Why Are They Scraped Booklet

8/11/2019 Why Are They Scraped Booklet

1/5

Hand

Scraping

faces,

but then you would have the

problem

of prevent ing the oil f rom

running out, or of repla cing it

fast

enough. (Oil grooving is normally

added

to

h e i p d i s t r i b u t e

the

oil,

whether or not the

ways

ar escraped.)

This d iscussion could lead one to

wonder about

the

e f f e c t

of the

amount

of

area

in

contact

on a set

of

ways.

Hand scraping

reduces

the

contact area bu t it 's d ist r ibuted even-

ly, and it is the distribution that's im-

portant.The

flatter

the two mating

surfaces

are, the more

even

the dis-

tribution of the con tac t

areas.

Bu t

there

is a

principle

o f mechanics that

says

friction is independent of area.

This

meansthat itwill takethesame

force

to

move

the

table whether there

are

10

square

inches

or 100

square

inches

in contact . (Wear is

another

load,th e faster the wear.)

Th e

main

point

is that it 's better

lubricationwe areaf ter,not moreor

less

contact area. If Jubricat ion were

perfect, the wayswould never wear

out. I f a t ab le mov es harder as i t

wears,it is probably

related

to

lubri-

cation and

c e r t a i n l y

not to the

amount

of

contact area.

How

Scraping

Is

Done

The

purpose here is not to teach

th e art of scraping, but only to give

an

idea of wha t is done. Although the

doing isdifficult, the actual idea be-

hind

it is simple enough.

be

scraped

off,a blue pasteis applied

to the

master

f ixture either aflal

surface or a

V-way that

is the

opposite

of the shape being scrapedso that

when

th e

blued master

is

rubbed over

the way surface, the bluing

transfers

to the high spots of theway.T heblue

high

spots

ar e

then scraped

off by

hand

with a

special scraping tool.

This operat ion

of

rubbing

th e

blued

master

an dscrap ingoff thehigh spots

s

repeated until the way

sur face

shows a uniform transfer of blue.

There

are,

of course, an y

number

of

techniques that

a

scraper

must

know.

To

me nt ion

just

two,

a

dull

file

is usually

rubbed very lightly over the

sur face

between bluing applicat ions

to remove an y

burrs.

An d

rags

ar e

never used to

wipe

the surface; a

brush or the

bare hands

ar e

used.

If

rags

we re used, t iny pieces oflint lef t

behindwould causemislea ding mark-

ings

at the next application of bluing.

A scraper does

his own

inspecting

s he works when he compares his

wa y

su r face with th e master. Regard-

ing inspection,allthe inspec tor could

when to

quit

scraping.

In the

past,

we had standards as to

ho w many

spots

of bearing su r face

pe r

square inch th e surface must

have

a ndwhatpercentage of the

total

found

that

.thepercentage

of

contact

area is almost impossible to

check-

and the

spots

p er

square

inchare now

le f t

to the judgment of ihe scraper .

H ow ev e r , h e n o rm a l l y s t r i v e s fo r

about

twenty to

thirty

spots per

square

inch.

In the

scraping process,

some of

the roughing work is now done with

powered hand

scrapers.

They arestill

the hard work out of the scraping

operat ion.

When you get down to the

f inal fitting though, there's still no

substitute for the. feel of straight

hand scraping. nuns

W H Y

A R E

THEY H A N D

S C R A P E D ?

R e p r i n t e d

from

MO ERN M CHINESHOP

600 Main

Street

Cincinnati

Ohio

452 2


2/5

W H Y A R E

T H E Y H A N D

S C R A P E D ?

Iy H Schanzenbach

Technical Editor

DoAll

Company

De s

Plaines, Illinois

is

nothing

like the

"tee "

of the

mpte

hand tool for the final finish.

/ays

that

ar e f lat

and Improved Lubrication

nd saddle ways that are flat a nd

match

within

tolerance, only

to find

ihat

the table is not quite parallel

withthe

spindle

to therequiredtoler-

ance. Can you imagine th eskill that

wouldbe

needed

to

remove just

the

right amount

of

metal from

the

right

placeswith a

scraper, without losing

the flatness, to adequately correct the

This

wa scertainly not the original

purpose of hand sc rap ingand should

not be

relied

on to

correct

any

large

misalignments. Nevertheless, a skill-

ed

scraper

can make

this

sortof

cor-

rection in a surprisingly short t ime.

Despite the skill required, this ap-

proach

is

less expensive than

the al-

ternat ives

of holding closer machin-

ing

to l e rances

on a number of parts,

or building in reliable and tamper-

proof ad jus tments designed to cor-

rec t

th e line-up.

