The post below is a summery I made from Stuart Batty's catalogue which may be helpful
http://www.dmwoodturners.com/?page_id=642
Overhang ratio
To ensure safe, comfortable control of a tool while cutting or scraping we recommended using the following ratios:
CONVENTIONAL SCRAPERS 7:1 GOUGES and SKEW CHISELS 5:1 PARTING TOOLS and BEDANS 5:1 NEGATIVE RAKE SCRAPERS 3:1
For every inch of blade overhanging the tool rest there should be at least the above ratio used when cutting or scraping on a lathe. This will ensure you have the correct leverage and help you stay in control. The ratio can be achieved using a combination of handle and any remaining blade still on the handle side of the tool rest. However, if you hold the handle towards the front you will affect the ratio and leverage
Negative Rake Scraping works best on medium density temperate woods and the densest exotic woods
Simply grinding the lower bevel pushes some of the metal up on to the top bevel creating a burr. It should be produced with a coarse wheel or belt, not with a burnisher. A sturdy grinding platform is strongly recommended so that you can create a burr at the same angle each time to get the longest life from your blade
The burr has a short life and will last approximately 90 seconds for each 1/2” width of blade in contact when using CPM 10V®, M2 only lasts about 20 seconds
Negative rake scrapping is an exceptional technique for working very thin walled pieces or broken surfaces, like square bowls or natural edge
It can produce an unmatched finish on end grain or around the mixed grain surface of a side grain bowl.
When cutting at the very centre of a box or bowl use caution not to cut past center.
Conventional Scraper
Ready to use out of the box with 70-degree included angle with burr
There are three main ways to sharpen scrapers: • Conventional Scraper with a burr • Conventional Scraper with the burr honed off • Negative Rake with a burr
I recommend creating a burr on a bench grinder. A coarse wheel creates a bigger burr than a fine wheel and, in most cases, this is an advantage. A bigger burr will last longer. I don’t recommend creating a burr with a burnisher. Burnishers don’t actually create a burr in the same way as a grinding wheel does; they tend to curl the edge over. This can make the scraper too aggressive.
Wood types:
• Softwoods – pine, cedar, spruce, alder or any wood with a Specific Gravity below 0.6
•Medium density or temperate hardwoods – ash, oak, walnut, hard maple or woods with an SG of 0.6-0.8
• Dense and/or exotic woods – ebony, boxwood, rosewood or any wood with a Specific Gravity above 0.8
Main grain types: • Side grain • End grain • Mixed grain – side grain bowls
There are many other grain types like burls, crotch figure, quilted and fiddleback; these should be treated as end grain
Leverage: It is critical when using Conventional Scrapers to ensure you have a long enough handle to cope with the amount of blade overhang from the tool rest.
Conventional Scrapers require a minimum ratio of 7:1.
Negative Rake Scrapers only require a ratio of 3:1 because they do not self-feed.
Conventional Scraper Angles
When using Conventional Scrapers it is critical to set the angle to suit the wood density and grain type. You also have the option to use these scrapers with or without a burr.
60° included angle is suited only to softwoods; this angle, especially with a burr, is too aggressive for temperate or dense hardwoods.
70° is the optimum included angle for medium density woods. Use with a burr for side grain but hone the burr off for end or mixed grain.
80° included angle allows conventional scrapers to work on dense and exotic hardwoods; the burr will need to be honed off for these types of woods.
Negative rake also requires a burr to be effective; once it has worn off the tool must be reground to create the burr again. A 40° included angle will create a bigger burr and last longer than a 70° included angle. A 90° or greater included angle cannot create a burr and is therefore not a Negative Rake Scraper. Negative rake also requires a burr to be effective; once it has worn off the tool must be reground to create the burr again.
To scrape softwoods a scraper needs a burr; without one it will tear the softwood fibers. Remember that there are three main densities of wood (softwoods, temperate hardwoods and dense exotics) and three main grain types (side grain, end grain and mixed grain/bowl blank). It’s important when working with a scraper to be able to change the way it functions in order that it best suits the wood and grain types you’re working with.
Steel blades can work easily with each of the three wood densities and the three grain types
Medium dense woods, like oak or ash can, be scraped with a burr on side grain. The burr will require honing off when working on end and mixed grain or it will be too aggressive
Medium and dense hardwoods work exceptionally well with Negative Rake Scrapers. The negative rake technique has been used by ivory and blackwood turners since the 1500’s. A Conventional Scraper—even with the burr honed off— will always be aggressive on dense woods. Scraping into end grain will more often than not cause the scraper to grab at the wood
A Conventional Scraper is best used at the very base of a side grain bowl. You wouldn’t use a Conventional Scraper at the bottom of an end grain bowl. Conventional Scrapers do not cut past center, making it easier to smooth through the very center of the bowl.
Negative Rake Scrapers cut both ways up and past center and therefore shouldn’t be used in the very center of a bowl if visibility is limited
Our Spindle Gouges are supplied with a 40° bevel and a 30° wing angle. The wing is ground straight giving a true included angle of 70°.
This is the optimum grind for a spindle gouge, helping with sharp entry cuts and making it proficient at coves and beads.
The bevel is ground on an 8” diameter CBN wheel and the angle was set with our Angle Gauge to ensure a 100% accurate edge angle.
All our gouges are supplied with straight wings, not convex or concave. This makes the gouge more proficient at entry cuts and able to produce better detail, as well as cut tenons and into corners without the wing damaging the surface ahead
Our “U” Flute Bowl Gouges are supplied with a 60° bevel and an 85° wing.
“U” Fluted Bowl Gouges are also excellent at cutting the face and shallow curves on platters. Its higher cutting edge angle of 60° helps when cutting platters made from highly figured grains, such as fiddleback and quilted maple.
The bevel angle on these gouges can be changed to suit the diameter and depth of your bowl. It can be as low as a 50° bevel (for very open shallow bowls) and up to 70° (for very deep narrow bowls.) The wing angle should remain a consistent 85° no matter which bevel angle is being used.
Skew chisel Ready to use out of the box with hollow ground 40° bevel
Parting tool 50° included angle
Beading and parting tool 40° included angle
Beading/Parting Tools, Bedans and Parting Tools
Parting Tools should have a minimum 4:1 overhang ratio.
Blades are supplied sharpened ready to use out of the box. All blades are sharpened on an 8” CBN wheel with hollow bevels. To resharpen these blades we recommend using our Angle Gauges to set and repeat the same angle; the angle gauges compensate for the curvature in the wheel.
Parting Tools are supplied with a 50° degree hollow ground included angle. When parting in the blade should be presented parallel to the floor so as not to lift the wood fibers. This is technically scraping, and therefore the 50° included angle edge will last longer than 40°. Once the blade has parted through the first fibers the handle can then be lowered to peel through the wood fibers rather than scrape when used on side grain. Never point a parting tool uphill on end grain or mixed grain.
Beading/Parting Tools and Bedan Series are supplied with the typical 40° cutting angle. Although any of these tools can be used as a parting tool they are best used to cut beads and V cuts.
Parallel side parting tools are much more effective, easier to sharpen and give a superior finish. They can always cut at least their own width and when newly sharpened they part in without binding in one pass, generating very little heat and do not requiring a double cut to avoid binding