Hand Scraping - How can it be more accurate than machining?

[MinnesotaHSM]

I have seen various threads on hand scraping and people wanting to learn hand scraping, but the question I always have is: How can hand scraping ever be more accurate than a machined part or a CNC machined part? When is scraping a better, more accurate technique than using a mill/lathe?

Thanks.

- T

[rotate]

Scraping if often employed when the resurfacing has to be performed in situ.

[dp]

I have seen various threads on hand scraping and people wanting to learn hand scraping, but the question I always have is: How can hand scraping ever be more accurate than a machined part or a CNC machined part? When is scraping a better, more accurate technique than using a mill/lathe?

Thanks.

- T

Hand scraping is better when the piece of work you have won't fit on the mill/lathe. The bed of your only lathe, for example. Another case is when it is too expensive to machine the part. The bed of your only lathe, to repeat the example.

Hand scraping 1-2-3 blocks, while an excellent exercise for the student, is probably not going to replace machined and ground 1-2-3 blocks. Certainly there are other examples though where scraping is the better solution for a particular problem.

[ptjw7uk]

Hand scraping is only as good as the surface you are using as the master.

In machine grinding the wear on the wheel has to be taken into account as well as the trueness of the slides on the bed which will change with use.
In hand scrapping the master surface has only minimal wear if any.

Hand scraped surface plates are made in 3's in which each in turn is used as the master so finally you end up with 3 plates true to each other.

I assume by now you could get a machine made hardened plate that is better than a scraped one but I expect that you would pay handsomely for it.

Just my 2p worth

Peter

[juergenwt]

Yes we now have grinders that can grind just about any length, but before you can build a grinder or mill or lathe bed you have to have something to work from.A standard has to be set up like a prototype on which all others are based. And like any prototype you have nothing to compare it to.
Some of the most famous machine tool builders have special clean rooms where they keep the best machines money can buy as a prototype for their product.
So the "Prototype" is most likely something made by highly skilled craftsmen and built to perfection. Scraping a surface is one of those methods used to achieve the results needed for a prototype.
As to your question, one answer is: a scraped surface on a machine bed besides being very precise (and costly) has the ability to maintain an oil film much better than a ground finish.

[Mcgyver]

I have seen various threads on hand scraping and people wanting to learn hand scraping, but the question I always have is: How can hand scraping ever be more accurate than a machined part or a CNC machined part? When is scraping a better, more accurate technique than using a mill/lathe?

Thanks.

- T

in most cases, there's not much advantage in scraping over grinding - I can grind as well as i can scrape, but both should be more accurate than work in the mill or lathe. The advantage comes in as others have noted, lots of work is too big for surface grinder and if making references, a scraped surface takes the blue better than a ground one.

End of the day though, probably the greatest appeal for many home shop guys is that very accurate stuff can be done without the need for grinding equipment. I view it as a basic shop skill, sort of like filing; I don't run around all day figuring out what i can file, but as a basic bench skill if you don't have it the work will suffer. There are times when its just right technique to deploy. That, and in small doses, its kind of pleasant zone-out-with-some-nice-music-on work.

scraping is accurate to the accuracy of your reference, ie surface plate that determines how flat the results are. a good surface place over a couple of feet would be more accurate than the average lathe or mill. Maybe the part you are missing is that the scraper makes a very shallow depth of cut, a tenth or so on light finish passes. Using the blue to identify where to scrape, you gradually bring the surface to the same flatness as the reference. Also, unlike machining, there is no clamping force that distorts the work.

[old-biker-uk]

End of the day though, probably the greatest appeal for many home shop guys is that very accurate stuff can be done without the need for grinding equipment.

That's me - no grinder, plenty of scrapers (and time).
Mark

[PackardV8]

Grinding or milling a large/long piece of cast iron is no more accurate than the mounting on the table. Then, the workpiece is removed from the table and moved several times and mounted to a work floor. All sorts of distortions occur during a machine move, especially a long lathe bed.

Alternately, adjust and level machine in place, shim the mounting as accurately as possible, lock it down and then scrape in situ and WYSIWYG.

thnx, jack vines

[.RC.]

I have seen various threads on hand scraping and people wanting to learn hand scraping, but the question I always have is: How can hand scraping ever be more accurate than a machined part or a CNC machined part? When is scraping a better, more accurate technique than using a mill/lathe?

Thanks.

- T

It mostly comes down to time...Grinders these days with CNC able to compensate for inaccuracies in the machine ways are incredibly accurate and very fast compared to scraping with is incredibly slow..

Machine tools used to be all scraped because labour was cheap, and machines expensive...Now it is the opposite..

[Evan]

Mitsubishi Heavy Industries used to have a picture on their machine tools web pages showing one of thier very large horizontal lathes in the finishing stages of production. In a spotlessly clean hall sat this machine with bedways 40 feet long. The only person in sight was a fellow in uniform sat on one of the bedways.

The caption read something to the effect, "Master craftsman Mr Hiro Mitagami is hand scraping the ways on this machine. It will take as long as he needs to be satisfied the job is done right"

[dp]

Yep - another case of no place to go for grinding a particular part as it's the biggest part in the building . I'll wager a lot that the grinding done on that machine brought things damn close to final dimension. As stated it almost sounds like the gentleman knocked the casting material away and started scraping the rough casting.

I was browsing historic sites recently and saw a very large lathe bed from a well known vendor sitting on the table of a huge precision ways grinder. If it can be avoided it is difficult to justify scraping to final dimension. For certain categories of machine it is also difficult to justify not hand frosting the ways after the grinding is done but frosting has a different purpose than dimensional scraping.

