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Define harmonic vibration......

Odie

Odie

Panning for Montana gold, with Betsy, the mule!
TOTW Team
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We've all heard the term "harmonic vibration" bandied about......but, what exactly does it mean to you?

Without using any lathe tools, the rpm can be adjusted for the least vibration......but, there are two types of vibrations that are strictly the result varying the rpm. One is an ever increasing vibration that are the result of an out-of-balance condition, which increasingly gets more pronounced with the increase of rpm. The other is a felt vibration that may come and go with the increase of rpm, and there can be certain speeds that are "sweet spots" in the felt vibration. This, to my thinking, is harmonic vibration, and is the interaction of two, (or more?) distinct influences......rpm and balance.

Then there are vibrations caused by the tool contact with the wood. Also to my thinking, there are many things that influence harmonic vibrations tuned to interactions between wood and tool. The overall characteristics of the wood itself, which includes physical geometry, wall thickness, rpm, tool handling, tool sharpness, presentation of the cutting edge, overhang distance, physical resistance, and much more (I think). I have come to the general conclusion that the most influential of these factors is the overall sharpness of the tool, in conjunction with the rpm.......but, what are your thoughts on this?????

ko
 
Harmonics are the interaction of vibration waves from different sources
These vibration waves add to each other or subtract from each other.
I don't know what all the components of vibration are. The earth is vibrating little bit.
The mounting point, the motor, the pulleys, the spindle, the headstock, legs, and bed all vibrate.
The wood vibrate too.
A 900 pound well made lathe does not vibrate as much as 50 pound lathe.

As to vibration of an out of round piece.
Very often there is speed of least viobration that is quite a bit faster than the then first unacceptable vibration found at a lower speed.

I find the proper speed by increasing the speed from zero. When I get to unacceptable vibration for what I'm turning I slow the speed down to what I think is acceptable. This i call the first acceptable speed. Then I increase the speed if the vibration gets scary I return to the first acceptable speed. But usually as speed increases, the vibration gets a little worse but not scary, then sort of levels out, then vibration decreases gets quite smooth, then vibration starts to get worse. The smoothest part of the higher speed is where I like to turn. This is spot where some of the vibration waves are cancelling each other out.

When I'm roughing more vibration is acceptable because the cutting tends to dampen the vibration.
For finish turning there should be no noticeable vibration the wood should be smooth under tool bevel,when the tool is not cutting.
 
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Pretty good overall analysis I think, Al.......:D

.....and, this abbreviated definition is a good one:
hockenbery said:
Harmonics are the interaction of vibration waves from different sources
Click to expand...

I'd add that I check, and adjust rpm for vibration every time I start up the lathe......even when roughing. There are more indicators than simply feeling for it with your fingers on the lathe. I have four lamps attached to the headstock, and two machinist's magnetic bases (one for flexible tip light, and the other for attaching a laser). All of these things add to the ability to fine tune the vibrations.

ko
 
Every piece of equipment big or small has a natural frequency determined by the mass of the equipment and the material it is made of. With a variable speed and a different weight and balance of the billet being turned can create a harmonic vibration which can quickly cause mechanical issues depending on the speed of the lathe.

Adding weight to the lathe usually changes the natural frequency of the equipment, this allows you to turn a larger billet without the lathe dancing across the floor.
 
I think vibration is a changing stress that will change with the speed this tress is changing, from where you can not easily detect it to where you can feel it and to where you can hear it and then go even beyond that.

The power that is causing this stress increase-decrease will make it easier or less so detectable, like where a sharper smaller tool will cause less vibration in that there is less power to cause the vibration.

Harmonic is where stresses will build on itself through flexibility and captured tension within..

Anyway that is how I see it,...............I think :confused::eek:o_O
 
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odie said:
We've all heard the term "harmonic vibration" bandied about......but, what exactly does it mean to you?

Without using any lathe tools, the rpm can be adjusted for the least vibration......but, there are two types of vibrations that are strictly the result varying the rpm. One is an ever increasing vibration that are the result of an out-of-balance condition, which increasingly gets more pronounced with the increase of rpm. The other is a felt vibration that may come and go with the increase of rpm, and there can be certain speeds that are "sweet spots" in the felt vibration. This, to my thinking, is harmonic vibration, and is the interaction of two, (or more?) distinct influences......rpm and balance.

