Looking for some scraping tooling tips

[Larry$]

I've watched the videos, got interested enough to see if I could do it. Just a practice piece. Having never seen the tool I just used what I had. A laminate file that I ground a recess in to hold a piece of a broken carbide from a molder head. I silver soldered the carbide in the notch. Thought it might not work since the carbide wasn't the kind intended for brazing. But it stuck w/o cracking. Using a diamond wheel on my tool grinder I shaped the end into a curve and ground the edge square in the up/down plane. After a couple of tries ground a tighter curve to reduce the contact area. I had some blue artist oil paint so tried that. It was too thick so added a bit of mineral spirits. I played with angles and the like and found a position that worked OK. My pattern never did look as uniform as the videos but it worked. I ended up getting a decent flat surface. I can sure see why a power scraper was invented.

So I'd like to know a bit more about the tool and the process. Is the tool flexible? What is the shape of the carbide cutting edge?

 

[benmychree]

Heh. If it was just a few chunks of dust or swarf I'd agree with you, John, but the batch I got was mostly grit. It was like someone had mixed a handful of sand into the ink.

It's not a foreign contaminant, though, it's something in the recipe that either crystallized or clumped up for some reason, or the stuff was never manufactured correctly originally (unsure which). If I take a small amount and mash it up good and keep stirring, it eventually seems to go away (hence the mortar and pestle comment). The individual grains almost look like glass shards (they aren't sharp, though). Never had a problem with hi-spot blue or Charbonnel, though.

 

[benmychree]

Yes, I have seen some Prussian blue that crystallized as you described, a friend was giving a bunch of tubes of it away, besides being gritty, the consistency of it was thin and did not mark well at all.

 

[Rex Walters]

I've watched the videos, got interested enough to see if I could do it. Just a practice piece. Having never seen the tool I just used what I had. A laminate file that I ground a recess in to hold a piece of a broken carbide from a molder head. I silver soldered the carbide in the notch. Thought it might not work since the carbide wasn't the kind intended for brazing. But it stuck w/o cracking. Using a diamond wheel on my tool grinder I shaped the end into a curve and ground the edge square in the up/down plane. After a couple of tries ground a tighter curve to reduce the contact area. I had some blue artist oil paint so tried that. It was too thick so added a bit of mineral spirits. I played with angles and the like and found a position that worked OK. My pattern never did look as uniform as the videos but it worked. I ended up getting a decent flat surface. I can sure see why a power scraper was invented.

So I'd like to know a bit more about the tool and the process. Is the tool flexible? What is the shape of the carbide cutting edge?

That's great that you were successful with just your own persistence. It may not be the most efficient way to learn, but the lessons sure stick!

I think the main thing with scraping is consistency and learning to read the blue. As long as you can make regular patterns of shallow rounded diviots (0.0002' to 0.0005" typically) and you learn how to read the blue-ups you can get there eventually, no matter what tools you use to do so.

The ideal pattern has equally spaced rows of equally spaced scrapes, where each scrape is roughly the same length. The longer the scrape, the deeper you're scraping and the faster the progress (so you shorten the line lengths as you progress). You want the spaces between the scrapes to be about the same width as the scrape marks themselves. Same thing for the spaces between the rows. It's very important to alternate the direction of the rows between passes: one pass with the rows oriented 45° to the work, the next pass with them oriented at 135°. It's also extremely important to stone off the burrs from scraping before bluing up. Every time!

Basically, after two passes to create a checkerboard pattern, you blue up to see where you are. Then scrape a pass or two to remove the blue. Then stone, blue-up, and repeat until you get the coverage you're after (typically 20 to 40 points per inch).

That's the basics, but there is some physical practice required to get consistent scrapes, and it takes a fair bit of practice to understand how to read the blue-ups.

The process is a LOT faster and less irritating with decent tools, though. In particular, a little flex in a scraper blade holder is definitely preferable (you'll want something stiffer for final flaking vs. scraping).

The carbide cutting edge is shaped into a crescent (half-moon) shape on the edge. The edge is beveled at about 5° from each face (for cast iron). The 5° rake and a normal stance while holding the blade creates significant negative rake at the cutting edge. In other words, if you look at the blade flat on the work, you'll see a half-moon shape at the front edge. If you turn the blade 90° and look carefully at the edge, you'll see a little tent shape rather than square corners all along the curved edge.

