28-08-2012, 05:42 PM
I'm not suggesting that anyone should read this end to end, but it might be useful as a reference.
In other threads, it's clear that Joe is having a great deal of fun and success in using his metalworking lathe for woodturning, despite the limitations of using a metalworking lathe in such a manner. I thought that a few comments about lathe speeds for woodturning might be of interest, and while not wishing to disuade Joe or anyone else from having fun using a metalworking lathe for turning wood, it's far from ideal, for reasons that I'll explain.
The main limitation in using a metalworking lathe to do woodturning is the slow speed(s) of the lathe as opposed to a woodturning one. A typical metalworking hobby lathe will have speeds from perhaps 50RPM up to perhaps 2,000 RPM, often much less. My little Lorch metalworking lathe has three speeds – 600, 1000 and 1600 RPM, pretty hopeless for turning small diameter objects in wood. My woodturning lathe has eight speeds, ranging from 214 to 3600 RPM. The slower speeds would only be used for turning large outboard items such as an 18” platter or bowl, which I’ll never do, or for sanding thin-walled bowls, where a faster speed would generate too much heat and risk cracking the bowl, or for thread chasing.
Newcomers to woodturning are often concerned of the dangers of a piece of wood flying off the lathe, and are scared of running the lathe at the optimum speed for whatever they're turning, so run the lathe at too low a speed and get poor results. Wood flying off the lathe is not the main danger in woodturning – dust is! Wood only comes of a lathe if it isn’t securely fixed, or a ‘dig-in’ occurs due to not using a gouge correctly, or having too large an overhang from the tool rest to the wood, or using a tool with too short a handle, so the wood snatches the business end of the tool as the tool can’t be held securely.
A woodturning gouge should always be held firmly on the toolpost, and for this, most turners use an overhand grip. When presenting a gouge to the wood, it should first be at such a steep angle that it will only rub - not cut. The gouge is then slowly lowered (by lifting the handle end upwards, pivoting on the toolrest) until the bevel of the tool starts to rub on the wood, then when lowered slightly more, starts to cut. This technique can be practised with the lathe switched off and the headstock turned by hand to see how the tool starts to take off shaving as the blade is slowly lowered. (The only exception to having the bevel rubbing is when using a roughing gouge).
When it comes to woodturning speeds, the faster the turning speed, the cleaner will be the cut, and the smaller the diameter of the wood, the faster the lathe speed needs to be. There is a’ rule of thumb’ formula for calculating the speed at which a woodturning lathe should be run, and that is: Diameter in inches x RPM = 6,000 – 9,000. So for example, a 10 inch diameter bowl blank should be run at a speed of 600 – 900 RPM. Most turners will start off at a slower speed when the blank is first mounted as it will be out of balance until trued with a roughing gouge, then when trued, will turn up the speed. More experienced turners run the lathe at the fastest speed that the wood seems happy with - 1,500 - 2,000 RPM. (To change the speed of my lathe means moving the belt on the pulley, and although it's only a minute's job, I tend to keep the lathe at 2,000 RPM for most things, but drop it to about 300 RPM for sanding).
Using the above formula, it follows that a 2” diameter spindle would have a turning speed of a minimum of 3,000 RPM, and a 1” one a theoretical speed of 6,000 RPM, well beyond the capacity of the lathe, so most turners would run the lathe at perhaps 3,000 RPM for spindle work. The longer the spindle, (stair spindle for example) the slower the lathe will need to run to prevent ther spindle from 'whipping', especially of not held midway along its length with a 'steady'.
At 3,000 RPM, a 1” diameter spindle would have about 3” of timber passing the tool for each rev, so that would be 9,000 inches of timber per minute passing the tool – about 750 feet. A 10” diameter bowl with the lathe running at 600 RPM will have 30 inches of timber per rev passing the tool 18,000 inches – 1,500 feet, twice that of a 1" spindle, even with the lathe for the spindle running at five times the speed of the 10" bowl. Thus, the smaller the diameter of timber, the faster the lathe needs to spin at to get a good clean cut, instead of the tool cutting the fibres, it tears them, then it’s necessary to resort to sanding by working through the grits to perhaps 600 grit for a good finish.
I’m not sure what speed Joe’s metalworking lathe spins at or if it has several speeds, but for small items up to say 6” diameter, ideally I’d suggest that it should be run at 750 – 1,000 RPM if at all possible, but for sanding, about 250RPM, moving the paper back quickly back and forth to avoid score marks around the item being sanded, starting perhaps with 220g then progressing to 320, 400, 600, to whatever degree of finish is desired. When sanding, it’s hopeless going from say 120 – 400 – 1000 as the sccore marks will never come out.
Necessity is the mother of invention, and as I said at the outset, I'm not wishing to disuade Joe or anyone else from using a metalworking lathe for woodturning, just seeking to explain some of the limitations, and to give some advice of speeds for woodturning and how to calculate them.
