Why scrape when you can grind?

[jscpm]

I don't completely understand the scraping thing.

Why not just grind the surface flat? Spending hours laboriously hand scraping something seems kind of pointless if it can be ground much faster. I don't see people scraping CNC guide rails.

In one book it talked about the scraped surface providing recesses for "lubrication". Well, first of all, you don't lubricate surface plates and a lot of other flat things. Secondly, if you do need some kind of recessing, you can always jewel the surface after it is ground or provide other texturing in a fast automated manner.

 

[AndySomogyi]

A huge reason to scrape is if you don’t have a grinder, and grind shops quote insane prices to individuals because they don’t want their business.

Also you never want a smooth on smooth contact, that will cause stick-slip. Scraping provides a slightly uneven surface for oil retention, you should have at least one surface scraped.


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[Limy Sami]

Scraping is the final mating of parts ........ even GOOD surface grinding can leave mismatches.

 

[eKretz]

Also, scraping is generally done in the free state. No clamping, no real heat generated. You will also be comparing to a high quality flatness standard if you are doing it right. Tough to beat for global (macro level) flatness. Grinding will get you pretty darn close, but won't get you as close as scraping (caveat: done correctly) unless you have a basically brand new, (read: unworn) very expensive machine, run by someone who knows what they are doing.

 

[jscpm]

Also, scraping is generally done in the free state. No clamping, no real heat generated. You will also be comparing to a high quality flatness standard if you are doing it right. Tough to beat for global (macro level) flatness. Grinding will get you pretty darn close, but won't get you as close as scraping (caveat: done correctly) unless you have a basically brand new, (read: unworn) very expensive machine, run by someone who knows what they are doing.

This kind of corresponds to some of the papers I have read on scraping. They are like "grinding does not actually leave the surface flat" it apparently mounds the metal around into shallow dunes apparently, which seems kind of counter intuitive because you kind of think of grinding as a flattening process, so how is it that it fails to process flatness.

 

[Tyrone Shoelaces]

When slideways grinders came out at first machine tool makers thought all their Christmas’s had come at once. That was until slideways started ‘firing up” because of the lack of lubrication. Then they realised that the moving surfaces still needed scraping to provide the space for the oil.

Regards Tyrone.

 

[DavidScott]

If you have ever done the two you would understand. When you are trying to get two parts in near perfect alignment scraping can't be beat. It is slow but being slow makes it harder to go too far and screw up.

 

[eKretz]

This kind of corresponds to some of the papers I have read on scraping. They are like "grinding does not actually leave the surface flat" it apparently mounds the metal around into shallow dunes apparently, which seems kind of counter intuitive because you kind of think of grinding as a flattening process, so how is it that it fails to process flatness.

It's not actually that it mounds the surface so much, but first, that it is hard to hold something down without distorting it in some way when you use either a clamp or a magnet or what have you to pull it down to resist the forces of the table and part moving back and forth or a grinding wheel acting on the part, etc. Once you grind as flat as you can and release those clamps or the magnet, guess what happens? Also, one false move with the grinding wheel that produces a burn mark, or in other words too much heat, can warp the part. That can't happen with scraping.

Second, a surface plate used for scraping should be approaching about as flat as is humanly possible to get. Easily as flat as an extremely high quality grinder's ways will be when brand new, if not flatter. The difference is the surface plate is used gently only for comparison and if it's used for commercial scraping, is generally checked every year and corrected if necessary by lapping. Grinders not so much, and they won't maintain that flatness. A rebuild on the ways of a grinder is not so quick, easy and inexpensive as a quick check, and if necessary, lapping of a surface plate. It is more economical to grind the parts as close as capable and do a precision scrape job to finish the part to a flatter level where and if necessary. There are some things where a good grind job is close enough and some that are not.

Edit: and I mean flatness in the global sense - averaged across the entire surface. At the local level there will of course be the highs and lows due to the scraping process that hold oil. For moving surfaces these help oil spread across the entire surface immediately as soon as the surfaces move across each other. They are generally tenths or less in depth, but that's a lot deeper than "jewelling" or engine turning. There's also flaking, which is even deeper.

And yeah, alignment is another important use of scraping - didn't think to mention that one since the main gist of the OP seemed to be geared toward flatness.

 

[michiganbuck]

I think grinding is a great way to rough something in, plus not many have a grinder big enough.

I know a few shops that have really big grinders that could do a nice/OK job of it.

 

[CalG]

I once worked for a very bright fellow. He came through the Swiss watch makers trade.

He was in the camp that one of the best surfaces for machine tool sliding members was obtained "right off the mill".

Of course, proper cutters and methods were part of the deal. I can find little to fault with the premise.

 

[52 Ford]

Scraped surfaces look perdy, too.

Much better than scrapped surfaces, at any rate.

 

[MCritchley]

There are dozens of reasons as why one would scrape vs grind. Let’s start with a static common bolted joint on a Horizontal boring mill between the column and bed. There isn’t a grinder on this planet that could grind the base of a column so they are milled. Once the erectors assemble the mill they would check the squareness of the column to the table. If the column is tipped back the scraper hand can disassemble the column and step scrape the bolted joint so the column tips forward a bit to compensate for tool pressure.

Hard to compensate for the castings free state with the weight of head and counterweight mounted and get the alignment perfect off the mill. It’s also good to have perfect contact between the two mating surfaces to have good contact, if they do not you will warp the casting when tightening mounting bolts. Scraping both mating surfaces can properly spread surface contact over the entire bolted joint.

