BELT POLISHING

by George J. Anselment PJorton Co., Worcester, Mass.

Basically, to run a coated abrasive belt require5 a power source, a tracking and tensioning device, and a method or unit to tranbfcr the power into driving the belt. A unit, therefol-e, consists of (Fig. I) the following: (A) a power source. (B) a drive wheel or contact wheel. (C) an idler for tensioning and tracking the belt, and (D) the proper coated abrasive belt.

POWER SOURCE

In the vast majority of off-hand belt applications (in this context. “off-h-hand” doe\ not imply “careless”: rather, it means with B hand-held workpiece). the amount of powa consumed is directly related to the operation being performed on the workpiece and/or the operator’s ability to apply pressure to the work. Naturally. an operation requiring grcatcr amounts of stock removal will require more pressure and, hence, consume more power than will a fine-finishing operation: however, since the same equipment with slight modification can accomplish both extremes (ix.. removing excess metal or fine polishing), the horsepouer (hp) requirement must be ample enough to encompass the most severe operating condition\.

Originally, the power source to drive coated abrasive belts was a converted buffing lathe. running at a fixed speed. As the industry progressed. proper belt speeds have been taken into consideration, and today a wide variety of variable-speed. single- or double-spindle “polishing jacks” are available to fit production needs.

It is seldom necessary to exceed 7% hp for driving a single-spindle unit or to use more than I5 hp for a double-spindle jack having a common power source and utilizing an operator simultaneously on each spindle. If the spindles are powered separately, 7% hp on each will normally suffice. Generally speaking, I to 2 hp per inch of belt width is sufficient; houevel-, special application conditions may need up to 5 hp per inch of belt width. Power-aaisted. work-holding devices WIII change horsepower requirements drastically, and 2.5. to SO-hp machines are not uncommon.

DRIVE WHEEL OR CONTACT WHEEL

Power is generally transmitted to the coated abrasive belt through a contact wheel, which is a multipurpose component and plays a crucial role (Table I) in stock removed per time

74

Tabl

e I.

Cont

act

Whe

els

Cog

to

oth

Rub

ber

70-9

5 D

urom

eter

V

ery

nggr

ewiv

e.

reta

rd\

dulli

ng

Slti

”di3

Kl

wrr

ated

Pla

in f

ace

“X”-

shap

ed

\rrra

tio”\

Flat

Flat

Flat

Rub

ber

Rub

her

Com

prw

ed

canv

a!,

Sol

id

sect

iona

l A

vaila

ble

five

dcns

ttvx.

cl

oth

SO

to 9

0 pl

ies

per

inch

Bur

r S

eCtlo

” cl

oth

T-95

D

urom

etel

40-9

5 rh

r0m

etet

35-7

0 D

urom

etct

Ava

ilabl

e se

vera

l de

nsiti

es

from

ex

tra

soft

to e

xtra

ha

rd

Med

un

to h

eavy

gr

indi

ng

Exc

elle

nt

stoc

k re

mov

al.

not

a\

seve

re

as c

og

toot

h

Ligh

t to

med

ium

gr

indi

ng

“MId

dIe-

of-th

e-ro

ad”ty

pe

whe

el,

l’mrr

\u

rl’xr

ro

ughn

cv

than

ab

ovc

Poh

shm

g to

lig

ht

grin

ding

Fo

r \e

ry

mild

co

ntou

r\ an

d hp

ht

stoc

k re

mov

al

Var

ies

\1 it

h de

nsity

fro

m

light

st

ock

rem

oval

to

fin

e po

lishe

s

Pol

ishi

ng

Con

tour

po

lishi

ng

Con

form

able

fo

r po

h4lin

g co

ntou

rs

and

irreg

ular

sh

ape7

For

heav

y st

ock

rem

oval

, su

ch

as

gate

s.

riser

r. et

c.

Not

as

agp

resi

ve

as c

og

toot

h.

depe

ndm

g on

la

nd t

o gr

oove

ra

t,<,

Mos

t co

mm

on

type

For

flatte

r su

rface

s or

w

here

be

lt m

ight

bc

pu

nctu

red

wth

w

ratio

ns

Mor

e ap

plic

ahlr

for

nonf

erro

w

parts

Ben

ch-ty

pe

grin

der

orie

nted

. A

ll-ar

ound

w

heel

Exc

elle

nt

for

all

type

s of

fin

ishi

ng.

