I have had a few deliveries of items I ordered recently.
Firstly a couple of ebay purchases – a 19 piece set of ER32 collets (2mm-20mm) and a bunch of 52 solid carbide 2, 3 and 4 flute end mills and slot drills ranging from 2mm to 16mm, including various ball nose items.
The mills are mainly brand new, although a few have been used, some are worth over £40.00 each, yet I picked up the lot for £59.00 + P&P, so just over £1.00 each.
I had a reply from Arceurotrade regarding the R8 spindle, and despite the web page showing it as out of stock, they did in fact still have one last one available.
Monday morning I was on the phone straight away to secure this spindle along with a drawbar, some ER32 collet blocks, a pulley and a couple of ball bearing collet nuts ER32 and ER11 as well as an ER11 16mm shank collet chuck.
Now I just need to obtain some suitable bearings. The 3MT spindle uses 2 x 6206-ZZ bearings (30x62x16mm), whereas the R8 spindle uses 1 x 6206-ZZ at the top and 1 x 6007-2RS (35x62x14mm) at the bottom.
ZZ means 2 metal seals, 2RS means 2 rubber seals, both are just some of the potential options on many different bearings.
Arceurotrade sell the 6206-ZZ for £4.64 and the 6007-2RS for £4.80, I can buy the 6206-2RS on ebay for £1.88, and the 6007-2RS for £2.25, all options come with free shipping.
I could also upgrade from the stock deep groove bearings to angular contact bearings. Arceurotrade sell the 7206 as a replacement for the 6206 bearings at £8.00 a piece (£7.33 on ebay so not much in it here), but they do not sell an equivalent for the 6007. The equivalent angular contact bearing would be 7007, which does not appear to be a very popular bearing and trying to locate a sealed one for a price under £130.00 is nigh on impossible.
I have found one from VXB with open sides for $16.88 (£10.27) which is a more sensible price, he also sells on ebay as irvineman, but I am also looking at a $12.75 (£7.76) shipping cost.
I am not at this point in time entirely convinced that there will be a huge improvement in performance or wear with the angular contact bearings over the deep groove standard types, the load ratings are not that different either (unless you start looking at the very expensive versions) values are from the VXB website for the various bearings.
| Bearing | Radial Load | Axial Load | Max RPM |
|---|---|---|---|
| 6206 | 20.3KN | 11.2KN | 12K |
| 7206 | 22.5KN | 14.2KN | 13K |
| 30206 | 41.5KN | 44.8KN | 6.5K |
| 6007 | 16.0KN | 10.3KN | 6.8K |
| 7007 | 17.5KN | 12.6KN | 9.5K |
| 32007 | 42.9KN | 54.0KN | 8K |
Dynamic load is the same as Radial load, Static load is the same as Axial load. All the bearings are more than capable of dealing with the 4K maximum rpm of the spindle.
According to the NSK bearing catalog (no, I don’t have a copy, but the values I am about to use are referenced on Bob Warfield’s site), stacking bearings increases their load ratings as follows:
Triple – 2.15 x Axial, 3 x Radial
Quadruple – 2.64 x Axial, 4 x Radial
The calculated figures in the following table have been rounded to 1 decimal place for clarity:
| Spindle | Radial Load | Axial Load |
|---|---|---|
| 3MT Stock Bearings | 40.6KN (2*20.3) | 18.1KN (2*11.2*0.81) |
| 3MT Angular Bearings | 45.0KN (2*22.5) | 23.0KN (2*14.2*0.81) |
| 3MT Taper Bearings | 83.0KN (2*41.5) | 72.6KN (2*44.8*0.81) |
| R8 Stock Bearings | 36.3KN (20.3+16) | 17.4KN ((11.2+10.3)*0.81) |
| R8 Angular Bearings | 40.0KN (22.5+17.5) | 21.7KN ((14.2+12.6)*0.81) |
| R8 Taper Bearings | 84.4KN (41.5+42.9) | 80.0KN ((44.8+54)*0.81) |
The load rating improvement for angular bearings over deep groove for both Radial and Axial is marginal: 10-20% at best compared with over 100% Radial and 200% Axial for the taper bearings.
So considering the combined cost for the angular bearings of £25.36 (7.33+10.27+7.76) over £4.13 (1.88+2.25) for deep groove on the R8 spindle for a 10% load rating improvement – I think I will stick with deep groove for now – I could replace them 6 times for the same cost.
I will discount the taper bearing option on the mini mill, despite the huge load rating improvements as this would involve machining the spindle housing on both ends for the deeper bearings.
Angular contact bearings are a more reasonable upgrade on the 3MT spindle as the cost increase although more than for the taper bearings at £14.66 (2*7.33) is still just under 4 times that of the stock bearings, they have the same dimensions so machining of the spindle housing is not required.
The machining aspect is not an issue for my 4th axis as I will be making the spindle housing from scratch and will simply bore out the bearing pockets to the required depths to accept the taper bearings so they still work with my 3MT spindle.