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Bowl turning Harmonic vibration adjustments for the "perfect cut."

Odie

Panning for Montana gold, with Betsy, the mule!
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There is the vibration your lathe will produce at any given rpm and combine that with the vibration a chunk of wood spinning will produce at any given rpm......along with the effect a sharp tool has on the vibration at that rpm. If you can control all those things, it's possible to get a perfectly clean tearout free cut......provided the tool is sharp and the presentation of that tool is correct.

It cannot be stressed how much rpm effects the cleanest of cuts, and rpm must be adjusted precisely.

It also cannot be stressed enough just how much sharpness contributes to the "perfect cut".

One very good method of infinitely adjusting the rpm beyond what us mere mortals can is with a vibrometer.
You can't buy one of these, but you can easily make one, and here is a thread that will tell you how:


1738338058571.png

=o=
 
There would be so many variables in this that makes it a quest of epic proportions. It seems everything would have a part to play in it, on top of species, wall thickness, toolrest, length of tool projection,tool thickness, sharpness, bearings, motor condition, belts, lathe construction,condition of lathe bed, stand construction, and floor to name a few. There probably would be a lot initial work to secure and remove other possible sources on the equipment before moving onto actually the cutting of the blank. This should prove to be a very interesting thread, a bit of a can of worms Odie :)
 
There would be so many variables in this that makes it a quest of epic proportions. It seems everything would have a part to play in it, on top of species, wall thickness, toolrest, length of tool projection,tool thickness, sharpness, bearings, motor condition, belts, lathe construction,condition of lathe bed, stand construction, and floor to name a few. There probably would be a lot initial work to secure and remove other possible sources on the equipment before moving onto actually the cutting of the blank. This should prove to be a very interesting thread, a bit of a can of worms Odie :)

Yeah, I hear ya, Hughie......

Eliminating as much vibration as possible is only a piece of the puzzle.....but, the puzzle isn't completed until all the pieces fit together!

=o=
 
I'm surprised there isn't more discussion about the importance of eliminating any vibrations a lathe, the spinning wood, or the interaction between the two have in getting a perfect tearout-free cut.

I can tell you that I'm using my homemade vibrometer on every bowl I make ever since I first made mine in 2019. The results I'm getting now should be some indication to other turners just how valuable this can be in pursuit of that ever-elusive "perfect cut".

The perfect cut allows a bowl turner to eliminate aggressive sanding.....and when that happens, it's possible to maintain geometric integrity of the bowl. Side grain bowl turning is particularly subject to the effects of aggressive sanding because of the alternating long grain and end grain as the wood spins. The resistance to the cut is pulsating for your final finish cuts. The tearout free cut is important, because it allows the turner to accomplish many fine details that look good to the eye.

When you eliminate the harmonic vibrations (a very basic requirement for getting the perfect cut), the only thing that can hold you back is your skill level in other aspects of tool preparation and tool handling.

=o=
 
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I think in some respects its a highly involved thread and not for the faint hearted. Back in the day I was involved in sound isolation and the national standard and it proved to be quite complex although not quite the same but there are some parallels I suspect, with mechanical isolation, equipment construction ie steel versus cast iron etc. We may have to assume that lathe, stand etc is in perfect condition and that we only need to concern our selves wood, tooling and the toolrest. I might start with sharpening, on a previous thread here it was discussed various types steels for gouge tips. The testing was based on duration of the cutting edge using known wood species and what we found that the finer the grind the longer the edge lasted and we ended up with 1000grit diamond lapping disc proved to be the most durable grind. I suspect much of this depends on artisans experience and ability as well as rpm's and whats being turned ie spindle or hollow vessel and wall thickness that this might be a nirvana type quest, but lets see what turns up - pun intended :)
 
equipment construction ie steel versus cast iron
Howdy Hughie.....I'm interested to hear of your conclusions regarding steel vs cast iron.
the finer the grind the longer the edge lasted and we ended up with 1000grit diamond lapping disc proved to be the most durable grind.
I'm currently using 600gt diamond hones, and I can concur with the edge lasting longer with a finer grit to create the edge. Also of note, is the edge seems to last longer with very fine cuts vs heavy cuts.