May

be

pra

hape

d

Adj

usta

ble

for

wtd

th

and

dens

ity

interval, finish generated, belt life, and hence. cost of operation. Since it play\ such an important part in the succesr or failure of off-hand metal finishing, careful v&xtion of the contact wheel is paramount.

Contact wheels have been made of practically ecery uorkahlc material imaginable. ranging from cloth or building hoard to steel. each hervine a definite purpose toward the end result\. Today, commercially available contact wheel\ are u~ally made from WC of three materials: (I ) rubber or synthetic compounds; (3) fabric (cotton cloth 01 can\a\): or (3) metal (either solid or in combination with [rubber inserts).

Since the latter i\ used primarily for special application\. mo\t of the following remark\ will he devoted to the rubber- and fabric-type wheel\.

Ruhbrr-covered or synthetic compound (urethane) contact wheels are availahlc in Gcs ranging from under I inch In diameter to special design\ in the RO- to -K-inch diameter I-an:‘. each having its mche in application. Normal contact wheel diametcl-s for off-hand worh have u~ally hren designed to penel-ate sufficient clearance for wet-kpiece\ in relation to hearlnf hou\inp< and, on fixed-speed machines, to apply a pencrally acceptable surface feet pet mlnutc (sfpm) speed for running the coated abrasive hell. Many widths. usually in ‘5 01 I -inch increments. are also available to accommodate convenient hclt width\ for the particular application.

The factor\ controlling the performance of rubber wheels are the tollowlng: ( I ) thichness of the covering: (2) hardness of the covering: and (3) wheel face design.

If serrated, additional factors are (I) angle of serration: (2) ratio of land to Froove: (3) shape of land, width of land; and (4) depth of groove.

All of the\c factors detcl-mint the ahtlity of a gibcn abrasive belt to rcmocc unwanted stock or produce a desirable finish.

Thickness of the Covering The thicknes of the rubher or synthetic covering. coupled with the density. \\,ill

determine the amount of “cushion” or “give” on a particular contact wheel. A thin COCCI- M ill not compres\ readily and may retard ihc capacity to develop flni\heh. Convesely. a COCCI- that is too thick may gibe too much cushion and retard the wheel’s cutting ability. Genera-purpo\r contact wjheels arc covered to a nominal radial thicknc\\ of :/1 inch, hut special-pul-po\c wheel% may vary somewhat.

Hardness of the Covering Unless otherwise stated. the hardness of ruhher contact wheel\ i\ \peciflcd in Shore “A”

scale durometer rcadingh. The higher number\ denote harder compounds, and loucr numbers represent softer compound\. Hnrdrr contact wheels will remove unwanted material at a fu\ter rate than will softer contact wheels hut produce a finish with a larger rms reading than a \trfter durometrr contact wheel used with the ume grit coated ahrasicc belt. and as would he expected. rafter contact wheels will product a superior finish at the expense of +\vct material removal. It is impos\ihle for one contact wheel to give the highest lratc of \toch removal yet generate the hut possible finish. Higher rotational speeds through centrifugal force caue contact wheels to perform harder and may have a detrimental impact on finishing.

Womiu,y: Contact wheels Should not he Irun at speeds !n excess of manufacturer\ recommendations and should always he used in conjunction with properly Guam-dcd quip merit. Operators should wear OSHA-approved safety goggle\ and protective equipment ach as leather apron\, safety rhoeh, and glove\.

Wheel Face Design Selection of the correct geometry of the contact wheel face is prohahly the most

important segment of contact wheel selection, at Ieat equally rated with hardness helection.

76

The surface configuration and finish produced by a rubber contact wheel may be varied by the use of serrations. Serrations are actually groove7 cut at an angle across the face of the wheel. Proper selection of the groove width and land width (the “land” is the remaining portion of the fxe that has not been cut away) can markedly increase stock removal rates, increase belt life, and even change the finish produced by a given gl-it-size belt. Wide grooves and narrower lands will make the cut more aggressive, and. as with hardnes, the opposite is al\o true (i.e.. an increase in the width of the land and reduction in the width of the groove will produce finer finishes). with a smooth or unserrated fact producing the best finish.

