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65. CERAMIC AND GLASS PARTS

65.1. DEFINITIONS

Ceramics are defined as inorganic, nonmetallic materials and are described

further in Chap. 2.4. They can be classified into the following groups:

1.Whitewares. These include, in addition to mechanical and electrical

components, earthenware, china, tiles, and porcelain.

2.Glass. Glass is a mutual solution of fused, inorganic oxides cooled to a

rigid condition without crystallization. It is made into a variety of hard,

transparent objects.

3.Refractories. These include heat-resistant and insulating blocks, bricks,

mortar, and fireclay.

4.Structural-clay products. They consist of bricks, tiles, and piping made

from natural clays.

5.Porcelain enamels. These are ceramic coatings on cast-iron, steel, and

other metal products.

65.2. MANUFACTURING PROCESSES

65.2.1. Ceramic Parts

To produce ceramic parts, refined powders of the basic raw materials are

first thoroughly mixed with some water and small quantities of selected

additives, normally metallic oxides that act as fluxing agents and inhibitors.

Then the basic fabrication operation, such as pressing, extrusion, or casting,

CERAMIC AND GLASS PARTS
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takes place. Depending on the shape and dimensions, machining or grinding

of the formed part also may be involved.

The green ceramic part is then dried and fired at a high temperature for a

specified period of time. This fuses the powders into a hard, dense, strong,

and homoge-

neous material. A typical temperature range within which alumina and other

common materials are fused is 1400 to 1800C (2550 to 3250F).

Pressing is the most common basic fabrication operation prior to firing. It is

similar to compression molding or powder-metal pressing in that the material

is compressed at high pressure into a mold cavity of the shape of the

workpiece. The pressed part is then trimmed as necessary and dried.

In wet pressing, the mixture is quite moist and flows somewhat as it is

pressed, similarly to the behavior of plastics being compression-molded. In

dry pressing there is a minimum amount of moisture, and the ceramic

powders behave very similarly to metal powders in the powder-metallurgy

processes.

Many ceramic parts can be formed directly to the final shape, with

allowances being made for shrinkage during firing. Often, however, turning,

drilling, boring, threading, tapping, and other machining operations take

place to meet some special requirement. Because of the highly abrasive

nature of ceramic material, carbide tools are used. Grinding also may be

employed. After firing, if dimensional tolerances are particularly close,

further grinding and lapping can be performed with diamond abrasives.

Glaze may be added to the part to provide a smooth, glossy surface. It is

applied soinetimes before firing and sometimes afterward, followed by a

second lower-temperature firing operation.

A more liquid mixture is used for casting and extrusion than for pressing.

Casting is performed with plaster-of-paris molds that absorb water from the

mixture, gradually building up a leathery cast that may be handled,

refinished, and fired with or without a glaze.

Jiggering, a process often used for dish- or bowl-shaped parts, involves the

use of a rotating form, usually of plaster, against which a putty-like clay mix

is pressed with a clay knife. Separate pieces may be joined together before


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drying and firing.

65.2.2. Glass Parts

Glass components are produced from a hot, viscous, homogenized melt. They

may be processed or formed by pressing, blowing, drawing, or rolling, after

which the glass is cooled at a controlled rate to anneal it (remove residual

stresses) prior to finishing. Figure 6.11.1 illustrates the pressing operation.

65.3. TYPICAL CHARACTERISTICS AND APPLICATIONS


Ceramic parts are hard, extremely strong in compression, highly chemical-

and corrosion-resistant, nonflammable, and suitable for use at extremely high

operating temperatures. Ceramic whitewares generally have good thermal

shock resistance and low thermal expansion. High modulus of elasticity and

high radiation resistance are two additional properties of importance in some

applications. Most ceramics are dielectrics and, except for ferrites, lack

magnetic properties.

Excellent abrasion-resistant surfaces are possible. These surfaces also offer a

pleasing gloss or patina and can be vitreous and nonporous. In addition to

their resistance to chemical substances and corrosive materials, ceramics are

relatively immune to fire, heat, and weathering.
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Figure 6.11.1. Pressing methods and mold types for glass. (From Errol

B. Shand, Glass Engineering Handbook, McGraw-Hill, New York.)

Generally, all ceramic materials are brittle. Tensile strengths are somewhat

limited. There also are some limitations in freedom of design because of

processing complexities and inherent mechanical properties. Because of high

firing temperatures, metal inserts cannot be molded in.