Then there are vibrations caused by the tool contact with the wood. Also to my thinking, there are many things that influence harmonic vibrations tuned to interactions between wood and tool. The overall characteristics of the wood itself, which includes physical geometry, wall thickness, rpm, tool handling, tool sharpness, presentation of the cutting edge, overhang distance, physical resistance, and much more (I think). I have come to the general conclusion that the most influential of these factors is the overall sharpness of the tool, in conjunction with the rpm.......but, what are your thoughts on this?????

ko
Click to expand...

Basically, when talking about mechanical systems, harmonic vibration is just a fancy way of saying vibration.

The term harmonic refers to integral multiples of a fundamental frequency. The term integral means integer or whole numbers. In music, even harmonics are called octaves. For example, A4 on a violin is 440 Hz. An engineer would say that the second harmonic is double that(880 Hz) and a musician would say that A5 which is one octave higher is 880 Hz.

A "pure" vibration is one that has a nice smooth sinusoidal waveform. However, mechanical vibrations are rarely "pure". Their waveforms may look like a square wave or a triangular wave or something else. A clever Frenchman named Fourier discovered that any periodic waveform is the combination of the fundamental frequency plus some unique combination of integral multiples of the fundamental. So that's it in a nutshell ... a harmonic vibration in a mechanical system is generally not a pure single frequency, but rather some unique combination of the fundamental frequency and its harmonics (integral multiples of the fundamental).

Some of the other vibrations that you talk about are manifestations of the same thing, As the lathe speed is increased, the amplitude of vibrations decrease, but there are various mechanical resonances that get "excited" when the natural frequently of some part of the lathe matches the frequency being generated by the spinning wood.

Wood is softer and far more flexible than steel (usually). So when a tool contacts spinning wood, the wood is vibrating much more than the steel ... unless the tool is hanging waaaay over the tool rest. But, you are right that it is a very complex interaction of many factors. This means that dealing with vibrations is to some extent dependent on the individual and his unique circumstances.
 
[QUOTE="Bill Boehme, For example, A4 on a violin is 440 Hz. "
Bill-thought violins were taboo!!!

ON the inside as I get the wood thin,( but also on the outside when not hollowed yet) I will get a different sound and there are tool marks that are ? can't describe unless I have it in front of me-curved along the outside of the bowl rather than parallel to say the rim. On the inside i can say maybe flex of the wood when it gets thinner, but not sure on the outside. It is near the rim during finishing cuts, but not sure why when the wood isn't hollow yet. Using sharper gouges helps but sometimes I just resort to scrapers to take them away. Is the duller gouge "riding the harmonic waves". and sharper cutting into the waves? Am I not applying enuf pressure, or applyiing too much???? Not sure if I am making myself clear. Get this on splindles in the middle too, assume flexing of the wood . . Can get them out but the wood is still flexing, no???Gretch
 
Every wood blank mounted on a lathe has hard knots, and dense wood grain and softer wood grain
throughout the piece. Depending on how well balanced the piece is mounted between centers will
dictate the potential harmonics and vibration the piece will create. One small crack or one hard knot
can easily create a harmonic when turning at a specific speed on the lathe. Increasing or decreasing
the turning speed can help reduce these vibrations/harmonics that can cause tool marks on the piece.
When your tool hits the hard knot or crack, your tool bounces, the blank flexes, the tool lands back on
the turning piece somewhere on the rotating wood blank depending on the speed it is turning. The thinner
the wall is on your vessel will also magnify the vibration/harmonics and flexing of the piece. Adding a support
to the turning vessel dampens the vibrations and changes the "natural frequency" of the hollow form. There are
many approaches to solving some of these issues.
 
We have thousands of vibration sensors installed on rotating equipment that we maintain at my work place.
One of the facilities monitors the entire structure of the building and monitors the "natural frequency" to reduce
the amount of harmonic vibration on the structure. This particular facility has a number of large rotating machines on one of the floors which when running can randomly set up a harmonic vibration and cause the floor and the building to "bounce". We can adjust the speed of the machines with VFD drives to mitigate this issue.
If these problems are not addressed it can easily destroy the floor, structure and building over time in an industrial setting. Tesla was said to have developed a machine that would amplify harmonic vibrations and could
use this device to cause a building to bounce/shake/vibrate violently.
 
Cars & motorcycles also suffer from harmonic vibrations that occur when they reach a
specific speed and RPM and the vehicle will suffer a violent range of acceleration which forces
you to speed through the harmonic range or slow down.
 
I have, on several occasions, had it happen that a slight increase in RPMs would cause a decidedly unbalanced piece of stock run slightly "less rough." Point being, vibration (not harmonic per se, simply vibration) does not always increase with increased speed. That being said, I test this possibility very, very carefully. If a slight increase in speed just makes things worse, back down it goes.
 
