Turning speed

[V8-108]

As a newbie to the hobby, I was laid in the bath thinking about the effect of turning speed on various things. I think I can probably guess the answer, but, what effect does turning speed have on end grain tear out in particular. Are you better at a slightly lower speed to give the chisel a "chance to cut", or will a higher speed "aid" the cut? I appreciate that a turners technique and the suitability of the chisel and it's sharpness are important, but when those things are equal, what effect does speed of turning have - if any?

[BrianH]

It is true to say that the higher the surface speed the easier it is to get an acceptable finish...BUT... it is also true that kinetic energy rises as a square of that speed (I think). This means that the price you pay for the ease of finish is the very real possibility of a tiny error putting you, and your project, at a much increased risk of injury or destruction.
I gave this matter a great deal of thought, some years ago, and made the decision that I would work at the SLOWEST speed at which I could obtain an acceptable finish. It took a lot of practice but my skills soon increased to fill the void I had chosen to live with. It is perfectly normal to me now to work at a speed others find surprising. I actually feel uncomfortable on the occasion that a problematic piece calls for me to increase speed.
There are an awful lot of woodyfolk out there who rattle on about safety... or lack of it.... without considering the most obvious way to reduce risk is to reduce lathe speed.
Im sorry I cant give you a straighter, yes or no, answer. There is no right or wrong only a personal choice between risk and reward.
All the best with whatever path you choose
Brian

[bodrighywood]

Speed (IMHO) is governed more by the nature of the wood, balance of it on the lathe etc than the need to get a decent cut. As Brian intimates, the best answer to the question is to start slow and build up to a sensible, safe speed.Turning an off centre lump of burr or a winged bowl will require different speeds to say a pen. for example.
Again we are back on the use of that  often missing factor 'common sense ' LOL

Pete

[Bill21]

I’m with Brian on this one. I don’t run the lathe any faster than I need to. The faster the lathe is running the quicker accidents unfold and the farther the pieces fly!

If it’s tiny spindle stuff though that’s when the speed control gets tweeked. 

[BrianH]

Im thinking that bath water might be getting a little chilly by now but, before you reach for the pink and fluffy, here's another aspect of turning speed for you to consider.
To the lathe operator, and every writer in Christendom, the revs per minute are the important consideration and certainly the most quoted. At the coal face, however, Mr Cutting Edge has no interest or understanding of that figure. He's only interested in the velocity of next passing splinter. The surface speed!
At any given RPM the rim of a platter blank (say) has a lot further to travel than the middle and so it's surface speed is much higher (in fact, Im thinking that the very centre of the blank must have a surface speed of zero! how can that be?). So in an ideal world the Woodyturner ought to increase the RPM as he works toward the centre of his blank but, until a voice-recognition lathe comes onto the market, he will need to be permanently seeking the best compromise available. Yet another area where there is no black and white solution and practice and experience comes to the fore.     
Byeeeeee....Dont forget to clean the tide mark off the bath.......
Brian

[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

[Twisted Trees]

All the answers are above... Except I just thought when reading Paul's answer the other factors that affect it is relative weight (wood to lathe) and how it is held / driven.

[Paul Hannaby]

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.

The energy in a moving object is proportional to its weight so big heavy lump = more damage on impact...

[Bill21]

It’s surprising what you see some folks do on a lathe.