The size of commercial ceramic components ranges from the very small

electronic components to large nose cones and radomes. Typical ceramic

parts for mechanical applications are bearings, turbine blades, cams, cutting

tools, extrusion dies, thread and wire guides, nozzles for abrasive materials,

wear plates, seals, valve seats, filters, pump parts, crucibles, and trays.

Typical parts for electrical and electronic applications include coil forms,

tubes, insulators, lamp housings, printed-circuit boards, radomes, resistor

bases, vacuum-tube-element supports, and terminals. Figure 6.11.2


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illustrates some typical ceramic components.

Figure 6.11.2. Typical technical ceramic parts. (Courtesy Duramic

Products Inc.)

65.3.2. Glass Parts

Transparency is the most important property of glass and accounts for most

of its applications. Other properties are similar to those of whiteware but

with less favorable strength and high-temperature characteristics. The poor

resistance of glass to thermal shock can be improved by tempering, which

also provides increased mechanical strength.

Glass products range in size from microspheres of fractional-millimeter

diameter used as fillers for plastics to large plate-glass windows. Normally,

pressed parts are about 9 kg (20 lb) or less in weight, while blown ware can

range up to 16 kg (35 lb).

Typical pressed-glass components are electrical insulators, baking dishes,

food blenders, stoppers and stopcocks for laboratory vessels, eyeglasses,

and ornamental pieces. Typical blown-glass components are bottles and

other containers, incandescent lamps, electron tubes, laboratory glassware,

and television picture tubes.

Tubing and piping of glass, made by drawing, are used for laboratory,

chemical industry, and high-temperature applications and thermometers. Flat

glass for glazing, mirrors, tabletops, and other purposes is made either by


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drawing or by rolling, which, in the case of plate glass, is followed by

grinding and polishing or by floating onto molten tin and drawing

horizontally. Glass powders are sintered to make filters and other porous

objects. Glass fibers are a major reinforcing medium for many products (see

Chap. 6.6), for insulation and for fiber optics.

Figures 6.11.3 and 6.11.4 illustrate typical pressed- and blown-glass parts.

Cellular glass is almost invariably black or dark-colored. Pore size can be

varied, depending on the method of introducing porosity. Thermal expansion

is the same as that of the base glass.

Figure 6.11.3. Examples of pressed glassware. (a) Block-mold

glassware. (b) Split-mold glassware. (c) Font-mold glassware.

(Courtesy Corning Glass Works.)

Color can be incorporated into most glass, whiteware, porcelain, and other
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ceramic materials by introducing the proper pigmentation medium to the

material before firing.

65.3.3. Refractories

Refractory products, being resistant to very high temperature and, generally,

to thermal shock, are used in such applications as furnace linings and similar

insulation. For the most part, they are molded in the shape of bricks of

relatively small dimensions. They also may be fusion-cast in large shapes

(e.g., 1 by 2 by 4 ft) and then cut into the required size and configuration.

Figure 6.11.4. Examples of blown glassware. (a) Paste-mold glassware.

(b) Hot-iron-mold glassware. (c) Press-and-blow glassware. (Courtesy

Corning Glass Works.)


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65.4. ECONOMIC PRODUCTION QUANTITIES

Excluding the art formsglass, pottery, porcelain enameling, etc. using

ceramic mediamuch of the true industrial portion of the ceramics field is

long-established, well stabilized, and geared for efficient large-scale

production. Factors such as adaptability to mass production, costs, setup

times, output rates, and equipment life are summarized for various branches

of the industry in Table 6.11.1.

Figure . TABLE 6.11.1 Economic Production Quantities

65.5. SUITABLE MATERIALS

65.5.1. Ceramics

Technical ceramics are normally dense bodies that contain steatite aluminum

oxide (alumina), beryllium oxide (beryllia), or related oxides such as mullite

(3A1 O 2SiO ), forsterite [(Mg Fe) SiO ], and cordierite (2MgO 2A1

O 5SiO ). Silicon carbide, silicon nitride, and boron nitride are other

materials of commercial use.

