Very helpful video Wes, thanks very much for making this! :) It's got to be just about the only real end-to-end power scraping project video out there; it's amazingly hard to find one where you can watch someone do a project like this end-to-end. I learned a lot! :)
Sure is quick with a Biax ! Nicely done Wes. I know how flippin hard it is to get the camera to pick up what you and I can see and show in the video. Reading the ink transfer and determining the 'too much ink down v's clump of hi spots - all takes time to learn. I know I am better now than I was when I started - lost count of the hours I spent scraping in the shaper ways by hand - but its the knowing where to scrape which saves the time. Thanks for uploading. All the best Mat
When I bought the Biax, I had a 36" parallel straight edge that I practiced on. I probably did 200 passes trying some different techniques. For such an expensive machine, the instructions are pretty much worthless.
Great video, and perfect info for me. I am just getting into scraping and have watched almost every video on RUclips about it. lol I also have a friend at work who is going to give me pointers, he has experience scraping. Thanks again for the video... I can see me wanting a powered scraper once I get the basics down.
I just had to watch this to see what your day was like when machining "was you". Most surprising thing was how your voice has deepened in just four years. Somehow, I doubt it was booze and cigs that did that, though your increased sarcasm might argue for that. 8-))
You show signs of genious, at least the ability to think outside the box as it were. Having the intelect and quality of charachter is what people that exell at their work have in common. I always thought I was pretty damn good at mechanics and operating machines, but watching you Wes and Mustie1 on here reminds me there are many or at least some who simply make me look FNG. ish. I Always enjoyed being one of the very best at whatever my trade was at the time. So you should be having a blast man!
It certainly does save a lot of time. Biax claims it's 50% faster than hand scraping, but I think it's more like 2 or 3 times faster. Plus it's much easier on the operator.
Never saw it done this way. Were I worked we hand scraped to a good bearing then flaked it just for appearance and improved oil retention between the working surfaces.
I spent 13 years working on rebuilding machine tools and I don't think I ever scraped a straight edge in. I always checked them, before scraping a set of ways, and have seen others do it, but everything we scraped, ended up with hand scraping to finish. Most of my time was doing machine work on tools, before the scraping process. I guess and guy can learn something new every day though, or they might as well start digging the hole for you. 😎 A guy could learn from this video a lot quicker than the way I learned, which was mostly trial and error.
Using a power scraper sure has it's place, what was model # and accessories you bought with Biax, they have a number of units, looking on there site. Thanks for the video.
I'm still learning the nuances of the Biax, but it really works the same as hand scraping. It just takes the really hard work out of it. It would be hard to go back to hand scraping now.
Cotton, red lead powder and all purpose grease. These are the 3 main ingredients. Put a little cotton and grease in a mixing bowl and knead both together with a screw driver to a pizza dough consistency. Now add a TSP full of red lead powder to the same mixture and continue kneading. Add more grease if it's too hard or more cotton if it's too thin. Once you have a lump comfortable to hold in you fingers, you are ready. Just rub this lump on the area to be applied and a thin coat of coloured grease gets coated. Hope this helps
Is there a process to minimize error on a surface plate that is smaller than the length of the part being scraped? 18” plate and 36” camelback or parallel?
You can scrape to a level,I have some small levels about 4" long and very accurate,you could take a rub/print off something shorter than you are scraping then map with the level, there's always a way.
Very good and informative video. When you talk about points per inch^2 how do you judge them? I mean the black high spots seem to be spread out nice and even on the surface but the individual spots look like they are so big that for example 30 or 40 do not fit in a inch^2. Do you count the spots or make some sort of estimate on the portion of area that is colored or are there some really high spots that punch through that black and polish that don't show in the video that you count?
There's a little too much black on the plate so it makes the points kind of smear together. With slightly less black, it's a little easier to see the individual points. The points look big on the camera, but remember that the whole surface is only 1.5 inches wide. I checked two areas and found about 30 bearing points in 1 square inch.
