Like a lot of things we do in wood turning - we get away with many slack practices, until one day we have an "accident." Unfortunately many turners don’t manage hazards and risk very well.
We may perceive the risk associated with using MT accessories in driven applications as low but some hazards do exist, & the potential injuries can be serious.
Just because a “tool” has a Jarno or MT1, 2 or 3 taper (I will just use MT as a generic term) does not necessarily mean it is suitable for use as an unsupported “driven accessory” on a wood lathe, or that the omission of a thread for the use of a draw bar correlates as “a draw bar is not required in all MT applications.”
MT drive & live centres, accessory arbors for items like cup chucks etc rely upon static friction to secure the MT taper in position. We want the mating MT surfaces to not move relative to each other i.e. not slip, and not to come apart so that we can drive a work piece and securely hold it where we want it.
The static friction is maintained by a positive axial force i.e. a force into the MT socket; negative force is away from the socket. On a drill press the press exerts the axial force/load on the MT, on a lathe the screw thread (or the lever in plunge/ram styles) in the tailstock delivers the positive axial force. MT drive & live centers rarely (if ever?) make provision for use of a draw bar, because on a lathe they are almost always used as a pair between centres so a positive axial load is maintained against the MT tapers unless the blank fails.
If for any reason that static friction is over come then the driven accessory/centre may slip (loose drive) or to move axially outwards, wobble and work its way out of the female MT taper (socket). Ever had a Jacobs chuck drop out of a drill press while there was no load on it; or the Jacobs chuck work loose from the tailstock while withdrawing a drill bit from the wood – negative axial load!
MT accessories are an entirely different proposition as they are often used by wood turners in unsupported mode i.e. no draw bar & no tail stock support. Tools like a MT Jacobs Chuck to hold/chuck small spindle pieces in the head stock; pendant chucks; as a mandrel for a collet chuck body, sanding discs, or friction/jamb chuck.
These unsupported applications present the additional hazard that they do not always maintain the positive axial force required to maintain the static friction fit and may work loose. In fact some wood turning tasks using buffs or certain cuts generate considerable negative axial forces!
The main causes of unsupported MT tapers working loose generally are
- dirty MT tapers, foreign objects, rust, oil, dust, shavings caught in the taper
- insufficient positive axial force to maintain the static friction
- particular wood turning cuts (RH Vee & Cove cuts) generate negative axial forces on the blank,
It is a relatively simple task to over come or eliminate the hazard,
- don’t use a MT accessory with out a draw bar in unsupported applications, or where negative axial loads are created,
- or choose MT accessories with provision for a draw bar, and install a draw bar,
- or choose an accessory that threads directly onto the head stock spindle.
Ultimately it is the individual turner’s decision. Is the perceived risk and the potential severity of an injury high enough to warrant addressing the potential hazard? How much time does it take to fit a draw bar?
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