This month our tip concerns Tail Center alignments on Horizontal Hobbing Machines.

Proper tail center alignment is absolutely critical to the quality of the gear or spline you are cutting. Improper alignment will result in size variations, bad lead, runnout, and possibly even finish problems.

The trick to doing a proper tail center alignment is that before you make any adjustments; check the level of the machine bed ways. This must be within the required .0005" per foot prior to making any adjustments. Once you have the bed ways square and level, it is time to check the alignment of the Overarm. This also must be done prior to making any adjustments to the tail center. The dovetail ways of the overarm must be parallel to the bed ways within .0003" low at the outboard end to .0000. It is imperative that the overarm not rise at the outboard end at all. Those of you that have machines with round overarms must hold these same tolerances, however, you must find the very bottom of the overarm to make these checks. Those of you with dovetail overarms should also make sure the dovetails are square to the work spindle. It is very unusual for this to be out and if you find this condition you should recheck the level of the machine.

Once you have verified the machine to the above settings, it is time to align the tail center. Insert a test bar known to be straight and round in the work spindle. This bar must be of the appropriate length for your machine. The length must be such that you can check over the entire length of travel of the machine. True the test bar to .0003" or less and put a .0001" indicator on the side of the test bar at the outboard end. Bring the tail center gently into the test bar center and check the indicator reading. This reading must be .0001" or less. If your reading exceeds this rotate the test bar to verify that you do not have runnout in the bar that is giving a false reading. If it is truly off, you must move the tailstock to compensate. First check that the gib on the dovetail is fit properly. If this is right, there is wear and the gib must be ground or shimmed. It is better to replace the gib than shim it, however if you are in a rush a shim will work temporarily. On a round overarm you only need to rotate the center bracket to a square position.

Once you have verified the side-to-side alignment place the indicator on the top of the bar after you have disengaged the center. Gently engage the center into the testbar and check the reading. This reading must .0003" low to .0000" high.

These checks should be part of your preventative maintenance procedure and checked at least once a month.

If you have any questions or have a particular topic you would like to see discussed here, please contact us.