Mike Johnson said:
Cars & motorcycles also suffer from harmonic vibrations that occur when they reach a
specific speed and RPM and the vehicle will suffer a violent range of acceleration which forces
you to speed through the harmonic range or slow down.
Click to expand...

Yep.......:D

Years ago, I had a Triumph 1969 650 TR6C motorcycle and the two pistons moved up and down together.....not alternating. The vibration was annoying, and one of the reasons I sold it! Trips over 100 miles were not pleasant at all! :rolleyes:

ko
 
I grew up sailing and living aboard sailboats. For a short period our wire rigging often vibrated audibly when winds came from unusual directions at certain wind speeds. On occasion it would wake up my folks, but not me because I was a teenager. My parents tuned their rig and over a little while successfully got rid of most of the harmonic vibration. Nobody wants to live on a 55 foot Aeolian harp!

A couple years ago I tried a related approach with my lathe—partly because there's nowhere for sandbags. I ran a pair of 1/8" stainless cable stays at the head stock and tailstock ends of my lathe that angle out and terminate in turnbuckles that are bolted into the floor. It made a significant difference in lathe vibration. That middle zone between the lower RPM sweet spot and the higher RPM sweet spot grew smaller and the sweet spots grew larger. Overall the lathe is just smoother.

Every several months my lathe seems to start vibrating more, and I retune the cable stays, just tightening until they all sound about the same when plucked like a guitar string. My lathe seems happier after a tune, or at least I am, since the vibration goes down.

I don't really know if I've changed/reduced the harmonic vibration or just reduced the side-to-side sway of the lathe, but it seems to work.
 
When turning a hollow form or delicate spindle form holding your hand on the opposite
side of the piece dampens the vibration and allows you to make a cleaner cut.
You are actually changing the "natural frequency" of the wood piece.
Adding rubber bands on the outside of a hollow form or using a wheeled steady rest
will change the "natural frequency" of the wood and allow for a cleaner cut.
Slowing or Increasing the speed of the lathe is the easiest solution to address the issue.
Securing the lathe with additional weight or bolting it to the floor or adding additional supports
like cable stays would be the next level of defense against the issue.
Dampening the vibration at the wood piece with additional support would be your next level
of defense to address the issue.
One other variable is the wood blank itself, you could stabilize the wood blank with resin and
this might reduce the amount of chatter and vibration during the turning process. A large wood
blank that has knots on one side will never balance out on the lathe. I have been forced to add
counter weights on several pieces over the years to get them turned round. It is easier just to
use a good quality balanced wood blank for your project. Finding large dimension wood blanks
for specific projects can be a challenge, sometimes you have to deal with what you have to work
with or can find on the open market.
The last variable would be the cutting tool, how sharp, hardness, tool approach and tool selection.
Adding weight to the tool also changes the "natural frequency" of the tool. A large heavy tool always
cuts cleaner then a small light weight tool that tends to chatter from vibrations when cutting hard woods.
 
"sinusoidal"
I get that effect from pollen during the warm months. o_O
BTW, seriously, interesting topic for thought. Will be aware of it when turning rough or out-of-round pieces. Thanks.
 
Zach LaPerriere said:
I grew up sailing and living aboard sailboats. For a short period our wire rigging often vibrated audibly when winds came from unusual directions at certain wind speeds. On occasion it would wake up my folks, but not me because I was a teenager. My parents tuned their rig and over a little while successfully got rid of most of the harmonic vibration. Nobody wants to live on a 55 foot Aeolian harp!

A couple years ago I tried a related approach with my lathe—partly because there's nowhere for sandbags. I ran a pair of 1/8" stainless cable stays at the head stock and tailstock ends of my lathe that angle out and terminate in turnbuckles that are bolted into the floor. It made a significant difference in lathe vibration. That middle zone between the lower RPM sweet spot and the higher RPM sweet spot grew smaller and the sweet spots grew larger. Overall the lathe is just smoother.

Every several months my lathe seems to start vibrating more, and I retune the cable stays, just tightening until they all sound about the same when plucked like a guitar string. My lathe seems happier after a tune, or at least I am, since the vibration goes down.

I don't really know if I've changed/reduced the harmonic vibration or just reduced the side-to-side sway of the lathe, but it seems to work.
Click to expand...


Interesting Zach.......

I've never heard of anyone doing that before......got any pics?
 