For scraping steel (to be avoided if you can) or other materials you might want positive rake on the cutting edge. One advantage of negative rake is you can bevel from each face, so you can flip the blade over when it gets dull and double the time between honing sessions.

Good luck!

 

[macardoso]

That's great that you were successful with just your own persistence. It may not be the most efficient way to learn, but the lessons sure stick!

I think the main thing with scraping is consistency and learning to read the blue. As long as you can make regular patterns of shallow rounded diviots (0.0002' to 0.0005" typically) and you learn how to read the blue-ups you can get there eventually, no matter what tools you use to do so.

The ideal pattern has equally spaced rows of equally spaced scrapes, where each scrape is roughly the same length. The longer the scrape, the deeper you're scraping and the faster the progress (so you shorten the line lengths as you progress). You want the spaces between the scrapes to be about the same width as the scrape marks themselves. Same thing for the spaces between the rows. It's very important to alternate the direction of the rows between passes: one pass with the rows oriented 45° to the work, the next pass with them oriented at 135°. It's also extremely important to stone off the burrs from scraping before bluing up. Every time!

Basically, after two passes to create a checkerboard pattern, you blue up to see where you are. Then scrape a pass or two to remove the blue. Then stone, blue-up, and repeat until you get the coverage you're after (typically 20 to 40 points per inch).

That's the basics, but there is some physical practice required to get consistent scrapes, and it takes a fair bit of practice to understand how to read the blue-ups.

The process is a LOT faster and less irritating with decent tools, though. In particular, a little flex in a scraper blade holder is definitely preferable (you'll want something stiffer for final flaking vs. scraping).

The carbide cutting edge is shaped into a crescent (half-moon) shape on the edge. The edge is beveled at about 5° from each face (for cast iron). The 5° rake and a normal stance while holding the blade creates significant negative rake at the cutting edge. In other words, if you look at the blade flat on the work, you'll see a half-moon shape at the front edge. If you turn the blade 90° and look carefully at the edge, you'll see a little tent shape rather than square corners all along the curved edge.

For scraping steel (to be avoided if you can) or other materials you might want positive rake on the cutting edge. One advantage of negative rake is you can bevel from each face, so you can flip the blade over when it gets dull and double the time between honing sessions.

Good luck!

I know this wasn't directed at me but I appreciate the knowledge. I think I may pick up a 20mm Sandvik scraper from MSC. $85 gets it to my door with a carbide insert included. Even if I don't immediately jump into it or take a course, it will be nice to have on hand. I've watched enough videos and messed with bluing enough that I just barely understand how to read blue and identify the existence of bow and twist. I can get down to a good flat surface with decent bearing, but I don't have a tool that generates the scraping marks I see others doing. I hope the correct tool can change that.

I also think I pretty much understand scraping for parallelism. Where my knowledge ends is understanding how to measure and scrape specific angles into parts, or scrape surfaces where there is a boss or feature that sits above the surface to be scraped (like the bottom of a lathe compound).

 

[Liljoebrshooter]

Buy yourself a 12" camel back straight edge and go to town on it. Check out Martin Mold and Pattern. He has a nice selection of straight edges that are quite affordable.

Joe

 

[macardoso]

Buy yourself a 12" camel back straight edge and go to town on it. Check out Martin Mold and Pattern. He has a nice selection of straight edges that are quite affordable.

Joe

If I were going to do this, what would be the best method for accurately scraping the 45 degree? I think I could get it flat, but what about the accuracy of the angled surface.

Would a 12" allow for printing of larger dovetails?

 

[macardoso]

One final question - for now. My machinery is a benchtop mill and a 12x36 lathe, hence any scraping I would do would be relatively small. Would you recommend a 20mm or 25mm scraper to start out with?

 

[Larry$]

That's great that you were successful with just your own persistence. It may not be the most efficient way to learn, but the lessons sure stick!

I think the main thing with scraping is consistency and learning to read the blue. As long as you can make regular patterns of shallow rounded diviots (0.0002' to 0.0005" typically) and you learn how to read the blue-ups you can get there eventually, no matter what tools you use to do so.

The ideal pattern has equally spaced rows of equally spaced scrapes, where each scrape is roughly the same length. The longer the scrape, the deeper you're scraping and the faster the progress (so you shorten the line lengths as you progress). You want the spaces between the scrapes to be about the same width as the scrape marks themselves. Same thing for the spaces between the rows. It's very important to alternate the direction of the rows between passes: one pass with the rows oriented 45° to the work, the next pass with them oriented at 135°. It's also extremely important to stone off the burrs from scraping before bluing up. Every time!

