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Negative rake scraper angles

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Chris152

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Having read around the net, I ground two conventional scrapers to a 45 degree angle top and bottom, meeting more or less in the middle:
IMG_2326.jpg

They seem to hold the burr pretty well given that apparently that's an issue compared to regular grind scrapers, but I'm wondering what angles others use and what the advantages of other angles are, esp in relation to the burr.
Very pleased I did it, much easier to use now.
C
 

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CHJ

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Remember using a negative rake scraper more or less horizontal on the tool rest is near enough equivalent to using a scraper in trailing shear mode as if you were lifting the handle.
Therefore to my way of reasoning the top rake angle is needed to be the equivalent of what you lifting the handle to present the cutting edge in trailing mode would achieve.
 

Chris152

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Is it the case that the more acute the angle at the cutting edge, the more fragile the burr?
 

CHJ

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On HSS tools sharpened on grinding wheel or linisher any resultant burr is minimal anyway and not a proper deformed edge burr so is very short lived in use.

Older style carbon steel tools will take a burr reasonably easily just like a cabinet scraper if you have the means to do it, but it is very difficult to form a deformation burr on HSS, needing something like a cobalt steel dressing rod and a great deal of pressure.



Since I Made this in 2011 I guess I've used it half a dozen times and find very little advantage in taking the trouble.
 

Dalboy

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CHJ":1xlikqvm said:
On HSS tools sharpened on grinding wheel or linisher any resultant burr is minimal anyway and not a proper deformed edge burr so is very short lived in use.

Older style carbon steel tools will take a burr reasonably easily just like a cabinet scraper if you have the means to do it, but it is very difficult to form a deformation burr on HSS, needing something like a cobalt steel dressing rod and a great deal of pressure.



Since I Made this in 2011 I guess I've used it half a dozen times and find very little advantage in taking the trouble.
Now that is something I keep meaning to make but not for turning tools
 

Chris152

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CHJ":m97j9i5m said:
Since I Made this in 2011 I guess I've used it half a dozen times and find very little advantage in taking the trouble.
That's an interesting thread - I have a carbide rod that I made a crude handle for, which does seem to make a difference to the burr on my hss scrapers after sharpening on the linisher.
 

Paul Hannaby

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A 45° angle top and bottom will give you a combined angle of 90°, which is quite an obtuse edge.

Only a slight angle is needed on the top bevel so the edge is trailing when the tool shaft is presented horizontally. A steeper top bevel has the disadvantage that you can't see where the cutting edge is when using the scraper in a recess - (box, small cup etc.) and that can make it difficult to cut exactly on center when you need to.
The bottom bevel should give a combined angle something less than 90°.
I would go for around 10-15° for the top bevel and around 40-60° for the bottom bevel. This gives a combined angle of somewhere between 50-75° which should give an effective cutting edge with or without the burr.

There are other ways to trail the edge without lifting the handle depending on where you are cutting. Toolrest height can be changed to help or the tool can be rotated in shear cutting fashion.
 

Chris152

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Thanks Paul - I'll take some more off the bottoms at 45 degrees (raising the point of contact) and put a more acute angle on the tops, and give that a go, then maybe take the bottoms down to 40 degrees to compare.
 

woodfarmer

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I know some of you will cringe when I tell you this. When I first started I couldn't find a use for the skew chisel that was in my set of chisels. Eventually I learned it was excellent used on its side as a scraper (negative edged) especially for breaking the edges of the rims on bowls. Gradually I extended the uses of it and eventually bought a scraper. I can now, with care use the skew properly a bit, but still very wary of it.
I haven't done any turning for five years now but hope to do a little from time to time in the future, although my days of big bowls is past. Sadly some of the bigger blanks I have are being cutup to use as firewood.
 

Chris152

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woodfarmer":375o71gz said:
Eventually I learned it was excellent used on its side as a scraper
I hardly ever turn between centres and as a scraper's the only way I use my skew, reground to 25 degrees.
 

Chico

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No problems at all, I found it to be very very informative and easy to follow . All very interesting too.
 
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