DEEREMEYER1 as you saw at the beginning, the straight edge was hinging on a point close to the middle, ie: it wasn’t flat. By scraping away until you have an even distribution of clustered points (high spots: spots that make contact with the mating surface) , then the piece overall will sit flatter on the mating surface than it did at the start. The amount of material removed with each scrape stroke is minuscule, so scraping just makes flat things flatter over their whole area instead of localized points
For a relatively newbie with a Biax, I think you have got it down pat!My scrape marks are nothing close to yours, I think it is time to check the wear in my toolholder and slide for wear. I may need to let the tool work, and not try to work the tool .....out to make a test run!
I did a thorough inspection on my Biax, and didn't have to look far to see the bushings are shot that guide the stroke rod.... new bushings, grind and shim the bottom plates and toolholder, it should be like new
FWIW .... I replaced the ram bushings with new 12mm bore bushings from McMaster, and ground .010" off my "ways" to clean up the wear, then shimmed .010" .to bring to original.....also ground .004" off the toolholder contact area to clean up.....it works fantastic
Very good work! You will have a fine tool there.I think you will find it less tiring and you get more control if you hold the scraper closer to your body and instead of moving your arms/hands as much, ie. use shoulder/elbow movement, rather swing in the hip joint from side to side. Anyway, this is how we learnt from Rich (King). PS! What is your scraping depth? I mean, the "typical" or average depth of the scraping marks..The chatter marks at the sides are hard to combat, but it may help if you stop the blade movement at halfway of width of the blade when you go outwards. I see that you sometimes go over the edge too, which can lead to rounding of the edges (which you don not want)
There are others, and one I would recommend to watch is Nick Müller and his restoration of the GM12 Myford grinder. That said, I had done a fair bit of hand scraping before the first Richard King class, but that class was an eye-opener! If you have ever thought about joining such a class, I highly recommend it. Especially for the learning on the BIAX. I benefitted immensely from this.. of course there are other methods, or i would risk to say variations on the theme, but the way he teaches is a proven method that gets you to the target (in the class this amounts to scraping a piece to 40 ppi (points per inch) with the correct coverage ie. what he calls pop (percentage of points) and scraping depth. As a brief toutorial, his DVD covers the use of the BIAX (available on Ebay)
+Wes Johnson only richard king can scrape, as to how the millions of machines in the world were made I dont know. In fact my apprenticeship in scraping must have been a delusion because those clever men who taught cant of existed. Sorry for rant the whole rk is god thing pisses me off
Hand scraping is the process of using either a manual or motor-driven scraping blade to remove very small amounts of metal from a precision surface to eliminate high spots left behind by the machining process. This provides a flat, accurate bearing surface onto which machine components can be mounted.
I like your technique. You keep the tool moving and don't dig any trenches. I see you hold the Bias with a slight up-angle which gives a more aggressive action; at the same time you rough and finish with a long scraper. I have to try that but I'll need to make some new shanks. I can't afford Dapra's prices. I'm concerned you leave your scraping colors in the path of the chips. I can't really tell but are the containers open? If I was going to critique your work I'd note the "railroad track" of heavy bearing near the edges and the un-printed roll-off at the very edge. I taught my scraping students the random print pattern should run consistently across the work right to the very edge. The reason is on box and dovetail ways the scraping relief in the inside corner is usually narrow. If the scraping reference has a rolled off edge it leaves a track of high metal right next go the relief. Those crisp scraped edges are important if false and confusing bearing indication are to be avoided. Have you worked with Richrad King and the technique of picking in the spots to subdivide them ultiplying the bearing points and making their distribution more uniform. He teaches what he calls "dive bombing" and I've taught the exact same thing calling it "hover and drop." Both terms are self explanatory. Please don't take my remarks as derogatory. I've been scraping on and off for 50 years and taught it for 5 until I got too old and creaky. I like how you work. You're quick and efficient and don't get ahead of yourself. I learned a couple things about a Bias and I thought I was pretty proficient. Goes to show: keep an open mind. Your red (more like vermilion, if my monitor is rendering chroma OK) scraping color you use on the work (contrast media) is that the stuff made in Italy? The ca looks right. A Polish scraper gave me a near used tin and I loved it while it lasted. It 1was opaque, spread well, non-abrasive, and stable. But I couldn't find a source. I don't scrape anymore but if you have a source for those Italian made scraping colors I'd like to know so I can pass it on to my proteges.