This afternoon, I was keeping my eyes closed and adjusting the lathe rpm for the least vibration while touching the bedways. I think I can fine tune my sense of touch perception to be a bit more acute.....need to play with this a bit more......:)
 
Odie,

You should know Zach also has a rudder mounted on the stern of his lathe. :-)
Cable stays would be a great way to "add weight" to the lathe and reduce the potential vibration issue.
Easy way to add several thousand pounds of ballast to your lathe!
 
Mike Johnson said:
Odie,

You should know Zach also has a rudder mounted on the stern of his lathe. :)
Cable stays would be a great way to "add weight" to the lathe and reduce the potential vibration issue.
Easy way to add several thousand pounds of ballast to your lathe!
Click to expand...


Yeah, a rudder would be his style!.......Zach is a most interesting fellow on here!......and brings an unusual mix of experiences to the table. :D

I'm trying to visualize what his cable stays setup looks like and does, but since the attachment points are connected directly to the headstock and tailstock, it seems like it might/would be more helpful than a simple addition of weight......but, I only speculate.

Regardless, I don't think I'd like to walk 20ft to one side of the lathe to get to the other side!:rolleyes: Heh,heh,heh!

Generally, I think there are two types of movement......that which originates with the bowl interacting with the tool, and those originating with the balance of the spinning bowl. The answers to eliminating/reducing the vibrations are entirely different.

ko
 
Odie,

Drilling (4) holes in the concrete floor and installing expanding lead concrete anchors would work
for the fastening points. Several years ago I was turning a large billet and used cargo ratchet straps
to anchor the lathe to the floor to keep the lathe from moving around. The steel guy cables would be
a better choice as they will not stretch out like the fabric straps.
 
odie said:
Generally, I think there are two types of movement......that which originates with the bowl interacting with the tool, and those originating with the balance of the spinning bowl. The answers to eliminating/reducing the vibrations are entirely different.
Click to expand...

I think that you did a pretty good job of analyzing the problem.
 
Mike Johnson said:
Odie, Drilling (4) holes in the concrete floor and installing expanding lead concrete anchors would work for the fastening points.
Click to expand...

Yes, the anchor bolts are what I'm using. They seem to be a pretty good solution for anything I do......however, some turners have a much larger lathe swing than I do, and yet others are more interested in very out-of-balance work than I am. Some turners face much more monumental problematic circumstances than I do.......

For those who want weight, there was the Serious Lathe, and it weighed 1580 lbs.....!!!! It never caught on very well. I think the 1 1/2" x 8 spindle size was the ultimate reason why it never was a success.......just massive, but not very many accessories available for it.

http://www.serioustoolworks.com/wood-lathes.php

GetAttachment.aspx3.5x3.5.jpg
 
odie said:
I think the 1 1/2" x 8 spindle size was the ultimate reason why it never was a success......
Click to expand...

Switching to 1 1/2 x8 or m33x 3.5 or no threads is a consideration but not a barrier.
There are lots of chucks, faceplates etc in 1.5 x 8
There are fewer options for #3 tapers but there are enough out there.
More big drill bits are available with #3 tapers than #2

The Serious lathe is for folks who turn big stuff. And sales are more related to the competition than the thread size.

People who turn big stuff mostly go with Oneway, the VB36, a used Nichols or a custom lathe.

I know quite a few people with Oneways, a few with VB36, and a few with Nichols.
Don't know of anyone with a Serious
 
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That is one big lathe! I don't need to drill holes in the floor as I don't turn anything big but would be reluctant as our house is built on a wet-weather spring.
 
Here you go, Odie. The definitive guide on how to jerry rig cable stays on your lathe. Red Green will be doing a feature on this soon... :) I promised to sweep the floor, but then I forgot.

Being a retired builder, I have a pile of left over 1/8" SS cable I used in lieu of spindles below deck rails. There was no logical attachment point, so I laid some 5/8" X 16" carriage bolts (left over from a boat job, Mike...) on the supports of the lathe bed. I wrapped Grace (the sticky bituminous weather flashing) around the bolts. Then I swaged eyes on the cables, and put some cheapo turnbuckles, also left over from a deck job where the owner only wanted the 99 cent turnbuckles. The cable eyes go through these bolts. (my apologies for the difficult picture.)

The turnbuckles go into aluminum angle (also left over from a boat job, Mike...) that are screwed through the flooring and into the floor joists.

The whole project took under an hour with a bunch of junk on hand and was just a trial, really. But heck, it seems to work quite well. My lathe is 630 pounds, but I really think it could benefit from extra weight—there just isn't anywhere to attach it easily.

