AWGB Woodturning Forum
General Category => General Discussion => Topic started by: GeordieTurner on April 28, 2019, 12:09:02 PM
-
As a newcomer I find lathe speeds confusing, many YouTube videos show good techniques but never mention lathe speed which I assume is important if not critical. What is the ideal lathe speed for using a roughing out gouge to turn a square spindle round? What other ‘ideal’ speeds are there for different tools or operations? Thanks for any advice
-
How long is a piece of string? if it just square then the best speed is the one you feel safest at. Depends if you are holding the spindle (only spindles with a roughing gouge of course)between centres or in a chuck. Also depends on the wood. Some woods like oak can chip and splinter others like beech are easier to round off. If you are roughing down a branch with bark etc then just remember that you can't always see what the wood is like and make sure you are wearing a mask as bits can fly off. In most cases it is actually safer a bit faster rather than slower as you are less likely to catch but bottom line as I said initially err on the side of caution and go at a speed that feels safe to you.
pete
-
pete is right and to be honest experience will answer this question for you in time.
we all start off a bit slow initially but as confidence grows you soon speed up. someone on facebook said that there was a formula for such things which is absolute tosh as it will all depend on the weight and size of your lathe and not how fast you are turning. a small late will need slower speeds but a big heavy one can safely turn large items without wobbling about.
the thing to remember is that you need to turn each piece when roughing out at least at around the fastest speed the lathe will allow for that particular bit of wood being turned.
i turn skittles for a living and turn 5 1/4" sq blocks 11" long at just under 2000rpm but a bowl blank that is a bit off center will need to be slower if the lathe starts moving around.
here is a good video on the subject.
https://www.youtube.com/watch?v=tDeo1qeV6Dg&t=737s
-
Without experience, how are people supposed to know where they should feel safe! Personally I take a more scientific approach...
The primary aim is to turn at a safe speed which isn't causing excessive vibration due to imbalance from the wood. If there is too much vibration, slow the lathe down.
Once you have taken that into account, a rough rule of thumb can be used to give the answer to how fast can be calculated by dividing the diameter of your turning blank in inches into 6000 (or into 15000 if working in centimeters). This figure is derived by starting with 26 feet/second as the "optimum" cutting speed for wood. This figure is put forward by some woodworking text books and wood cutting machines often cut at around this speed.
The figure of 6000 is arrived at by rearranging the formula for circumference and peripheral speed to give rpm as a function of diameter.
For example, for a 2" diameter blank, the rpm can be up to 3000rpm and for a 12" diameter blank, the speed would be 500rpm.
I have used this rough rule of thumb for many years and find it a simple way to estimate turning speed because all I have to remember is 6000.
I recall in one of David Ellsworth's books, he suggests dividing diameter into 6000 and 10000 to give a safe working range. There was no explanation where the numbers were derived from but they at least agreed with my rough guide at the lower end.
Remember not all types of wood are the same so as with any cutting process, you might find certain pieces of wood respond better to slightly slower / faster speeds.
It should also be remembered that any rotating object is subject to centrifrugal force and if you double the speed, you quadruple the centrifrugal force (it's an exponential relationship). Excessive speed can result in the blank tearing itself apart with potentially catastrophic results so always check your blank is sound and free of major cracks, bark inclusions etc. and wherever possible, don't stand in the line of fire when turning.
A final suggestion - if you are turning an non-circular blank (such as a square block you are roughing to a cylinder), turning too slow will lead to the tool bouncing and will make it more difficult to get a good cut. Speeding the lathe up (within the safe contraints) will reduce the bounce and will give a cleaner cut.
-
Basically you are saying the same thing Paul but adding numbers. I can't argue with your scentific approach but wonder how many of us actually work that way. New turners IMHO should try and get alongside a more experienced turner to start with anyway be it getting some tuition or just a mentor as however much we follow the rule of numbers it only applies in ideal circumstances. Try turning a 2" lump of oak with knots in it at 3000rpm would, I think be looking for trouble. Personally I can't remember the last time I had my lathe cranked up to that speed, certainly not when roughing out. There are far too many improbables to try and turn by a formula. OK my opinion for what it's worth. Obviously it works for you but I wouldn't personally advise it as an approach for a new turner.
Pete
-
Pete, The formula takes into account the balance and centrifugal force issues so I would suggest it is safe as long as common sense is applied regarding balance. Knots will affect the cut but not the balance to any great extent (although varying density can affect balance if there is enough of it). Also bear in mind that for a 2" blank at 3000 rpm and a 12" blank at 500 rpm the wood is going past the tool at exactly the same speed. It's a simple linear relationship.