You are correct that the edges are looking a bit high. I've found that when scraping narrow sections such as this (1.5 wide), there is a real tendency to "roll" the edges and scrape them too far. Hand scraping has the same risk. It's tough to find a balance. You can hear the scraper sometimes hit the side of the casting as I come off. The pigments I use are probably a type of iron oxide. I suppose it's somewhat abrasive, but the particle size is sub 1 micron and it's at least 50% oil. I get the occasional chip in the marking media. I'm careful in how I apply it, so it's never caused any harm. Surely a bad habit though.
again one who has no idea cockroaches has to do with lubrication to do when sharp mountains and valleys in the valleys accumulates the oil and during the movement, the oil is taken and it smears the surfaces of machine tools
I dont know why so many guys use WD-40 as a cutting and cooling solution. It is shit for cutting and keeping tools sharp. It was never designed for that. It is a solvent not a lubricant. It is used for displacing water and cleaning surfaces.
Maybe because it works for him? If it works who gives a shit what the proper definition is. People use Coca Cola to clean their toilet bowl which if you pick up a can was never the intended use but it works.
Your edges are rolled off. I see 1/8" to 1/4" wide strips along both edges showing no indications or very few. The opposite condition, raised edges (railroad tracks,) are only a few over-compensated cuts away. The happy medium, where the pattern of indications continues un-interrupted from edge to edge, is difficult to attain but essential if the end use of this straight edge is to validate work scraped flat to a raised shoulder or the apex of an internal dovetail. What is your red contrast medium made of? Some recipes for shop made scraping pigments call for concrete pigments. These are mineral oxide pigments very abrasive in nature. That handsome red, for example could be red iron oxide. It provides a dense opaque color but don't forget iron oxide is also used to polish glass and in buffing rouge for polishing metal. Pigment particle size is graded much like abrasives. When red iron oxide, ochre (yellow iron oxide,) manganese black oxide, etc are used in concrete pigments they are often milled quite coarse. These abrasive pigments if rubbed between reference surface and work to be scraped are equivalent to lapping compound. The traditional pigments used for precision scraping are carbon black (lampblack,) Prussian blue, white lead carbonate, and red lead oxide. They are soft and friable with very low abrasive properties. So if you wish to prolong the usable interval of your reference tooling between calibration and scraping, be aware of the abrasive properties of your scraping pigments.
@TakeDeadAim I disagree. I've scraped lots of reference tooling with a Biax (40 points once on a bet) and the trick to avoiding rolled-off edges and "railroad tracks" boils down to technique: scraping off the edge (never into it), varying the downward pressure, and above all keep moving. I used a hand scraper to get places I couldn't get with my Biax; maybe 15% in the course of a rebuild. You have to practice with the Biax and that takes a few hours over each of several days to gain the "second nature" sort of familiarity needed to machine scrape consistantly. It's a developed skill. You can't pick it up, work with it a while, get discouraged, and denounce a Biax power scraper as no good for high precision work. Not when many others all over the world over the last 60 years have Biax scraped to small tenths and single digit arc seconds. Early-on, the biggest problem I found with the Biax was operator fatigue. Ten pounds gets to feel like fifty after a few hours humped over a vibrating power tool and that makes you clumsy. Suspending the Biax scraper elimnates 85% of the effort required to use it. My go-to trick was to suspened the Biax level on a screen door spring by its center of gravity (I used a string loop around the barrel in a basket hitch) so the scraper tip was an inch or two above the work. You don't have to support the weight of the tool for hours and that saves the back and arms plus it makes for sensitive control for hours of use. Guide it with your fingertips if you want. I prefer to hang the Biax from a crane hook but pipes, building structure, anything overhead works. But you have to be able to shift the suspension point to cover the whole work surface. A section of garage door track and a trolley for a sliding door works great. Anyway the cure for rolled-off edges is scrape the entire surface down until the print extends to the edge. If you're scraping a single surface reference tool, it's a lot of work so rolled edges are best avoided in the first place. If you're scraping to achieve a fit or alignment, rolled off edges could mean going back and rescraping the other surfaces to compensate for the addition stock removal. The prep machine work may have allowed you 0.002" scraping stock, for alignment, fit, and bearing so don't overshoot. Then again, a small amount of edge roll-off may not matter. It depends on the end use. A straight edge with a bevel for getting into the interior angle of a dovetail is probably the worst case. In the beveled straight edge, the reference surface HAS to bear all the way to the beveled edge or the there will be a slight ridge between the flat and the dovetail relief. And that's bad bacause it greatly complicates fitting the mating slide.
