General Category > General Discussion

Turning speed

<< < (2/3) > >>

Bill21:
That’s a very good point Brian and one that as a metal machinist as well I fully appreciate, but I expect many don’t. It is fascinating to watch CNC metal lathes use constant surface speed to alter the speed automatically as the cutter moves.

https://youtu.be/LLOBZVM7y64

Twisted Trees:
In wood turning we have a number to work with RPM, but it is completely wrong! you will often hear turners say that it gets slower towards the middle, especially when taking about sanding.

The measure that actually matters is inches per second, that is why people run the lathe faster on small spindle work than on larger diameter pieces, that inches per second needs to align with the tool speed of movement along the toolrest, which in turn ties in to your stability in holding the tool in correct presentation.

Bottom line is RPM is mostly irrelevant, but it is what we know, the 3 speed related items that make a difference to cut are unmeasured and can only be achieved through feel and practice.

Which is part of the contribution to the ask 10 turners get 11 answers philosophy!  :)

BrianH:
Back in my engineering days, Bill, it took two peoplefolk to increase a machine's speed..... The turner to shout and a bloke with a shovel to sling more coal into the furnace. My my aint fings changed?
Brian

Paul Hannaby:
Back in the dim and distant past, I attempted to come up with a definitive answer on what speed (rpm) to turn wood. Where this led was that there were references to a speed of 26 ft/s being used for most wood cutting machines so I based what followed on that. Wood isn't uniform and different species have different densities and grain structures etc. so there is considerable leeway as far as that goes.

Converting the linear speed of 26 ft/s to something meaningful for turning involves using juggling the formula for the circumference of a circle (pi*D) as all things being turned are circular at the point where the cut is being made.

To cut a long story short, the result is the diameter of the workpiece in inches can be divided into 6000 to give the rpm. For example, a 6" spindle would be 1000rpm, a 20" bowl would be 300rpm etc. [no doubt some smart a*** will point out that for a 1/2" pen blank this gives 12000 rpm but not many lathes work at that speed ;) ]

In one of David Ellsworth's books, he mentions dividing into between 6000 and 9000 but gives no explanation of where the numbers were derived but they are in the same ballpark.

A little faster or slower than the "optimum" won't cause any problems and may improve the cut but bear in mind that increasing the rpm also increases the centrifugal force and that one is an exponential relationship so doubling the speed quadruples the centrifugal force, tripling the speed would result in 9 times the centrifugal force and so on. It's this one we need to be most careful of to avoid exploding workpieces which are effectively torn apart by the centrifugal force.

bodrighywood:
As seen by the posts so far this subject can get very technical. Not sure how many of us actualy want to measure the diameter of the work (I turn msinly spindle work so do the formulae still apply for something that is 1" diameter and 35" long? No because then you get the whip effect the faster you go. It is horses for courses and to a large extent depends on experience alone. Listen to the wood, work with it not try and force it to obey your will, Above all, as Paul points out, the centrifugal force (speed something is likely to come off and hit you) is roughly 4x faster than the rpm you see on your readout. The biggest factor to think about is not whether it will cut better at a certain speed but is it safe.  A pen blank turned at 3000 rpm will still hurt if it comes off and hits you though not as much as a 36" burr which at that speed is likely to kill you. Be safe people, follow your instincts rather than the rules and remember you are dealing with machines and materials that couldn't care less about whether the cut is a nice one or not.

Pete

Navigation

[0] Message Index

[#] Next page

[*] Previous page