I also feel the very tip of the cutting edge is finer when the hone forms the edge from multiple directions. This pretty much makes a statement about the quality of the edge while hand honing vs from a spinning wheel.

=o=
 
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Howdy Hughie.....I'm interested to hear of your conclusions regarding steel vs cast iron.

I'm currently using 600gt diamond hones, and I can concur with the edge lasting longer with a finer grit to create the edge. Also of note, is the edge seems to last longer with very fine cuts vs heavy cuts.

I also feel the very tip of the cutting edge is finer when the hone forms the edge from multiple directions. This pretty much makes a statement about the quality of the edge while hand honing vs from a spinning wheel.

=o=
Cast has the ability to absorb vibration due to its microstructure, which contains graphite flakes that act as natural dampeners. Steel on the other hand transmits as it carries vibrations through its structure due to its high elasticity and rigidity as a metal. Interesting though if you fabricate a tool rest with several tack welded layers of mild steel it will act like cast iron.
 
Howdy Hughie.....I'm interested to hear of your conclusions regarding steel vs cast iron.

I'm currently using 600gt diamond hones, and I can concur with the edge lasting longer with a finer grit to create the edge. Also of note, is the edge seems to last longer with very fine cuts vs heavy cuts.

I also feel the very tip of the cutting edge is finer when the hone forms the edge from multiple directions. This pretty much makes a statement about the quality of the edge while hand honing vs from a spinning wheel.

=o=
one of the reasons why many if not all professional turners will highly polish the flute. Potentially, the finer the surface the finer the edge.
 
Maybe I am missing something, but what is the "perfect cut"? I think Ulf Jansson in Sweden has the perfect cut, as he can start sanding at a very high grit, and often the finish is glossy off the tool. He does sharpen tools past 600 grit I believe. Is this necessary for all woods? A wood such as a mountain mahogany, with nearly no pores, extreme hardness, fine grain, etc. is easy to cut. With a screwdriver (which I use often) I can get a polished-looking surface with no sanding. I would then assume there would be no incentive to put much effort into sharpening, lathe speed, lathe construction, tool rest, or other factors. On the other hand, a wood-like western redcedar is nearly impossible to cut with a freshly sharpened skew. But I don't see anyone turning western redcedar, so why is this a struggle? On most woods, even softwoods like yellow cedar, I haven't noticed a problem with tear out on a freshly sharpened tool, even with fine details. What species are you using that this is a concern?

That being said, from the miniature carving I do, I have grown to notice details that everyone else who sees the final product doesn't notice, so am I not noticing what the perfect cut is? Have I been striving for a mediocre cut, without realizing that I could be achieving the "perfect cut"? Is the polished surface off the tool possible with most woods?

As someone who doesn't care for sanding (I don't really have sandpaper to sand properly), I am interested in a "perfect surface". My 10x18 lathe doesn't have variable speed, and it stays at about 2000 rpm. Is it possible that I could get a better cut by reducing the speed with certain woods? Possibly on thinner pieces? Is there more to a "perfect cut" than having a freshly sharpened tool, taking a minuscule cut extremely slowly with a perfect technique?
 
Maybe I am missing something, but what is the "perfect cut"? I think Ulf Jansson in Sweden has the perfect cut, as he can start sanding at a very high grit, and often the finish is glossy off the tool. He does sharpen tools past 600 grit I believe. Is this necessary for all woods? A wood such as a mountain mahogany, with nearly no pores, extreme hardness, fine grain, etc. is easy to cut. With a screwdriver (which I use often) I can get a polished-looking surface with no sanding. I would then assume there would be no incentive to put much effort into sharpening, lathe speed, lathe construction, tool rest, or other factors. On the other hand, a wood-like western redcedar is nearly impossible to cut with a freshly sharpened skew. But I don't see anyone turning western redcedar, so why is this a struggle? On most woods, even softwoods like yellow cedar, I haven't noticed a problem with tear out on a freshly sharpened tool, even with fine details. What species are you using that this is a concern?

That being said, from the miniature carving I do, I have grown to notice details that everyone else who sees the final product doesn't notice, so am I not noticing what the perfect cut is? Have I been striving for a mediocre cut, without realizing that I could be achieving the "perfect cut"? Is the polished surface off the tool possible with most woods?

