RISK & Dale Nish's bowl speed formula
Geoff Whaling
RPM x diameter (in inches) = 6,000 to 9,000
Dale Nish recommended this as a basis to determine a safe speed to turn bowls. When you consider the bowl size & rpm some turners work at and then calculate the rim speed of a bowl it becomes quite significant at times.
Dales formula gives guidance on a recommended speed range of 750 to 1125 rpm MAX for a sound 8" bowl blank. Unsound blanks or blanks with "natural features" require careful appraisal and stabilization or modified turning & work holding techniques before proceeding & lower speeds.
If you ask those same turners who recommend relatively high lathe speeds to front up to a major league baseball pitcher or a top line cricket fast bowler without any protective gear their response would most likely be - NO WAY!
Lathes potentially can generate objects with significantly higher speed & mass than a baseball or cricket ball. Potentially the "struck by object" event produces severe head & facial injuries that render the injured turner unconscious & incapable of calling for assistance.
Several wood turners have died in similar circumstances in the past ten years. How many received serious life changing injury? A - unknown. How many have had black eyes or significant facial injury? A - many & every club has several examples in recent years. How many turners have lost a bowl off the lathe? - ALL! & it is exceptional to meet one who hasn't - yet ;-)
Yet many turners are quite prepared to accept that level of risk when turning an unsound bowl blank or sanding one at very high lathe speeds. They regularly expose themselves to hazards that are known to produce quite serious even potentially life threatening injuries. The more they expose themselves to hazards with high risk without implementing suitable hazard reduction controls generally means a much higher probability of a failure and injury.
Their response "I've been doing this for X years & nothing has happened" - yet!
Philip,
The others have identified a number of hazards in your particular bowl turning scenario, which in combination present quite a serious risk to your safety.
As a quick summary of hazards,
1. Unsound blank - punky & cracks with high potential for failure.
2. Technique - tool control is producing significant tear out - high potential for severe catches.
3. Tool sharpening - may require attention?
4. Blank mounting technique - between centers at least initially with a soft blank.
5. Unsound tenon - cracks & dia of tenon.
6. Turning lathe speed - unknown but likely high given next point.
7. Sanding lathe speed - very high - 2 times MAX recommended.
8. Use of protective equipment?
9. Potentially turning alone?
10. Emergency plan? - potentially no safety backup plan.
The first five generate a high probability of dislodging the bowl from the lathe or ripping relatively large objects from it.
Combine that probability with high lathe speeds capable of generating objects with high momentum & the risk increases dramatically and so does the severity of a potential injury.
As mentioned above these typical scenarios have caused serious head injuries & brain trauma and often render the injured turner unconscious an unable to call for help.
Turners may "get away with it" for a long time before such a scenario plays out fully & results in the turner actually receiving a direct hit. I liken the risk to fronting up to a blindfolded pitcher or fast bowler at close range. If you do it often enough one ball is eventually going to hit you in the head!
Hand turning is a safe pursuit IF we follow recommended safe turning practices.
p.s. If you look carefully at the 4? cracks near your thumb in photo 2 you may notice that the tear out pattern changes abruptly across the cracks. This indicates a couple of things to me
1. the bowl is flexing across the crack.
2. maybe a fair bit of force is being used to produce the cut.
3. tool presentation & sharpening requires adjustment.
4. it is unsafe to continue in this condition.