United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

Research

Paper

FPL 369

May 1980

Does Gang Ripping

Hold The Potential

For Higher

Clear Cutting Yields
ABSTRACT

Cutting yields from gang ripping

hardwood lumber graded by the National Hardwood Lumber Association

standard grades are determined using

the technique of mathematical modeling. The lumber used is the same as

that in an earlier mathematically

modeled determination of cutting

yields from traditional rough mill procedures. Mechanical cutting factors

such as kerf, cutting lengths, and

minimum salvage size are also the

same in both studies. A comparison

of yields between the two systems is

made. While gang ripping produces

higher total yields in all grades, the

gain tends to be in the medium and

shorter cutting lengths.

Key Words: Gang Ripping, Furniture

Cutting Yields, Rough Mill,

Mathematical Modeling, Hardwood

Lumber
United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory1

Research

Paper

FPL 369

May 1980

Does Gang Ripping

Hold The Potential

For Higher

Clear Cutting Yields

By

HIRAM HALLOCK, Research

Forest Products Technologist

and

PAMELA GIESE, Computer Programer

Introduction

When clear, one- or two-face furniture cuttings are the objective from

factory grades of hardwood lumber,2

the cutup procedure traditionally

begins by crosscutting to maximize

the desired cutting lengths in areas

between knots. Crosscutting is

followed by ripping the crosscut sections to desired cutting widths that

are free of unacceptable defects.

Residual from this step in the operation is then further defected by

crosscutting and ripping either to

smaller acceptable cuttings or to

random-width strips of specified

length and some specified minimum

width for edge gluing into panels.

These panels are then resawn to

desired widths.

Although a few high-priced furniture lines require some of the cuttings, such as drawer fronts, to be

one piece with no glue joints, this requirement is becoming the exception

rather than the rule. Generally,

random-width cuttings can be

assembled and glued to form the final

cutting item.

If one accepts edge gluing in all

panels and for all cuttings, the

possibility of another system to produce furniture cuttings can be considered. This system begins by

mechanically gang ripping all the

rough or skip-dressed lumber to some

predetermined width presumably

related to the lumber grade. From the

ripped strips, cuttings of the desired

lengths would be developed by

crosscutting in the process of removing unacceptable defects. In most

cases during gang ripping an edging

strip less than full ripped strip width

would be developed on the edge of

the board farthest from the fence. If

this strip were equal to or more than

the minimum acceptable width for

salvage, it would be included with the

strips for crosscutting to desired cutting lengths. In some cases defective

pieces removed in the crosscutting

operation would contain a randomwidth strip less than the full gang ripped strip width that could be salvaged by additional crosscut and/or ripped to a narrower but acceptable

width (fig. 1). Panels would then be

edge glued from cuttings of the same

length. These panels would be resawn

to the desired final cutting width. Any

part of the panel width remaining

following the removal of all specified

width cuttings would be recycled into

the next panel to be glued up.

The gang ripping of hardwood

lumber for certain products, such as

flooring, is in widespread use. In the

flooring industry all lumber is ripped

to the desired width for a strip flooring blank and this operation is followed by crosscutting to remove unacceptable defects. No salvage of

secondary cuttings is ordinarily made.

Gang ripping of hardwood lumber for

narrow moldings is also common.

Ripping first is almost universal in

the softwood molding, millwork, and

sash and door industry. Here,

however, rip saw spacing is normally

1

 Maintained at Madison, Wis., in cooperation

with the University of Wisconsin.

2

 National Hardwood Lumber Association

(NHLA) “Rules for the Measurement and Inspection of Hardwood and Cypress Lumber.”
of cuttings found.

Figure 1.–Graphic representation of a board processed by the gang rip model illustrating the various types

M 148 535

variable and reset for each board

sawn. The reason for this results from

the general industry practice that requires full-width cuttings (no edgeglued stock) and a much wider

average width of softwood lumber as

manufactured. Short lengths are

finger jointed for increased recovery.

size from 1 by 10 inches to 5-1⁄2 by 96

inches. Complete descriptive data

defining the board and all its defects

on both faces were reduced to a

Cartesian (x,y) coordinate system and

stored on cards.

When gang ripping hardwood

lumber several factors are involved

that affect the final recovery in comparison to the conventional crosscut

first system. The sawdust factor in

most cases will be slightly higher

with gang ripping. This is especially

true if the ripping is to narrow widths.

In theory, at least, total recovery (ignoring sawdust loss) increases as the

cutting width is reduced because of

lower defecting losses during the

crosscutting operation. This becomes

less a factor as cutting length is

shortened since the chance of encountering a length-limiting defect is

reduced. Compared to the conventional method, when sawdust losses

are included in actual practice the

theoretical gain in long length cuttings resulting from ripping narrow

widths might be more than offset by

the increased sawdust loss from

more rip lines. As one would expect,

there is a “best” rip width for each

cutting length and lumber grade.