And finally Mike, for your benefit, I have included a rudder I salvaged off a beached fishing boat. 80 pounds of silicon bronze. Still looking for a lathe attachment point.;) And I thought it also appropriate to show you the steering wheel that came off a gillnetter that I attached to our beachfront garden deck some years ago for our youngest son. :)

Edit: tried to clarify how the cable eyes attach to the bolts.
 

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Zach LaPerriere said:
Here you go, Odie. The definitive guide on how to jerry rig cable stays on your lathe. Red Green will be doing a feature on this soon... :) I promised to sweep the floor, but then I forgot.

Being a retired builder, I have a pile of left over 1/8" SS cable I used in lieu of spindles below deck rails. There was no logical attachment point, so I laid some 5/8" X 16" carriage bolts (left over from a boat job, Mike...) on the supports of the lathe bed. I wrapped Grace (the sticky bituminous weather flashing) around the bolts. Then I swaged eyes on the cables, and put some cheapo turnbuckles, also left over from a deck job where the owner only wanted the 99 cent turnbuckles.

The turnbuckles go into aluminum angle (also left over from a boat job, Mike...) that are screwed through the flooring and into the floor joists.

The whole project took under an hour with a bunch of junk on hand and was just a trial, really. But heck, it seems to work quite well. My lathe is 630 pounds, but I really think it could benefit from extra weight—there just isn't anywhere to attach it easily.

And finally Mike, for your benefit, I have included a rudder I salvaged off a beached fishing boat. 80 pounds of silicon bronze. Still looking for a lathe attachment point.;) And I thought it also appropriate to show you the steering wheel that came off a gillnetter that I attached to our beachfront garden deck some years ago for our youngest son. :)
Click to expand...

Thanks for the photos, Zach.......:D

I had envisioned a setup quite different, with a very big "footprint".......dumb me! That really does look like it ought to do some good. :cool:

ko
 
Very nice, Zach. I predict that the epicenter of the next earthquake in Alaska will be right under your lathe when you turn some huge out of balance cedar tree. I can't see how the cables are attached to the underside of your lathe, but I would recommend using something to distribute the load over a wide area because cast iron can crack if there is a high stress concentrated at a single point.
 
Bill Boehme said:
Very nice, Zach. I predict that the epicenter of the next earthquake in Alaska will be right under your lathe when you turn some huge out of balance cedar tree. I can't see how the cables are attached to the underside of your lathe, but I would recommend using something to distribute the load over a wide area because cast iron can crack if there is a high stress concentrated at a single point.
Click to expand...

Thanks, Bill. I distributed the weight as best as I could under the circumstance. I know it's not very easy to see in the picture—I have 16" bolts that span three cast supports that connect each side of the bed, one bolt for each cable stay. You can just barely see the cable looped around the bolt if you look in the middle of the black Grace.
IMG_1285.JPG

I should also note again that I have stays on the tailstock side as well, but didn't include pictures.

I'll check with the Alaska Earthquake Center next time I turn something big and out balance!
 
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Harmonic vibration is one of those things that you always hear about, but do very little to help. Even if you do know exactly what it is, how can you use it to your advantage? Speeding past the initial vibration is something that I do not tell beginners anymore!! One had a close encounter with a flying bowl blank, lol. I'm lucky to own the Cadillacs of lathes, the Omega Stubby is super heavy, and to top that I added 250 Lbs of sand, solid. It did make an interesting thread, might use the question if and when the AAW gets the Facebook group started... Aloha from Maui.
 
odie, if you want to add weight to a lathe the best way I've found is to skip the whole concrete and sand thing and head on off to your local auto tire place and offer to take a coupe five gallon pails of discarded lead weights. The lead in those suckers can be melted down and reshaped into anything you want. I used green wood to make the molds . Or don't melt 'em just cast 'em in epoxy or concrete. I recall a wooden boat issue where they melted the lead and poured it into a mold the shape for which they just dug in the dirt.
 
In my former life as an engineer I was involved in vibration testing and analysis on a number of projects and we had various categories of vibration, but don't recall using the term "harmonic" used with vibration because it was either superfluous or insufficient and wouldn't have added anything useful to the description.

As a woodturner, when I see the word "harmonic" used to describe vibration, it seems a bit presumptuous as if using the word adds an air of authority. From my perspective, the word "vibration" is completely adequate.
 
Harmonic vibration.......remember the pictures of the bridge that was caught in a wind storm......1930s or 40s or so?
 
That was the first Tacoma Narrows Bridge, AKA "Galloping Gertie". It was opened in mid 1940 and collapsed within a few month on a very windy day. It has become an engineering textbook example of forced resonance.
 
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