Negative Rake - Downsides on wood?

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EricRN

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Hi everyone. I'm curious if there are any downsides to using negative rake carbide cutters (or a regular negative rake scraper for that matter) on wood? I know they are far superior and less likely to catch for acrylics and plastics, at least for someone of my abilities. But I was wondering if there are any downsides to using them on wood. In other words, why not simply use a negative rake cutter all the time and not just on acrylics?

From what I can tell, I see two downsides. Many negative rake cutters have a radius edge, meaning they are not perfectly square at the corner. So, if you're looking for a perfect right angle, you won't be able to get it with a negative rake cutter. Second, negative rake cutters make a less aggressive cut. In theory, that means your turning will go slower and take longer. But in practice, I don't really notice much of a difference.

Anything else I'm missing? Any reason to use a regular rake cutter on wood other than what I'd flagged above?
 
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Lots of woodworkers swear by negative rake scrapers, as they provide a much smoother cut and often are less agressive. The key is the actual angle of the negative rake versus the cutting edge. Finding the sweet spot is a matter of taste because speed of cut and smoothness of cut surface seem to be opposed when talking negative rake changes. Sharpening a negative rake on a chisel versus using new carbide cutters allows you to set the angles, something nearly impossible to do on purchased inserts.

I like negative rake carbide cutters when turning alumilite or other acrylics because they seem to consistently deliver less chipout. However with wood, I don't use carbide cutters as frequently as I do classic chisels, because I prefer to do the sharpening myself (setting my own edge and sweep angles) and you can't get any carbide insert that can be shaped like a bowl or spindle gouge - the carbide tools are limited to scraping edges. You can fake it with a circular carbide cutter, but cost goes up fast due to the frequency of blade changes.
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EricRN said:
Hi everyone. I'm curious if there are any downsides to using negative rake carbide cutters (or a regular negative rake scraper for that matter) on wood? I know they are far superior and less likely to catch for acrylics and plastics, at least for someone of my abilities. But I was wondering if there are any downsides to using them on wood. In other words, why not simply use a negative rake cutter all the time and not just on acrylics?

From what I can tell, I see two downsides. Many negative rake cutters have a radius edge, meaning they are not perfectly square at the corner. So, if you're looking for a perfect right angle, you won't be able to get it with a negative rake cutter. Second, negative rake cutters make a less aggressive cut. In theory, that means your turning will go slower and take longer. But in practice, I don't really notice much of a difference.

Anything else I'm missing? Any reason to use a regular rake cutter on wood other than what I'd flagged above?
Click to expand...
I've been watching a a lot of bowl turners lately and the ones turning wood seem like the negative rake scraper. Seems they produce a smooth finish which might work out good pen turners, cut down some time sanding. Might be worth trying. Just my opion.
 
So, I think it depends on what kind of cutter we are talking about. A negative rake scraper tool, like a French Curve Negative Rake Scraper that you might use on bowls, is designed in such a way that you want a burr (curved up edge) so that the tool will in fact actually CUT the wood, rather than scrape it. Keeping such an edge on a negative rake scraper is tough, and you will usually need to pull a burnishing tool across the edge numerous times to keep the burr in place while cleaning the bottom of a bowl or platter or what have you.

Once that burr is gone, you are scraping, not cutting.

Now, with negative rake carbide cutters...you have a negative rake edge, so the angle of the metal that is in contact with the wood has that downward slope, which does reduce the aggression of the tool. But, you don't have that burr, so you are just scraping, not cutting. I have a full set of EWT tools, and I use them primarily for hollowing small vessels and turning resins, including resin pen blanks.

I don't use them on wood pen blanks, though, because they scrape, they don't cut, and the impact to the surface of the wood is quite obvious by the time you turn it down to the bushing diameter. You'll usually find the wood is either fuzzy, and requires additional cleanup, or even torn out (particularly if you are turning sidegrain blanks where you have to turn through endgrain on two sides of the blank).

Regarding the radius edge...you really don't want a square carbide cutter to be in full contact with the wood along the whole edge...that would be much more aggressive, and compound the problems. The radius edge ensures that you contact the wood over a manageable amount of metal, reducing the chances of catches. A strait edge with strait edge with sharp corners would be really hard to move along the wood and would catch a lot. The slight radius effectively eliminates this problem. Personally, I normally use round cutters with resins.

