Here's a general question for the forum to ponder.....

[odie]

What is more likely to result in the best tooled surface quality......IF it's a given the tool is sharp, and there is good tool choice, presentation, and technique..........?

A) The best vibration free spindle rpm.

B) The best speed you would choose, if the rpm were not factored in.

Enjoy the discussion.....

-----odie-----

 

[Emiliano Achaval]

I had to read your post several times to understand it, blame it on my spanglish... For the best cut you need the most speed within reason and obviously no vibration. I'm still trying to figured out choice B... Isn't speed and RPM's the same?
Its simple, crank up the speed until your lathe starts walking, chase her around the shop, dial it down till she stops, then with a sharp tool take your best cut.

 

[john lucas]

I think when we talk about speed it's really more about taking a small bite per revolution. That's the biggest difference between running the lathe fast and running it slow. When you run slower it's easy to take a deeper cut. I have done a test where I purposely ran the lathe very slow and tried to take the same depth of cut I did when it was running faster. I basically got the same cut. What is different to me is being able to move your body in a smoother arc when you do it more quickly. So running at higher speeds does 2 things for me. It lets me get a better arc and with light cuts you do get a cleaner look. I think someone said it the other day. You get a screw thread with every cut. When turning slow you tend to get a deeper thread with fewer threads per inch. When turning faster you get a shallower thread with many per inch. That's an exageration of course because when turning faster the threads are so close you can't really see them. Now that's assuming you aren't forcing the tool. If you push the tool obviously you get fewer threads per inch and at the same time you get more tearout. Of course safety is the most important issue.

 

[Fred Belknap]

The larger the bowl the greater the rim speed will be. I also do a lot of bowls that are similar to the one in my avatar, so the speed has to be reduced to get a vibration free spin. To get a good cut on the wings is rather difficult as the shape rules out the use of a steady rest. I seem to get the smoothest cut on the wings with the highest vibration speed and using a thick scraper with a negative rake grind on the top side. On smaller NE bowls I can get a rpm around 1000 which lets me use a gouge more effectively. A large gouge and really sharp seems to work pretty good. I have a Glaser 5/8" gouge that I like for doing this.

 

[odie]

I had to read your post several times to understand it, blame it on my spanglish... For the best cut you need the most speed within reason and obviously no vibration. I'm still trying to figured out choice B... Isn't speed and RPM's the same?
Its simple, crank up the speed until your lathe starts walking, chase her around the shop, dial it down till she stops, then with a sharp tool take your best cut.

Well......just blame it on my coming up with this thread just before hitting the sack. I can remember thinking I needed to tweak the question some, but left it as is.

Yes, I was considering speed and rpm to be the same thing, but John Lucas does bring up some interesting aspects to the question......being that the amount of cut per revolution needs to be factored into the discussion.

I basically do the same thing as you do, but since I have so many things on my headstock that vibrate subject to different harmonic input, I do feel it's an advantage that allows me to fine tune the rpm to a greater extent than many turners who are looking for the lathe to "walk". Now, just how far you can "fine tune" is a question for me. Is there a point where fine tuning for vibration does not result in a better cut? I'm asking myself that because the act of the tool cutting the wood is going to be the source of some vibration, no matter what the rpm is.

I think when we talk about speed it's really more about taking a small bite per revolution. That's the biggest difference between running the lathe fast and running it slow. When you run slower it's easy to take a deeper cut. I have done a test where I purposely ran the lathe very slow and tried to take the same depth of cut I did when it was running faster. I basically got the same cut. What is different to me is being able to move your body in a smoother arc when you do it more quickly. So running at higher speeds does 2 things for me. It lets me get a better arc and with light cuts you do get a cleaner look. I think someone said it the other day. You get a screw thread with every cut. When turning slow you tend to get a deeper thread with fewer threads per inch. When turning faster you get a shallower thread with many per inch. That's an exageration of course because when turning faster the threads are so close you can't really see them. Now that's assuming you aren't forcing the tool. If you push the tool obviously you get fewer threads per inch and at the same time you get more tearout. Of course safety is the most important issue.

John, you make a very good point about how rpm necessitates cutting less per revolution......and, ultimately will result in a better cut.......but, I feel the better point you're making, is that higher rpm does allow the flow of the tool to be a little faster, and it definitely does result in a better allowable body movement. There we are speaking about the relationship between the interaction of your body and how "the dance" effects the level of smoothness in the final results. This is an extremely relevant point to this discussion.