Basically, after two passes to create a checkerboard pattern, you blue up to see where you are. Then scrape a pass or two to remove the blue. Then stone, blue-up, and repeat until you get the coverage you're after (typically 20 to 40 points per inch).

That's the basics, but there is some physical practice required to get consistent scrapes, and it takes a fair bit of practice to understand how to read the blue-ups.

The process is a LOT faster and less irritating with decent tools, though. In particular, a little flex in a scraper blade holder is definitely preferable (you'll want something stiffer for final flaking vs. scraping).

The carbide cutting edge is shaped into a crescent (half-moon) shape on the edge. The edge is beveled at about 5° from each face (for cast iron). The 5° rake and a normal stance while holding the blade creates significant negative rake at the cutting edge. In other words, if you look at the blade flat on the work, you'll see a half-moon shape at the front edge. If you turn the blade 90° and look carefully at the edge, you'll see a little tent shape rather than square corners all along the curved edge.

For scraping steel (to be avoided if you can) or other materials you might want positive rake on the cutting edge. One advantage of negative rake is you can bevel from each face, so you can flip the blade over when it gets dull and double the time between honing sessions.

Good luck!

Thank you. I was doing the 45/135 passes and stoning. If/when I try this for real I will need to position the work & my body in better relationship. My body was wearing down faster than the iron. (Old, worn out body!)

 

[Rex Walters]

If I were going to do this, what would be the best method for accurately scraping the 45 degree? I think I could get it flat, but what about the accuracy of the angled surface.

Would a 12" allow for printing of larger dovetails?

Please keep in mind that I'm just a hobbyist who barely knows what he's talking about at the best of times. I'm a long way from an expert! Please read all of the following accordingly,

The angle of the bevel on a straightedge isn't important, you just want a narrow enough angle that it clears going into dovetail ways. You only use one side or the other as the flat reference to mark up the ways. You want both sides of the angle scraped flat because the bulk of the straightedge will interfere with the work in one orientation or the other, but the actual angle just needs to be smaller than the dovetail angle. You only print and scrape one side of the dovetail at a time.

Even the exact angle between the mating dovetails on the work doesn't really matter. Who cares if it's off a degree or two as long as they are mating perfectly and nothing gets tighter or moves off-axis as the table slides? In practice, you usually scrape the moving part to match the fixed part, first getting the fixed geometry right with a straightedge, then bluing up the fixed part to mark up and scrape the moving part.

Getting the actual geometry right for sliding prismatic or dovetail ways is far too complex to write about in a paragraph or three, refer to the Connelly book mentioned above for a full explanation (or if you just need a nap).

Despite my joke above about needing every measuring tool, the absolute best tool to own (I don't, sadly) is the one Richard King's dad designed, the King-Way alignment tool. It's a clever arrangement of precision levels, sliding joints, indicator mounts, and hemispherical and ball feet to assist in measuring any misalignment of machine tools. It's not a mandatory tool, but you'll be cobbling together custom jigs without one.

The King-way was designed with sound kinematic principles. Not every jig I've made has been!

I think I'm replying to multiple people here, but in my opinion, the only truly mandatory metrology tools are a (hopefully calibrated) surface plate, a decent surface-gauge/indicator-stand, a 0.0005"/graduation dial test indicator, and a precision machinist's level (on the order of 0.0005"/12in per graduation). I'd really want to add a 0.0001"/graduation indicator, too, but it's only useful for the particularly persnickety stuff. Other things like feeler-gauges/shim-stock, parallels, 1-2-3 blocks, and jo-blocks/gauge-pins can also come in very handy.

Lastly: either a 20mm or 25mm blade is fine for scraping flat, unobstructed surfaces. In general, the wider the blade the easier it is to handle, but the more likely you'll have trouble fitting into tight spaces. The compound on a small lathe will require a much narrower tool to get into the dovetail than 20mm. I use a 20mm blade for almost everything, but a 10 or 15mm blade when I need to get into tight spaces. A 10mm blade is much more difficult to handle well — it's all too easy to tip the blade accidentally and catch a corner. (It's worth rounding the corners of all your blades for exactly this reason.)

The most important tool is your brain and patience. Rich calls it "detective work" for a reason.