RPM as a function of diameter question

[Bruce Schoenleber]

I am aware of the maximum safe rpm = 9000 / diameter rule (guideline?). I usually take no notice of that and use the crank up the speed until it vibrates, empirical algorithm.

My question is this: what is the logic of the rpm=9000/D? Who came up with it and is there any valid physics behind it? Seems more of a one size fits all, bureaucrat solution. Also, not all lathes have rpm indicators, and each chunk of wood is different.

TIA, have popcorn and awaiting physics lecture

[Reed Gray]

I think they came up with that number more recently rather than a long time ago, especially with the new variable speed lathes. Most of the older lathes didn't have rpm read outs, so we just did what you do, turn it up till it shakes, then turn it down. Beginners need guide lines, and that formula works. I generally turn spindles a bit slower than that number, and bowls faster. You don't realize it till after it happens, but some times we turn faster than we should. Not a huge problem if you stand out of the line of fire, and know what you are doing, but it can be a lethal problem if you don't. One formula for sure is higher speeds make 'accidents' much more spectacular....

robo hippy

[John K Jordan]

I am aware of the maximum safe rpm = 9000 / diameter rule (guideline?). I usually take no notice of that and use the crank up the speed until it vibrates, empirical algorithm.
My question is this: what is the logic of the rpm=9000/D? Who came up with it and is there any valid physics behind it? Seems more of a one size fits all, bureaucrat solution. Also, not all lathes have rpm indicators, and each chunk of wood is different.
TIA, have popcorn and awaiting physics lecture

I agree with Reed, some kind of guideline is probably needed for beginners. I've never heard of the 9000 rule-of-thumb. (BTW, it's meaningless without specifying the units - is D in inches or centimeters or what?) If diameter is in inches then I should be turning 1" spindles at 9000 rpm? (Perhaps D in CM would make more sense for spindles.) I turn most spindles as fast as the lathe will spin, around 3200 on my PM lathe. But even turned much faster would be perfectly safe - if I snapped the spindle it would be harmless having very little mass and energy.

I think I work much like you - mount the piece well, use good wood, and use a speed that "feels" right. I suspect you are spot on about the 9000 number - perhaps it is a made-up number with no solid testing or consideration of the variety in the real world. For example, I turn 8" pieces with wings at well over the 9000 number (with D in inches), BUT, the pieces are solidly mounted good dry wood without cracks or voids, well balanced, I don't stand in the line of fire, I wear a good face shield, and the pieces are fairly thin with much lower mass (and energy) than an 8" solid blank, an 8" hollow form, or even a 8" bowl. Other factors in addition to mass, energy, and geometry are the holding method, the integrity and type of wood, and the skill level - far too many variables for physics lab. The holding method is a big factor - for example compare an improperly sized cross-grain tenon in weak wood to a faceplate or a properly sized tenon in a solid end grain blank. I once watched a sloppy turner throw the same bowl from the lathe three times, each time was because of a bad catch and probably poor mounting.

Over the years I've heard all sorts of guidelines from turner/demonstrator experts and in books and I suspect they made them up or are parroting numbers someone else made up. I've never read or heard solid testing or reasoning behind the numbers. I recently heard one such professional at a symposium give an RPM number below which he said any piece would bounce harmlessly off the floor instead of flying dangerously through the air. No conditions other than speed were given. ??? I suspect numbers such as those given by chuck manufacturers are driven at least partially by legal concerns.

JKJ

[Mick Fagan]

I've never heard of the 9000 rule, do you have a link?

I've just come in for a late lunch after turning some Camphor Laurel, as I have a belt speed lathe I have to judge the starting speed based mainly on experience, with a bit of luck thrown in. I generally work on cutting speed, unfortunately for you lot in the northern hemisphere I work in metres per second (m/s).

The blank was a bandsaw circular cut of 200mm, with a thickness of 50mm (8" by 2"). The wood was camphor laurel, which is quite easy on the tools, but you do need sharp tools to get shavings not sawdust, this timber in my experence crumbles when presented with blunt(ish) tools. Best speed for this timber is at my higher end of cutting speed between 8m/s to 11m/s. With this in mind I started out with 1200rpm within seconds and with a very sharp tool, I knew this was too fast. A quick look on my phone and I realised I was doing 13m/s. I changed the belt to 850rpm which is 9m/s and I was cutting cleanly and easily.

If this had been a hard hardwood, then I would probably be running closer to 5m/s, otherwise the tool isn't able to make smooth clean cuts with shavings curling off; if you know what I mean.

When I was quite young I helped the old man doing plumbing work, my main job at the start was to cut the galvanised water pipes in as straight a cut as I could get. At first when I attacked the pipe with the hacksaw, I ran it back and forth at a great rate of knots. The old man told me to let the tool do the work, don't force it. Above a certain speed the tool cannot work any faster or better, but you will expend a lot of energy.

Essentially I use that guide when applying cutting wood on a lathe; sharp tool presented to the wood with light pressure, wood travelling at a speed commensurate to its inherent characteristic and watch the shavings curl off. I know it isn't that easy, but that is how it sort of works for me.

