On later production model 41B and 42B machines of 3-bearing construction, the Front Main Spindle Bearing is mounted in a removable casting wall called the Head Front Cover. Sometimes, this wall gets 'shifted' out of proper location, resulting in Spindle Runout or machining problems.
It is my experience that this is caused most frequently by 'punching' into a part at Rapid Traverse with a large drill instead of the more commonly expected 'side-blow' to the Spindle caused by striking the side of the Spindle with the side of the part. The issue seems to be not so much the 'size' of the cut taken, as it is the sudden application of the load. In this type of 'crash', the 'front' end of the Spindle momentarily 'stops' while the 'rear' end of the Spindle Sleeve has full torque applied to it. I believe that the applied torque from the motor and the inertia of the gearing are what really causes the cover to move 'off location'.
Here's how to see if this is really the problem:
- Start the Spindle rotating at its SLOWEST RPM.
- Slowly, hand feed the Spindle in and out several times using the Turnstile (the dial mechanism that looks like it has 'bull horns').
- Attempt to 'feel' if the Spindle Feed alternately 'tightens up' and 'loosens up'. It most commonly gets tight once-per-revolution, however, it might be twice-per-revolution. If the Head Front Cover is off-location, you cannot 'miss' this. It is very distinct and noticeable. If you can't feel it - that's not what wrong - leave the cover location alone!
Here's how to fix an 'off-location' Head Front Cover:
- Check the 'double' bolt circle that is at approximately a 9-inch diameter abound the front end of the Spindle Sleeve to verify that the bolts are tight. If the inner pattern of bolts is NOT tight, do not proceed with this re-alignment and DO NOT TIGHTEN THEM. You may cause expensive damage! Look in other Lucas technical literature for what to do to correct this situation before you re-align the Head Front Cover. If the bolts are already tight, proceed with cover re-alignment.
- Remove the 4 taper pins from the 4 corners of the Head Front Cover. You will probably need to replace them with new taper pins. If they have the slightest amount of 'upset', they will pull the cover back into mis-alignment when they are put back in.
- Loosen up all of the bolts around the outside edge of the Head Front Cover.
- Start the Spindle rotating at its SLOWEST RPM.
- Using approximately a 4-pound rawhide hammer, begin striking the Head Front Cover at cover center - directly above the Spindle. The 'idea' is to make the cover 'vibrate'. The precision tapers of the Main Spindle Bearings will cause the cover to move such that the tapers of the bearings are precisely aligned to each other.
- While the Spindle is still rotating, tighten the cover mounting bolts - using a 'back-and-forth' 'wringing' pattern.
- When the bolts are tight, test for proper cover alignment as described above. If the cover is now aligned correctly, ream the holes and install new taper pins. If its not properly aligned yet - repeat the realignment process.
