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Why not scrape in reverse?

jamiesaun

Plastic
Joined
Mar 7, 2019
Hey guys, how's it hangin?

So let me start by saying I'm not a professional by any stretch, as will no doubt become obvious soon enough. Im just a hobbiest with too many hobbies.

Having said that, When watching the scraping process, it seems pretty reasonable if the surface is already pretty close to flat. But when it's way out, why not lap the thing flat with sandpaper on a surface plate or something, then blue it up and scrape for points? In other words, scrape for low points as opposed to scraping away the high points?

Just seems way easier. Thanks for any responses on this.
 
Hey guys, how's it hangin?

So let me start by saying I'm not a professional by any stretch, as will no doubt become obvious soon enough. Im just a hobbiest with too many hobbies.

Having said that, When watching the scraping process, it seems pretty reasonable if the surface is already pretty close to flat. But when it's way out, why not lap the thing flat with sandpaper on a surface plate or something, then blue it up and scrape for points? In other words, scrape for low points as opposed to scraping away the high points?

Just seems way easier. Thanks for any responses on this.

Not sure I'm following how that's reversed. Sanding down an area is still removing metal. If an area has enough wear or damage (anything deviation greater than .001"-.002"), it is always advised to start by milling or grinding it flat, this is part of roughing it in. Trying to hand grand or lap it down is going to take much much much longer though.

Often, surfaces are scraped when you can't get to them with anything else because the process is so versatile. If you don't have the ability to haul a mill table to a machine that can mill or grind it down, many guys will just scrap it down. It takes longer (each pass with a scraper will remove aprox. .0002"), but it works.

Scraping individual points is done because most other process's (milling, grinding, etc.) work in an entire plane and leave errors. Scraping removes those errors while leaving the rest.
 
I see. I was going to mill it first, but then lap it flat so it's blued basically over the entire surface, then scrape low points in it. But you're totally right about a surface grinder doing the same thing.

So thanks for that. I think I'll press on with the idea.
 
I've inhaled a lot of Canode blue fumes in the last two weeks. Even with that excuse, I don't understand the OP.

L7
 
I've inhaled a lot of Canode blue fumes in the last two weeks. Even with that excuse, I don't understand the OP.

L7

Not surprising.

Another question, is canode easier to spread than dykem high spot? Because I'm having a hell of a time spreading the dykem out. Thinning a little bit helps, I'm just wondering if canode is just easier.

Edit: yes I have a brayer, but it's a piece of crap, it's not flat at all and doesn't even kind of work.
 
I use a rubber brayer with canode blue which works well. A lot of guys use the yellow neoprene foam paint rollers.

I haven't had a chance to break the seal on my tube of dykem, but my understanding is that they spread and print the same, but canode is much easier to clean up.

IMO, whatever medium you use to print with, be it canode, dykem, or a mix-your-own concoction, and however thick or thin it spreads, whither or not it "works" will depend on the results you get. If you're printing an unscraped surface, you'll get a lot of smudges and streaking, but a scraped surface with 2 or 40 points-per-inch should show a variety of blue points. You should have some light, some dark, and some dark with shiny spots in the middle. That variety will help you understand what needs to be done and which points to scrape.

When you are doing your initial roughing and scraping, your points-per-inch are not really a concern so much as getting an even bearing across the surface and knowing that surface is square to other reference surfaces.
 
I've inhaled a lot of Canode blue fumes in the last two weeks. Even with that excuse, I don't understand the OP.

L7

He's asking why parts can't be ground flat(or lapped) and then flaked for relief instead of scraping the high spots coplanar.

I would say if you did an exceptional lapping or grinding job I don't see why it wouldn't work but I am not a rebuilder by any stretch of the imagination.
My gut feeling says it would be harder to get the small contact points with your method without risking creating voids in whole areas.
 
He's asking why parts can't be ground flat(or lapped) and then flaked for relief instead of scraping the high spots coplanar.

I would say if you did an exceptional lapping or grinding job I don't see why it wouldn't work but I am not a rebuilder by any stretch of the imagination.
My gut feeling says it would be harder to get the small contact points with your method without risking creating voids in whole areas.

Thank you, that is indeed what I was asking.
 
Somewhat the rough equivalent of what you're suggesting is done when oil grooves are cut or tiny pockets are put (flaked) into a ground surface.

A lot of precision surfaces today are things like ground rods with linear bearings or even ground surfaces with a thin layer of oil under pressure -- no scraping (+ / -) required.

I'm going to guess you don't have a surface grinder capable of getting something flat to .0001" (with all the problems of clamping, heat warping, etc. ) -- able to end up with a fine, flat surface finish ready for flaking.

In that case you get it flat as you can by milling or planing -- then scrape away the high points. Trying to hand lap a surface flat over a large area and with thousandths of inches to go -- and then scraping for an oil retention pattern -- is a far harder and less likely to be successful path. Particularly with sandpaper laid over a surface plate.
 
Somewhat the rough equivalent of what you're suggesting is done when oil grooves are cut or tiny pockets are put (flaked) into a ground surface.

A lot of precision surfaces today are things like ground rods with linear bearings or even ground surfaces with a thin layer of oil under pressure -- no scraping (+ / -) required.

I'm going to guess you don't have a surface grinder capable of getting something flat to .0001" (with all the problems of clamping, heat warping, etc. ) -- able to end up with a fine, flat surface finish ready for flaking.

In that case you get it flat as you can by milling or planing -- then scrape away the high points. Trying to hand lap a surface flat over a large area and with thousandths of inches to go -- and then scraping for an oil retention pattern -- is a far harder and less likely to be successful path. Particularly with sandpaper laid over a surface plate.

Yes sir, understood. It won't be easier is what you're telling me. Got it, I will to take your word on that. Thanks.
 
Read this "why are they scraped" It's part of my work booklet I give to my students. A simple pamphlet written by an Engineer from Do-All Grinder company. It was written before Power Scrapers were perfected.
You may have to sign up for a free 30 day trial. Why Are They Scraped Booklet | Machine Tool | Machining

(some of the questions remind me of my old buddy bill may have written them)

Much obliged, thank you Richard. I will read it and report back.
 
Actually the thread title made me think of something else - a tool that would let you add metal to the low places in very small increments. I thought, what a wonderful idea for worn machines, reverse scraping where you go blip, blip, blip and start building up to the level of the unworn parts. Even there you could overdo it and would then have get your regular scraper and take a little off the top here and there.

Maybe that's in the future for 3D printers. Map the existing wear, plug it into the computer, set up the machine and let it go back and forth spitting a tenth or two of plating where it's needed. When the guy's patent comes out, say you saw it here first.
 
Actually the thread title made me think of something else - a tool that would let you add metal to the low places in very small increments. I thought, what a wonderful idea for worn machines, reverse scraping where you go blip, blip, blip and start building up to the level of the unworn parts. Even there you could overdo it and would then have get your regular scraper and take a little off the top here and there.

There is sort of tool like that. Brush-on electroplating:
SIFCO ASC : What is Brush Plating? by Derek Vanek
 
Not surprising.

Another question, is canode easier to spread than dykem high spot? Because I'm having a hell of a time spreading the dykem out. Thinning a little bit helps, I'm just wondering if canode is just easier.

Edit: yes I have a brayer, but it's a piece of crap, it's not flat at all and doesn't even kind of work.


Jamie
I use what I call the SNOW PLOW technique to spread the dykem.

Angle your braver over 30 to 40 degrees (just like a snow plow) which imparts a sliding motion to the braver. This will spread the dykum in one tenth the time.
 








 
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