As someone who doesn't care for sanding (I don't really have sandpaper to sand properly), I am interested in a "perfect surface". My 10x18 lathe doesn't have variable speed, and it stays at about 2000 rpm. Is it possible that I could get a better cut by reducing the speed with certain woods? Possibly on thinner pieces? Is there more to a "perfect cut" than having a freshly sharpened tool, taking a minuscule cut extremely slowly with a perfect technique?
lol more worms :) Yes species play a huge part. I have some 100 year lignum Vitea it turns like plastic and a glass finish every time. Details, yes! What I have found about details is that most folks cant see them, maybe. But if have too identical pieces one is finished to the 'nth degree we shall say the other is finished well. I have found often they will choose the 'nth degree over the other but explain why
 
Maybe I am missing something, but what is the "perfect cut"?
Good question......What I feel is the perfect cut, is a surface straight from the tool, without tearout, that can be start-sanded at 180/240gt, or higher.
That being said, from the miniature carving I do, I have grown to notice details
The "details" I speak of are those intersecting edges and grooves that are done while spinning on the lathe......not carving. As previously stated, the quality of these details can shift dramatically once the geometric integrity is altered. This "geometric integrity" can be further understood when considering perfect concentricity within multiple perfect circles
My 10x18 lathe doesn't have variable speed, and it stays at about 2000 rpm. Is it possible that I could get a better cut by reducing the speed with certain woods?
Absolutely!......My 33-year-old Australian made Woodfast lathe originally wasn't a variable speed....had to change belts for quite a few years before I converted it to variable speed. After converting it to variable speed, one thing I noticed almost immediately, is the vibration resulting from the combination of wood and lathe could vary with small rpm adjustments.....much more pronounced with major rpm adjustments. By these rpm adjustments, the quality of the tool finish was also a variable. This tool finish variable is directly related to fine tuning of rpm in order to minimize any vibration. The rpm can change with species of wood, size, and denseness......but, the lathe vibration at any given rpm is static, until the block of wood of the moment enters the equation.

=o=
 
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Cast has the ability to absorb vibration due to its microstructure, which contains graphite flakes that act as natural dampeners. Steel on the other hand transmits as it carries vibrations through its structure due to its high elasticity and rigidity as a metal. Interesting though if you fabricate a tool rest with several tack welded layers of mild steel it will act like cast iron.

Hughie.....I have heard similar explanations concerning the value of cast iron vs steel from other sources. I tend to think there is something to the notion that cast iron is a better material for wood lathe construction, but I have no personal experience to validate the concept.

=o=
 
If you could convert your vibration sound into an audible harmonic or a visible sine wave that should do. I just use my fingertips and the feel of the tool against the wood.
Kim, that could very well be the case. Whether it is a better method of determining the vibration level than my homemade vibrometer is a question unanswered. I'd say it is about 15' the laser beam transmits to the opposite wall, and thus makes even the smallest rpm adjustments very apparent.....and very applicable to my purpose in searching for the best rpm, and therefore the perfect cut......

=o=
 
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Good question......What I feel is the perfect cut, is a surface straight from the tool, without tearout, that can be start-sanded at 180/240 or higher.
Slightly off of the original topic of this thread, but I’m interested in your thoughts on something I’ve observed.

All else being equal, I find that using a shear scrape or negative rake as the final pass makes initial sanding easier. If my last pass is a clean push cut it leaves a nicer looking surface than the shear scrape, but I think that surface is burnished a little and it takes longer for my initial sanding to ‘bite’ or often I drop down a grade for that initial sanding just to cut quickly.

Stated more succinctly: for the same surface (no tear out) a final push cut looks nicer than a shear scrape but requires me to start a grit lower to sand than the shear scrape. Does this make sense?
 
Slightly off of the original topic of this thread, but I’m interested in your thoughts on something I’ve observed.

All else being equal, I find that using a shear scrape or negative rake as the final pass makes initial sanding easier. If my last pass is a clean push cut it leaves a nicer looking surface than the shear scrape, but I think that surface is burnished a little and it takes longer for my initial sanding to ‘bite’ or often I drop down a grade for that initial sanding just to cut quickly.