There is one potential use case for carbide cutters, although not negative rake. That is as a sheer cutter. Instead of putting the cutter edge flat to (in line with) the blank, you would use a non-negative rake square cutter (which you can get without a radius, BTW) at a sheer angle, and use that to sheer cut. This IS a way to CUT the wood, rather than SCRAPE the wood, with a carbide tool. In order to do this, you need a tool that can hold the cutter and keep it at an appropriate angle. That means either a round bar, or a diamond bar, rather than a square or flat bar. A square or flat bar (i.e. EWT) are designed to keep the cutters flat, and trying to hold them at an angle with just a point of the bar contacting the tool rest is...well, don't do that. ;P

There are alternative tools out there that have round bars that can make sheer cutting an option, but you would normally want to use a normal square cutter for that, and keep it very sharp, rather than using a negative rake tool. You also wouldn't want a radius on the square cutters. You can get a pretty clean cut on wood blanks with sheer cutting with normal carbide cutters this way.

I have tried sheer cutting with resins...but so far, I haven't quite figured out how to make that work well. I get a rougher finish when I try to sheer cut than when I simply use a negative rake cutter flat. Resins are actually pretty easy to turn with negative rake cutters. Once you get the blank round, you just get that nice stream of curling resin coming off the tool. For sheer scraping, I've thought maybe a diamond bar tool (basically, a square bar, but turned 45 degrees) would help...that would give you a flat surface to put on the tool rest (rather than round), giving the cutter more stability while at an angle... I have such a tool, I think it was a Sorby, but it takes a round rather than square cutter. You can get a little bit of sheer cutting done with the round carbide cutter, but its really light. I haven't found such a tool that holds a square cutter yet.
 
jrista said:
That means either a round bar, or a diamond bar, rather than a square or flat bar. A square or flat bar (i.e. EWT) are designed to keep the cutters flat, and trying to hold them at an angle with just a point of the bar contacting the tool rest is...well, don't do that. ;P
Click to expand...
While a round bar is easier for sheer cuts than a square bar, I routinely sheer cut with a radiused carbide cutter on a square bar. It's not really that much different than holding a round bar in place.

A sheer cutting carbide tool would be trivial to make. Take a square bar and grind a shoulder at one end at an angle so that when the side of the bar is flat on the tool rest, the shoulder is at the angle. drill a hole and tap for a screw and add a cutter. The cutter is then presented to the piece at an angle. Or take an existing tool with a square bar and grind a small flat on the corners of the tool.
 
I tried grinding my scraper to a negative rake a few weeks ago. It didn't work very well. After doing more research I don't think I had the small bevel angle flat enough. I had it somewhere around 50 degrees but it needed to really be about 30. So I'm going to try it again.
 
crokett said:
While a round bar is easier for sheer cuts than a square bar, I routinely sheer cut with a radiused carbide cutter on a square bar. It's not really that much different than holding a round bar in place.

A sheer cutting carbide tool would be trivial to make. Take a square bar and grind a shoulder at one end at an angle so that when the side of the bar is flat on the tool rest, the shoulder is at the angle. drill a hole and tap for a screw and add a cutter. The cutter is then presented to the piece at an angle. Or take an existing tool with a square bar and grind a small flat on the corners of the tool.
Click to expand...
Yeah, it probably wouldn't be too hard to make one... Where would you source the metal bar?
 
jrista said:
Yeah, it probably wouldn't be too hard to make one... Where would you source the metal bar?
Click to expand...
Well my dad's a blacksmith so I just get stock from him. :) If he wasn't I would look at home depot or lowes. They have both 3/8" and 1/2" square steel rod. You can use mild steel for the bar. I have 3 shop made carbide tools, all use mild steel. A couple summers ago someone sent me some EWT carbide tools to try and compare to mine. I couldn't tell a difference in cut quality, other than the EWT have an admittedly better fit and finish. If you want to try this, you should grind a small bevel on the end of the bar where the cutter is attached. This will provide clearance for the end of the bar so it doesn't hit the work. Also if I were doing this again I would grind the top of the end of the bar down a bit to create a shoulder on the end of the bar so that the edge of the cutter sits against the shoulder. This would prevent it from spinning when you tighten the set screw down and keeps the cutter edge square to the bar. On a round cutter that doesn't matter. on a square or radiused cutter it does.

