Bridgeport Spindle 4 Bearing Conversion
Looks like I may have to rebuild a BR2J spindle, so in the interests of rigidity I am thinking about using 4 bearings and not the 2 normally used, something like the spindle on the right:
I see that Babin Machine offer this service in the US but cant find anyone who does in the UK so I intend to do it myself.
I suspect it may simply be 2 pairs of Abec 7 Duplex bearings back to back.
Has anyone done this type of conversion and if so can you confirm how the bearings were set up and was any additional shimming required?
Any and all info appreciated.
Last edited by _Paul_; 05-01-2014 at 01:38 PM.
Well, this is an example from SKF for a general milling spindle:
You could replace the spacer with another bearing. I'd then probably put three in the same direction and one in the other. You could get general matching bearings (GA) or a set (QBT), depending on how accurate you wish the preload to be defined. Or you could get the two front and two back version QBC, if you expect a lot of pulling load as well.
If you buy a heavier preload setting than light, you have to derate speed and life, so it depends how you want to trade this off.
My thnking is that four bearings will add little rigidity to a spindle with designed in limitations.
An R8 spindle has a 1 1/4 big end on the collet taper. That is the factor controlling rigidity for every tool installed in it.
So taking the 35mm bearing bore and the additional stiffness the bearing sleeves afford, the lack of a sufficient tool regristration feature and the overhang of tooling installed the limited space in a 3 3/8 dia quill and the typical turet mill's spindle rigidity can't be significantly improved by adding bearings where spacers used to be.
I've toyed with a design where an extension is added to the quill itself using the retainer threads. Inside is a #32920 taper roller bearing whose 40mm ID accepts about half the length of a #40 taper allowing a bout 1/4 wall. The #40 taper is cmplete with the 4" Dia spindle nose. The old R8 spindle is cropped a the shoulder of the lower bearing fit. It's shrunk into aspindle extension leaving space for a tandem pair of stock spindle, etc etc. It's do-able and provides in theory about 3 x the (my) calculated spindle rigidity. BUT: now the spindle is rigid the rest of the machinie is still a lightly constructed turret mill.
The problem is a Bridgeport is a superbly balanced design. It's like "The Famous One Horse Shay". Improvement of one key part requires upgrading of all others until it's no longer a Bridgeport.
Well that's speculation and philosophy.
My suggestion is: save your money. Two bearing pairs are certainly beter than one but you still have to contend with the limitations of the spindle. I don't think you will realise much benefit from the greater cost.
I suggest if you want a stouter turret mill you move up to a Cincinatti Tool Master, a BP Series II, or a Ramboudi.
I'm a big fan of a BP Series II with its $40 taper and more robust construction. It's what a BP turret mill should have been in the first place.
I hear what your saying Forest, though it would be nice to find someone that has opted for the Babin 4 bearing spindle conversion to see what if any difference they think it has made.
Thanks Ikdor I hadn't considered the various effects the different configurations might have.
C&M Precision Spindle here in oregon does that conversion too. They do real nice work, we just had a HMC spindle rebuilt by them and it is virtually silent at 8k.
They are pretty helpful. They have given me advice in the past. You might ask them about tips on a conversion.