Hard Fact of Hard Rack Milling

The following paper concludes that hard rack milling for finishing hardened racks provides the same level of accuracy as rack grinding, with lower costs and shorter cycle times.

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Rack gears used for rack and pinion assembly are often hardened to increase durability. In most cases a finish grinding process is necessary for correcting the distortion of racks after heat treatment. If the hardened gear racks are finished by milling instead of grinding it will increase productivity and cut production costs considerably.

The major benefits of a ground rack are long life, low noise, and low dusting, but grinding machines are expensive and the process, as well as the dressing process, takes considerable time. Also, the high running costs—the actual cost of grinding and dressing wheels, and the electrical costs for coolant and a temperature-controlled room—raise the price of ground racks. Ground racks therefore become much more expensive than non-heat treated gear racks.

If high-accuracy hardened gear racks become available at a much lower price it is expected that more users would start to use them. The rack finishing method that we are going to introduce here is a new concept in the milling process that utilizes CBN cutters. We at the Saikuni Manufacturing Co. call it the “Hard Rack Milling Process,” for which a patent has been applied. We also applied for a patent on the hob cutter design used for hard rack milling.

In the first two sections of this article we talk about CBN characteristic and the cutter design used for rack milling. In the remainder of the article we report the milling test data of the CBN cutter.

CBN and Intermittent Cutting 
Cubic boron nitride (CBN) was developed by General Electric, bringing remarkable progress to the field of cutting tools. The hardness and thermal conductivity of CBN are near to that of diamond, but it has the unique characteristic of insolubility in iron that diamond does not possess. Because of these characteristic CBN is widely used for cutting high hardness steel and other hard materials. Until recently CBN cutting tools were not considered suitable for intermittent cutting since it is adversely affected by impact load due to its poor toughness [KA1]. CBN tools, therefore, have mostly been used for lathe turning, and it is still in the trial stage for use in intermittent cutting.

The CBN cutter we developed for hard rack milling has superior wear resistance and toughness for machining hardened steel or cast iron because this CBN cutter contains a unique ceramic bond and is sintered under high pressure and high temperature, resulting in a high retention of CBN grains.

CBN Hob Cutter Design
We made two different sizes of CBN hob cutters (hard rack cutters) for this test. The CBN cutters were made with carbide bases (see “1” in Figure 1) and sintered CBN compact chips (“2” in Figure 1) that are brazed to the SCM cutter body. The carbide base acts as an impact-resistant material that absorbs the shock applied to the CBN chips.

Although sintered CBN compact itself has superior characteristics against the heavy intermittent cutting load, we tried to enhance its performance as much as possible by refining the cutter design. This is to obtain long cutter life under the optimum cutting conditions with low running costs. Since there was limited data available on intermittent cutting with CBN cutters we had designed the cutters based on our own experience and using a trial and error process. Although we had to try many different tool designs to correlate with cutting conditions, we’ve come to the point where we’re satisfied with the cutting test results and tool life.

Some of the important factors in tool design are the rake angle A, tooth tip angle B, and relief angle C. We can’t publish the detailed specifications, but we are proud that we have found an optimum tool design for efficient milling and long tool life (Figure 1, Figure 2).

Hardened Rack Milling Test
For rack milling before hardening, high speed cutting tools with and without a coating are generally used. For hard rack milling, CBN tools were used with different cutting conditions.

For CBN milling, the feed rate 0.03-0.3mm and cutting speed V=100-200m/min. are generally recommended, and our new hard rack mill used to cut the hardened racks meets these cutting conditions. The cutter spindle was designed to be more rigid in order to minimize cutting shock on the CBN tools, and each of the machine’s slides were also designed with high rigidity for high speed and high feed milling. Figure 3

Module 3 Rack Milling Test
Module 3 rack milling test condition and results are as follows:

Test piece
Module: 3.0
Dimensions: 30mm wide x 35mm 
high x 1000mm long 
(Total cutting length will be 3.0m)
Heat treatment: High frequency 
hardening HRC 53-57

Milling conditionsCutter: Module 3.0 single cutterCutter speed: 500 rpmFeed: 600mm/minMilling stock: 0.2mm each flank

Test Results: The milling test with the hard rack cutters showed excellent results. Pitch accuracy and surface roughness are as follows (also see Figure 4, Figure 5). At the total milling length 600m under module 3 rack milling test one of 12 cutter chips showed small chipping. We continued the test and that cutter chip broke off at the total milling length 700m.