For mot off-hand finishing operations, it is impractical to change the contact wheel yc\eral time5 a day. Thi\ results in selection of a wheel (hat may not give the utmost in cut or helt life nor the best finish, but, when coupled with the proper coated abrasive belt selection. will produce satisfactory results. As conditions vary from shop to shop and job to .job, it is nearly imposGble to make one contact wheel recommendation that will give the utmost in results for all possible work combinations.

One of the final considerations in contact wheel selection is land and groove angle. Angle is always measured from the \ide of the contact wheel. The most aggressive angle and. hence, the poorest finish generator, would be 90”. or with the lands and grooves parallel with the bore: however, as air is trapped between the helt and contact wheel during rotation of the wheel. and forcefully expelled during the process, a severe, sharp whistling noise ih generated. far above comfortable or safe levels. This high-decibel noise Icvel, coupled with the severe hammering or chattering effect produced by the sharp, sudden contact of the lands with the workpiece, makes the selection of this angle of serration an unwle choice.

For off-hand polishin g, the angle of serration seldom exceeds JS” and may be as low as 8”. Angles in the 60” range are not uncommon but are usually reserved for roll\ incorporated in the higher horsepower finishin, m of a machine-held and controlled workpiece. il ~~rtl O/ ~NU~~OH: The herration angle should never match the angle of a helt ,joint but should be cut \o that the serration angle crosses the angle of a belt joint. Example: A 35’ belt joint and a 45” serration angle would form a 90” angle when the belt is placed over the wheel face.

For most general-purpose work. when removal of excess Stock i\ the prime considcr- ation, contact wjheels of 70 durometer. land-to-groove width of one part land to two parts groove. will normally give excellent results. If the w’heel muyt produce commercial finishes as well as light stock removal, a land and groove ratio of I: I at a JS” angle built into a h@durometer wheel will usually do an excellent job.

Cloth Contact Wheels Contact wheels constructed of cloth are available in three designs: (I 1 \olid buff

type-made to desired width: (2) sectional, pleated buff sections; and (3) sectional finger buff. The solid buff-types wheels are manufactured to a desired width and diameter and are not

available with serrations and normally are not “ganged” together on a spindle to achieve a

greater width for running wider belts. They are available in five densities, ranging from #SO on the soft side to #YO on the hard end of the scale. The hardntxs measurement on cloth wheels should not be confused with the durometer reading\ by which rubber or synthetic compound wheels are measured. For cloth wheels. the numbers are relative to allow more variations in manufacturing and use than merely the description soft. medium, or hard.

Solid buff-type wheels in the #SO range are the softest and most conformable to part radii, and, hence, the #90 are the hardest. mo<t aggressive, and least conformable. All do an excellent job ofdebeloping fine finishes, run smoothly without excess vibration, and the softer densities allow moderate contouring. The same rule of thumb exists with solid buff wheels as with rubber wheels (i.e., the harder the wheel, the greater the aggressivenes; the softer the wheel, the finer the finish). Cloth wheels usually allow greater conmuring ability than do rubber wheels, with the possible exception of sponge density (under 20 durometer).

Sectional, pleated buff sections offer the versatility of “ganging” two or more sections side by side to quickly build a wheel to the desired width. Care should be exercised to ensure

77

that all sections in a wheel have been used or worn equally or a streaking condition of the workpiece may result. One or two sections of greater diameter will cause poor belt trackin@. because the high sections will act as a crown. with the coated abraivc belt trying to center itself on the crown. This wheel offers somewhat more cutting ability than a solid buff-type wheel but usually at a lower wheel cost per inch of face width.

Sectional finger buffs offer the most flexibility of the cloth wheels and although somewhat lower in cutting ability. offer added contourability. Sectional finger buffs. too. have the advantage of lower cost and the capacity to quickly construct a wheel of needed face width.

All of the cloth-type wheels arc excellent for developing fine finishes or prebuff finishe%.

TRACKING AND TENSIONING DEVICES

Tracking and tensioning devices are mure commonly called “backstand idler\” 01 “idlers” when used in conjunction with a contact wheel or drike pulley mounted on a powered shaft. The idler’s purpose is to (I ) keep the proper temion trn the abraive belt so the driving unit can transmit horqx)wer to the belt; and (2) provide rhc operator with a practical man\ of establishing the running path of a coated abrasive belt in relation to the contact wheel 01 platen ober which the belt travels.