At my wood turning club I use machines with variable speed, I too turn up rough blanks until one feels a tremor from the hand sitting on the lathe bed, then back it off a bit. For rough stuff this is a reasonably good approach, but once you have a balanced bit of timber, cutting speed does come to the fore; I really would like to know more about that 9000 rule.

Attached is my pre-lunch effort, Camphor Laurel blank 200mm x 50mm, end product 195mm x 45mm. finish is Ubeaut EEE followed by Ubeaut Glow. Perimeter cutting speed of 9m/s reducing as one gets closer to the centre...

Mick.

Camphor_Laurel_23_04_2018.jpg

[Thom Sturgill]

The rule was speed should be between 6000 and 9000 divided by the diameter in inches. So a 10" bowl should be turned between 600 and 900RPM, while a 12" bowl is 500 to 750RPM and an 8" bowl is 750-1150RPM. I always heard it attributed to the late Dale Nish, founder of Craft Supply. This rule was for bowls and obviously breaks down quickly for spindles .

[Bruce Schoenleber]

Thanks Thom.

My question was sloppy; pertains to safe turning speed for bowls based on blank diameter in inches. It is just something I have seen posted on this and other turning forums over the last couple of years. I saw it mentioned again in a reply post yesterday morning, I can't recall what the original subject was. It just got me wondering, as, again seeing it a number of times, is there any reasoning behind it. Apparently, and not surprisingly, there is not.

Thanks again to those of you who took the time to respond.

And Reed, I love your Robo Rest and was very impressed by the promptness. I ordered late on a Friday and received it on the following Monday, Amazon has nothing on you.

[Dan Hunkele]

I agree with Mick. I have always adjusted the RPM according to the cut rate. Different species of wood have different cut rates determined by their hardness and grain characteristics. I liken it to the machine shop industry. A piece of brass would have a different t rate than a piece of steel. Diameter also comes into play in both industries.

[Michael Mills]

My question is this: what is the logic of the rpm=9000/D? Who came up with it and is there any valid physics behind it? Seems more of a one size fits all, bureaucrat solution. Also, not all lathes have rpm indicators, and each chunk of wood is different.
)

It may have been Nish or it may have been Green as here...
http://www.docgreenwoodturner.com/lathespeed.html
I think it is just a guideline for new turners just as Nova has a chart in their manual for speeds based on diameter and whether it is balanced or not. I think most manufacturers offer a suggested speed.

Stuart Batty offers guidelines in his "fundamentals" videos. I assume they are just his ideas and not that of a committee. They may not be right (tool type, overhang, length of handle) but go ahead and use a spindle roughing gouge six inches over the tool rest with an eight inch handle if you want to.

[Pat Scott]

It was Dale Nish that came up with the formula years ago, and it is intended as a guideline for new turners. The formula only applies to bowls, not spindle turning.

Diameter x RPM = 6,000 to 9,000

Note that 6000 to 9000 is just a number, not RPM! The formula also assumes sound wood, sharp tools, good technique, etc. Those of you have have more experience than a beginner may turn faster which is fine, but again the formula is intended for beginners.

Dale Nish formula.jpg

Smaller diameter bowls and spindle stock can be turned faster:
-Six inch bowls will require a speed from 1000 to 1500 RPM
-Five inch bowls or projects require a speed from 1200 to 1800 RPM.
-Three inch stock from 2,000 to 3,000 RPM
-Stock 2" or smaller in diameter usually will be turned at speeds 3,000 or higher, often at the topspeed which the lathe will turn. However, caution must be practiced at all times and if the lathespeed seems too high, slow it down.

Stuart Batty said in one of his demos that if a bowl comes off the lathe at 1,000 rpm or less, it will come off and go to the ground. If turned at 1,000 rpm or higher it will be launched and that's when it can hit you in the face. I don't remember if he said this applies to a certain size or depth of bowl or what any of the other details were, I just remember the 1,000 rpm comment. Of course always use common sense and stand out of the line of fire, use a tail stock, wear a face shield, and all the other safety tips when turning.

[Randy Heinemann]

I have only been turning for a little over a year and have a lot to learn. However, until I get the bowl blank essentially round, I usually pick a speed that makes me feel safe. (Since I usually cut my blanks round on the bandsaw, I don't have many that are out of round a lot for very long.) I would guess that this sometimes results in a speed slower than it "should" be to get the best results, but it's a speed that makes me feel the most comfortable and a speed that keeps the blank from vibrating significantly. My Nova lathe manual had some guidelines based on diameter but I only used those at the very beginning of my turning. I really don't pay that much attention to exact rpm's now; just what makes me feel safe and doesn't make the lathe jump around. Does this method of determining speed present any turning quality or safety issues I might not be thinking of?

[Michael Mills]

I think your approach is about as good as it gets. I do the same, based on my experience I "guess" about what speed I should be at. I believe it was Bill Grumbine who said of speed.. "If the lathe isn't shaking and you'r not shaken (afraid)..."
Another consideration may be the soundness of the blank for potential problems so I try to inspect it well upfront and if I suspect something then stopping to recheck while turning.