I found this on
http://www.practicalmachinist.com/vb/showthread.php/why-matched-bearings-bridgeport-110661.htmlAnd have added it here for reference ...
Posted by Paul Hooper on the Chaski forum-
Now I know that this may sound like a stupid question but why do I need to buy a "matched" pair of angular contact bearings for the BP spindle, not just 2 off the shelf ones?.Isn't it the case that modern manufacturing methods would produce standard abec7 bearings of far better quality than the matched pair on my 1965 Bp?.I'm not looking for high accuracy just a useable mill for general fabrication at home.I realise this may create a little "interest" but I must be feeling brave today!
Response from Forrest Addy
Conventional ball bearings have a certain amount of radial clearance. This clearance will doom a machine tool spindle. The spindle requires a positive means of center the axis of rotation.
Consider the construction of an angular contact bearing: the line of action of the balls in their races forms a shallow cone. If the inner race is moved axially the clearance is taken up against the outer race. If angular contact bearings are installed as opposing pairs on the machine tool spindle they support it rotates freely without clearance providing a stiffer assembly than any other means except tapered roller bearings.
Ball bearings will deflect under load thanks to the elasticity of steel. If an angular contact bearing pair is axially preloaded a specific amount this deflection can be reduced to 1/5 that of an un-preloaded bearing pair. The preload ensures there's a certain amount of resistance to bearing deflection.
You can install a pair of ABEC grade 1 angular contact bearings in your spindle but the assembled spindle wouldn't bore a hole very round. In an extreme case you could figure an eccentricity of 0.0005" maybe greater.
The bearings may also run hotter or have end play depending on the "stand out" of the inner race with the wide end of the outer race. The amount of standout determines the bearing preload. In ABEC grade 1 bearings the standout is uncontrolled except as more or less flush with the outer race. If you installed them back to back (DB that is duplex back) there's be no good control over their preload except by experiments with shims.
ABEC Grade 7 angular contact bearings are selected for a number of characteristics to closer tolerences than ABEC grade 1 bearings. They are matched for OD and ID to within 0.0001. The concentricity and face runout of the races are held to 0.0002 or better to their registration features. The balls are matched within 4 millionths of a size and sphericity.
And as the final touch the inner and outer races are ground to produce the designed preload at the factory and the races marked to indicate the point of greatest run-out. Most ABEC Grade 7 and 9 bearings these days are ground duplex universal (DU) so they can be installed back to back (DB), duplex tandem (DT), or face to face (DF) as the situation warrants. They are ground as a pair and may nt be separated without loss of a controlled preload and consequent loss of performance.
ABEC grade 7 bearings have the usual factor marks plus an additional code usually hand inscribed on the outer race detailing in a series of code letters the bearing grade, preload, date of manufacture, and serial number. Yes, if you contact the manufacturer, he can look up the manufacturing data and the names of the techs and the inspector.
This is a world standard: ISO standards for their equivalent angular contact spindle bearings are nearly the same as ABEC Grade 7. A millwright repairing a Bridgeport spindle in Siberia can with confidenceuse bearings made in USA, Isreal, Indonesia, South Africa (or for that matter Senegal or Beliese if they have the industrial base).
Machine tool makers are in a competitive business. If they could install cheaper bearings and still satisfy their own specificaions they would. Downgrade your spindle bearings at your own risk. If you do, believe me, you will regret it sooner or later.
