I like many others have followed along with many recommendations found on this forum and in others while progressing my CNC build on the G0704. I don't claim to be any sort of expert but have learned a lot from experience and breaking things. There is a ton of information available for this mill and mills in this class to draw from. You may even say there is more information than is necessary which makes it difficult to sort out what to do with your own conversion. Hopefully this will consolidate some information I have gathered through my experience with bearings.

As I looked to replace the stock spindle motor for one with more power and higher speeds I naturally upgraded my spindle bearings to accommodate those speeds. Like most I did the AC spindle bearing upgrade. The main reasons were that it was well documented and the cost was very reasonable to achieve the performance I desired. Unfortunately the results were not what I expected and I spent more time messing with the AC bearings then making parts. The TIR was also .0015" no matter how much I tinkered with the internal alignment. So I did some research and found an acceptable tapered bearing replacement.

I started out with the stock tapered roller bearings and measured spindle TIR of .0007". This is on a used machine which I am sure was never broke in as the rest of the machine still had cosmoline all over it. The stock tapered bearings were UBC brand 25x47x15mm 32005X & 35x62x18mm 32007X They are rated at 7500 rpm and 5800 rpm respectively per the UBC catalog here.

The upper bearings on the barrel were NWH(?) brand 35x62x14 6007RZ & 45x85x19mm 6209RZ. I was unable to find much info about these but installed the popular replacement Nachi 6007ZZE & 6209ZZE rated at 12000 rpm and 7800 rpm respectively (Links provided, click on the part number). I did not spend much time shopping around for these as they met my requirements. There is an option if you are trying to achieve speeds over 7800 rpm that I will not get into. But you can check out this link to get the general idea, this guy got 13000rpm: link

As for tapered roller bearings on the spindle most of what I have seen is using the Nachi bearings which will limit your max rpm to 6000. Not much of an improvement above stock but a much better bearing than the UBC. I did some searching and found SKF bearings that were capable of higher rpms. The part numbers and specs are as follows:

32005 X/Q Reference speed 11000 rpm, Limiting speed 14000 rpm

32007 X/Q Reference speed 8000 rpm, Limiting speed 10000 rpm

The reference speed is based on a max operating temperature of 250 degrees Fahrenheit. If you wish to go above to the limiting speed you must have a system to dissipate the extra heat generated (ie oil bath). Without going into it further there is a ton of information on the SKF website to explain what each of the numbers and ratings means. The suffix "X" at the end of the designation indicates "Boundary dimensions changed to conform to ISO".The suffix "Q" at the end of the designation indicates "Optimized contact geometry and surface finish".

I ordered the bearings from 123bearing.com which is based in France. This was an economical decision that I do regret a bit. They had the lowest price at $38 for the set and shipping was quoted at "3-7 days". My bearings arrived 14 days later after 6 days in customs that according to FedEx was due to incorrect paperwork. So if you are not in a rush you can save a couple bucks but the bearings are available elsewhere.

The bearings installed easily with a drift pin and some light love. I used Kluber Isoflex NBU15 from the cnc-specialty-store.com this grease is rated to 266 degrees Fahrenheit. SKF recommends that grease fill 30% of the cavities of the bearing. I was not able to find specs on how much volume the bearing had so I used the old wheel bearing packing technique of rubbing grease into the bearing from the palm of my hand. The two bearings probably sucked up 15 cc of grease, considerably more than the AC bearings. On the initial install I "pre-loaded" the bearings to a snug fit. On the G0704 spindle there is not much difference between free spinning and snug. I scribed a line on the spindle and retainer nut to mark this location for reference as needed. After the spindle was installed I checked run out and happily noticed I was barely wiggling my .0005" indicator. I would say in the neighborhood of TIR .0002".

I did a break in incrementally using a heat gun and my spindle drives load meter. Starting with 1000 rpm for 10 minutes then 2000 rpm for 10 then 4000 rpm for 10 followed by 10 minutes in reverse at 1000 rpm. When you do this break in I would recommend extending the quill out all the way. You will need to take a temperature about halfway up the quill for the upper bearing and for the lower you will be reading about 1 inch up from the lower retainer nut. If you can't do this you can get a reading from inside the casting but I have noticed the cast iron builds heat faster and holds it longer. A few minutes into the 4000 rpm segment I noticed the load meter creeping up and once it broke 10% I shutdown the spindle, allowed it to cool for a moment and then ran it in reverse at 1000 rpm for 10 minutes. The temperature when this happened were 155 upper and 130 lower.

I let the spindle cool off and removed it. I adjusted the pre-load to about 1/32 off from where it was checked TIR at .0005" and then ran another cycle. This time was 2000 and 4000 rpm for 10 minutes each. Temps reached 145 and 120. I let the spindle cool for 10 minutes then ran 3000 and 6000 rpm for 10 minutes each followed by 1000 in reverse for 10 minutes. Temperature spiked at 150 and 125 during this set and the load meter never moved off 0. At the conclusion of the test I checked TIR at .0002".

Overall I am happy with the results and will run one more break in lap tomorrow to double check things. I would like to give credit to lcvette and russtuff for starting down this path already. I am open to suggestions if anyone notices issues with how I did something or a better way to go about it. I will update this as time progresses to see if it holds the run out and how much maintenance is required.