Idler\ should be mounted on the lloor where \pace and workpiece shape permit (Figs. I and 2).

In certain instance\, where workpiece contours arc such that the operator need\ clearaxe below the contact wheel. the idler may be pedestal or wall mounted: however. it should he kept in mind that the more wraparound the coated abrasive belt ha\ on the contact wheel before contacting the workpicce. the better the tracking control over the belt will bc.

Wall-mounted idlers (Fig, 2) should be avoided wherever practical, provided that the belt is running in the normal, toward-the-floor. counterclochwiae dir-ection when vicwcd from the end of the drive shaft and contact wheel. With ;I belt Irunning in thi\ mode and utililinp a wall-mounted idler. only a small portion of the belt is in contact with the contact ti,hccl brforc rcachinp the workpiece area and can easily be forced to truck-off through work pressure alone. especially when \oft contact wheels are u\ed. If the belt i> running in the opposite direction

(i.e., clockwise or up and away from the operator when viewed firom the shaft end positmn). this would have the revcr\e effect and provide the most stable belt traching.

Id& may be tensioned by air. hydraulically, by sprin, (1 load, counter-\\,eighted. or through a simple operator-actuated screw adjustment (Fig. 3). The idler selection should he viewed carefully: too little or too much tension can have detrimental effects on the results received from the contact wheel. Belt tension can harden the face of soft contact wheel\ and, if excessive. can defeat the purpose of selecting a soft wheel. Reduced conformability. poorer finish. and tracking problems can be r&ted directly to excessive belt tension. If thel-e is a variety of workpieces to be finished, requiring contact wheel and belt changes. an idler incorporating fully adjustable tension is a must.

Tracking is accomplished hy the majority of idlers by displacin, ~7 the alignment of the idler shaft and contact wheel shaft, either vertically (Fig. 4) or horizontally. in a controlled manner. Tipping or pivoting of the idler shaft by the operator, through a mechanical Imkqe. causes the belt ta track to the right or left and even partially off the edge trf the contact wheel for work on radii.

Most idlers are crowned to varying degrees, depending on the type of pivoting mechanism used to track the belt. If a straight taper from each edge toward the center of the idler is used, the center diameter should be on the order of %J inch larger than the pulley edges (Fig. 5). This gives enough variation in sfpm to keep the belt centered on the idler. Should thih crown be worn off after much use, tracking trouble\ will result. and the crmvn should be regenerated.

It ih not uncommon to have the lapel- wear so that the crown increases. Too much crown will also contribme to tracking problems and will cause the belt to depres\ the center of ;I soft contact wheel excessvely. contributing to finish variation and folding of flexible belt\.

78

WALL MOUNTED

/ / / PEDESTAL MOUNTED /

/

/ / / FLOOR MOUNTED

THE PROPER COATED ABRASIVE BELT

After proper .selection of equipment, coated abrasive belt selection must be given

stringent consideration. To better select the proper product combination, the components of a belt should be understood.

The term sandpaper is a misnomer, because the grit hears little actual resemhlancc to sand and the backing is much more complicated than the name “paper” implies. Various types

of abrasive grain, adhesive, and backing materials make coated abrasive products very versatile tools for metal finishing. To achieve a better perspective for the complete product.

79

----- --____..______- - ___---

CIA BELT

80

finishing, is normally relegated to wide-belt use, which cxcludcs handheld opcrationa: however. a sizable quantity is used in the development ofcoametic finishes on stainless steel trim and tlat surfaces where heavy grinding pressures are not normally used and there i\ little or no danger of snagging an edge on a hole or projection.

Fiber-backed products find their natul-al end USC in heavy-coated abrasive disks or as covcr’r for drum sanders. Since the advent of belt sanders. little metal i\ finished using the drum-sander type of equipment. The largest end use of fibcr-backed coated abrasives is in disk form. and the popular sires are 5, 7. and 9 inches. These are u\ed with right-angle grinders in conjunction with a backing pad to support the disk during the grinding cycle. A typical use of a coated abrasive disk would bc removal of excess weld bead.

