinding, a coarser surface roughness of the wheel is advantageous, as the grinding wheel grinds cooler and more aggressively, which reduces the risk of grinding burn.

In synchronous mode, the penetration angle of the diamond is much steeper than in asynchronous mode, which results in a coarser surface roughness on the wheel. Therefore, it is generally recommended to dress in synchronous mode whenever possible. Asynchronous dressing should only be used if the surface requirements cannot be met with synchronous dressing. The main difference lies in the influence of a single theoretical diamond during dressing. In reality, countless diamonds act on the grinding wheel with "hammer blows." However, the tendency for the wheel to become rougher during synchronous dressing remains the same, regardless of whether one diamond or many diamonds are used.

During synchronous dressing, the individual diamond strikes the grinding wheel vertically. This "hammer blow" makes the grinding wheel sharper and more aggressive. With asynchronous dressing, on the other hand, the contact path of a single diamond causes a "glancing impact" on the surface of the grinding wheel. This "glancing" impact results in a finer grinding wheel surface.

Speed ratio q_d: This parameter describes the ratio between the surface speeds of the grinding wheel and the dressing roll. 

qd = (surface speed of the dressing roll in m/s) / (surface speed of the grinding wheel in m/s) = vR / vc = (ratio)

Calculation of the surface speed in m/s of the dressing rollers and the grinding wheel:
v = (n x d x π) / (1000 x 60)

Calculation of the revolutions per minute (n) of the dressing roll or grinding wheel based on the surface speed in m/s (V):
n = (V x 1000 x 60) / (d x π)

• V = surface speed in m/s of the grinding wheel or dressing roller
• d = diameter in mm of the grinding wheel or dressing roller
• n = revolutions/min of the grinding wheel or dressing roller

The following formula can be used to calculate the dressing feedrate:

After the effective width has been derived, the dressing feedrate v_d is calculated the same way as for dressing with single-point diamonds.

The effective width b_d is a function of the radius r on the circumference of the form dressing roller and its depth of infeed a_d.

Vd = (n_s × b_d) / u_d

• a_d = infeed depth of the form roller per dressing pass (mm)
• b_d = effective width of the form roller (mm)
• n_s = revolutions/min of grinding wheel (min )^-1
• u_d = degree of overlap (ratio, usually between 4 and 6)
• v_d = dressing feedrate (mm/min)

Select the overlap ratio u_d between 4 and 6 with a dressing depth per pass a_d between 0.005 and 0.02 mm/pass. For vitrified-bonded aluminum oxide grinding wheels, the depth of infeed a_d per dressing pass should be a maximum of 20 µm for rough dressing strokes. The depth of infeed a_d per dressing pass should be limited to 10 µm for finishing dressing strokes.

The infeed depth a_d per dressing pass for vitrified-bonded CBN grinding wheels should not exceed 3 µm.

CNC form rollers offer the user a high degree of flexibility. They can be used on all modern surface and cylindrical grinding machines from the UNITED GRINDING Group. Compared to stationary dressing tools, CNC form rollers have the advantage that the dressing process can be better controlled and offer more consistent dressing values.

Please contact us. We will be happy to advise you.