Re: [mill_drill] Re: Harbor Freight Mill/Drill 33686 Bearing ?
- I'm sorry, but you will need to cite something to make that claim
because I believe it to be patently false. There is more than a
simple clearance---able to be adjusted in installation---involved in
bearing class. Other factors of tolerance include out-of-roundness,
running accuracy, sphericity of the balls, and radial runout. Here is
a table giving detailed breakdowns on the individual tolerance
differences courtesy of SKF: http://tinyurl.com/b3l5qj6 Here is a
gateway to precision classes by same: http://tinyurl.com/aflzvbh
Furthermore, the OP cites a running class accuracy of P5. This is a
separate and distinct class from ABEC-7 which relates to the assembled
bearing and the tolerances to which the assembled bearing runs as a
unit. Pre-load is a separate issue altogether. Under load, the ball
bearings will minutely deform. An analogy could be made with the tire
on an automobile. The tire is more uniformly round where it is not
resting on the pavement. The section in contact with the pavement
flattens to an extent based on the weight of the car (load applied).
If the clearance in the bearing allows this deformation to affect only
a select number of the ball bearings in the unit as a whole, you loose
accuracy. Pre-load is used to counteract this phenomenon and apply a
modicum of deformation to all the balls in the bearing evenly when
installed. Then, when the load is applied, the accuracy is not lost.
Not all applications require a pre-load. Some require more or less.
The pre-load can be ground into the bearing race thickness so when
they are assembled flush, the necessary pre-load is "built-in".
> That is true for ball bearings but makes no difference on tapered bearings
> or thrust bearings. The number has to do with the clearance between the
> ball and the 2 races. Consequently, it is a meaningless number for any
> bearing where you can set the pre-load. Ball bearings in machines are
> typically ABEC7. Automotive bearings are ABEC3 or 5.
- I am not sure what your milling table looks like but would you be able to
mount your tools on an adapter plate? I have this small vise attached to a
3/8 steel plate. It makes for very quick angle set-ups on the mill and
will bolt down to many of my other fixtures.