Very helpful video Wes, thanks very much for making this! :) It's got to be just about the only real end-to-end power scraping project video out there; it's amazingly hard to find one where you can watch someone do a project like this end-to-end. I learned a lot! :)
The only decent Biax videos I know of are from Jan Sverre Haugjord in Norway. I learned a lot from watching him.
found another old one!! love all of the vids Wes!
It’s mind boggling how jabbing a bunch of holes in something makes it flat. I think scraping is pure witch craft
Sure is quick with a Biax ! Nicely done Wes. I know how flippin hard it is to get the camera to pick up what you and I can see and show in the video. Reading the ink transfer and determining the 'too much ink down v's clump of hi spots - all takes time to learn. I know I am better now than I was when I started - lost count of the hours I spent scraping in the shaper ways by hand - but its the knowing where to scrape which saves the time. Thanks for uploading. All the best Mat
When I bought the Biax, I had a 36" parallel straight edge that I practiced on. I probably did 200 passes trying some different techniques. For such an expensive machine, the instructions are pretty much worthless.
Great video, and perfect info for me. I am just getting into scraping and have watched almost every video on RUclips about it. lol I also have a friend at work who is going to give me pointers, he has experience scraping. Thanks again for the video... I can see me wanting a powered scraper once I get the basics down.
Great job Wes! .... Really enjoying the scraping vids! They are few and far between out there.... keep them coming!
I just had to watch this to see what your day was like when machining "was you".
Most surprising thing was how your voice has deepened in just four years.
Somehow, I doubt it was booze and cigs that did that, though your increased sarcasm might argue for that. 8-))
You show signs of genious, at least the ability to think outside the box as it were. Having the intelect and quality of charachter is what people that exell at their work have in common. I always thought I was pretty damn good at mechanics and operating machines, but watching you Wes and Mustie1 on here reminds me there are many or at least some who simply make me look FNG. ish. I Always enjoyed being one of the very best at whatever my trade was at the time.
So you should be having a blast man!
I like watching his videos too. There's something satisfying about digging something out of the trash and making it run.
I can see why he wanted you to do this as part of the sale! Nicely done.
Very nice Wes, Thanks for taking the time to make this video, you done a great job. That Biax saves hours and hours.
Thanks Again, Steve Summers
It certainly does save a lot of time. Biax claims it's 50% faster than hand scraping, but I think it's more like 2 or 3 times faster. Plus it's much easier on the operator.
I'm ignorant about the Biax and what is going on here? What is that for?
For do this operation like on video,can do with hands,is a same time if use biax.different is the nice hand scraper can do it witout biax
Very cool. Thanks for sharing this video.
Its a pure art to know how to read those patterns, computers do it all now
great stuff!
Nice work!
Excelente y muy interesante!!!
Thank you again Wess.
Never saw it done this way. Were I worked we hand scraped to a good bearing then flaked it just for appearance and improved oil retention between the working surfaces.
I spent 13 years working on rebuilding machine tools and I don't think I ever scraped a straight edge in. I always checked them, before scraping a set of ways, and have seen others do it, but everything we scraped, ended up with hand scraping to finish. Most of my time was doing machine work on tools, before the scraping process.
I guess and guy can learn something new every day though, or they might as well start digging the hole for you. 😎
A guy could learn from this video a lot quicker than the way I learned, which was mostly trial and error.
Me too i use biax if i have to remove a lot of material,but for ended i use hand scraper.now is same cool use biax for all...
Pretty cool.
Hi Wes that's a nifty tool you have there
Using a power scraper sure has it's place, what was model # and accessories you bought with Biax, they have a number of units, looking on there site. Thanks for the video.
Very nice scraping work, Wes! It is obvious this is not your first rodeo...
I'm still learning the nuances of the Biax, but it really works the same as hand scraping. It just takes the really hard work out of it. It would be hard to go back to hand scraping now.
