Card scraper hook - how long does it last?
I've reviewed on-line resources until my fingers are blistered, and I finally got a half-backsided edge on my card scrapers.
I'm filing down with a fine single cut file, using a wood block as a template to remove imperfections;
I then smooth the edges with a 4000-ish grit DMT diamond plate, being as close to perpendicular as I can - sharp edges on both sides.
I then smooth the sides of the plates using the same 4000 to try and make a proper 90 degree surface.
First question; in drawing out the steel (it's on the granite top of my Rigid table saw) how much pressure do I need to put on the scraper? I use a Veritas Tri-burnisher, and I swipe it in my mineral oil soaked rag before using. It applies to my 2 scrapers, a Bahco and a generic Lee Valley.
Second; I do get a very small hook, but I'm working with Oak, and I'm not getting the kind of shavings the on-line videos show. I'll keep working on technique, obviously.
Third; again, using Oak, how long should to hook last. It seems I scrape one or two small areas, and I have to go to the next edge. Is this a function of the poor hook, or not enough pressure in working the initial edge, or what?
I know it's nearly impossible to diagnose this long distance, but any hints would be appreciated.
File: maybe, maybe not, depends
Quote:
Originally Posted by
Jim Matthews
Try using one, straight off the file.
Getting a stable, durable hook achieves best results but a sharp arris is often sufficient.
Polishing on a stone will produce a continuous shaving, but it's not necessary.
Most of the effect is achieved "springing" the steel into a curve that has a progressive cutting angle.
Yes, I agree it is useful and educational to try and evaluate different ways of preparing and using scrapers.
The result may depend on the intended use and method of working.
I file, hone, and burnish and use curved hand scrapers for smoothing curved woodturnings before sanding then sand with fine grits either by hand or with very gentle ROS action. The scraping leaves the surface so smooth I am often able to start with 400 grit and even finer. I show one piece in demos that was touched only by 600 grit.
While teaching a private class at Arrowmont a few months ago, one person said honing and burnishing was a waste of time and showed his technique of going straight from a fairly coarse file to the wood. This certainly did create shavings and appeared useful. However, in testing later at home I found that filed edge left a very rough and relatively weak burr, similar to a burr directly from the grinder. The scraping left such a rough surface I had to resort to much coarser sand paper to remove the scratches left by the scraper. I tested with with hard, fine-grained wood.
My take-away is yes, I could scrape straight off the file. But no, I could not get as smooth a surface. Also, burr did not last as long as the burnished cutting edge. Scraping with a coarse burr may be OK to remove wood to shape the part, but in my use the part is already well shaped when I take it off the lathe and and my use for hand scraping is to remove remaining tool marks and prepare the surface for sanding with fine paper.
The file burr seems somewhat analogous to woodturners who prefer to use negative rake scrapers with the burr directly off the grinder. This works, but the life is so short (reported to be 20 seconds or less) that the tool must be returned to the grinder often. (Keep in mind that negative rake scrapers used at the lathe are subject to MUCH higher abrasion since the much more wood is passing the scraper, perhaps 1000 inches per second or more.) In contrast, a NRS that is honed and burnished properly will last much longer and provide a smoother surface to boot.
Just my experience.
JKJ