Once the tool is done you can use it to turn a handle for itself. You can fit a square peg into a round hole by drilling the hole in the handle with the next size up bit, so a 3/8" bar drill a 1/2" hole. A 1/2" bar drill a 5/8" hole. File or grind a slight radius on the corners of the bar to create a tang. Fill the hole with epoxy then push the bar in. Epoxy will fill squish around and fill any gaps. A 1.5" copper pipe coupler makes a great ferrule.
 
jrista said:
Yeah, it probably wouldn't be too hard to make one... Where would you source the metal bar?
Click to expand...

I think I would start out buying something like a 3/8 steel hex rod that way the angles would be on both sides to facilitate both a left to right shear cut as well as a right to left along with the standard flat scraper cut.

Dave
 
jrista said:
So, I think it depends on what kind of cutter we are talking about. A negative rake scraper tool, like a French Curve Negative Rake Scraper that you might use on bowls, is designed in such a way that you want a burr (curved up edge) so that the tool will in fact actually CUT the wood, rather than scrape it. Keeping such an edge on a negative rake scraper is tough, and you will usually need to pull a burnishing tool across the edge numerous times to keep the burr in place while cleaning the bottom of a bowl or platter or what have you.

Once that burr is gone, you are scraping, not cutting.

Now, with negative rake carbide cutters...you have a negative rake edge, so the angle of the metal that is in contact with the wood has that downward slope, which does reduce the aggression of the tool. But, you don't have that burr, so you are just scraping, not cutting. I have a full set of EWT tools, and I use them primarily for hollowing small vessels and turning resins, including resin pen blanks.

I don't use them on wood pen blanks, though, because they scrape, they don't cut, and the impact to the surface of the wood is quite obvious by the time you turn it down to the bushing diameter. You'll usually find the wood is either fuzzy, and requires additional cleanup, or even torn out (particularly if you are turning sidegrain blanks where you have to turn through endgrain on two sides of the blank).

Regarding the radius edge...you really don't want a square carbide cutter to be in full contact with the wood along the whole edge...that would be much more aggressive, and compound the problems. The radius edge ensures that you contact the wood over a manageable amount of metal, reducing the chances of catches. A strait edge with strait edge with sharp corners would be really hard to move along the wood and would catch a lot. The slight radius effectively eliminates this problem. Personally, I normally use round cutters with resins.

There is one potential use case for carbide cutters, although not negative rake. That is as a sheer cutter. Instead of putting the cutter edge flat to (in line with) the blank, you would use a non-negative rake square cutter (which you can get without a radius, BTW) at a sheer angle, and use that to sheer cut. This IS a way to CUT the wood, rather than SCRAPE the wood, with a carbide tool. In order to do this, you need a tool that can hold the cutter and keep it at an appropriate angle. That means either a round bar, or a diamond bar, rather than a square or flat bar. A square or flat bar (i.e. EWT) are designed to keep the cutters flat, and trying to hold them at an angle with just a point of the bar contacting the tool rest is...well, don't do that. ;P

There are alternative tools out there that have round bars that can make sheer cutting an option, but you would normally want to use a normal square cutter for that, and keep it very sharp, rather than using a negative rake tool. You also wouldn't want a radius on the square cutters. You can get a pretty clean cut on wood blanks with sheer cutting with normal carbide cutters this way.

I have tried sheer cutting with resins...but so far, I haven't quite figured out how to make that work well. I get a rougher finish when I try to sheer cut than when I simply use a negative rake cutter flat. Resins are actually pretty easy to turn with negative rake cutters. Once you get the blank round, you just get that nice stream of curling resin coming off the tool. For sheer scraping, I've thought maybe a diamond bar tool (basically, a square bar, but turned 45 degrees) would help...that would give you a flat surface to put on the tool rest (rather than round), giving the cutter more stability while at an angle... I have such a tool, I think it was a Sorby, but it takes a round rather than square cutter. You can get a little bit of sheer cutting done with the round carbide cutter, but its really light. I haven't found such a tool that holds a square cutter yet.
Click to expand...
Superb Review of my observations also!
 
Glad I clicked on this thread. I have two pieces of leftover square stock that I bought a couple of years ago to make a Bedan tool and a tool for hollowing bowls; has a radius on one side. Will get ambitious and photograph it FYI.
I could grind the end to accept a carbide tip. Know of a fellow who has access to a full machine shop so that will help.
 
I think this sounds a lot like the the Magical Skew from Speakeasy pen emporium (formerly TShadow).
If you are not metally inclined this might be something worth trying as well.

Just a thought.
 
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