Scraping

Hi

Can someone advise how to perform the scraping process in matching two sliding surfaces?

Also, why is this method used ? the machines used to grind a lathes bed would or at least I thought create a suitably smooth uniform finish, yet the process of rubbing blue onto the saddles and use a scraper to fine tune the mating surfaces is still necessary.

Thank you

George

There are plenty of articles and videos available to give the information and the how. But for simplicity, when two ground faces interact they create a stick and slip situation, and the surfaces can be completely devoid of lubricant. Scraping one surface leaves small pockets in which oil can reside. Bluing and scraping the surfaces allows you to match the two surfaces, removing high points. Typically the longest length, the bedway, is ground along with the matching part surface, but the this being shorter is blued and scraped. Scraping can also be used to correct small geometric errors on other non contact surfaces.

Some people take classes to learn the art of scraping and it is not a quick process unless you just want to remove the worst of the error.

George,

Have a look at this book: **LINK**

https://archive.org/details/whitworthmeasur00whitgoog

MichaelG.

There's a vast amount of info available on the 'net (as well as books, of course). Look for YouTube videos - far more useful than words for providing insight into the physical process. The philosophical justification for this tedious (but addictive) process is that one is working the scraped surface to match, as accurately as possible, a highly-accurate, non-wearing, reference surface, such as a surface plate, straight edge, etc. One may also be scraping a part to fit as well as possible, another part. It is even possible to scrape gas-tight joints which need no gasket or sealant. Machining operations are limited in accuracy by the intrinsic accuracy of the machine, which won't be as accurate as a reference standard. By accuracy, in this discussion, I mean as close as possible to the chosen reference standard - flat, straight, cylindrical, a given angle, etc.

The other advantages of scraping are that one can control the quality of the surface finish (oil pockets, etc.) and, given large enough reference standards, one can bring dauntingly awkward things like long slideways to high levels of accuracy with the use of simple hand tools - and a lot of patience. There's quite a few videos showing how cheap machine tools made of butchered chineesium can be made far better than as-bought. The factory 'scraping' that may appear on them is usually shown to be decoration to fool the gullible...

Before the advent of high performance sealants the company I worked for scraped all horizontal gearbox halving joints, checking them with blue, despite them being ground. The grinding process was somewhat crude so a better joint was required.

It was also standard practice to bed in shafts into white metal bearings by scraping and blueing as well. The gearboxes were in the order of three tons upwards...

I still have my "Golden Virginia" tin of engineers blue and some small scrapers made from old files from when I was an apprentice but could do with a decent scraper, the modern replaceable blade ones were so much easier to use I remember.

As already mentioned there is much to learn on youtube.

Here is a good place to start

BTW his channel is excellent!

Regards.

David.

Not really necessary in most cases. Are you referring to a new lathe you have just bought? Unless there are problems, should be good to use as is. Beginner level amateur scraping may or may not improve it. Most new lathes today have ground ways but they are sufficiently rough finish to retain oil. Oil retention would only be a problem on superfine polished finishes, which you almost certainly don't have.

Edited By Hopper on 25/06/2019 09:10:46

Isn't Scraping the process of achiving truly flat mating surfaces and Frosting the process of creating minute shallow pockets to hold oil? So really a ground surface would not be scraped but frosted (even though you do it with a scraper).

Just saying ;0)

regards Martin

Once upon a time scraping was the only way of making a flat surface. Took skilled men a long time to get right. Not so today: precision grinding, lapping and micro-finishing all produce high-quality finishes. Gauge blocks, arguably the best surface to be found outside a laboratory, are not scraped. Nor are granite surface tables, or gas turbine blades.

My feeling is scraping is only worth doing in two circumstances today:

1. To improve oil retention on an already good surface that needs to take exceptional wear.
2. To manually correct minor flaws on cheaper machines. No doubt at all that hobby lathes and mills benefit from fettling. If they can be improved, why not?