Stated more succinctly: for the same surface (no tear out) a final push cut looks nicer than a shear scrape but requires me to start a grit lower to sand than the shear scrape. Does this make sense?
Very interesting, Ron......

My experiences are similar, but not exactly the same.

Let me contemplate this before I respond in depth.

Thanks for bringing this up.....

=o=
 
Slightly off of the original topic of this thread, but I’m interested in your thoughts on something I’ve observed.

All else being equal, I find that using a shear scrape or negative rake as the final pass makes initial sanding easier. If my last pass is a clean push cut it leaves a nicer looking surface than the shear scrape, but I think that surface is burnished a little and it takes longer for my initial sanding to ‘bite’ or often I drop down a grade for that initial sanding just to cut quickly.

Stated more succinctly: for the same surface (no tear out) a final push cut looks nicer than a shear scrape but requires me to start a grit lower to sand than the shear scrape. Does this make sense?
Sorry for the delay, Ron......had other things going on.

For my bowls, I hardly ever use NR scrapers on the exterior, but do extensively use them for finish cuts on my interiors. For finishing cuts on exteriors, I've come to rely on manually raised burs on scrapers used in the shear mode. (For exteriors generally, I use scrapers flat to the tool rest to bring a roughed bowl to round. I then transition to gouges for general shaping and then go to the scraper in a shear mode for the final cut just prior to sanding.)

For interiors, I use NR scrapers quite a bit for a final sand-able surface, and whether it's a push or pull cut depends on the bowl shape and species of the moment. As the cut proceeds closer to the exact center, where the speed of the cut slows down considerably, there seems to be a point where a push cut is better controlled by reversing the NRS direction to a pull cut. There doesn't seem to be a hard fast rule about what direction is best, but listening to the cutting action tells me when it's time for a change in direction. On the side walls of the bowl interior, the best cut seems to generally be a push cut, but there is always exceptions.......for instance, my extreme undercut rims where directions can change, as well as NR vs standard scrapers. (For initial shaping of the interior, I generally rely on gouges.....prior to transitioning to NR scrapers.)

Note: On interiors, if I could get my tool rest closer to the cut without a long overhang, I could possibly use scrapers in a shear mode much more than I do.......and, that seems to be the reason why I don't very often. :)

=o=
 
Thanks Odie.

It sounds like your process doesn’t often (ever) end with a gouge push cut (floating the bevel) which I think is where I’ve notice a slight burnishing that makes sanding take longer. I’ve really mostly noticed this on the exterior of bowls.

FWIW - my process on the exterior usually ends with a shear scrape using the gouge I turned the exterior with, lately I’ve added a negative rake (flat on tool rest) over the exterior often.

On the interior I used to similarly use the gouge upside down to shear scrape before sanding, but a “surprise” doing this at the transition a few years ago where I got too much edge involved made me a little more nervous about this. I still do this for shaping, but virtually all of the time now I finish with a negative rake, whichever direction is cutting better (usually push down the side to the transition, and pull across the bottom into the transition). As often as not I use a nearly flat regular scraper in the dead center area, just because it does better than the very rounded negative rake that I have.

After adopting the negative rake I seldom shear scrape with normal scrapers anymore, some but not often.

I grind the wings on my gouges with pronounced convex wings which allows me to easily use them to shear scrape.

More info than you wanted, but presented in case you have some advice to offer :)
 
It cannot be stressed how much rpm effects the cleanest of cuts, and rpm must be adjusted precisely.
Think about how as a young kid I was able to get the cleanest of cuts barely needing very fine sanding on my dad's old metal lathe with a max of only 500 rpm on the spindle. Two words come to mind for me.
 
Hughie.....I have heard similar explanations concerning the value of cast iron vs steel from other sources. I tend to think there is something to the notion that cast iron is a better material for wood lathe construction, but I have no personal experience to validate the concept.

=o=
The theory is cast iron will absorb the vibration and if we look at metal turning lathes are all cast iron for the same reason as well as the mass.
 
Some of the species I turn have problems with scraping. Some require the razor sharp shear cut with a burr, others only negative angle will work , it sort of horses for courses. I have some curly Blackwood [ Acacia Melanoxylon] that requires turning the external from each end with razor sharp tools and some scraping in the centre to get a good finish, inside is a whole different kettle of fish.
 
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