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Discussion in 'MACHINE RESTORATION & WAY SCRAPING' started by Rex Walters, Jun 20, 2017.
Thoughts on the process: the actual scraping
There is a bit of a rhythm to scraping: scrape off the high spots, stone the burrs down, dull the surface with yellow, prep the plate with an ink brayer (one tool I forgot), blue up the part, repeat.
The only optional step is dulling the surface with red or yellow ink, and it's only optional during the initial roughing, in my opinion.
Scraping is not hard. Anyone can learn the knack, but there is no substitute for several hours of practice. Getting a part roughly flat to 5 or 10 PPI over the surface is particularly easy once you get the hang of it. Finishing is much harder, but is all about evenness of strokes and consistent depth of scrape. It's kinda like learning how to strum a guitar with a pick. It looks deceptively easy (and it is when you acquire the knack) but it takes an awful lot of practice before you get very good at it.
I'm convinced I can actually feel the blade clipping off a high spot when I hit my mark just right, which is pretty amazing when you consider how little metal you're removing (just a couple tenths!). Your fingers are amazingly sensitive.
It's very important to keep the rows of scrapes angled at 45 degrees to the part, and to reverse direction every alternate pass. Otherwise, you'll create ruts and cause chatter.
It's easy to forget which direction you were just scraping, though, when you get back from the plate.
TIP: I've made it a habit to consciously set down my scraper on the bench so it's angled in the proper direction for the next pass. When you get back from the plate and pick up the scraper, you'll automatically be facing in the right direction. The only hard part is training yourself not to unconsciously set it down in any old direction.
Some people scrape in circle or Z patterns, but I always just use crosses or half-crosses, even with the power scraper. I find it helps me keep a consistent depth of cut.
When roughing, I'll put down a row of scrapes in one direction, then immediately put down a crossing row in the other direction (before bluing up). This makes a pattern of crosses over areas where there was blue. Each individual scrape is about an inch long during the roughing stage. Initially, the scrape marks can bleed into one another, but as you begin finishing it becomes increasingly important to make separate scrape marks clearly distinct from one another.
Once I've roughed out any big holes and have ink at about 5 PPI over the entire surface, I stop making crosses each pass. Instead I just make one arm of all the Xes in a single bluing. Next time I come back in the other direction. That is, roughing makes rows of "X X X X", finishing alternates rows of "/ / / /" with rows of "\ \ \ \". The rows themselves are also angled at roughly 45 degrees to the part.
Everybody will hav different body mechanics, but one thing that's universal is to use your legs and body to power the scrape as much as possible, not your arms. It does not take much strength at all to scrape, but by using your larger muscles you'll be more consistent and won't fatigue nearly as quickly.
Anyone that has ever used a wood plane (or played sports) should immediately grasp the balanced split stance.
I'm right-handed, so my left hand is forward, palm facing down. I grasp the scraper about six inches back from the tip (just behind where the blade holder meets the scraper). My right hand is curled underneath.
I've got fairly long forearms, so I nestle the handle of the scraper in the inner crook of my elbow. I've got a little piece of foam that I attach to the handle to make it more comfortable.
I don't press down very hard, but you do want to be firm and create chips, not dust. Hard to describe the pressure, but if you imagine trapping a sheet of copy paper under the blade, it would be just enough pressure to prevent someone from pulling the paper out from under, no more.
I also use a bump scraping technique for the final passes, but I'll cover that separately.
Hope this is useful to someone!
Rex, you did a superb nice job on everything you presented!
Could you start a new threads on the subject of slow speed sharpening/lapping devices if you don't mind. I see treadmill motors to be in high demand soon!
Our new residence expert on scraping?
Can everyone say Rex!
Yikes, I'm hardly an expert! Just overly verbose and spending a fair bit of time trying to learn myself.
If I start a thread on a lapping device, I'll end up having to build one.
These are excellent points to keep in mind. I try to follow this every time when I scrape.
Again, excellent writeup Rex!
We'll let Thunderdog do that since he brought it up!
