Precision, Hardened Bearings and Ways
Helmel’s present CMM design utilizes both radial and recirculating bearings. High-class radial ball bearings are made especially for us by the bearing company, both crowned and flat with shields and special grease film. We inspect every bearing before use to insure that runout does not exceed specifications. The precision recirculating bearing block we use is commercially available.
Sellers of air bearing machines will claim that mechanical bearings wear out. They do not. Our radial bearings are used at 10% of their rated load, the recirculating ones at about 2% in medium sized CMMs. They may need a drop of oil after several years of use. Machine No. 2 we built in 1975 still has the original bearings and is used on the shop floor at a Carboloy plant.
All ways are alloy steel, hardened to RC 55-62 and precision ground. Therefore wear is not an issue either. We have inspected ways of older machines and did not find a noticeable wear pattern when measured to the millionth with an LVDT.
Air bearing CMMs do not need hardened ways. The gyrations they go through to justify their material selection is amazing. One outfit advertised “Space Age Material”, which turned out to be anodized aluminum.
figure 1, Bearing Example
Some competitive mechanical CMMs have appeared on the market as of late, employing bolt-on guide ways for recirculating bearing blocks (e.g. THK) see Figure 1. Small cross section rails are used to keep the weight down. Well – these rails do not stay in position. We have used this approach on some specials and prototypes and abandoned it very quickly. After one year or sooner they need readjustment. For CMMs that rely on 3-D error mapping for accuracy, this is disastrous.
Remapping is expensive, $4000 plus on a standard sized CMM. If these rails will not stay straight, they certainly will not stay parallel! Therefore the bearing away from the main guide should be floating, which is not the case. Bad design! The ways should be an integral part of the structure and be ground in place. Yes, this is expensive – but it is the only way to build a good and accurate mechanical machine. Period! That’s what we do.