65.5.2. Glass

The major portion of the glass industry uses as its raw materials oxides and

carbonates of silicon, calcium, and sodium, mainly as sand, limestone, and

soda ash. Numerous other oxides are added to obtain special properties such

2 3 2 2 4

2 3 2


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as radiation resistance, hardness, controlled expansion, etc. The principal

types of glass are as follows:

Silica Glass. Silica (silicon oxide) or silica quartz (sand), when fused, forms a

glass with very-high-temperature resistance, high strength, chemical

resistance, and resistance to thermal shock. Unfortunately, it is extremely

difficult to form into useful shapes, and articles made from it are therefore

expensive.

96 Percent Silica Glass. This type has somewhat easier formability and

slightly reduced properties compared with silica glass because of the

presence of small amounts of boric oxide and other ingredients.

Borosilicate Glass. This type contains silica as the chief ingredient but has

from 13 to 28 percent of boric oxide for low thermal expansion and other

oxides that provide further improvements in workability. Mechanical,

electrical, and chemical resistance properties are still good, and borosilicate

glass has wide usage for electrical insulators, laboratory glassware,

cookware, and sight and gauge glasses.

Lead Glass. This type contains a portion of lead oxide in addition to silica

and other oxides. Normally, the lead oxide content is less than 50 percent,but it can be as much as 90 percent for glass used for radiation shielding. In

portions below 50 percent, lead oxide enhances the workability of glass, and

lead glass is normally called for when intricate forming is required. Optical

and electrical properties are also excellent, although mechanical properties

(strength and abrasion resistance) are low. Lead glass is used for

thermometer tubing, neon and fluorescent lights, television tubes, art

glassware, and jewelry.

Soda-Lime Glass. This type contains appreciable quantities of soda, Na O,

and lime, CaO, in addition to the chief ingredient, silicon oxide. Soda and lime

lower the melting point of the glass, reduce its viscosity when melted, and

thereby improve its workability. Soda-lime glass is a good general-purpose

glass and is used for window and plate glass, containers, and electric-lamp

bulbs. It is economical to melt and to fabricate.

Table 6.11.2 summarizes the prime characteristics of these common glasses

on a comparative basis.

2


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Table 6.11.2. Properties of Principal Types of Glass

65.5.3. Other Ceramics

Porcelain enamels, or frits, are low-melting, lead oxidebased glasses.

Ground-coat enamels, which cross-bond a metal substrate to a topcoat

porcelain enamel, always contain cobalt oxide.

Lime

glass

Lead

glass

Borosilicate

glass

96%

silica

glass

Silica

glass

Weight Heavy Heaviest Medium Light Lightest

Strength Weak Weak Moderately

strong

Strong Strongest

Relative cost Lowest Low Medium High Highest

Resistance to

thermal shock

Low Low Good Better Best

Electricalresistivity

Moderate Best Good Good Good

Hot

workability

Good Best Fair Poor Poorest

Heat

treatability

Good Good Poor None None

Chemical

resistance

Poor Fair Good Better Best

Impact-

abrasion

resistance

Fair Poor Good Good Best

Ultraviolet-

light

transmission

Poor Poor Fair Good Good


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Whiteware used for dinnerware and other nontechnical applications is

normally similar in composition to that used for mechanical and electrical

parts. Combinations of clay, feldspar, and flint are used with minor variations

to impart desired characteristics.

Refractory materials produced from aluminum and chromium oxides are used

for large tank furnaces. Silica is used when acidic atmospheres are involved,

and fireclay is employed for general nonnoxious high-temperature

environments.

65.6. DESIGN RECOMMENDATIONS


Although technical ceramics can be fabricated into complex shapes, it is

always desirable to keep shapes as simple as possible for economic reasons.

Tolerances also should be as liberal as the function of the component

permits. It is important, from a structural standpoint, to avoid problems that

result from the low tensile strength and lack of ductility of ceramics.

Specific design recommendations for technical ceramics are as follows:

1. Edges and corners should have chamfers or generous radii to minimize

chipping and stress concentration and aid forming. When parts are

machined, outside radii should be 1.5 mm (1/16 in) or more and inside radii at

least 2.4 mm (3/32 in). For dry-pressed parts, outside edges should be

beveled in a manner similar to that employed with powder-metal parts; 0.8

mm by 45 is a desirable minimum. Inside

Figure 6.11.5. Design rules for corners of ceramic parts.

radii should be as large as possible: 6 mm (1/4 in) unless the height or width

of the smaller surface is less than 6 mm. (See Fig. 6.11.5 for an illustration of

these rules.)


