Scrapers and understanding the forces behind them?

[David Gilbert]

I recently reread the recent thread on negative rake scrapers, https://www.aawforum.org/community/threads/negative-rake-scraping.25073/#post-284343
There was only one comment by Dave Landers on the forces that are at play with straight (standard) scrapers and negative rake scrapers (NRS). For the record I use NRS almost exclusively. They are easily controlled and almost never catch on wood. (If you want a catch with a NRS, use it on a PVC plumbing fixture at high speed. It is soft enough that it will catch.) We all understand that straight scrapers can easily catch. I have heard several respected demonstrators say that tilting a straight scraper down to a NRS angle makes it perform just like a NRS scraper. This is not my experience.
My question is has anyone figured why straight scrapers tend to self feed and NRS don't? Can anyone generate a force diagram for these? If I can understand the forces behind the scrapers I might be able to set up straight scrapers to be as safe and easy to use as a NRS.
Cheers,

 

[hughie]

I find lifting the handle to make the scraper negative puts it in a trailing mode and the chances there of catch minimal to say the least. The straight scraper is higher up at the centre, here you can easily slip into catch mode by letting your attention slip, especially if you have a large round scraper and the leading edge is allowed to come in contact with the work.
I have several flat and thin scraper 3mm thick that are curved for working inside hollow vessels, shear scraping the bottom half of the scraper works well but if I engage the upper half in the slightest way its all over. The top edge is not supported and over it goes, bang! I'm in trouble.