Experience hasproved that scraped

ways reduce friction through better

lubrication,

but

opinions

as to why

differ somewhat. The mos t

popular

theory seems to be that th e scrape

pockets(or more specifically the flake

pocketsextra deep pockets

deliber-

ately added for lubrication) provide

many pockets

of oil

that

are

shear-

ed off by the microscopic bearing

surfaces.

Another logical theory is that we

comeclosetofloatingthe top

surface

continuously

on an oil film,

which

is

the goal

in all lubrication. The main

reason this happens

is

that

the ir-

regular patternof the oilpockets pro-

v ides avery difficult

escape route

fo r

the oil. Theideal situation for lubri-

cationwould

be to

have

afilmof oil

between two perfectly smooth sur-


3/5

os t of

the

mughing-ln

work is made

With

hand

scraped

ways

this surface

easier by

using

a powered hand

tool,

grinderproducesaf latness of0.0005inch.

Hand scraping is an artistic

skill

more difficult than wood carving. It

is

the

starting point

to

obtain

the

basic

accuracy required

for

today's

precision

machine tools. Handscrap-

racy

of other machine tools andelim-

i na tesdistortions caused

byclamping

forces

and the generation of heat.

Hand-scraped

waysare

more friction-

freethanks to better lubrication.

There are

many

techniquesthe

scrap-

willgive him that feel hemust

have

to

obtain precise

flatness.

When you walk

through

a machine

tool builder's shop and see men

scraping ways

by

hand,

you

can't

helpwondering, Can they really

im-

prove on today's machined

surfaces

by

handscraping "TheDoAII Com-

pany

says

the answer is no, if you're

talking about measured surface fin-

ishbut there are

still

some excellent

reasons for hand scraping. For one

eliminated. Machine tools are

used

to

manufacture other machine tools,

but

ca n nevermakea reproduction more

accurate than the original.So there

must be

a

starting point where the

first machinedoesnot rely onanother

machine

for its

bas ic accuracy.

In

other words,

you

have

to

start

with

human

effort;

inthis

case,

in the form

of hand scraping.

Hand scraping is not, as it may

seem, free-hand work. Rather, it's

a

method of making a near-perfect

copyof a masterthatwas

a lso

ahand-

generated surface.

Scraping, thoughhard work, is an

artistic skill.

It

would probably be

easier

to makewood

carvers

out of

most

of us

than hand sc r aper s .

Not

much published material

seems

to be

available on the subject especially

on

why. This

is

probably because

it is considered an art.

Where

Do You

Start?

If a manufacturer decided to grind,

rather

than hand

scrape, theways on

a

su r f acegrinder,he must have abet-

ter c lass of accuracy in the parent

grinder thanhe

plans

to have on the

newone.Sowhere doesthe

first

ma-

chine get its accu r acy? It has to be

or it has to rely on someother meth-

od that

will

generatea truly flatsur-

face,or that

will

come close to copy-

ing a

generated

flatsurface.

Theideaof ageneratedsu r f acecan

be

illustrated by comparing th ree

methods of making a circle. (Acircle

isaline

rather than

a

surface,

but it

illustratesthe idea.)Withan

ordinary

compass

a draftsman can theoretical-

ly generateaperfect circle.If he runs

his pencilaround an existing hole in

a plastic template,

he

copies

any in-

accuracies

in the

existing

hole. If he

draws the circle free-hand, the accu-

racy

will

depend upon

his

limited

skill.


4/5

Scraped

saddle

wa y shows oil pockets,"

oil

feeder

hole,a nd

short

feeder

grooves.

A

scraperusinga

master

fixture to trans- A partly scraped way with a fresh appl i-

ler

bluing

onto

the

ways

of a workplece. cation of bluing to

reveal

the high

spots.

Perfectly flat sur faces cantheoreti-

callybe generated by alternately rub-

bing

(lapping) three

sur faces

together.

Forsimplicity,let'ssayt's

th r ee

rocks,

each of which has a fairly flat

side.

If you

rubbed these flat

sidestogether

alternately in some

kind

of random

sequence,

you

would

come

closer

and closer

to getting all th ree sur-

faces

flat.

If you

used only

two

rocks,

youcouldget a matched set, but one

could

beconcaveand the

other

1

con-

vex.

In

practice, scraping,

instead of

lapping,

is

used

and a

definitepairing-

off

sequence must

be

followed.This

principle has

been used

to

make

the

master

fixture

that the

hand

scraper

uses.

In using thefixture,thescraperputs

bluing

on the

mas te r

and slides it over

the

su r f ace that

he is

working

on to

reveal where

additional

scraping

is

needed.

As he

repeats this process,

he

comes closer

and

closer

to

making

aperfect copy of the mastersurface,

which wa sa truly generated

surface.

Most grinder

castings

that

are

sc r aped

are

milled

to

within

a few

thousandths

of the final dimensions

and f la tness and then st ress relieved

( hea t

treated to relieve residual

s tresses).

