turning techniques
#11
I am new to turning and very new to turning bowls. I have made 4 bowls 3 of which are segmented in the past month. The latest one is purple heart with the top ring out of walnut. I have noticed it seems to be a lot harder then the maple and mahogany one I made before it. My question is I bought a few carbide tools and 2 bowl gouges. The carbide tools work awesome getting the bowl to shape quickly but on the inside I seem to get a lot of catches. So I have a few questions. What speed do you guys normally turn at for roughing and finishing? I feel like I am not turning fast enough when finishing. Can turning to slow increase the odds of a catch? Are carbide tools more prone to catching? Thanks
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#12
Rather than use scrapers to work your surfaces, I'd recommend switching to bowl gouges. You'll find, with proper technique, that catches will rarely happen.

You can also buy a curved tool rest to get the tool overhang to a minimum so that the reaction load at your hand will be minimal when using scrapers.

IIRC, the rule of thumb is RPM x diameter = 6000 - 9000. Even at those speeds you can have catches depending on the wood used, but, in general, it's a good range to be in for any tool whether you are roughing or finishing.

IME, you are more likely to get a catch with scrapers, particularly for internal work, regardless of the tool material. I suspect it is more a function of the tool geometry.
Cellulose runs through my veins!
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#13
The most common exotics used in segmenting purpleheart, bloodwood, yellowheart, etc, are harder than the domestics. The other issue you will face that most turners don't is all the glue lines you are cutting. That alone will require more sharpening of HSS tools. I'm assuming your carbide tools are used on a scraper. For the inside you want the cutter at the center line and the handle elevated. If the bar the cutter attaches to has a rounded bottom try rotating the handle slightly for a shear scraping effect.
Speed would be determined by the size of the piece and the holding method. But, with that said theirs nothing wrong with slower speeds. My turnings rarely see anything over 750 rpm, the big ones never above 450.
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#14
What size motor does your lathe have? I found I got a lot of catches when turning bowls on my mini-lathe. The motor just couldn't power through some of the cuts. I upgraded to a full-size lathe with a 1-1/2 HP motor and that helped tremendously. Use the speed formula cited above as well. The momentum of the bowl will help power through some of those cuts. Doesn't mean you can take off more wood - just means you can get better cuts. You are turning dry wood - not green wood, so you will need to take light cuts and be patient. Catches often happen when one tries to take more than the tool/bowl/lathe will allow.
Still Learning,

Allan Hill
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#15
Thanks for the tips I have a curved tool rest it isn't long enough to get to the bottom of the bowl though. As far as speed goes I was in that range it's probably due to user error. Probably trying to take to much off. I noticed switching to the round cutter was easier on the inside the n the square cutter. The lathe is a 1hp and I was turning a 12" bowl 5-6" deep I think I had enough power. I would like to upgrade to a 2hp or 3hp whatever I have in the rathole that will fit. It is a Powermatic 90 lathe that will have steel riser blocks by the end of the week to give it a 20" swing. I am having a lot of fun messing around with it though. It's been a nice to get out in the shop a lot lately. I have been using bush oil to finish everything but I am almost out. What do you guys like to use?
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#16
In my opinion, catches are almost always due to technique. There's nothing wrong with using scrapers, but I prefer gouges, especially for the finishing cuts. We all had LOTS of catches starting out, particularly on the insides of bowls. I had so many I felt a little scared every time I had to turn the inside of a bowl. My advice is to study and practice. Understanding the mechanics of the tool and the cut goes a long way towards minimizing catches.

As for speed, I don't think it's possible to prescribe one speed. My first lathe had a minimum speed of 600 rpm, so that's what I learned on. Now that I'm more comfortable, I prefer higher speeds. There are a lot of factors that go into determining speed, though. I'd start out at a lower speed until you feel comfortable making your cuts. Then you can try speeding things up safely to experiment.
I'm slow, but I do poor work.
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#17
Somebody already mentioned this but its worth repeating - always keep your scraper handles a little above horizontal so the tool is aimed slightly downward. On bowl interiors, the scraper cutting should happen at the center line of the work or even a hair above. If you go below, you're looking at a catch because the wood is now moving toward the cutter rather than across it like it would be if cutting above the center line. What used to get me on this is that I'd start with the tool horizontal and the cutter at the center line, then while cutting I'd raise the handle a little and that would drop the cutter below center. Then the bang.
I find videos really helpful for getting an understanding of how this stuff works. Richard Raffan's bowl video is excellent and I'm sure you can find other good ones online. Good luck.
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#18
Tilting your tool very much depends on the geometry of your cutter. If you are using Easy Wood tools scrapers, then the preferred method is to keep the tool level when turning. The video below is from Easy Wood Tools. If you have a different tool or handle, some of the same principles might apply (cutter contacting along the center line), but you'll need to figure out what's best. For example, I bought a Hunter carbide tool awhile back and it had catches like crazy. The cutter on a Hunter tool is WAY different than EZ Wood's cutters. After watching a video produced by Hunter, I understood why I was getting so many catches. It shears. It doesn't scrape.

https://www.youtube.com/watch?v=LsRrfl0gvSg#t=130
Still Learning,

Allan Hill
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#19
AHill said:


It shears. It doesn't scrape.




Scraping is shearing. It's a common misunderstanding of mechanics. Removing material with any tool at any angle is always cutting by shear.
Cellulose runs through my veins!
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#20
Steve, I think I will disagree with you on that. Especially in woodturning, a scraping cut is one with the cutting edge at 90 degrees to the spin/rotation of the wood. So, a scraper flat on the tool rest, or a gouge with the flutes rolled over on the side (3 or 9 o'clock), and the handle level. The 'shear' comes in when the cutting edge is at an angle to the rotation. This would be a scraper on its edge rather than flat, most bevel rubbing cuts with gouges. A shear scrape is another term, that doesn't make sense to me since it is usually referring to any 'non bevel rubbing' cut, and called a 'scrape' just because the bevel is not rubbing. Don't know what else to call it though. So, scraping cuts, while 'technically' a shear cut, like on a punch press, isn't considered a shear cut in woodworking.

Shear angles are far better at getting a clean cut than scraping cuts, except in end grain turnings because the shear does a better job of gently lifting the fibers as you cut. A scraping cut tends to pull more at the fiber rather than slice.

robo hippy
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