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2. Since parts may sag or be distorted if not properly supported during

firing, it is preferable to avoid large overhanging or unsupported sections.

Otherwise, supporting-fixture costs may be excessive.

3. Pressed parts should be designed with as uniform a wall thickness as

possible. Differential shrinkage of sections of nonuniform thickness during

drying and firing causes stress, distortion, and cracking. Sections should notexceed 25 mm (1 in) in thickness. (See Table 6.11.3 for wall-thickness

information.)

Table 6.11.3. Thicknesses of Ceramic Products

Thickness range, mm (in) Maximum practical

thickness buildup

within individual

part, ratio

Minimum Maximum

Technical

ceramics

Standard types

0.5 (0.020) 25 (1.0) or

more

4:1

Glass

Glass containers Blown: 1.5 (1/16)* 9.5 (3/8) 4:1

Pressed: 2.4

(3/32)

9.5 (3/8)

Flat glass Picture glass: 1.1

(0.043)

Preferably

none

Doors: 2225

(7/81)

Technical glass 1.5 (1/16)or as

required

4:1

Cellular glass As desired (cast

material)

As desired;

machinable

Whiteware

Vitreous sanitary

ware

6.3 (1/4) 50 (2) 2:1


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4. Other factors being equal, simple symmetrical shapes without deep

recesses, holes, and projections are preferable. Gently curved surfaces

without abrupt break lines or angularity are normally preferred with most

ceramic-forming processes.

5. When hollow pieces are cast against a male mold (e.g., cup-shaped parts),

a draft angle of at least 5 must be provided to facilitate removal of the green

body. If the part is left in the mold too long, drying shrinkage will draw the

material against the mold, resulting in cracking. Dry-pressed parts do not

require draft on either outside surfaces or the walls of through holes. Wet-

pressed parts should have at least 1 on exterior surfaces and 2 on interior

surfaces. (See Fig. 6.11.6.)

6. Undercuts should be avoided in ceramic components if possible. Although

some undercuts can be incorporated through the use of mold cores,

machining is the normal method for producing them. With dry pressing,

*Throwaway bottles; returnable bottles are slightly thicker. Light bulbs = 0.020 in thick.

Or less, as in Japanese rice or Irish Belleek ware.

Vitreous

dinnerware

1 (0.040) 3 (1/8) 3:1

Semivitreous

dinnerware

1.7 (0.065) 9.5 (3/8) 3:1

Floor and wall

tiles

6.3 (1/4) 13 (1/2) Raised ridges: 1.2:1 to

1

Porcelain

enameling

Cast-iron

plumbing

3 (1/8) 4.8 (3/16) Preferably none

Steel plumbing 1.5 (1/16) 3 (1/8) Preferably none

Appliances 1.5 (1/16) 2.4 (3/32) Preferably none

Refractories

Standard types

As required;

bricks and heavy

cast shapes

Preferably none


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machining is essential if undercuts are required. In all cases, costs are

added.

7. Dry-pressed ceramics are subject to other design rules of powder-metal

parts (Chap. 3.12) also but cannot match their close dimensional tolerances.

8. Cavities, grooves, and blind holes in pressed parts should not be deeper

than onehalf the part thickness and preferably only one-third the thickness.(See Fig. 6.11.7.)

Figure 6.11.6. Draft angles for ceramic parts.

Figure 6.11.7. In pressed parts, blind holes and cavities should be as

shallow as possible.

9. Extruded parts should be symmetrical, if possible, with uniform wall

thickness. The minimum wall thickness for extrusions should be 0.4 mm (1/64

in) or, for round sections, 10 percent of the extrusion diameter. For long

extrusions, 150 mm (6 in) in length or more, the wall should be thicker, at

least 20 percent of the extrusions outside diameter. (See Fig. 6.11.8.)

