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Simulating a Negative Rake Scraper (Read 343 times)
 
Ray Stubbs
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Simulating a Negative Rake Scraper
Jun 10th, 2019 at 7:00am
 
I don't have a negative rake scraper and never used one. but if I take my regular scraper, freshly sharpened with a burr on top, and raise the handle to simulate the same angle as the negative rake scraper as it is introduced to the wood, is this the same effect as a NRC?
It seems to cut pretty good, and seems to not have the tendency to dig in. On numerous occasions I have had the regular scraper, when introduced to the wood at a 90 degree angle, to want to dig in, and in some cases cause a catch.
I don't know if I have made myself clear on this or not. Has anyone any ideas on this technique, or am I all wet?
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Ed Weber
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Re: Simulating a Negative Rake Scraper
Reply #1 - Jun 10th, 2019 at 8:00am
 
Ray Stubbs wrote on Jun 10th, 2019 at 7:00am:
if I take my regular scraper, freshly sharpened with a burr on top, and raise the handle to simulate the same angle as the negative rake scraper as it is introduced to the wood, is this the same effect as a NRC?


Yes,
The N in NRS is for Negative  angle. This means that the only difference is how (at what angle) the cutting edge is presented to the wood. If you change the angle of the handle you do indeed wind up with the same thing.
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David Moeller
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Re: Simulating a Negative Rake Scraper
Reply #2 - Jun 10th, 2019 at 11:20am
 
For 40 yrs been working up a burr on my scrapers and then raising the handle to get a shaving cut. Lately "neg rake" became the latest flavor and I couldn't figure why I would want to grind the top surface and blunt the chisel.  The burr is still there but the handle angle changes relative to the work. Neg rake allows working in deeper narrower vessels. thanks for the diagram Ed.
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Ed Weber
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Re: Simulating a Negative Rake Scraper
Reply #3 - Jun 10th, 2019 at 1:16pm
 
The diagram usually helps people see what they can't visualize
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Ray Stubbs
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Re: Simulating a Negative Rake Scraper
Reply #4 - Jun 10th, 2019 at 3:03pm
 
Ed, thanks for clearing this up for me.
The only draw back is the awkward way the tool is being held. I don't believe it could dig in unless you forget and pull down on the handle.
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Ed Weber
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Re: Simulating a Negative Rake Scraper
Reply #5 - Jun 10th, 2019 at 3:29pm
 
Like most things, it's a trade off.
You can't remove as much material as quickly as with a standard scraper but a NRS is  much more forgiving and requires much less effort to operate, from you and your lathe.
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Mike Nathal
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Re: Simulating a Negative Rake Scraper
Reply #6 - Jun 10th, 2019 at 5:24pm
 
I understand the argument that a NRS is the same as a standard scraper tilted down, but many prominent turners disagree.  I can't reproduce it,  but there is a video out there where Stuart Batty explains the difference in the physics.   One thing I don't see mentioned very often is that a NRS allows me to get smooth, flowing curves on a vase shape much easier than other tools.  BY being so easy to use, I can concentrate on the shape more than tool presentation.
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Ed Weber
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Re: Simulating a Negative Rake Scraper
Reply #7 - Jun 10th, 2019 at 6:39pm
 
This subject is going on in another thread as well, I will try to answer what I can.

Mike Nathal wrote on Jun 10th, 2019 at 5:24pm:
I can't reproduce it,  but there is a video out there where Stuart Batty explains the difference in the physics. 


One point I will disagree with Mr Batty on, is that you need a burr on a NRS, you do not.
This is a provable fact not simply my opinion. The other thing is that Mr Batty has decided what angles should be used to be considered to be classified as a positive and negative rake scraper.
In the diagram I posted the included angle of the standard scraper would be too great to be considered a NRS by Mr Batty, even though it is clearly cutting in a negative rake angle.

People have been using everything from a piece stone, to a broken piece of glass to scrape wood for thousands of years without a burr.
Look at a scraping plane which can be used with or without a burr or the scrapers recently discussed Multimedia File Viewing and Clickable Links are available for Registered Members only!!  You need to Login or Register. These do not have a burr by design and are used at a negative rake angle. As mentioned in the other thread there are NRS inserts for replaceable tip tools like Easy Wood Tools, these are carbide and do not have a burr.

I don't intend to start an argument but the physics (geometry) of Standard (positive rake) and Negative rake scrapers is clearly visible in the diagram I posted.

Mike Nathal wrote on Jun 10th, 2019 at 5:24pm:
One thing I don't see mentioned very often is that a NRS allows me to get smooth, flowing curves on a vase shape much easier than other tools.  BY being so easy to use, I can concentrate on the shape more than tool presentation.


This is what I have been trying to convey.
A NRS is by design less aggressive, which in turn lends itself to be easier to control.

I want to be clear, a NRS can have a burr, if there is a burr, it is usually just an artifact created by the grinding/sharpening process. Not a work hardened or burnished burr like you would find on a standard scraper.

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Don Stephan
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Re: Simulating a Negative Rake Scraper
Reply #8 - Jun 10th, 2019 at 7:13pm
 
Confused a bit I am.  A regular scraper, my understanding, HAS to be used with the handle higher than the "cutting edge" to avoid nasty catches/digs.

