Negative Rake Scrapers

[Will Hunt]

Why do Neg. rake scrapers positioned at horizontal work better than regular scapers positioned with handle up? I've made a couple and they certainly work very well, particularly on finish cuts.
But Why?

Will Hunt:confused:

[J.D.Redwine]

Don't know if this will answer your question but here is an interesting discussion: http://www.sawmillcreek.org/showthread.php?148939-Negative-Rake-Scraper

[Jon McElwain]

I had the same question until I tried one. My lathe is set the the correct height for me and when I lift up the handle of a regular scraper, I find that I have less control than I do with the negative rake scraper. I like to hold my tools at the ferrule and have the rear of the handle along my arm; with a regular scraper, I have to lift my shoulder way up to get the negative rake effect.

I also grind my negative rake scrapers such that the angle between the two ground surfaces is greater than 90 degrees. I have not measured it, but I would guess that it is 110 degrees or so. I suppose you could accomplish this angle with a regular scraper if you ground it upside down, but in order to get the burr on the cutting edge, you would have to put the scraper on the grinder at a dangerous angle.

Hope that helps. I'm sure there are much more informed folks than me who can comment - these are simply my observations from trial and error.

Jon

[Stuart Batty]

Why do Neg. rake scrapers positioned at horizontal work better than regular scapers positioned with handle up? I've made a couple and they certainly work very well, particularly on finish cuts.
But Why?

Will Hunt:confused:
Negative rake scraping benefits from a secondary angle on top of the blade. This has a very different effect to a conventional scraper with only one angle.

The secondary angle places the burr on a different plane to the bottom of the blade that sits on the tool rest. By having the burr of the scraper on a different plane to the bottom of the blade it prevents it from self-feeding and actually makes negative rake scraping the only neutral way to form wood on a lathe.


Gouges tend to push the wood away when cutting. An example of this would be a thin wall bowl. When cutting thin wood on a lathe the wood will flex away from the gouge edge.


Conventional scrapers are the opposite to a gouge, when the wood is thin it will pull the wood in to the edge. If the wood is thicker, then the scraper can self-feed.


Negative rake scraping works between gouges and conventional scrapers and neither self-feeds or pushes the wood away.


If you are a member of the AAW, you can go online and look at an article I wrote on this technique in 2006 Journal issue # 1. It covers some of the rules about negative rake scraping and the history of this very old ivory turning technique.

It is important to note that negative rake scrapers should rely on using a burr for wood (This was not essential on Ivory). It is therefore essential that the included angle be below 90 degrees, any angle above 90 will not create a suitable burr. I use an included angle of 40 degrees when possible but up 70 degrees still creates a useable burr. The burr is best created on a coarse grinding wheel, both regular wheels and CBN can create a burr on the edge. It will not last long about 15-20 seconds for M2 and about 60-90 seconds with CPM 10V. However, this short life is worth it for the end results.

Stuart Batty

[John Keeton]

Stuart, welcome to the creek!!! Great to have you participate. Most folks here will be familiar with you, and your input will be a great contribution to this forum. In addition to informational postings, I am sure these folks would love to see examples of your turnings from time to time.

[Steve Schlumpf]

Stuart - Welcome to the Creek! Hope you have time to get to know us here! Like John said - would be great if you could post some of your work for us!

[Roger Chandler]

Welcome Stuart...........please do chime in every chance you get! Thanks for posting info on negative rake scrapers.......I learned something from your post!

[Ted Calver]

Welcome to the creek Stuart!! Hope you drop in and share with us often.

[Harry Robinette]

Thanks for the info Stuart that will help me when I make my Neg scraper.

[Bernie Weishapl]

Welcome Stuart. Thank you for the explaination. Please drop any as often as you can.