Sometimes there is a need to groove the outer ring of the bearing along the diameter or end. Given the high hardness of bearing steel, which cutter should be selected for this operation on a lathe?
A bit of theory ...
After hardening and subsequent low-temperature tempering, the hardness of bearing steel should be at least HRC 62. Moreover, it contains approximately 1% carbon and 1.5% chromium, which ensures the quality of hardening of bearing steels throughout the entire volume of the product.
The hardening of this metal is performed at a temperature of 830 ° C in oil, followed by tempering for at least two hours at a temperature of 160 degrees Celsius.
It is clear that not every tool will be able to process bearing steel with the above characteristics. For example, the hardness of tool steels of grade P6M5K5, P9, P9M4K8, P18 is no more than HRC 59, and grades U7, U8, U10, U12, P6 depending on the tempering regime after quenching ranges from HRC 59 to HRC 63.
Considering that some tool steels, having less or comparable hardness, are clearly not suitable for machining bearings, we will try to use two cutters for these purposes: from T15K6 steel and with an elbor insert (other names: borazon, cubonite, kingsongite, kiborite).
In order to install the sharpened cutter more accurately in the tool holder, we’ll “cut through” its base and fix the bearing ring in the machine chuck as precisely as possible in skew.
We will try to process the end face of the ring at a speed of 315 rpm in several passes with a minimum chip removal thickness and without the use of cutting fluid (coolant).
Visually, the processing quality looks pretty good, but the cutter is dull and requires a new sharpening.
For the second time when sharpening the cutter, the main angle is negative, we use coolant and reduce the speed of rotation of the cartridge from 315 to 250 rpm. We also use, as for the first time, the minimum flow.
At first glance, the machining surface looks quite smooth, but the cutter again failed and at the same time not only became dull, but the cutting edge of the insert was chipped.
Now try a cutter with an insert from the Elbor at a speed of 315 rpm without using coolant.
We will process the inner and side surfaces of the bearing ring using several passes.
The surfaces treated with elbor look very high quality, and the insert itself has not suffered at all, although chips and crevices are visible in some places on the metal framing it.
Let’s try after the third regrinding to use the T15K6 cutter at a low speed of 115 rpm, since the diameter of the bearing ring is still 105 mm. But already at the very beginning of the groove, the cutter plate received damage - it had to be chopped off and the operation had to be stopped.
If only we didn’t come across a defective T15K6 cutter, then it is clearly not suitable for machining bearing steels under any cutting conditions.
Another thing is a cutter with an insert from elbor. He ensured the quality of processing and at the same time did not suffer at all.