Thread: Does Speed Matter?

Just an amateur, so take my comments lightly. But perhaps those that are experienced and confident enough to spin a bowl fast are also the ones with better tool control, so their cuts appear smoother?

For what it's worth, John Ewert (turning instructor from Trend Timbers Sydney) recommends as fast as possible without excessive vibration. Obviously with an odd shaped bowl blank, vibration is going to be pretty serious at first.

Hi Brendan

I'm reading Mike Darlow's "Fundamentals of Woodturning" at the moment and I'm sure I read something there as to why. Will have a look when I get home.

cheers
WH

Originally Posted by brendan stemp

This question may have been dealt with before but I'm sure there are a few who have come to the Forum later that might find the issue worth a re-visit.

If you are like me the thought is that any turning should be done AFAP; as fast as possible. 'Possible' is the important word because I am not suggesting all lumps of wood should be spun at 3000rpm rather they should be spun as fast as the circumstances allow. 25mm spindles can go at 3000rpm but an irregular buloke bowl blank might need to go a lot lot slower. Keith Rowleys Laws of Woodturning.
Law 1. The speed of the lathe must be compatible with the size, weight, and length of the wood to be turned. This is a given.



If we agree on this then the question is 'why should speed matter"?

Now if you are like then everything needs to spin flat chat 'cause time is money. I do not spin everything flat chat.
I understand that. But do you get a better (cleaner) cut off the tool if the wood is spinning faster. Generally speaking yes but there are some timbers that will not cut at a higher speed because the hardness of the timber will not allow the cutting edge to "bite" so one has to slow down.

Anecdotal evidence would suggest this is the case but this then raises the question of "WHY"? I can't see a reason why this should be the case so my observations confuse me.

I am interested in an explanation if anyone is prepared to tackle this question.

The main reason "faster is better" is that more timber passes under the cutting edge for a given pass of the tool and you tend not to leave hollows.

It is also painfully too slow.

Some timber refuses to cut at a high speed and just wears out tools. See above answer.

My general recommendation is the same as John Ewarts: Speed up until it vibrates and then back off 1 speed in the case of belt drives or back off a little on a variable.

BTW, my usual answer for how fast should it spin is, adequately.

Funny Story
I wanted to make a lazy susan for the dining room table. I had some 3" X 1.5" Cypress Pine and arranged the blocks in a pattern about 400mm diameter, like parquetry, glued it up (PVA) Was going well right up until the last step. I found the speeds I had used to get to this stage were a little slow for sanding as I was making scratches rather than fine dust. You guessed it..... it flew apart like a hand-grenade. Bits went all over the place but none hit me. I found out later that PVA doesn't really stick to Cypress Pine.
Lesson learned....be careful how you go

I once bothered to work out the tip speeds of various sized circular saws. The smaller the diametre of the blade the greater the revs in order to attain the actual metres per minute rate. this metres per minut is the near ideal speed at whi the blade will produce a a near optimal cu.

Surely this must also apply to lathe speed. I turn pens at somewhere near the max revs of the lathe, if they are timber blanks, that is.This is because they have a small diametre and need revs under their belt for efficient cutting by the tool. Acrylics won't stand this as too much heat is generated.

Heat is also generated when turning wood. Surely it follows that too much speed equals too much heat and there the possibility of surface cracking.This is very true when sanding bowls and platters.

You should sand at a lower speed for the reasons above and most sandpaper is made for hand sanding so it cuts better.

Originally Posted by brendan stemp

But do you get a better (cleaner) cut off the tool if the wood is spinning faster. Anecdotal evidence would suggest this is the case but this then raises the question of "WHY"? I can't see a reason why this should be the case so my observations confuse me.

I am interested in an explanation if anyone is prepared to tackle this question.

An example from today. Turning a bowl at 's that had lumps in the side I wanted to get out, But it was uneven. Only in part of the wall. I guess it had develpoed a bit of ovulation overnight as well. Only a mm or so but still. At a slower speed the chisel just bounced and missed most of the lump only to land just before the next lump digging the hole deeper then missing the lump again. A bit like driving down a corrugated dirt road. The car/chisel keeps landing on the front of the corrugation making them worse. Going faster so the chisel can skip across the top of the corrugations means the chisel can gradually cut them away without bouncing.

I bet there is something on youtube. Here is Dr Karl talking about them. Although he doesn't talk about going faster to get a smoother ride.