Very neet techniques To re flatten The Plate @Watch Wes Work
How to make a red brick paint that you apply to the work to make contrast?
Cotton, red lead powder and all purpose grease. These are the 3 main ingredients.
Put a little cotton and grease in a mixing bowl and knead both together with a screw driver to a pizza dough consistency. Now add a TSP full of red lead powder to the same mixture and continue kneading. Add more grease if it's too hard or more cotton if it's too thin.
Once you have a lump comfortable to hold in you fingers, you are ready. Just rub this lump on the area to be applied and a thin coat of coloured grease gets coated.
Hope this helps
Do you still have your Biax / scraping gear
Very interesting and instructive . 2 hours is very fast!
It depends entirely on the condition of the surface before scraping. If it was out .005, it might take twice as long.
Is there a process to minimize error on a surface plate that is smaller than the length of the part being scraped? 18” plate and 36” camelback or parallel?
You can scrape to a level,I have some small levels about 4" long and very accurate,you could take a rub/print off something shorter than you are scraping then map with the level, there's always a way.
Very good and informative video. When you talk about points per inch^2 how do you judge them? I mean the black high spots seem to be spread out nice and even on the surface but the individual spots look like they are so big that for example 30 or 40 do not fit in a inch^2. Do you count the spots or make some sort of estimate on the portion of area that is colored or are there some really high spots that punch through that black and polish that don't show in the video that you count?
There's a little too much black on the plate so it makes the points kind of smear together. With slightly less black, it's a little easier to see the individual points. The points look big on the camera, but remember that the whole surface is only 1.5 inches wide. I checked two areas and found about 30 bearing points in 1 square inch.
How are you determining "bearing points" and how does putting 30 raised points per square inch make something "flatter"?
DEEREMEYER1 as you saw at the beginning, the straight edge was hinging on a point close to the middle, ie: it wasn’t flat. By scraping away until you have an even distribution of clustered points (high spots: spots that make contact with the mating surface) , then the piece overall will sit flatter on the mating surface than it did at the start. The amount of material removed with each scrape stroke is minuscule, so scraping just makes flat things flatter over their whole area instead of localized points
For a relatively newbie with a Biax, I think you have got it down pat!My scrape marks are nothing close to yours, I think it is time to check the wear in my toolholder and slide for wear. I may need to let the tool work, and not try to work the tool .....out to make a test run!
If you have a good handle on hand scraping, the Biax comes pretty naturally. The biggest thing is just controlling the machine to prevent gouging.
I did a thorough inspection on my Biax, and didn't have to look far to see the bushings are shot that guide the stroke rod.... new bushings, grind and shim the bottom plates and toolholder, it should be like new
FWIW .... I replaced the ram bushings with new 12mm bore bushings from McMaster, and ground .010" off my "ways" to clean up the wear, then shimmed .010" .to bring to original.....also ground .004" off the toolholder contact area to clean up.....it works fantastic
Very good work! You will have a fine tool there.I think you will find it less tiring and you get more control if you hold the scraper closer to your body and instead of moving your arms/hands as much, ie. use shoulder/elbow movement, rather swing in the hip joint from side to side. Anyway, this is how we learnt from Rich (King). PS! What is your scraping depth? I mean, the "typical" or average depth of the scraping marks..The chatter marks at the sides are hard to combat, but it may help if you stop the blade movement at halfway of width of the blade when you go outwards. I see that you sometimes go over the edge too, which can lead to rounding of the edges (which you don not want)
Yes, I had all of the problems you describe! I think you are the only guy with any videos of the Biax in action on youtube.
There are others, and one I would recommend to watch is Nick Müller and his restoration of the GM12 Myford grinder.
That said, I had done a fair bit of hand scraping before the first Richard King class, but that class was an eye-opener! If you have ever thought about joining such a class, I highly recommend it. Especially for the learning on the BIAX. I benefitted immensely from this.. of course there are other methods, or i would risk to say variations on the theme, but the way he teaches is a proven method that gets you to the target (in the class this amounts to scraping a piece to 40 ppi (points per inch) with the correct coverage ie. what he calls pop (percentage of points) and scraping depth. As a brief toutorial, his DVD covers the use of the BIAX (available on Ebay)
If Richard King is anything like his posts on the practical machinist forum, I will pass. That's all I will say.
