Machine spindle bearings - how delicate are they actually?

Hi Hopper,

You are partly right in so far as the top bearing being there to hold the spindle in place more than anything else.

Small correction however which I hope you wont mind: Most Chinese mills and lathes upto the X/SX2 mini mill and C2/C3 mini lathe size contain ball raced bearings top and bottom as standard. Larger mills and lathes around the X3 mill and C4 lathe sizes start having combinations of ball race at the top and angular contact ball or taper roller bearings at the bottom and so on, until we get to the Bridgeport size machines where the bearings become far more heavier duty combinations of angular contact ball/taper rollers all the way through.

For small mills and lathes, it is a compromise between cost, max speed, and some technical assembly issues, which govern the type of bearings being fitted. Then it starts getting complicated - slim bearing/housing capacity, introduction of 'C3 clearance ball bearings' etc., and their effects on the various attributes of a small machine spindle assembly.

Ketan at ARC.

I used to use a brass rod and tap the top end wiv an 'ammer, but later fitted a simple self-ejecting captive drawbar device to my X2. It's very easy to do if you have a machine with a thread on top of the quill (bear in mind it may be a left hand thread!). The only disadvantage is that the spindle end cap I made can get stuck and need a tap wiv an 'ammer to get it loose...

Neil

I feel that he use of morse tapers on milling machines is bad design, to use one and not provide a self ejecting drawbar is very poor indeed, it would be nice to blame the Far East for this but even the venerable Tom Senior was guilty of this crime. My mechanical sympathy hates me hitting shafts mounted in ball bearings as a ball is close to a point contact, taper rollers at least have a line contact. The moment you succeed in denting a race then it is only a matter of time even though the dent may be off the normal running line of the bearing the distortion is probably larger and the bearing will be be toast eventually.

Mike

Question, as a self ejecting drawbar will apply X force until it breaks the hold of the taper and hitting with something will also apply the same X force as the taper lets go what is the difference between the two?

A self ejecting drawbar should put zero force through the bearings as it is attached to the rotating part of the spindle as is the taper to be ejected. If the design does not do this then yes it would be pointless but I have only seen designs that do not put thrust through the bearings.

Mike

My son's milling machine has a cap fixed to the belt cover which retains the drawbar when it is unscrewed. The force is therefore acting through the quill (which will move down unless locked) and the spindle bearings. I assume this is done for safety reasons as the ejection cap is non-rotating and in operation encloses the rotating drawbar.

Brian

Mike - presumably you are describing a captive drawbar. Thus will bear against the spindle at the top end in both directions ie when tightened and also when untightened. So unscrewing it will force the tool out of its taper without loading the bearings. This is what I have on my CNC machine although it's hardly required for my 40 taper tooling which pretty much self-releases.

Murray

I was thinking of the type Brian describes. One where the bar bears against something within the bearings will indeed not put any strain on them.

I suppose we need to make the distinction between milling tapers which are pretty much self releasing and require minimum force to eject and the morse taper which is a self holding taper principally designed for drilling and designed to be released by means of a taper drift. To use the morse taper certainly requires a drawbar to hold a milling chuck or cutter, the trouble with using a drawbar on a morse taper is finding the happy medium where it is tight enough to retain the cutter or chuck reliably and not so tight it needs beating out.

Before I bought a VMC mill I had a Rodney attachment for my Myford, the drive was from a morse taper in the headstock retained by a drawbar, not wishing the taper to spin in the headstock I tightened on the tight side, I had to whack it harder than I was comfortable with to release it. This was some years ago and the bearings are still good so far. I spent quite a long time mulling how to make a self releasing drawbar fo the Myford and even posed the question on here. Myford came up with the collets with a groove for a nut on the nose thread to eject the collet without any need to hit anything, it also allowed the possibility to feed through the collet. I chose R8 for the mill after the Rodney experience and this releases with a gentle tap well within my mechanical sympathy limits. There are a few designs for self releasing drawbars for the VMC and if I had gone for the morse taper option I feel that it would be essential to make one.

Mike

Mike beat me to it but I agree completely with his analysis, industrial mills have self ejecting tapers, international, Newall, R8 etc where as a Morse taper is intended to drive the tool thus considerable force is required to eject the taper. A self ejected mechanism is to be recommend, not say it will damage the bearings but there is a more than reasonable possibility of doing so.

I have machines with both systems and have no problems with the one with the morse taper -- it has an eject mechanism for the tool release and now is around 30 years old.

John

Thanks for the clarification Ketan. My experience on the smaller stuff is limited so good to have the voice of expertise in that field here.

Wouldn't putting the steady thrust of an ejecting drawbar through the bearings still be better than the repeated impacts of pounding on the drawbar wi' big 'ammer?

I started this thread out of idle curiousity thinking there would be a definitive answer (I'm that naive!), but obviously not. Personally I still think that JasonB's (been knocking out tapers from a X3 for 11 years without detriment to bearings) and Ketan's (been selling these machines for 17 years without detriment to bank balance) posts are convincing - it's probably OK. Though not with 'opper's big 'ammer perhaps!

My Warco WM14 mill has a self ejecting M10 drawbar with an 8mm square at the top to get a spanner on. My 8mm spanners are say 100mm long and I don't think I've ever had to put more effort into ejecting the MT2 than I would in lifting a 10kg weight. So that's - erm - 10 Nm ish torque. Bunging these numbers into the calculator at engineersedge.com gives an axial force of 5kN, or half a tonne in weight terms. Can this be right? Seems a quite a lot! I've had a pint or two, so might have gone horribly wrong with the calculation!

Rob