Here is tips on driving on corrugations. But they say going faster, although smoother, means your wheels aren't in contact with the road all the time so you loose control. Well, at least with the chisel we also have the support of the tools rest so can skim off the top of out bumps and smooth out our surface.

Anyway. That's my theory. There is also something about the length of tool overhang giving different resonances to the corrugation-making potential of a tool. In those instances you might just have to be going at a more "out of resonance" speed. Faster OR slower might do the same job. Although the momentum of the work going faster means you don't have to use so much of your own strength to make shavings.

Originally Posted by wood hacker

Hi Brendan

I'm reading Mike Darlow's "Fundamentals of Woodturning" at the moment and I'm sure I read something there as to why. Will have a look when I get home.

cheers
WH

OK According to Darlow

"If you use a lathe speed which is too low, you will produce an excessively rippled surface because of the lack of reference support and the large pitch of the spiral which your tool cuts. Using a low lathe speed also increases subsurface damage for two reasons:
1. The shavings will have less momentum and therefore will separate less readily.
2. Wood is weaker at higher temperatures. For example at 160°F (71°C) the loss of strength is 10-50%. At low lathe speeds the temperature of the cutting edge will be lower, the adjacent woods temperature raised less, and the wood will therefore fail less readily and cleanly"


There are a couple of diagrams to illustrate his points but I can't reproduce them here. Persoanlly I think there is more involved than what Darlow covers, most of it mentioned in the posts above.

cheers
WH

I remember whilst down at 's last time i was turning something flat out and I was having trouble getting a decent cut (and for a change it wasn't just me and my technique ). I couldn't believe it when said try turning the lathe down So does say it sometimes, and ps, yes it was better.

Originally Posted by wood hacker

OK According to Darlow

"If you use a lathe speed which is too low, you will produce an excessively rippled surface because of the lack of reference support and the large pitch of the spiral which your tool cuts. Using a low lathe speed also increases subsurface damage for two reasons:
1. The shavings will have less momentum and therefore will separate less readily.
2. Wood is weaker at higher temperatures. For example at 160°F (71°C) the loss of strength is 10-50%. At low lathe speeds the temperature of the cutting edge will be lower, the adjacent woods temperature raised less, and the wood will therefore fail less readily and cleanly"


There are a couple of diagrams to isllistrate his points but I can't reproduce them here. Persoanlly I think there is more involved than what Darlow covers, most of it mentioned in the posts above.

cheers
WH

Thanks WH. That's the sort of thing I was after although some of it makes no sense to me; ie "lack of reference support".

I raised this issue because I use a metal lathe to cut wood quite regularly. Often I will engage the automatic feed and have noticed that its not necessarily the speed of the lathe that makes the difference but more the rate the cutter travels in relation to the speed of the lathe. THis is along the lines of what was saying but it seems there is more to it. Interesting.

Originally Posted by dai sensei

I remember whilst down at 's last time i was turning something flat out and I was having trouble getting a decent cut (and for a change it wasn't just me and my technique ). I couldn't believe it when said try turning the lathe down So does say it sometimes, and ps, yes it was better.

I had a similar situation recently with a student. He simply couldn't get a cut to happen so I had a go. I too had exactly the same trouble. I then realised the lathe was spinning in reverse

Another point to consider is the timber being used. Black and yellow walnut are notorious for their silica content. These can be worked easily with hand tools so the obvious solution to turning is to slow the lathe down. Mango wood and Queensland maple can also have a high silica content, there are others as well.
Most of my larger laminated bowls are turned at what I call "mid range" speeds. Faster and slower speeds do not produce a satisfactory cut. My lathe has evs but does not have a rev counter, so I can't give numbers for the rpm. I usually turn at a speed where everything is happening sweetly.
How fast should you turn? At a speed where the lathe is happy and just as importantly, when you the operator is happy. Sand paper can fix some chisel marks..
Jim

Originally Posted by brendan stemp

Often I will engage the automatic feed and have noticed that its not necessarily the speed of the lathe that makes the difference but more the rate the cutter travels in relation to the speed of the lathe.

Dalow does talk about this issue with lots of pretty pictures that do better than words. It does boil down to a few things effecting reference support and the rate of traverse as well as the lathe speed are among them.

Originally Posted by brendan stemp

I had a similar situation recently with a student. He simply couldn't get a cut to happen so I had a go. I too had exactly the same trouble. I then realised the lathe was spinning in reverse

Nah - you were just standing on the wrong side of the lathe