+Wes Johnson only richard king can scrape, as to how the millions of machines in the world were made I dont know. In fact my apprenticeship in scraping must have been a delusion because those clever men who taught cant of existed.
Sorry for rant the whole rk is god thing pisses me off
Do you have a video doing the 45,and how you keep it true to the base?
No I don't. Typically you do not need to keep it true to the base. You just scrape it flat and use it as an independent surface.
Dedicação ❤
Hey Wes, what is this tool your working on used for. Never heard of "scraping". Educate me. Thanks
Hand scraping is the process of using either a manual or motor-driven scraping blade to remove very small amounts of metal from a precision surface to eliminate high spots left behind by the machining process. This provides a flat, accurate bearing surface onto which machine components can be mounted.
He wes how much you payed for the tool?
I find this beyond cool..who thought this up?
What rpm do those accufinish grinders run at?
Pretty slow. I think it's 300 RPM.
I like your technique. You keep the tool moving and don't dig any trenches. I see you hold the Bias with a slight up-angle which gives a more aggressive action; at the same time you rough and finish with a long scraper. I have to try that but I'll need to make some new shanks. I can't afford Dapra's prices.
I'm concerned you leave your scraping colors in the path of the chips. I can't really tell but are the containers open?
If I was going to critique your work I'd note the "railroad track" of heavy bearing near the edges and the un-printed roll-off at the very edge. I taught my scraping students the random print pattern should run consistently across the work right to the very edge. The reason is on box and dovetail ways the scraping relief in the inside corner is usually narrow. If the scraping reference has a rolled off edge it leaves a track of high metal right next go the relief. Those crisp scraped edges are important if false and confusing bearing indication are to be avoided.
Have you worked with Richrad King and the technique of picking in the spots to subdivide them ultiplying the bearing points and making their distribution more uniform. He teaches what he calls "dive bombing" and I've taught the exact same thing calling it "hover and drop." Both terms are self explanatory.
Please don't take my remarks as derogatory. I've been scraping on and off for 50 years and taught it for 5 until I got too old and creaky. I like how you work. You're quick and efficient and don't get ahead of yourself. I learned a couple things about a Bias and I thought I was pretty proficient. Goes to show: keep an open mind.
Your red (more like vermilion, if my monitor is rendering chroma OK) scraping color you use on the work (contrast media) is that the stuff made in Italy? The ca looks right. A Polish scraper gave me a near used tin and I loved it while it lasted. It 1was opaque, spread well, non-abrasive, and stable. But I couldn't find a source. I don't scrape anymore but if you have a source for those Italian made scraping colors I'd like to know so I can pass it on to my proteges.
You are correct that the edges are looking a bit high. I've found that when scraping narrow sections such as this (1.5 wide), there is a real tendency to "roll" the edges and scrape them too far. Hand scraping has the same risk. It's tough to find a balance. You can hear the scraper sometimes hit the side of the casting as I come off.
The pigments I use are probably a type of iron oxide. I suppose it's somewhat abrasive, but the particle size is sub 1 micron and it's at least 50% oil. I get the occasional chip in the marking media. I'm careful in how I apply it, so it's never caused any harm. Surely a bad habit though.
Do you still have your scraping tools yet and Biax
No. Not really.
Don't you miss doing stuff like this, when you are working on rusted out farm trucks these days, Wes?
A shame you don't do scraping vids anymore.
why are u doing this,what is that thing used for ,excuse me for not knowing .But a straight edge for what..?
I reccomended to you to watch a Brian Ives video.
'Power scraping' is a contradictio in terminis to me.
I never heard of a Biax Scraper, have no idea what that is.
again one who has no idea cockroaches has to do with lubrication to do when sharp mountains and valleys in the valleys accumulates the oil and during the movement, the oil is taken and it smears the surfaces of machine tools
I see your not into scraping by hand.
I’m not really into scraping at all.
Why do people scrape before marking? Seems like a waste of time and material...
If you blue up a smooth surface it will just look like a smear of blue that is hard or impossible to read.