[Forrest Addy]

I teach a class on basic hand scraping. It's an obsolete crafy now having only niche application but it does have its virtues whenever some one with time and dedication comes up with a jewel of a machine tool in need of reconditioning. So for most home shop people and small operators hand scraping can restore machine tools to original fits and accuracies if cunningly applied and a definite plan is followed.

Most of my students discover they can work to an easily verified 50 millionths of flatness, parallelism, squareness, and eventually dimension on the 1-2-3 block project pieice I issue at the start of the class. So why spend 30 hours on a project that when completes results in a set of four matched 1-2-3 blocks when hardened and ground blocks can be purcchased for $10 a pair? Beats me. Personal satisfaction? A start on skill building? A preface to grander projects where precision hand scraping rescues a worn out machine tool from the foundry ladle?

I make this stunning pair revalation: precison hand scraping witlessly applied can be a huge waste of time and effort. Precision hand scraping shrewdly applied can equip someone with few resources but bulldog determination with a shop full of resurrected older machine tools having their original accuracy, linearity, geometry etc restored via hard work and a hand scraper.

The problem with precision hand scraping is that in ineitably requires ancillary tooling that is either hard to find, expensive, or needs to be built from scratch. I refer to 10 arc second levels, straight edges, scraped triangles, and of course the 55 gallon drum of elbow grease and bax car load of dedication to pursue skill building to competency in machine tool reconditioning.

Thene there is the narure of a scraped cast iron surface as a linear bearing. Cast iron, because of the free graphite in its substance, has a limited self lubricating property. This coupled with the faint undulations of a scraped surface, and the effect of boundry lubrication results in a durable linear bearing having high lateral stiffness. The secret of this bearing is oil lies in the spaces between the bearing points and the points them selves are not points but tiny areas randomly distributed. No one pair of opposing bearing poing has to carry load over more than its own area. The load is reansferred from point to freshly lubricated point

Until hardened steel ways and bonded plastic composite bearing materials came along in the '70's scraped cast iron was the linear baring of choice in machine tool construction. A proliferation of rolling element bearings, way baring materials, and high precision way grinders has made a dinosour of the scraper hand, once a foundation stone of the industry.

There are other subtle points that could be made but I've ranted long enough.

[Evan]

Knowing how the Japanese work he wasn't doing it all by himself. He would have a small army of assistants to speed the work but because he is entirely and solely responsible for the finished product he alone gets the credit. When he retires/d then everybody moves up the ladder one rung.

[Liger Zero]

From what little I've learned of scraping you can take the scraper and master with you into inaccessible locations and spot-scrape rather than dismantle the gigantic machine and mount the part on a grinder.

[pcarpenter]

As Forrest pointed out, you can, in fact, scrape a surface flatter than *most* grinding will produce over a long way surface...and way flatter than a mill will produce. Still, few have answered the original question about *how* it can produce a flatter surface than say milling (or grinding which is even more precise). In scraping milling machines, I learned and read again in Connelly's book on the topic that typical way surface flatness for a knee mill will be on the order of .0005" per foot. That means that you can't expect better product than the machine tolerances. Since you scrape the machine to that tolerance, each subsequent machine made by the chain of milling machines would get slightly less precise.

Milling and grinding have as a minimal limiting factor, the flatness of the machine ways and table (plus the tolerances in the spindle etc.) Scraping relies only on the flatness of the master. Going a step further, you don't rely on the flatness of any one area of the master, but instead, you get the *average* flatness of the scraping master because it is intentionally moved around. In scraping a large flat surface, you even change the total orientation of the master in making a rub over the entire scraping process so as to create this "aggregate" flat reference. In the case of say dovetail way surfaces, you often cannot flip your scraping straightedge (it likely only has the angled surface to reach under the dovetail on one side). However, you do work it back and forth so as to not continually have only one set of bearing points on the reference master marking the same spot on the way surface.

To add to the "why" answers you got already....the guy who tought me to scrape had been pulled (many decades ago) from his original role as a machinist to be a "mechanic" (machinery mechanic) by Caterpillar. They had big Devlieg boring mills that they scraped in house. He was left handed. As such, he was able to work from a direction they needed to work on the huge way surfaces on those machines and could reach "from the other side". So, scraping is often a process that is practical in a situation where grinding is not an easy reach. It also saves moving some absolutely huge machinery completely out and then back into its production location with the accompanying re-levelling and setup work.

Edit-- In re-thinking about how well (or not) I may have answered the OP's question, I hit on this. I remember struggling years ago with understanding how any (seemingly crude) process like this could result in some of the most amazingly flat surfaces. The answer is in the process and the geometry. While three points can define a plane, you have the ability to put literally millions of points in that same plane by setting a reference on a few points and working until more and more of those points touch the same flat reference. You may start with very few bearing points in a scraping job and you keep knocking them down until those and still others contact the reference. Then you knock that set of points down until more are touching the reference. Think of it like slicing the peaks off of mountains in a mountain range until the tallest points are at the same level as the lowest ones. After a while, you have a bunch of plateaus. The reason that the seemingly crude act of scraping at metal by hand with no way surfaces to define the cut can result in a fairly even cutting of these high points comes from careful technique, but also because you are cutting a difficult to cut material with limited force. You have to really work to cut one part deeper than another. Stoning of the scraped surface knocks off the burrs between passes. *Many* passes produce the desired end result while any one pass taken by itself may seem to have been a step backward the next time you mark your work to see where you are at.

Paul