10. Holes in pressed parts should be large and as widely spaced as possible.

Thin walls between holes, depressions, or outside edges should be avoided.These walls should be at least as thick as the basic walls of the part,

especially if the part is small and thin-walled. In any case, the minimum in

internal areas should be 0.8 mm (0.030 in) and, in the case of outside edges,


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for holes that are designed to accept fasteners. To compensate for variations

in hole spacing, multiple holes that are to be aligned with corresponding

holes in other parts must be further enlarged (or elongated in the direction

of the other holes). The amount of the enlargement or elongation depends

on the allowable hole-to-hole tolerance of the two parts.

12. Molding of screw threads in ceramic parts is not feasible. Screw threads

can be machined in green ceramic workpieces, but they constitute a

potential problem, and it is better to design parts without screw threads if

possible. If incorporated, threads should be coarse and not smaller than 6-

32. Internal threads should be considered acceptable if they accept a Class

1A mating metal screw; external threads should be considered acceptable if

they accept a Class 1 nut. Holes should not be tapped to a depth greater

than six threads because dimensional variations in the thread pitch from

firing shrinkage may cause fit problems if too long a thread is used. All

tapped holes should be countersunk. (See Fig. 6.11.9.) External threads also

should be as coarse as possible and have a well-rounded thread form to

reduce edge chipping and stress cracking. Coarse-pitch threads with a

truncated form also can be used to increase the strength of the threaded

ceramic part. As with internal threads, it is recommended that the number of

threads in engagement be limited to six.

13. Ribs and fins should be well rounded, wide, and well spaced and have

normal draft. Figure 6.11.10 illustrates design rules for ribs.

14. Grinding after firing can produce ceramic parts of high accuracy, but

stockremoval rates are slow, and the operation is expensive. When the

operation is nec-

Figure 6.11.9. Internal screw threads in ceramic parts.
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Figure 6.11.10. Design rules for ribs in ceramic parts.

Figure 6.11.11. Minimize the area to be finish-ground after firing.

essary, it is advisable to reduce the area of the surface to be ground as much

as possible and to provide clearance for the grinding wheel at the ends of

the surface. (See Fig. 6.11.11.)

15. Ceramic parts can be permanently joined to metal components by

adhesive bonding, soldering, brazing, and shrink fitting. Shrink fitting ishighly satisfactory as long as the metal is on the outside (in tension) and the

ceramic on the inside (in compression). Brazing is stronger than bonding or

soldering and more temperature-resistant but requires a metallized layer as

a base for the brazing alloy.

65.6.2. Glass Parts

Guidelines for the design of pressed- and blown-glass components are shown

in Tables 6.11.5 and 6.11.6. Note that tolerances and minimum desirable

production quantities also are shown.


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Other points to bear in mind when designing glass parts are the following:

1. Holes, cavities, and deep slots can cause molding problems and should be

included in a part only if absolutely necessary. Holes are normally not

punched through in the pressing operation but are machined from a thin

web or hollow boss, as shown in Fig. 6.11.12.

2. As in the case of whiteware parts, best results are obtained when walls

are uniform in thickness, when the part is designed for compressive rather

than tensile strength, and when gently curved rather than sharp-angled

shapes are employed.

Figure . TABLE 6.11.5 Manufacturing Tolerance and Design

Recomendations for Pressed Glassware

Table 6.11.5. Manufacturing Tolerance and Design Recomendations for

Pressed Glassware (Continued)


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Figure . TABLE 6.11.6 Manufacturing Tolerance and Design

Recomendations for Blown Glassware

Table 6.11.6. Manufacturing Tolerance and Design Recomendations for

Blown Glassware (Continued)

*Maximum dimension.

Source:For Errol B. Shand, Glass Engineering Handbook, McGraw-Hill, New York.


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Figure 6.11.12. Two designs for holes in pressed glassware. (From

Errol B. Shand, Glass Engineering Handbook, McGraw-Hill, New York,

1958.)

3. Lettering or other irregular surface features may be incorporated as long

as they are aligned in the direction of, and not perpendicular to, the mold

opening.

4. Ribs and flanges can be incorporated in pressed-glass components, but

they are not practicable in blown parts.

*Weight based on density pf commercial glasses.

Tolerence on diameter for circular pieces is on mean diameter and does not include out-

of-round.