Scraping isn’t just about making a flat surface, it’s very much about alignment also.

Scraping a sliding surface also is done for many reasons, such as reducing stick slip, matching way angles, scraping hollows to anticipate wear, or rebuilding a machine to restore its accuracy. Most builders do what they can to get around scraping. The table of my new Okuma mill has shims between the linear guideway cars and the table. I was quite pissed when I seen the shim not completely covering a linear guideway. The salesman that sold me the mill was surprised It was shimmed. That’s a commodity mill that is fine for what I am doing.
There are shops and builders that need a higher level of rigidity and accuracy that mandates a cast iron way. I believe Mitsui Seki’s are hand scraped cast iron hence their slow rapid speeds. Scraped ways offer incredible accuracy and rigidity due to the the more contact area vs a linear rail.

I scrape, machine, and grind. Each process has its place. Good question to ask for sure.

 

[crickets]

such as reducing stick slip

Stickiness between precision ground surfaces can be a big problem for sliding parts. Additionally there is no room left for the lubricants, so wear will increase. Which is where scraping is advantageous - peaks provide for the reduced contact between the parts, while valleys hold the oil.

 

[CalG]

Stickiness between precision ground surfaces can be a big problem for sliding parts. Additionally there is no room left for the lubricants, so wear will increase. Which is where scraping is advantageous - peaks provide for the reduced contact between the parts, while valleys hold the oil.

Most of the work I put out off the surface grinder has a VERY NICE oil holding pattern. ;-)

 

[CarbideBob]

Take two gauge blocks and wring them together.
Is this good or desired for a surface plate or machine ways?
Bob

 

[jscpm]

As I said in my original post, I don't get the lubrication/stiction argument because any texture, such as jeweling for example, can always be applied to a surface after it is ground.

 

[Mechanola]

To add a texture to a ground surface will not deliver the surface made by scraping. Huge difference. The main difference lies in the materials. You scrape cast parts and everything not hardened. Grinding applies preferred to hardened parts, say, for bearing rollers when you want to omit the inner ring.

There’s a process that goes beyond what’s feasible with grinding and scraping, lapping. Gauge blocks are lapped. Still, a scraped surface can be made to a defined balance between lands and hills. Nothing replaces that. If you just flake a ground surface or make a perlage you don’t know anything about the contact areas.

Each process has, like everything, two sides. The third dimension, lastly, is economy. Scraping costs wages for labour, no machine involved. Grinding can be done by hand, but is generally understood as machine-oriented. Lapping can be done manually as well, above all, if you have one-offs and or smaller parts. Nothing replaces the feel of a trained crafts/wo/man.

 

[boslab]

2 lapped or superfine surfaces if left in contact will weld, I’ve seen slip guage stacks left for several months that when broke apart tore lumps out of each other by cold welding, smooth and flat is different.
Mark

 

[MCritchley]

As I said in my original post, I don't get the lubrication/stiction argument because any texture, such as jeweling for example, can always be applied to a surface after it is ground.

By jewel it do you mean engine turning? I feel that modifying a ground surface would still have too much wringing potential. The oil would also have a hard time moving from swirl to swirl.
I am certain in the last 100 years companies have tried a lot of different techniques, we just have not seen them as they most likely failed after some time such as Tyrone suggested.

The beauty of a scrapped surface is that very little of it is truly flat. You want depth between the high spots for the oil film to spread out. It would be difficult to achieve a good oil flow pattern with out going back to scraping. The scraped gouges communicate with every gouge on the surface, much like a flow path.

 

[DMF_TomB]

on printing press cylinders (rolls) with a mirror finish about 12" dia 6 foot long I often hand sanded with
hand held 180 grit sheet a 45 degree cross hatch pattern in the surface the X lines might be .00005" to .0001"
deep but helps hold some liquid for better printing, (it wear off in a few months)
.
that is the mirror surface but with 180 grit cross hatch sanded areas didnt transfer liquid but
the microscopic amounts in cross hatch helped somehow, the areas that print had a diamond engraved
looked like sand blasted finish this was deep enough to hold liquid and transfer it onto to plastic film
.
doesnt take much to grind a mirror surface with coarser wheel but lines all straight
.
fine milling leaves a .0001 to .0004 TIR wavy surface that is waviness about .0050 to .1000" spacing if you
hand stone surface the shiny or shiner high spots appears, it takes a lot of lapping or stoning to remove .0001"
on surfaces 40" to 120" long. its easy to hand lap or sand a curve into a long >40" long surface, it requires
a way of measuring or marking up surface every maybe .0001" so you dont not increase the flatness error
.
if a block is only 4 square inches it takes a fair amount of lapping to remove .0001", i usually use a
magic marker and hand lap it off 2 times to remove .0001" I do not normally lap more than .0001"
.
i use StikIt adhesive backed sandpaper on a ground steel block, they also sell lapping film or abrasive coated
plastic rolls (adhesive coated or not coated comes either way) this is for wet lapping as plastic not as
effected by water. if you use 400 to 600 grit it removes maybe .00001" have to do maybe 10 times to remove .0001"
.
... lapping blocks come in different shapes you can lapped angled surfaces, many photographic paper film coating
is done where a "water" fall of 8 layers of chemicals is pouring over a sharp lip so 8 liquid layers are not disturbed
this is poured on to paper or film moving 1000 to 2000 feet per minutes, the coating lip needs hand lapping
periodically to clean and restore the sharp lapped edge. hard to describe