Cloth-backed coated abrasives are the type with the largest end uhe in metal finishing. They combine the necessary strength, flexibility. and dumbilitv in a carrier for the abrasive grain. By varying the size, number. and type of threads in different woven constructions, many end-use requirements can be met.

A cloth of extreme flexibility will differ markedly from one used to provide a ha\e for heavy grinding. So, to accommodate the wide needs of industry. the following different weights and constructions have been developed:

I. “J.” or Jeans weight. is a very tlexible, lighter weight cloth. used where conformation to contours and finish are prime requisites.

2. “X” weight. or drill cloth. is heavier than Jeans and for many year? was the staple backing for what was then heavy stock removal.

3. “Y” weight is a newer. even stronger development for today’s market, which is demanding stronger, more rugged abrasive product\.

4. 3” weight is special-purpose cloth designed specifically for the manufacture of wide sectional belts.

Cotton has been historically the basic fiber used m producing these cloth backings: however, polyester fibers, which offer greater tensile strength and more resistance to fraying and tearing, are becoming the standard in coarse-grit sizes and are moving into the fine-grit areas.

Abrasive particles of natural materials, such a\ garnet, flint. and emery, at one time accounted for the largest portion of the grit used on coated abrasives but have been replaced for metalworking by the newer man-made abrasives of aluminum oxide, silicon carbide. zirconia alumina, and ceramic aluminum oxide.

Aluminum oxide, primarily Al,O,, is a tough, somewhat chunky-shaped grain ideally suited for most metalworking applications. It has the ability to withstand considerable grinding pressures before fracturing to expose new cutting edges.

Silicon carbide is a harder, more brittle abrasive than aluminum oxide; each grain ha, a longer axis and fractures more readily than A1203. In fact. on most metal applications, it fractures too readily, so that its use is relegated to very hard metals or very tough ones, such as titanium, or to the development of special stainless steel finishes.

Zirconia alumina, ZrO,-Al,O,. is known for its exceptional durability under lighter grinding pressures and its ability to resharpen under heavy grinding pressure. Ceramic aluminum oxide is the newest manmade abrasive grain and is designed to break down and resharpen under lighter pressure than zirconia alumina in the 60.finer grit range and may be superior to rirconia alumina in the 50.coarser grit range, depending on the alloy.

The backing material and abrasive grain must be incorporated into the final product by a bonding process, using an adhesive such as glue, phenolic resin. urea resin, individually or in combination.

This bonding process is a two-step operation, consisting of a base coat, which may be glue or resin, and an additional layer of adhesive applied after the abrasive grain is deposited on the base coat. The second coat is commonly referred to as the size coat and may also be of glue or resin, depending on the intended market for the product.

81

THE ABRASIVE BOND

Glue bonds are used where the utmost in finish is the prime requisite of the operation.

When they arc combined with a “J”- weight cloth, excellent tlexihility ~rcsults. These products are used for fine finishes on parts with contour5 and arc not designed for heavy stock removal.

Glue bond product5 cut cooler than do rc\in bonded hut are also less heat resistant. If an “X”-weight cloth is used as the car!-lcr rather than “J” weight. a dccrca\e in Ilrxihility will bc

encountered, hut finish should remain equal. To increase the aggrc\sivene\\ trf a product and retain maximum flexibility. a coat of

resin may he suhstitutcd for the siLc coat of glut. This will detract slightly from the finishing capability of the product hut will increase it!, cutting capacity and life. The same comments

on hacking weight apply to thi\ product type (i.e., “J” weight is more flexible than “X” weight), These resin-ober-glue products are used whcrc good llexihility is desired and finish

i\ not of prime concern. When an all-resin bond is coupled with an “X”- or “Y”-weight cloth, the maximum in

durability. aggressiveness, and productivity of the coated-abrasive grain i\ achieved. Because of the heavier hacking and stronger, tougher bond, it would he reasonable to expecr a marked decrease in tlexihility. Generally this is true. but on certain specialized products. techniques have been developed to retain an excellent degree of flexibility. When selecting an all-rc\in product, if the operation to be performed requires moderate flexibility in the belt. a representative from the manufacturer of the coating should hc consulted.