Unfortunately because hand scraping is obviously expensive it's widely imitated as an indicator of 'quality'. Pretty sure the frosting on my milling machine wasn't put on by a master craftsman! Looks good but may not be worth paying extra for. Scraping to life-extend hard working professional machine tools and giant bearings is one thing, but for most of us precision grinding is more than 'good enough'.

Dave

Scraping is most certainly worth doing in a most important third circumstance - the serious re-building (or improvement) of machine tools - not just minor fettling of cheaper machines! It is certainly practicable for the determined amateur (as well as the professional, of course) to bring well-worn, old machines back to as-new, or better, standards of accuracy. And all with simple hand tools (and power scrapers for the very well-heeled).

Edited By Kiwi Bloke 1 on 25/06/2019 11:26:53

Edited By Kiwi Bloke 1 on 25/06/2019 11:33:00

Yes indeed. Scraping cuts are typically .0002" deep. Flaking (or frosting) cuts five to ten times that.

Deleted bragging.

Edited By Mark Rand on 25/06/2019 23:36:39

Who let the cunning linguists in? Frosting is a variety of scraping. It's done with a scraper. It's covered in every good book or training course on scraping.

Edited By Hopper on 26/06/2019 00:24:17

My experience of scraping is that it doesn't need to be pretty to work - you can make a right pig's ear of the appearance while achieving exactly what you want.

For most machine tool applications scraping is overkill, it's possible to finish machine surfaces to any required surface quality and accuracy. Bear in mind gauge blocks are ground and lapped not scraped - you won't want your slides to be finished to that level of accuracy as your machine would lock solid!

Scraping these days is chiefly a useful approach for improving fits by hand.

Neil

Machines are still rebuilt by scraping, presumably because of a combination of the accuracy achievable and the ease with which long dovetails, etc. can be attacked. No need to hoist heavy machines into awkward set-ups for machining or grinding. Slideway grinders are thin on the ground, and you'd still want to scrape. It's quite possible to scrape several thou off slideways, to remove the effects of wear, and then bring the surface to a few tenths all over, and to a similar level of accuracy of alignment to another surface. And all with hand tools...

It's hard and tedious work, although fascinating. I scraped all the sliding surfaces of a late model Senior Universal. It took ages, and much metal was shifted. It was hardly worn, but alignment was all over the place. Perhaps it was a Friday job, but it made a bit of a joke of the frequent advice to buy British if you want quality. For the next rebuild, I'll buy a Biax power scraper - to hell with the cost, I'm not getting any younger, damnit!

There are videos around showing Swiss commercial rebuilders of Schaublin (etc.) lathes, scraping away. At the other end of the scale, there are videos showing how bad slideways may be, when roughly hewn out of chineesium, and how readily they may be corrected by hand-scraping. There's actually a lot of it about...

On my Taiwanese lathe the bed is hardened and ground, the saddle etc are scraped to fit, this forming the oil retention area.

Scrapers, a good second life for old files, or you can braze or clamp an old carbide tip on the end of a bar with a comfortable handle on it.

Ian S C

I did say that.

I was answering the question why scrape a prescision ground surface. Because you are not scraping to achieve flatness you are frosting to hold oil. Calling both scraping(which I agree they both are to some extent) is what caused the confusion so in order to indicate the purpose it's better use the term frosting.

I would be correct to call you an ape but more accurate and descriminating not to say polite to call you a human being.

So sometimes the employment of accurate terminolgy is about better communication and not just showing away etymologigal prowess (.....he said tongue in cheek).

regards Martin

.

Martin,

Have you considered the wide range of available definitions of 'frosting' ?

MichaelG.

Yes but you do should always take into consideration context. It is language after all the ambiguity of which is demonstrated by it's ability to create a niche market for lawyers to argue meaning.

"I know that you heard what I said but you don't seem to have heard what I mean" is an oft used cry.

regards Martin