It was not clear to me in the first posting if it was always being used that way.
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Ed Weber
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Re: Simulating a Negative Rake Scraper
Reply #9 - Jun 10th, 2019 at 7:46pm
 
Don Stephan wrote on Jun 10th, 2019 at 7:13pm:
Confused a bit I am.  A regular scraper, my understanding, HAS to be used with the handle higher than the "cutting edge" to avoid nasty catches/digs.


This isn't entirely accurate
Here is a section on scrapers cut from Multimedia File Viewing and Clickable Links are available for Registered Members only!!  You need to Login or Register I feel this explains the topic well

Pointing a Scraper Downward


It is often suggested that a sure-fire way to avoid the unpleasantness of an aggressive scraper is to raise the handle so the tool points downward at a significant angle. This supposedly puts the cutting edge in trail and makes the tool easier to control.


However, . . . if the burr has a significant curl, pointing the tool downward will have just the opposite effect. The burr will be aimed directly into the wood which will make the problem worse. A heavy cut is likely to follow.


A secondary effect is that a force will be exerted on the tool that tends to pull it across the tool rest toward the workpiece. This is the self-feeding tendency. I think the chances of it causing a serious mishap are small, but it is a bit unsettling to feel.



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Re: Simulating a Negative Rake Scraper
Reply #10 - Jun 11th, 2019 at 9:59am
 
I was discussing the 'if you raise the handle enough with a standard scraper will it cut like a NRS?" on this forum a while back with Ed, and he commented that it doesn't work because what you get is a 'trailing cut'. I, as a scraper psycho, still don't quite get the concept, but do know it doesn't work. See my video 'Scary Scrapers' on You Tube or in the video section here. Anyway, the 'trailing cut' is what happens if you are using a card scraper, and tilt the blade too far. The edge wants to tear rather than cut.

As for having to have the handle on a standard scraper elevated for it to work, that is a 'kind of' answer. I hold mine pretty much level. On the outside of a bowl, you want to be cutting at or slightly below center. On the inside, you want to be cutting slightly above center. This is so that if you have a catch, the edge drops out of the wood rather than digging in deeper. You do not ever want to do a peeling type cut on bowls, which with a scraper would mean dropping the handle till you are rubbing the bevel while cutting. If you come off of the bevel, even a tiny bit, you dig in while pointing into the spin, and can not drop out. There is an infamous video by Ian 'Robbo' Robertson about why you don't use spindle roughing gouges on bowls. In it, he is doing the standard peeling cut which works on spindles, a bevel rubbing cut with the handle dropped. He cuts fine for a bit, but to get the catch, he extends way out off of the tool rest and then raises the handle. Big catch, bent tool, and he donates a little blood....

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Don R Davis
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Re: Simulating a Negative Rake Scraper
Reply #11 - Jun 11th, 2019 at 10:31am
 
Here is a video that may answer some of the questions you have. I sure hope so.

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Ed Weber
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Re: Simulating a Negative Rake Scraper
Reply #12 - Jun 11th, 2019 at 12:06pm
 
One thing I would point out is that when making/choosing a NRS, it doesn't have to have such an extreme included angle as the one in the video.
What I mean is that the Negative rake comes from the additional bevel on the top, making the cutting angle (level tool at center-line) less than 90 degrees. The bottom bevel doesn't need to be equal or even close, A similar profile as the NRS in the diagram

If you are using a NRS with a burr as in the video, the more support below the burr the better. This will help the burr last longer having more support directly below the cutting area. A similar profile as the NRS in the diagram
If you are using a NRS without a burr, just a Keen edge, you would want the top and bottom bevels to be similar, if not the same. This is to achieve the more acute point that does the cutting.

Additionally, if you want your burr to last longer;
1. Grind/sharpen
2. Hone away burr from grinding/sharpening
3. Use burnisher to raise burr.
A work hardened burr that is drawn out with a burnisher will last many times longer than a burr produced by grinding.

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Re: Simulating a Negative Rake Scraper
Reply #13 - Jun 11th, 2019 at 12:43pm
 
Ed Weber wrote on Jun 11th, 2019 at 12:06pm:
One thing I would point out is that when making/choosing a NRS, it doesn't have to have such an extreme included angle as the one in the video.
What I mean is that the Negative rake comes from the additional bevel on the top, making the cutting angle (level tool at center-line) less than 90 degrees. The bottom bevel doesn't need to be equal or even close, A similar profile as the NRS in the diagram

If you are using a NRS with a burr as in the video, the more support below the burr the better. This will help the burr last longer having more support directly below the cutting area. A similar profile as the NRS in the diagram
If you are using a NRS without a burr, just a Keen edge, you would want the top and bottom bevels to be similar, if not the same. This is to achieve the more acute point that does the cutting.

Additionally, if you want your burr to last longer;
1. Grind/sharpen
2. Hone away burr from grinding/sharpening
3. Use burnisher to raise burr.
A work hardened burr that is drawn out with a burnisher will last many times longer than a burr produced by grinding.



Like on a card scraper?
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Ed Weber
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Re: Simulating a Negative Rake Scraper
Reply #14 - Jun 11th, 2019 at 2:53pm
 
Yes a burnished edge like a card scraper.
Veritas sells a tool designed just for this although you can do it with a standard hand burnisher,  Multimedia File Viewing and Clickable Links are available for Registered Members only!!  You need to Login or Register
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