-----odie-----

 

[odie]

The larger the bowl the greater the rim speed will be. I also do a lot of bowls that are similar to the one in my avatar, so the speed has to be reduced to get a vibration free spin. To get a good cut on the wings is rather difficult as the shape rules out the use of a steady rest. I seem to get the smoothest cut on the wings with the highest vibration speed and using a thick scraper with a negative rake grind on the top side. On smaller NE bowls I can get a rpm around 1000 which lets me use a gouge more effectively. A large gouge and really sharp seems to work pretty good. I have a Glaser 5/8" gouge that I like for doing this.

Fred.....lately, I've been experimenting with a 3/4" gouge. Even for smaller bowls, it does seem like the added weight and heft of the tool adds to more resistance to vibrating.......so, even where taking smaller/lighter cuts is a paradigm, the added weight can (but not always) be a factor in arriving at that one best cut possible.

-----odie-----

 

[odie]

Even if all the variables with rpm, vibration, tool selection, sharpness, technique, body movement, etc., are perfect.......the act of turning will result in the source of some amount of vibration. This will be different between spindle turning and bowl turning. The main difference here is grain orientation......specifically as it applies to my own turning. The fact that the grain alternates between end grain and long grain is a source of vibration in itself. This alternating of grain orientation means there is less resistance to the cut with the long grain, than there is on the end grain. No matter how well you can hang on to your tool, there will be an alternating difference in the residual pressure applied to the tool directly from the wood being cut. It just isn't humanly possible to hold your tool as steady, and function as smoothly as a tool being manipulated in a metal cutting lathe.

Because we wood turners have to deal with issues that are irrelevant in metal cutting, the importance of understanding all the interacting aspects and influences are important determining factors in just what level of performance we can, and expect of ourselves to achieve.

-----odie-----

 

[Doug Rasmussen]

Along with the discussion of vibration versus rpm, I wonder if any of you have had luck weighting your lathes as a vibration dampener. I'm not talking about adding a couple 60 pound bags of sand. More like many hundreds of pounds.

I remember a local metal spinning shop made six foot diameter spinning forms on a Wadkin face plate lathe (no bed or tailstock) with a weight of probably several tons well anchored to the cement floor. Vibration was not such a consideration because of the machine's mass. In comparison a PM 3520 has a listed shipping weight of 682 pounds making it a light weight machine in comparison.

Back in the day it seemed a good many commercial turners were using converted metal lathes for columns where the machines were massive enough to overcome unbalanced workpieces. One turner I knew built his bowl lathe using a diesel engine block (pistons removed) as the headstock and the flywheel as a mount for the work.

A limitation on a good many lathes in adding weight might be whether the headstock bearings are able to handle unbalanced loads over time.

 

[robo hippy]

Still waiting for the coffee to kick in.... Part of the vibration issue when turning, bowls or spindles, is the bevel rub. Still trying to master this one, but "The bevel should rub the wood, but the wood should not know it." As I told Ashley Harwood after a demo on her sea urchin ornaments, "I have to go home and practice my dainty skills." She was doing cuts on long finials without her finger on the back side as a steady rest. This is part of the reason why I use a shear scrape to clean up inside and outside of the bowl, and the mating surfaces of my recess. It takes the bounce out. Other than that, shear angle plays a big role. Most of the time, with both gouges, and the skew, the shear angle is around 45 degrees. I go up to 70 or so for 'difficult' pieces.

No real idea if the rpm makes any big difference other than at slow speeds, you cut more slowly, and at high speeds, you cut faster. Well, maybe you remove more wood at a faster rate at high speeds.

robo hippy

 

[Charlie Knighton]

what about how tight the grain and punkyness....ie splat??????? They must have an effect on the cut.

 

[john lucas]

If you want to reduce vibration of the lathe one of the best ways is to increase the lathe footprint. I have a friend who did that using 4" angle iron. He went to the extreme and used 6 foot pieces but then he was turning hollow forms off center and had a hole in the side. He had to have no vibration at all because you could see the inside and there was no way to sand a vessel like that.

 

[odie]

what about how tight the grain and punkyness....ie splat??????? They must have an effect on the cut.

You're right, Charlie.......I've found that each piece of wood is an "individual" with a unique personality all of it's own. Even seemingly identical pieces of wood from the same species can turn differently, simply because Mother Nature doesn't always create trees equally!

-----odie-----