Locking Oneway Chucks to Spindle

The forward spindle rotation direction and cutting forces tighten chucks and faceplates in normal operation. The opposite is true when the lathe is run in reverse and for this reason lathe chucks and faceplates may have threaded setscrew holes for locking the chuck onto the spindle to keep it from un-threading.

For the purposes of this discussion chuck usually means any device that can be threaded onto the lathe spindle. Although there is a danger of the chuck un-threading and literally flying off the lathe many turners sand with the lathe in reverse. Tightly screwing the chuck onto the spindle for sanding may or may not be sufficient but cutting the chucked workpiece with a tool while running in reverse affords a much higher risk by increasing the forces that will unscrew the chuck. This is particularly true if a catch occurs. At 480 rpm a caught workpiece will completely un-thread in only 1 second or less!

To prevent un-threading Oneway chucks have two setscrew holes positioned in the chuck spindle adapter such that they contact a machined groove in the spindle of Oneway lathes. Other lathes typically don't have this groove thus letting the setscrew contact the spindle thread where it may cause varying degrees of thread damage depending on the hardness of the spindle.

It is possible to put a piece of brass, plastic, or other material into the setscrew hole before inserting the screw to minimize spindle thread damage from the setscrew. Another way is to buy soft-tip setscrews with a brass or plastic insert on the tip.

Craig Daymon recently ordered (January 2006) some soft-tip setscrews for his Oneway Talon chuck from McMaster-Carr and provided the full specification from his order which is listed below. They are available from other suppliers as well.

Oneway Thread Size

The setscrew size information, not provided by Oneway in their documentation, for locking the Oneway (Oneway, Talon or Stronghold) chucks onto the spindle is metric M8 with a thread pitch of 1.25mm (M8–8 threads/cm). The 4 mm hex key provided with Oneway chucks for changing the jaws fits the M8 setscrew. Ed Kelle reports the same screws are used for Oneway faceplates.

The setscrews are available in various lengths and almost any length is suitable for this application. The depth of the Talon adapter is about 0.375" or 9.5 mm whereas the Stronghold adapter depth is about 0.5" or 12.7 mm. The screws are well shielded by the chuck body and if they are a little proud it won't matter and may provide a visual reminder of why you can't get the chuck unscrewed! Since the adapter supplied with the Oneway rotary joint for vacuum chucking is the Stronghold taper-lock adapter the M8 screws are used as well. In this case the adapter is not shielded and the setscrews should be below the surface. Oneway provides the regular setscrews with the rotary adapter to aid vacuum sealing regardless of any locking requirement.

Plastic or Brass

I asked a machinist about his preference for brass or nylon in this application. He didn't like the practice of screwing anything down onto a thread even if it was brass which is more brittle than commonly thought. However, if any piece of brass or plastic comes off it is unlikely to jam in the thread when removing the chuck because there are no adapter threads in the region of the setscrew hole. The spindle thread should be examined for damage and cleaned if necessary before placing another device onto the spindle. This suggests plastic may be preferred but is likely to have a shorter useful lifetime. A solution is to grind away the thread in the appropriate places. Since woodlathe threaded devices tend to use the spindle shoulder as a reference for alignment the ground locations would have to be very carefully selected to ensure the chuck remained tight against the shoulder when the screws are tightened.

Caution

The Oneway adapter design is designed for locking on the Oneway spindle. It was not specifically designed for non-Oneway lathes so be careful when using the setscrews to lock it on your lathe. For maximum grip use both setscrews and frequently check chuck for tightness and immediately after any unexpected event like a catch.

Nylon tip set screws fitting pretty much the same description (and in other lengths and sizes, as well) are available for about $10 for 10 from McMaster-Carr.