Some comments on spindle bearings
Guys,
Boy, lots of discussion here on spindle bearings. Some of the points made in the many postings are correct, but a many of the technical points are not really applicable to the PCNC 1100. More expensive machine tools commonly have 30,000 to 50,000 RPM spindles with accuracy tolerance better than 0.00005". In those applications the balance, temperature fits, and clean room type procedures are essential. A spindle repair on many machine tools will cost 4 or 5 times the price of a PCNC mill.
The PCNC mill design does not push the envelope on bearing technology, running only 70% of the rated speed of our bearing design. The process is done in a clean area, but it's not a controlled clean room. Probably the most important part is that only a few experienced certain people are allowed to do the spindle assembly.
In April of 2008 we conducted an audit of the spindle assembly process. I wasn't there, but our China based employee conducted the audit and send us photos, videos, and a written report of the audit. Our only modification to the process was to require better control of the amount of grease used. They were doing OK with the grease, but it was based on experience and not on any volumetric tools. They now use a couple simple tools to measure the volume of grease used in the upper and lower bearings
Here are some comments on a few of the issues discussed earlier in this thread:
- By our estimate the spindle design is fine for anything up to 7000 RPM. Taking it higher is certainly possible, but will lead to shorter bearing life.
- Preloads should be matched to a speed range. As noted, a preload suitable for 10,000 RPM is going to be loose at 500 RPM.
- We work hard to follow quality issues at Tormach, including tracking issues and frequency. Spindle bearing issues have affected less than 0.5% of all machines and most incidents were associated with much earlier machines. The incident that started this whole thread is one of those documented problems. Nearly every problem we see like this is brought up for discussion with the people at the factory.
- The service bulletin mentioned in msg #79 of this thread had not gotten past draft. This is the same doc attached to msg #85. It's available on demand, but we wanted to improve the document before we published it. A final version of it should be posted pretty soon.
- We now stock the spanner needed to adjust the preload. 42 mm spanner, PN 31038. Nobody ever asked for one, we just thought it would be a good thing to stock.
- The cartridge style spindle is not common on low cost machines, but we designed the machine with a cartridge to make it easy for people to replace. While someone can rebuild their spindle, it's easier just to buy a new one.
- Spindles just have not been much of an issue, but it is a wear item. As years go on we expect spindles to be wearing out, needing repair or replacement. If it ever becomes frequent we'll probably sell rebuilt spindles as part of a trade in service.
- We agree with one of the posts, 180F to 200F is a fine range for the spindle bearings at top speed. If the spindle never gets warm you might have too little preload. If the spindle gets hotter than 200F you might have too much preload.
- If anyone ever does need bearings, we recommend buying them from us. We have been fully satisfied with the quality and you'll generally find that Tormach sells the bearings much cheaper than local over-the-counter sales.