Why do you think some small lathes such as pen lathes have upper speeds up to 5000 rpm? These lathes are designed to turn small diameter blanks. It's a simple relationship between diameter and RPM.
I just watched the video Steve suggested. A couple of points on that one - the "wobble points" Mike refers to is resonance. With variable speed lathes, even with no blank on the lathe you might find at certain speeds there are small amounts of vibration due to resonance. With these, you will find they also happen at multiples of that speed - for example if you get resonance at 200 rpm you will see it happen again at 400, 600, 800 etc. These resonances are rarely significant and you can usually increase the speed to get through them to the point where the vibration settles down. However, serious imbalance doesn't stop as you increase the speed, it just continues to get worse until the lathe shakes itself apart or throws the blank from it's mountings. You can't increase the speed through these because it won't get better.
At one point in the video, Mike is turning a 11" blank at 1400-1500 rpm. In my opinion, that is excessively fast to the point I wouldn't feel safe. You would probably get away with it with a perfectly sound blank but as I said earlier, doubling the speed beyond the safe limit will quadruple the centrifugal force and quadruple the risk of catastrophic failure.
Different people have different approaches to risk and different perceptions of danger so using a simple baseline that works the same for everyone, regardless of experience or , in my opinion reduces the risk and potential for accidents.
-
All the replies are correct...
BUT in these wonderful days of variable speed rather than fixed pulley systems, when rounding off stock the less time your gouge is wobbling about in free air the better, so set the lathe off slowly turn the speed up gradually, don't let the lathe vibrate preferably at all, but if you have a small light weight lathe then not too much.
Feel the speed, don't set it to a number. once the corners are off then everything should be in balance anyway so no need for slowing down. If it's branch rather than square stock, balance can be more of an issue but the vibration will still tell you if it is too much speed, then you should concentrate on the balance before rounding the whole project..
-
I have just completed the AWGB Tutors Course(Let's Teach woodturning, free down load from the website) and the advice there is always start slowly and raise the speed as you feel comfortable. Finding the right speed foy you is experience, so start cautiosly and gently increase the speed.
Ron
-
Have to agree. When teaching, one thing I always stress is to start the lathe on a low speed. If you don't have variable speed then still start slow and move up if you want later. I had a load of 'oak' I picked up recently, covered in dirt and old paint. All 24 x 4 x 4. Most when cleaned and blanked into spindles turned out to be spalted beech. Some was spalted heavily to one side where it had been lying on the ground for a few years and despite being square was quite off balance. If I had put that on the lathe at 3000 rpm I would have had to swithch off pretty darn quickly. Wood is far too variable to take the chance. You don't really make things any quicker at 3000 than at 1000 so why risk it. I do a lot of bobbins and other small spindle work and quite honestly I don't go much over 2000 rpm, why do I need to?
Pete
-
If you have enough time to spare, you can cut wood at the lowest speed you want to!
When I teach someone, I want them to have a basic understanding of how to arrive at a safe upper limit for speed before they go on their way because the few hours spent in a lesson doesn't really give anyone enough experience to be able to make an educated guess - which is what most of us do for the rest of the time!
-
I don't and never have had a RPM meter on a lathe so have no idea what the actual RPM is, what I can say is I now, within reason, turn at higher speeds than when I started out. Start slow, be safe and you will soon get the "feel" of the right speed for each individual blank be it spindle or face plate turning. try and get some tuition from local turners, it can save a lot of time and frustration. Best of luck.
-
Oh and keep your tools sharp
-
I am strongly in favour of NOT giving formulae for or tables of appropriate lathe speeds for various diameters of work pieces. I believe this approach to be fundamentally flawed. It is hypothetical and treats wood as though it is some sort of uniform commodity, which we all know to be untrue.
There is no predetermined safe speed for any given size of timber, rather, there are basic guidelines which must be adapted to take the nature of each individual piece of wood into account.
Les
-
I take the opposite approach because terms such as "slow", "fast", "safe", "comfortable" etc. mean different things to different people and often rely on a degree of experience that a novice can't be expected to have. I'm confident that the method I use can be used by a turner at any level and is ultimately safer than using wooly terms to avoid the question.
I agree that wood isn't uniform and there are other factors such as balance which override any calculation on safety grounds but as long as all that is made clear, a consistent approach can be presented.
-
Hi GeordieTurner
The very first response you received, from Pete of bodrighywood, was "How long is a piece of string?" which indicates the subjectivity of your question. You've had many good replies with several points to try and understand, take on board and absorb.