Pitch Accuracy
Single pitch error: 5.7 micron
Adjacent pitch error: 5.4 micron
Accumulative pitch error: 27.3 micron

Surface roughness
Ra 0.10 micron
Ry 0.63 micron
Rz 0.43 micron

Module 5 Rack Milling Test
Module 5 rack milling test conditions:
Test pieceModule: 5.0Dimensions: 30mm wide x 35mm high  x 1000mm long 
Heat treatment: High frequency hardening HRC 53-57
Milling conditionsCutter: Module 5.0 single cutterCutter speed: 500 rpmFeed: 600mm/minMilling stock: 
0.2mm each flank
Test results: Milling test data of the pitch accuracy and surface finish are as follows (also see Figure 6):

Pitch Accuracy
Single pitch error: 2.9 micron
Adjacent pitch error: 3.9 micron
Accumulative pitch error: 13.7 micron

Surface roughness
Ra 0.09 micron
Ry 0.60 micron
Rz 0.39 micron 

Conclusions

It was confirmed that hard rack milling for finishing hardened racks provided the same level of accuracy as rack grinding and its cycle time was shortened to about 1/10 of grinding time, thus productivity was drastically improved.

For the rack grinding process, the work and the grinding wheel are in contact all the time. Plenty of high-pressure (5Mpa), temperature-controlled coolant needs to be provided for cooling the heat generated by grinding and to remove the sludge from the wheel surface. Whereas hard rack milling is high speed, intermittent hob cutting and cutting chips take the heat away from the work, so there is no need for a high-pressure, temperature-controlled coolant system. The basic coolant system (0.5Mpa) is good enough.

Also, for the high-pressure coolant system, a totally enclosed cover is needed to keep the coolant splash inside, and for safety reasons as well. However, for the basic coolant system there is no need for the total enclosure cover, and a simple safety cover is enough.

Generally, the closed loop control with the optical linear scale is used to keep close tolerances in the rack grinding process. With this hard rack milling test we confirmed that the ground rack accuracy was achieved by the NC pitch compensation on the master ball screw without the optical linear scale. The dresser used for the grinding process is not needed for hard rack milling. The grinder spindle (max. 5000rpm) is much more expensive than the hard rack mill spindle (max. 600rpm). When those costs are considered, the hard rack mill is much less expensive than the grinding machine, and this cost advantage will be reflected in the price of ground quality racks made by hard milling.

Still, there are issues to be addressed. Since CBN hob cutters are used for hard rack milling, tool cost is relatively high. It is difficult for the user to resharpen the solid hob cutters as a special technique is required, so the user needs to keep some cutters in stock, which brings up the cost and is not economical. We believe throwaway-type cutters with cutter chips and a body would be a solution. This type of cutter is under development and will be ready to introduce soon. 

To achieve the production cost reduction it is important to improve the technique for correcting the distortion caused by hardening, and also for the heat treatment itself. If the hardening depth is not even, or hardening distortion can’t be corrected within the certain level, it will increase the process time and the production cost, as more material has to be taken off. As this hard rack milling method was developed only recently, our extensive cutting tests are ongoing. The hard rack milling method will be improved with the help of its end users, whose results will be added to the extensive cutting data we are accumulating.  

Reference:
Reference: Y. Kuroda, A. Kukino, M. Goto; “Development of Solid CBN tools BN800,” SEI Technical review No.162 2003/03