Hope these notes might be of interest and use to someone out there!
In other threads, it's clear that Joe is having a great deal of fun and success in using his metalworking lathe for woodturning, despite the limitations of using a metalworking lathe in such a manner. I thought that a few comments about lathe speeds for woodturning might be of interest, and while not wishing to disuade Joe or anyone else from having fun using a metalworking lathe for turning wood, it's far from ideal, for reasons that I'll explain.
The main limitation in using a metalworking lathe to do woodturning is the slow speed(s) of the lathe as opposed to a woodturning one. A typical metalworking hobby lathe will have speeds from perhaps 50RPM up to perhaps 2,000 RPM, often much less. My little Lorch metalworking lathe has three speeds – 600, 1000 and 1600 RPM, pretty hopeless for turning small diameter objects in wood. My woodturning lathe has eight speeds, ranging from 214 to 3600 RPM. The slower speeds would only be used for turning large outboard items such as an 18” platter or bowl, which I’ll never do, or for sanding thin-walled bowls, where a faster speed would generate too much heat and risk cracking the bowl, or for thread chasing.
Newcomers to woodturning are often concerned of the dangers of a piece of wood flying off the lathe, and are scared of running the lathe at the optimum speed for whatever they're turning, so run the lathe at too low a speed and get poor results. Wood flying off the lathe is not the main danger in woodturning – dust is! Wood only comes of a lathe if it isn’t securely fixed, or a ‘dig-in’ occurs due to not using a gouge correctly, or having too large an overhang from the tool rest to the wood, or using a tool with too short a handle, so the wood snatches the business end of the tool as the tool can’t be held securely.
A woodturning gouge should always be held firmly on the toolpost, and for this, most turners use an overhand grip. When presenting a gouge to the wood, it should first be at such a steep angle that it will only rub - not cut. The gouge is then slowly lowered (by lifting the handle end upwards, pivoting on the toolrest) until the bevel of the tool starts to rub on the wood, then when lowered slightly more, starts to cut. This technique can be practised with the lathe switched off and the headstock turned by hand to see how the tool starts to take off shaving as the blade is slowly lowered. (The only exception to having the bevel rubbing is when using a roughing gouge).
When it comes to woodturning speeds, the faster the turning speed, the cleaner will be the cut, and the smaller the diameter of the wood, the faster the lathe speed needs to be. There is a’ rule of thumb’ formula for calculating the speed at which a woodturning lathe should be run, and that is: Diameter in inches x RPM = 6,000 – 9,000. So for example, a 10 inch diameter bowl blank should be run at a speed of 600 – 900 RPM. Most turners will start off at a slower speed when the blank is first mounted as it will be out of balance until trued with a roughing gouge, then when trued, will turn up the speed. More experienced turners run the lathe at the fastest speed that the wood seems happy with - 1,500 - 2,000 RPM. (To change the speed of my lathe means moving the belt on the pulley, and although it's only a minute's job, I tend to keep the lathe at 2,000 RPM for most things, but drop it to about 300 RPM for sanding).
Using the above formula, it follows that a 2” diameter spindle would have a turning speed of a minimum of 3,000 RPM, and a 1” one a theoretical speed of 6,000 RPM, well beyond the capacity of the lathe, so most turners would run the lathe at perhaps 3,000 RPM for spindle work. The longer the spindle, (stair spindle for example) the slower the lathe will need to run to prevent ther spindle from 'whipping', especially of not held midway along its length with a 'steady'.
At 3,000 RPM, a 1” diameter spindle would have about 3” of timber passing the tool for each rev, so that would be 9,000 inches of timber per minute passing the tool – about 750 feet. A 10” diameter bowl with the lathe running at 600 RPM will have 30 inches of timber per rev passing the tool 18,000 inches – 1,500 feet, twice that of a 1" spindle, even with the lathe for the spindle running at five times the speed of the 10" bowl. Thus, the smaller the diameter of timber, the faster the lathe needs to spin at to get a good clean cut, instead of the tool cutting the fibres, it tears them, then it’s necessary to resort to sanding by working through the grits to perhaps 600 grit for a good finish.
I’m not sure what speed Joe’s metalworking lathe spins at or if it has several speeds, but for small items up to say 6” diameter, ideally I’d suggest that it should be run at 750 – 1,000 RPM if at all possible, but for sanding, about 250RPM, moving the paper back quickly back and forth to avoid score marks around the item being sanded, starting perhaps with 220g then progressing to 320, 400, 600, to whatever degree of finish is desired. When sanding, it’s hopeless going from say 120 – 400 – 1000 as the sccore marks will never come out.
Necessity is the mother of invention, and as I said at the outset, I'm not wishing to disuade Joe or anyone else from using a metalworking lathe for woodturning, just seeking to explain some of the limitations, and to give some advice of speeds for woodturning and how to calculate them.
Hope these notes might be of interest and use to someone out there!
Regards, David.
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