So you don't get confused or frustrated, I'd suggest you follow Paul Hannaby's good advice and formulae and stick with that quite religiously as a guide to the working lathe speed to run your lathe up to. This will give you a line in the sand to not exceed until you gain more experience BUT, each and every time you switch on and run the lathe up to the guide speed, if you feel it's too fast, unsafe or things such as vibration start to happen that scares you then simply slow the speed back down bit. Take your time and work slowly. You may never feel the need to go above that guideline speed.
That's the approach I took and now feel accomplished at selecting a safe lathe speed for a particular turn at the lathe. I always kept in mind it wasn't a race and, after all, pole lathe turners turn out some nice work at a slow speed. I now don't look at the speed dial very often, mostly only when I'm about to abrade, but just run the speed up while watching the wood spin and stop when I feel 'it's fast enough'.
Excuse the above puns.
Enjoy safe turning.
-
... terms such as "slow", "fast", "safe", "comfortable" etc. mean different things to different people and often rely on a degree of experience that a novice can't be expected to have. I'm confident that the method I use can be used by a turner at any level and is ultimately safer than using wooly terms to avoid the question....
Am I missing something Paul...I don't recall having used any such terms at all in my post? I certainly do not use woolly terms, either.
Les
-
As you can see from the various replies lots of advice and all can be correct for different turners but without doubt in my opinion is that of Pete start slow and increase speed as comes into balance , but Les is right as no piece of timer is uniform and as turners we want nice grains , burrs etc so treat each piece with care and look at the wood and let the wood be your guide. I very rarely use a rouging gouge a 3/4 bowl gouge just moves as much timber and gives better control.
Do make sure with bark on full face guard on £14.00 .
happy turning
-
surely a formula could only work to simply state the speed of the wood going past your tool at a particular rpm. it can't tell you if that bit of wood is safe at that speed.
as i said before it will all be down to the weight of the lathe being used and it's center of gravity which will be different for every single set up.
you could work out some sort of formula for your own set up if ose so wished but by the time a new turner has worked one out said turner will have quickly gained some experience and probably some common sense and realised that their own intuition will be the best option.
-
Les, it wasn't a direct reply to your last post, just a general one to some of the others and to other advice I have heard which doesn't really pin down what can be considered a safe approach to speed selection on the lathe.
Steve,
Yes the formula simply gives the rpm to achieve a given peripheral speed of the wood past the tool for a given diameter and as I said at the beginning of that explanation, the overriding factor is balance and safety and it is just a rough guide for a generic piece of wood but one that gives a safe upper limit in normal circumstances.
The formula is the same for every lathe. The bit that differs is how a particular lathe can cope with vibration and imbalance.
What I don't think is good advice is to suggest to increase the speed until there is vibration because for a well balanced piece of wood, there is a good chance you can considerably exceed a safe working speed before there are any noticeable vibrations so in my opinion, a method of determining a safe upper limit is needed which isn't reliant on someone's limited experience or perception.
Digressing slightly - balance is a complex subject in its own right. I'm not an expert on that but from what I have learnt, you can't always treat something as a simple mass. The actual balance at different points along any object might vary, that's why when they balance car tyres they sometimes end up with one weight on the outside of the wheel and one on the inside. It is possible to balance a piece of wood with counterweights, I have done it in the past with eccentric turning but for most occasions, we just have to remember to limit speed to below the point where the vibrations caused are more than the machine and mounting points for the wood can safely cope with and a decent cut can be achieved.
-
Although I tentatively agree with Paul's comments I have to point out that there are those that will be unable to do the maths, some are scared of numbers and always get it wrong. There are many factors that should be taken into consideration, on spindle work (with the grain running inline with the bedways) length and diameter play a part. If the wood is too thin or too flexible reducing speed can help take any "whip" out of the workpiece which may account for vibration over an off balance piece of wood. You must also take into consideration the straightness of the piece as that will also affect vibration as will the position of any spring wood or summer wood present. Another thing to consider is which way up has a wet piece been stacked as the moisture will sink to the ground and that too can cause an unwanted vibration.
Advice has been given here about starting the lathe on a slow speed and then speeding it up but more importantly you should start your turning journey slowly and learn the correct methods of doing things before you start to put larger off balance pieces of wood on the lathe, this is the biggest failing of all wood turners, they try to run before they can walk. As an apprentice I was not allowed to touch the lathe for the first six months I just stood and watched, then as I progressed I was only allowed to turn small bits to begin with eventually being allowed to turn six inch diameter porch supports. May I suggest that you start by turning nothing bigger than two or three inches diameter, learn your tooling presentation and sharpening and master it before moving on to more complex stuff, you will be a better turner for it and the quality of your work will improve immensley.