Notes:Wall thickness cannot be positively controlled. It depends on glass distribution in

the blank and on the shape into which it is blown. Note in table that wall thickness of blown

ware can be much lighter than for pressed ware. A pear-shaped piece is ideally suited to

blowing. An inverted cone is undesirable. Long, thin necks make it difficult to handle the

blank during the the blowing operation. In hot-iron ware, a circular section at the cutoff

point permits a flame burn-offwhich is considerabely cheaer than other methods.

Source:From Errol B. Shand, Glass Engineering Handbook , McGraw-Hill, New York.


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5. While bosses may be incorporated in some items like electrical insulators,

they are normally not practicable for general-purpose design and

manufacture.

6. Threads for bottle caps or similar connecting devices may be incorporated

in blown-glass parts as they are with blow-molded plastics, and the same

screw-thread designs (see Chap. 6.5) are recommended.

65.7. DIMENSIONAL FACTORS AND TOLERANCES


These parts are affected dimensionally primarily by drying shrinkage and

firing shrinkage, which can total as much as 25 percent for high-clay

ceramics and about 14 percent for porcelains. Other factors affecting the

accuracy of ceramic parts are mold accuracy and mold wear. Processing

variables, such as the amount of material pressed, pressing time, and

pressure, affect the dimensions of pressed parts. Machining variations affect

green-state machined and finish-ground parts.

Table 6.11.7 presents recommended dimensional tolerances for technical

ceramic parts.

65.7.2. Glass Parts

These parts are dimensionally affected by gob weight, temperature of the

melt and mold, mold tolerance and wear, and shrinkage of the glass on

cooling. Shrinkage rates vary from the equivalent of that of steel [92 10

in/(in C)] down to that for pure silica glass [7 10 in/(in C)].

Table 6.11.7. Recommended Dimensional Tolerances for Technical

Ceramic Parts*

7

7

Porcelain; cast

ceramics

Standard

tolerance

for

technical

ceramics

Tightest

tolerance for

precision

electronic and

mechanical

applications
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Recommended tolerances for various dimensions of pressed- and blown-glass

parts are shown in Tables 6.11.5 and 6.11.6. Tolerances for locations and

*NLT = not less than.

As-fired lengths

and widths,

unglazed

1/2%, NLT 0.38

mm (0.015 in)

1%, NLT

0.13 mm

(0.005 in)

1/2%, NLT 0.08

mm (0.003 in)

As-fired lengths

and widths, glazed

3%, NLT 0.75

mm (0.030 in)

2%, NLT

0.30 mm

(0.012 in)

1%, NLT 0.

13mm (0.005 in)

Angles 2 2 1

As-fired thickness 10% 10% 5%

Ground thickness 0.10 mm

(0.004 in)

0.025 mm

(0.001 in)

0.025 mm

(0.001 in)

Other ground

dimensions

0.10 mm

(0.004 in)

0.025 mm

(0.001 in)

0.013 mm

(0.0005 in)

Hole diameter,

unglazed

0 to 13 mm (to 1/2

in)

0.13 mm

(0.005 in)

0.08 mm

(0.003 in)

0.05 mm (0.002

in)

Over 13 mm (over

1/2 in)

2% 0.13 mm

(0.005 in)

0.10 mm (0.004

in)

Hole diameter,

glazed 0 to 13 mm

(to 1/2 in)

0.30mm

(0.012 in)

0.20 mm

(0.008 in)

0.10mm (0.004

in)

Over 13 mm (over

1/2 in)

2% 1% 1%

Hole locations

center-to-center

2%, NLT 0.13

mm (0.005 in)

1%, NLT

0.08 mm

(0.003 in)

1/2 %, NLT 0.08

mm (0.003 in)


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diameters of holes are shown in Table 6.11.8.

Table 6.11.8. Recommended Location and Diametral Tolerances for

Holes in Pressed-Glass Components

Recommended tolerance,

mm (in)

Method Hole

diameter

Location

within

Diameter

Drilled or pressed and

ground

36 (1/81/4) 0.8 (1/32) 0.4 (1/64)

625 (1/41) 0.8 (1/32) 0.8 (1/32)

Burned through and

punched

36 (1/81/4) 0.50 (0.020) 0.25 (.011)

Citation

James G.Bralla: Design for Manufacturability Handbook, Second Edition. CERAMIC

AND GLASS PARTS, Chapter (McGraw-Hill Professional, 1999, 1986),

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Documents

ceramic_and_glass_parts.pdf