EFFECTOFSURFACESPEED

Assuming that the correct components have been selected and properly installed (i.e., correct power. contact wheel. idler. and coated-abrasive belt). two added factors that now must be considered (Table II) are surface speed of the belt and grinding aids.

If each individual abrasive particle on a helt is thought of as a single-point cutting tool. perhap\ a better perspective of the abraslke action can he e\~abliyhcd. The speed of the helt or number of times a minute each particle contacts the work in a given time frame. will determine the rate of removal and the working life of the indicidual particle, assuming that the conditions of pressure and workpiece consistency remain constant. An increase in the surface speed will cause the pressure to he applied to many more particle\ in a given time frame and will reduce the penetratmn of a given particle into the workpiece. When thi? speed reaches beyond a critical point, rapid dulling of the abrasive grain takes place. the rate of cut decreases. and excess heat is generated.

To resharpen or restore the cutting capacity of the abrasive. added pressure mu\t he exerted to cause fracture of the grain. This added pressure will generate added heat and. shortly. part burning or operator sensitivity will result in the removal of the belt. Conversely. as speeds are slowed, more pressure ib exerted on each grain, causing the grain to resharpen with less heat generation. To put this in better perspective, refer to Table II. where suggested speeds, abrasive. contact wheel, grit size, lubricant. and operation have been coordinated into a quick, accurate reference chart.

BELT LUBRICANTS

Lubricants may serve several purposes when the proper one is used. They may he used to retard metal welding to the grain (as in the grinding of aluminum). improve the finish. extend the belt life, increase the aggressiveness, or a combination of any or all of these

benefits.

82

Tabl

e II.

Su

gges

ted

Surfa

ce

Spee

d an

d Ab

rasiv

es

for

Vario

us

Met

als

Light-bodied prease sticks are normally used to prevent loading or metal welding on the abrasive grain. Lubricant should bc applied before work i\ prerented to the belt and at frequent intewals during its life cycle. This is especially important when working with nonferrous material\. Mo\t light-bodied grca\e sticks do not retard the rate 01‘ cut ~-ecri~~I from a belt. and after long vxvice, the spaces hetwcen the abrasive grain\ may hecome clomped with ground-off material cntrupped in the lubricant. Degreasinp the belt will Ircmove this type of loading. and added w+ce may be t-eceiwd from the abrasive.

Heavy-bodied grease <tick\ perform a dual role. They will alightly retard grain pcnctration into the workpiecc. producing a finish actually finer than would be generated hy a dry belt: they will retard loading and will act :I a “heat sink.” causing the part to grind somewhat cooler. Grease stichh containing sulfur and chloride will aid cut and life on chrotnlurn-nickel alloys but should not be used on material\ that will I-ext nepatively and discolor metal!, huch as brass. aluminum. and copper.

The finish produced by a dry belt may require twice the time to buf‘ out. compared to the \amc grit belt that has heen lubricated. As an example, a dry belt may, under a given \c‘t of conditions, produce a finish in the 3% to 35.rms range. but by greasing the wme belt. finishe\ in the range of IO ttr 12 rms arc not uncommon.

If the part is to he buffed, il carcl’ul cost study should he done on the preparation and ensuing time to but’l’. For example, it wwn belt may produce the wme rms as a finer grit but newer belt; however, the WI-face produced by the coarser gj-it hclt wjill have wider wratch lines than the liner grit and may need up to twice the buffing time to remove.

The finish recei\,cd l’l-om a hclt will WI-~ from the first part produced to the la\t pal-t produced. Usually, the rms on ;I given belt will improve rapidly during the first IO to 25% of its life. depending on preywre. workpiece material. etc.. then 4owly drop over the next 75 tn 90% of its life. In this area. most of the productive litc of the belt is obtained and the most uniform finish is produced. Frequently, an operator will “break in” a new belt or wt aside the first parts produced to he rerun after the \tahle portion of rm\ has hcen reached.

ABRASIVE BELT PLATEN GRIBDERS n Heavy-duty Construction n Wet or Dry Operation w 1 to 10 HP n Manual or Semiautomatic n 4” to 10” Wide Platens n Choose from 10 Models

-- 1600 Douglas. Kalamazoo, Ml 49007e(616) 345-7151

84