4th Axis on My Taig
The screw drive of the standard rotary table has a lot of benefits in this 4th axis application. There are a few limitations in the area of backlash (controllable) and rapid speed (not really necessary.)
With small mills like the Taig and the Sherline, weight (mass) is a required parameter to consider. Most rotary tables are designed to have a lot of weight as part of their design. It adds stability for normal machining. However for miniature CNC machining, it is undesirable to abuse your drive system with a lot inertial mass to start, stop and reverse perhaps up to a hundred times a minute.
The winner in my selection is the Sherline 3700-CNC rotary table with motor mount. At $320 it is not the least expensive of my 4th axis schemes but I think it is the best in this case.
First point is the weight. At 8 pounds it is heavy enough and when you look at it, you see it carries no extra weight in a heavy case or mounting system flanges.
Second point is it is specifically designed for CNC operation internally (Sherline says in the worm housing) as well as the included #23 motor mount and coupling.
Third big point with me is the drive is 72/1 turns. Some rotary tables are 40/1 (yuk!). At 72 turns and 1/4 stepping, each step is 0.00625 degree. (Sherline uses 1/2 stepping.) A 90/1 would have been nice but I didn’t find small tables with that ratio. There probably wouldn’t be much “meat” in the gear mesh on 4” tables.
Forth point is it is a 4 inch table which fits well with the intended use in mills of the micro variety.
I intend to make my own mounting plates and accessories. Of course this adds weight as does the stepper motor. I am considering using the Taig tailstock (when needed) I as see more options using it.
Of course I will be posting everything here as my 4th axis project progresses. Right now the table and a 130 oz/in stepper motor are on order. I have the material on hand to make the base. 2012 looks like a good year!