@@WatchWesWork
Was wondering where you got your cast iron wheel on your bench grinder set up?
@@johncollings6799 I made it.
@@WatchWesWork
So you machined it out of stock on your lathe?
@@johncollings6799 Yes.
I dont know why so many guys use WD-40 as a cutting and cooling solution. It is shit for cutting and keeping tools sharp. It was never designed for that. It is a solvent not a lubricant. It is used for displacing water and cleaning surfaces.
Maybe because it works for him? If it works who gives a shit what the proper definition is. People use Coca Cola to clean their toilet bowl which if you pick up a can was never the intended use but it works.
Why not use a ball peen hammer? Every body thinks they can scrape.
Your edges are rolled off. I see 1/8" to 1/4" wide strips along both edges showing no indications or very few. The opposite condition, raised edges (railroad tracks,) are only a few over-compensated cuts away. The happy medium, where the pattern of indications continues un-interrupted from edge to edge, is difficult to attain but essential if the end use of this straight edge is to validate work scraped flat to a raised shoulder or the apex of an internal dovetail.
What is your red contrast medium made of? Some recipes for shop made scraping pigments call for concrete pigments. These are mineral oxide pigments very abrasive in nature. That handsome red, for example could be red iron oxide. It provides a dense opaque color but don't forget iron oxide is also used to polish glass and in buffing rouge for polishing metal. Pigment particle size is graded much like abrasives. When red iron oxide, ochre (yellow iron oxide,) manganese black oxide, etc are used in concrete pigments they are often milled quite coarse. These abrasive pigments if rubbed between reference surface and work to be scraped are equivalent to lapping compound.
The traditional pigments used for precision scraping are carbon black (lampblack,) Prussian blue, white lead carbonate, and red lead oxide. They are soft and friable with very low abrasive properties. So if you wish to prolong the usable interval of your reference tooling between calibration and scraping, be aware of the abrasive properties of your scraping pigments.
Excellent information Forrest!
ATB, Robin
@TakeDeadAim I disagree. I've scraped lots of reference tooling with a Biax (40 points once on a bet) and the trick to avoiding rolled-off edges and "railroad tracks" boils down to technique: scraping off the edge (never into it), varying the downward pressure, and above all keep moving. I used a hand scraper to get places I couldn't get with my Biax; maybe 15% in the course of a rebuild. You have to practice with the Biax and that takes a few hours over each of several days to gain the "second nature" sort of familiarity needed to machine scrape consistantly. It's a developed skill. You can't pick it up, work with it a while, get discouraged, and denounce a Biax power scraper as no good for high precision work. Not when many others all over the world over the last 60 years have Biax scraped to small tenths and single digit arc seconds.
Early-on, the biggest problem I found with the Biax was operator fatigue. Ten pounds gets to feel like fifty after a few hours humped over a vibrating power tool and that makes you clumsy. Suspending the Biax scraper elimnates 85% of the effort required to use it. My go-to trick was to suspened the Biax level on a screen door spring by its center of gravity (I used a string loop around the barrel in a basket hitch) so the scraper tip was an inch or two above the work. You don't have to support the weight of the tool for hours and that saves the back and arms plus it makes for sensitive control for hours of use. Guide it with your fingertips if you want.
I prefer to hang the Biax from a crane hook but pipes, building structure, anything overhead works. But you have to be able to shift the suspension point to cover the whole work surface. A section of garage door track and a trolley for a sliding door works great.
Anyway the cure for rolled-off edges is scrape the entire surface down until the print extends to the edge. If you're scraping a single surface reference tool, it's a lot of work so rolled edges are best avoided in the first place. If you're scraping to achieve a fit or alignment, rolled off edges could mean going back and rescraping the other surfaces to compensate for the addition stock removal. The prep machine work may have allowed you 0.002" scraping stock, for alignment, fit, and bearing so don't overshoot.
Then again, a small amount of edge roll-off may not matter. It depends on the end use. A straight edge with a bevel for getting into the interior angle of a dovetail is probably the worst case. In the beveled straight edge, the reference surface HAS to bear all the way to the beveled edge or the there will be a slight ridge between the flat and the dovetail relief. And that's bad bacause it greatly complicates fitting the mating slide.