Harold Hall

Bull Wheel Dividing Device for a Myford S7,  Harold Hall

Methods of dividing on the home workshop lathe are many and varied, but one that has been very popular, especially on Myford lathes, is to use a detent which locates  into  the lathe's bull wheel. This is the gear in the back gear assembly that is fixed to the lathe's spindle.


Apart from a very recent version of the series seven, where it was an accessory that could be obtained separately,  it was always a facility that had to be made by the lathe owner. The photograph showing my version.


The method works well as the bull wheel on the Myford has sixty teeth and can therefore provide divisions of 2, 3, 4, 5, 6, 10, 12, 15, 20 and 30. With a forked detent the following additional divisions are possible 8, 24, 40.


If though you are considering adding the facility to your lathe, it is my understanding that Myford, on one of their very late versions, produced lathes with other than a sixty tooth bull wheel. Do therefore check before making the accessory. I think that the move by Myford was greeted with complaints and they soon returned to the sixty tooth wheel..


Making the assembly

The photograph and drawings should give all the detail needed to make this useful addition to the series seven with just two M5 tapped holes needing to be added to the lathe itself for fitting it. Their position is not critical so should not present a problem. Probably the easiest method is to drill and tap one hole and then use this to fit the Detent Carrier Base, using it to act as a template for the second hole. Mark the position of the hole using a 5.5mm drill and then drill a tapping size hole and tap for the second screw.


However, as the assembly is not permanently mounted I do add two screws to keep the tapped holes clean.      


Workshop Projects

Most of the machining required is straight forward and normal workshop precision being adequate. The detent does need a little extra care.


Do turn the 7.5mm diameter to be a close sliding fit in the detent carrier. Then, with this done part off, turn end on end and fit in a four jaw chuck setting the diameter to run true. Alternatively, use a collet if you have one as their accuracy should be adequate.


Face the end, lightly centre drill and drill a 2mm diameter hole just 3mm deep.  Its purpose is to give an indication of the width of the detent when filing the shape and more importantly getting the tip central. It can then, also be used as a guide for shaping the fork. If you wish to be extra careful with the detent the following process, after assembly, can be carried out.


Locate the detent into the bull wheel, making sure it is fully engaged, and then carefully retract, turn through 180 degrees and relocate. If the detent engages on one side only, remove, file a very little off that side and repeat the test. The same process can then be used to test the fork.


A compression spring is required to hold the the detent firmly between the gear's teeth, the stronger the better whilst not making it difficult to operate.

Dividing on the lathe using its Bull Wheel

Click on the picture to provide a larger view.