Hollow form question?

[Reed Gray]

I have always been amazed at some of the carved hollow forms, and how smooth and clean they are on the inside. I have always wondered at the skill level that it takes to tool it smooth enough so you don't have to spend a week hand sanding it, but still just can't figure out the secret, well other than '10,000 more times'... So, I am wondering if any one used the tear drop scrapers with a NRS (negative rake scraper) grind? I have been doing a few end grain pieces, and using a 70/25 grind NRS, and it works excellently. I keep thinking about applying that to one of the tear drop scrapers, but maybe a 30/30 grind. True, the NRS burr off the grinder doesn't last too long, but I have been playing around with Eric Loffstrom's idea of using about a 1/8 inch carbide rod in a handle and burnishing a burr on my NRS. As long as I don't over burnish, it seems to cut almost as cleanly, and lasts a lot longer. Nope, haven't tried it on a tear drop scraper yet, if I can even find mine.... Just wondering..

robo hippy

[John K Jordan]

I use a teardrop scraper inside a closed form but don't see a need for the negative rake - I simply hold it a an angle. I have two sizes, both made by Sorby. Adding a negative rake might be useful if the tearddrop was not angled but held firmly horizontal.

I don't know what kind of "carved hollow forms" you are thinking about (carved on the outside?) but those hollowed through holes so small you can't reach in and feel the surface tend to be pretty rough inside. If the opening is large enough to reach a finger well inside, I do make it extremely smooth but not from a week of hand sanding - the teardrop scraper can leave a beautiful surface that doesn't require a lot of sanding. I hold the scrapers firmly but in very light contact with the inside surface, a little above center, most often on the Sorby swan neck tool.

BTW, I think we talked about this before: for almost 15 years I've used a carbide burnisher on all scrapers, from NRS to conventional to card and cabinet scrapers. The carbide rods I have are 1/4" or so in diameter, mounted in wooden handles. I grind, remove any burr, then burnish. It doesn't take much burnishing or much pressure. This smooth "burr" is more durable than the one from the grinder.

JKJ

[Adam Petersen]

That's just great. Reed Gray doesn't know how to get the inside of a hollow form smooth. What hope I had to someday do so just went out the window...

Adam

[john taliaferro]

I use a belt sander on an extension .

[Alan Trout]

Reed,

I have used negative rake scrapers on the interior of pieces and it helps and it is of course less grabby. My pieces are very smooth and have finish on the interior, and yes it always involves sanding. I don't burnish but have tried it and it to me does not seem to make a huge difference. If I am charging big bucks for a piece 3 or 4 hours of sanding is not going to break the bank and gives me the level of quality I feel the client deserves.

Alan

[John Hart]

I make all my own hollowing tools (mainly because I'm cheap ) But I've found that it is control that is my greatest friend to achieve a smooth wall.
this scraper is about 40 inches long with a teardrop head that I made out of the mainshaft of a laser printer. Nice steel but I don't know the metallurgy.
The length of the tool creates a high control zero bounce environment and I can just feel my way.

HollowingStick.jpg

[Reed Gray]

The carved hollow forms I am talking about are the ones that look like fingers and spiral up, so as much negative space as positive space. I do like shear scraping as it leaves a cleaner surface on flat/bowl grain than a NRS, but most of the time I have to use a pull cut on it, and if I want to push with it, I have to reverse the angle. So on a hollow form, to cut with the grain, you would have to angle it one way for part of the cut and then angle it the other way for going the other direction. With a NRS, you could push or pull, with no changing of the angle, since it sits flat. The tear drop shape also is better for smoothing out the little bumps and waves than a shear scrape. So, without having tried it, I am wondering about this approach for this tool.... Dangerous thing thinking....

As for the burrs, and this is still in experimental stages, the burnished burr on a NRS does last considerably longer than the grinder burr. I think I like it better on the 25/70 NRS, and still experimenting with the 30/30 NRS. Main thing it to make sure you don't 'over burnish' so the hook curls over like a breaking wave. I generally don't hone off the grinder burr, but turn it back and forth a couple of times. The burnished burr can be burnished down, then back up a couple of times before I need to go back to the grinder. I need to work with the 600 grit CBN wheels, and the 1000 grit CBN wheels and the burrs from them, both straight from the grinder and burnished.

I may have to play around more with the burnished burr on my scrapers. For roughing work on bowls, I don't know if it will be any advantage since it is rough work. There is a huge difference in how a fine burr works for shear scraping... Never forget my first AAW Symposium up in Portland. A bowl turner up there stated there was no way you could hand burnish a burr on HSS because HSS was too hard..... Even a standard burnishing rod for card scrapers works, but I prefer the triangle ones, and the 1/4 inch is a size I don't have. I may have to convert one of my dull carbide router bits..... More toys!!!

If it ain't broke, take it apart and fix it anyway....

robo hippy

[John Sincerbeaux]

I have been turning and selling large HF's for about three years now. In my (short time) turning experience, it doesn't matter what the inside looks or feels like. I don't try to sell my vessels to wood turners. I sell vessels to people who are looking for a piece of art work/sculpture for their beach home in Hawaii or their mountain home in a resort. This clientele does not care or even look inside a vessel.
I certainly care what the inside looks and feels like but I view it differently than most wood turners. I bet I don't spend 30 minutes on finishing the inside. I use a scraper then a sanding mandrel. I sand the inside lip as far as my fingers can reach. I then pour epoxy in the vessel and coat the entire inside. I do this for structural and a "balance" to the external finish which is also a "resin" based. Where I spend a very short time inside the vessel, I make up in time tenfold on the outside.
Finally, even if someone was interested in buying a vessel and actually tried to get some light inside and see what they could.... I believe tooling marks on the inside is actually a positive thing. To me, the contrast from a glass like outside surface to the dark, inside that gives a hint of just how that vessel was produced is a very positive thing.

[Mike Nathal]

Charles Farrar, a pro turner from North Carolina, demonstrated for us in Cleveland last year. For his final cuts he used a tear drop scraper with a negative rake, held horizontal.

[Peter Blair]

Reed, I'm not sure if this is helpful but yesterday i turned a Hollow Form from Silver Maple the wood is still mostly wet. After hollowing with my captured arm system (Kobra) I was scraping just to get the bumps out, I mostly agree with John S above but like the inside sort of smoothed but not sanded much. I use a tear-drop shape on both a cranked arm or a modified one I insert into my Kobra. It was a touch grabby and I did ponder if I should modify it as a negative rake. My issue would be that the cutter would then be below center. I suppose I could raise it with a washer which I plan to try in the next day to two. Incidentally I was working about 9" deep and control is at least for me a little difficult at that length for me.
I'm not sure what angles to try and the scrapers does not extend too far past my arm and as a result I don't have much steel to play with.

[Reed Gray]

I am still playing around with the NRS. I have a couple that are 70 on the face, and 25 degrees on the top. If you have a spare tear drop scraper, just grind a 25 or so bevel on the top, and then burnish. I have tried the tear drop scrapers inside forms and found them to be grabby. I probably need to get a bunch of tear drop scrapers and try it out. I don't like my scrapers to be at dead center height. They just don't 'feel' right. Also, by the time I get a 5 or so inches off the tool rest, just like with the McNaughton, the pressure puts them below center. Only place I need dead center is at the bottom dead center of the form. If I am shear scraping that part, I can be off a little to either side.

robo hippy