The last few days I have been busy putting together a fourth axis unit for my mill. It is now finished enough to operate although there is more to do, quite a bit more. It uses a headstock from a mini-mill with a MT3 spindle. I replaced the bearings with smoother ones. The base is a chunk of 7075 aircraft aluminum ( ) and it has been milled on all sides for parallelism and flatness. When I built the controller for the mill I included all the circuits etc for a fourth axis as this has been part of the plan all along.
This is however more than just a 4th axis drive. It slots into the mill table with a underneath rail and is fully aligned without further fuss. I will be adding a rotating mount so it can be angled. The drive system uses a 280 oz stepper and a 100 tooth worm wheel driven by a double enveloped worm. It requires 222.222 steps per degree in 1/4 microstep mode giving a resolution of appoximately 16 arc seconds. That can be increased at the expense of speed by using 1/8th step mode. It currently rotates at 3400 degrees per minute.
The drive is a self adjusting zero backlash design. By using a double enveloped worm the gears have a large contact area making it possible to use a spring loaded worm with the worm wheel. Because of the way a double enveloped worm meshes with the wheel it won't skip out of mesh except under very heavy load.
The design is simple. On the left is a needle bearing to locate and control the worm and on the right is a pair of miniature ball bearings face to face and captured by the mount, the worm and the pulley. These two bearings absorb the thrust loads, one for each direction. They are rated for about 50 lbs static axial thrust and near double that when rotating. The worm shaft is hardened 1040 steel and the worm wheel is 660 bronze. I hobbed the wheel with a hob I made a couple of years ago for this sort of purpose. I will build a clear lexan protective housing for the unit to keep chips out of the gears.
Now, about the 'more than just a 4th axis part'. You may have noticed the HTD drive belt pulley. This unit will also serve as the headstock of a CNC mini lathe. By flipping a cam (not yet installed) the worm is disengaged from the wheel allowing a DC servo motor (with encoder, not yet installed) to drive the spindle. A fully functional lathe style tailstock is under construction as well and will be adaptable to the mill as a 4th axis accessory and for use as the CNC lathe tailstock. All that remains to make it a lathe is a bed with carriage ways and a pair of servos, which I have. The headstock and tailstock are simply placed into the waiting bed assembly for use as a lathe. All parts are self aligning to angular tolerances of better than .001" per foot.