4gsr, whatever you do don't pay more than $50 for a treadmill. That's the highest I'll ever go when buying them. I've picked up two for free and even scored a legitimate 1 h.p. Leeson DC motor.
On the design of the lapping "machine". I was at work and errdaydreaming, I mean taking some real notes and drew up the sketch below. The idea that the table could slide across linear bearings and the table could also be rotated. Pretty basic, and I know I've seen setups as simple as a 5° block of wood.
Here is my general idea:
Tilt feature for grinding table(side view)
That's definitely worth starting another thread for, TD.
I just realized that none of the photos clearly show the shape of the finished tool, so here are two more:
You can clearly see the chowdered edge from the power scraper (or, more accurately, from the incompetent operator of the power scraper).
The third side is left unscraped as it isn't intended as a reference surface.
I've still got one more to make, the 12" model. These were basically practice runs. I'm intending to really do my best with the last one.
Scraping thoughts (addendum)
Just remembered a few more things. Might as well document them for completeness.
I have two bad habits, but I'm getting better at recognizing the problems they cause and correcting quickly.
One is I tend to grip the scraper too tightly with my right hand. I tend to tense up over time and this causes me to miss my marks. When I relax, the results are much improved.
The other is that I start to press down too hard with my left hand, causing short stuttering scrape marks. The trick is to press down lightly but consistently (try not to get that guitar pick caught up in the strings).
Finally, remember that there is a complex relationship between the angle of the scraper and the amount of downward pressure required. Sometimes it's better to adjust the angle rather than the downward pressure, but this is a "feel" thing you have to learn for yourself.
These straightedges are a perfect project to develop that feel!
The blade is curved in a half moon shape. The more pronounced the curve (the tighter the radius) the narrower but potentially deeper the scrape. Roughing usually uses wide blades with a tight radius. Pinpointing uses a very narrow blade with a wider radius.
(Edit: wrong, wrong, wrong. Rough with narrower blades and a more pronounced radius, the finish with wider blades and a larger radius. The wider radius won't dig as deep a trench when finishing.)
With the scraper held at too shallow of an angle, it's hard to discern if you're tilting the blade. Tilting makes you miss your mark at the least. Far worse, a slight tip with a shallow angle risks gouging a scratch from the corner of the blade. A scratch may take hours to remove. (Scratches are places where crud can collect and rust can start. They should be avoided like the plague.)
The steeper the angle, the more obvious it becomes when the center of the curve is contacting the work. It also makes narrower scrapes, though, and drastically affects the amount of downward pressure required. Steep needs less pressure, shallow needs more. With enough practice you develop a feel for what's best.
As the cookbooks so helpfully say: "Cook until done. Do not over cook!"
All of this makes it sound impossibly difficult. It's really not (I'm just obsessing over details). Anyone with a modicum of patience can acquire the knack. All it takes is a couple afternoons with a hand scraper and any old hunk of cast iron (day one of Richard's class, by the way!).
Time to call it a night. I'm imagining Elvis singing about a hunk'a hunk'a burning iron.
Anyone following this thread should read this separate thread I created regarding a real epiphany. In a nutshell, metal scraping and hand planing are far more similar than you might think.
I now need to go back and work on those first too straightedges a bit more with a wider, wide radius blade to smooth the overall surface. Even my gnarly fingers can detect a pretty substantial roughness to the surface. My pinpoints were far too deep.
Again: beginner here, not an expert.
I made a straight edge and finished it with a 3/4" wide pull scraper.
This produced a very flat surface without the low spots generated using normal procedures.
I found that the blue had to be very very thin when spreading it on the straight edge or it would give
bad readings and the straight edge wouldn't hold blue because it had no low spots.
Blue had to be applied and carefully spread with each check.
While this allowed some very good finish readings it was slow and not so good for most of the process.
Another member pointed out the flaws in having such a flat tool surface.
Because the straight edge bowed over time it was re-scraped with a rougher surface and is better to use.
The rough surface holds blue and is much faster to use.