They

are

then

sent

back

for

lightclean-up machiningbeforehand

scraping. Although hand scraping

takes a lot of

time

with

consequent

high labor costs, it

takes

the place of

alternate

p r ocesses

that

would

entail

high equipment costs.To

eliminate

scraping,

the partswouldhave to be

routed

to

expensivemachinery

of the

highest

precision available for final

machining.

Besides the

high

equipment

costs

thatwould be involved in the

latter

process, another consideration ap-

plies, in machining a part, especially

a

large

casting, some kind of heavy

clamping

or holding is

necessary.

When machining to a fractionof a

thousandth, this clamping force can

cause

enough distortion in thework-

piece to jeopardize the accuracy of

the

workpiece after unclamping.The

heatgeneratedduring machiningcan

also causedistortions.

That's

one of the

beautifulJhings

about

scraping. Thereare noclamp-

ing

forces

and the

heat generated

is

practicallynl. Thecastingis support-

ed atonly three points to make sure

itdoesntdeflect from its own

weight.

.When 3

machine

tool with

scraped

ways gets

worn,

it can be

recondi-

tionedby rescraping.

This

is agreat

advantage compared to the alterna-

tives

of discarding the

machine

or

sending it back to the factory to be

completely torn down, remachined,

and so on-

When the ways on a machine tool

need rescraping it can be done by

service

men from the factory, but it

is

often possible to finda man locally

to do the

rescraping.

Hand scraping can be used,within

limits,toobtain finalgeometric line-

upwith

other machine elements.

Visualize a hand-scraped set of table


5/5

Perfectly

flat

sur faces can

theoreti-

callybe

generated

byalternately rub-

bing(lapping) threesu r f aces

together.

For

simplicity, let's

say

t's

(hree rocks,

each of

which

has a fairly flat

side.

If yourubbed

these

flat

s ides

together

alternately in some

kind

of random

sequence,

you would come closer

and closer

to getting

all three

sur-

faces

flat.

If you used

only

two rocks,

youcouldget amatched set,but one

could

be

concave

and the

other

con-

vex.

In

practice, scraping,

instead of

lapping,

is used and a

definite pairing-

off sequence must

be

followed.This

principle hasbeen usedto

make

the

master

fixture

that the

hand

scraper

uses.

In

using

the

fixture,

thescraper

puts

bluing

on themasterandslidesit

over

the

su r f ace

lhat he isworking on to

reveal where

additional scraping is

needed.

As he

repeats this

process,

he comes

closer

and closer to

making

a perfect

copy

of the master

surface,

which

was a

truly

generated su r f ace .

Scraped saddle

wa y shows oil pockets,"

oil

feeder

hole,

a nd

short feeder grooves.

Most grinder

castings that are

scraped

are

milled

to

within

a few

housandths of the

final dimensions

nd f la tness and

then

st ress

relieved

hea t

treated to relieve residual

:resses).

They

are

then

sent back for

ght

clean-up machining before hand

:raping. Although hand

scraping

jkes a lot of time

with consequent

high laborcosts, tttakes the place of

alternate

p r ocesses

that

woutd

entail

high

equipment

costs.

To

eliminate

scraping, the partswouldhave to be

routed toexpensive

machinery

of the

highest

precision

available for final

Besides the high

equipment

costs

thatwould be

involved

in the latter

process,

another consideration ap-

plies.

In

machiningapart,

especially

a large

casting, some

kind of heavy

clamping or

hoding

is

necessary.

When machining to a

fraction

of a

thousandth,

this clamping force

can

cause

enough distortion in

the

work-

piece to

jeopardize

the

accuracy

of

A

scraper using amaster

fixture

to trans- A partly scraped way

with

a fresh appl i-

fer

bluing

onto

the

ways of a

workplece. cation

of

bluing

to reveal the

high

spots.

the

workpiece

after

unclamping.

The

heat generated

during machiningcan

also cause

distortions.

That's

one of the

beautiful Jhings

about

scraping. There

are no

clamp-

ing

forces

and the

heat generated

is

practicallynl. Thecastingis support-

ed at

only three points

to make

sure

it doesn't deflect from its own weight.

.When a

machine tool

with scraped

ways

gets

worn,

it can be

recondi-

tionedby rescraping.

This

is a

great

advantage

compared

to the alterna-

tives

of

discarding

the

machine

or

sending it

back

to the factory to be

completely torn down,

remachined,

and so on-

When the

ways

on amachinetool

need rescraping it can be done by

service

men from the factory, but it

is

often possible

to finda man

locally

to do the

rescraping.

Hand

scraping can be used,

within

limits,to

obtain

final

geometric line-

upwith

other machine elements.

Visualize

ahand-scrapedset of table

Documents

Why Are They Scraped Booklet