The availability of this extensive

board-descriptive data bank and the

development of a mathematical

model of the gang ripping process7

has made possible the determination

of cutting yield by this method. Since

both the earlier modeling of the traditional method (program “YIELD”)

3

 and

the current study use identical data

and mechanical processing values,

very valid yield comparisons are

possible.

“MULRIP” -The Gang

Ripping Model

Mathematical modeling by computer has been used to determine the

furniture cuttings from the various

National Hardwood Lumber Association (NHLA) standard grades of hardwood lumber when cut up by the

traditional method.3456 Hard maple

was the species chosen. Actual Iumber chosen was selected to provide a

statisticaly reliable representation Of

the quality and board size range

found within each of the standard

lumber grades. Yields were determined for all combinations of cutting

Beginning at the lower edge of the

board, with reference to its position

when the board and defect data were

recorded, a full-length 1/4-inch-wide

strip is removed. A similar 1/4-inch

strip is also removed from the upper

edge. This procedure is identical to

“YIELD” and is intended to account

for the loss that would result from

straightening up the cuttings adjacent to the edges of the board. The

board is then “ripped” into

lengthwise strips of a specified width.

All strips are the same width except

that part of the board outside the last

sawline (edging) usually will be narrower than the other strips. If this

strip is 1 inch or wider, it is saved.

Each strip is separated by a 1⁄4-inch

kerf allowance, which is also the

same as used in “YIELD.” Thus, each

ripped strip becomes a narrow, fulllength board.

cutting length, full strip-width cuttings are placed in the ripped strips

wherever sufficient length exists between defects or the ends of the board

and defects. When all possible cuttings of this length have been placed,

the next longest cutting length is used. This is repeated until no full-width

areas are left that are at least as long

as the shortest cutting.

Next, to locate cuttings of less

than full strip width a search is made

of all the remaining areas that contain defects. A cutting is taken if the

clear area is equal to or larger than

the minimum size and can be removed

by no more than one crosscut and

one rip operation. (In removing the

cutting up to two crosscuts and two

rips are allowed if they can be performed without more than one change

of operation between rip and crosscut

stations: both crosscuts followed by

both rips or vice versa.) When these

steps have been followed the model

assumes all available cuttings have

been found. At this point the data are

summarized and categorized by size

of cutting; yield of full-length, fullwidth cuttings; cuttings salvaged

from the edging area less than rip

strip width; and cuttings salvaged

from the defect areas.

A series of cutting lengths is

selected: Beginning with the longest

3

 C. Wodzinski and E. Hahm, A Computer Program to Determine Yields of Lumber (Madison,

Wis.: USDA For. Serv. Unnumbered publication,

For. Prod. Lab., 1966).

4

 D. R. Schumann and G. H. Englerth, Yields of

Random Width Dimension from 4/4 Hard Map/e

Lumber (Madison, Wis.: USDA For. Serv. Res.

Pap. FPL 81, For. Prod. Lab., 1967).

5

 D. R. Schumann and G. H. Englerth, Dimension Stock: Yields of Specific Width Cutting

from 4/4 Hard Maple Lumber. (Madison, Wis.:

USDA For. Serv. Res. Pap. FPL 85, For. Prod.

Lab., 1967).

6

 G.H. Englerth and D.R. Schumann, Charts for

Calculating Dimension Yields from Hard Maple

Lumber. (Madison, Wis.: USDA For. Serv. Res.

Pap. FPL 118, For. Prod. Lab., 1969).

7

 The unpublished computerized model known

as MULRIP was developed by A. Stern of FPL.

2
The Study

One of the objectives of this study

was to compare yields of cuttings by

the gang rip method with yields when

the traditional method was used.

Thus, all the cutting lengths for each

of the lumber grades FAS, Selects,

No. 1 Common, No. 2 Common, and

No. 3A Common were held the same

in this study as in the previous

study.4 5 6 These were as follows:

greatest for the longest and shortest

cuttings in each grade and are appreciably lower in the midlength

range. For example, in FAS grade for

96-inch cuttings, the best rip width is

9.1 percent better than poorest, and,

for 10-inch cuttings, 15.5 percent better

(table 1). However, for 60-inch cuttings

the difference is only 4.2 percent.

Comparable values for No. 2 Common

are as follow: 40-inch cuttings, 8.3

percent; 10-inch cuttings, 11.4 perGrade

FAS

Selects

No. 1 Common

No. 2 Common

No. 3A Common

Cutting Lengths-inches

96, 90, 80, 70, 60, 50, 40, 30, 20, 10

96, 90, 80, 70, 60, 50, 40, 30, 20, 10

80, 70, 60, 50, 40, 30, 20, 10

40, 30, 20, 10

30, 20, 10

All cuttings were clear on two faces.

ripping widths from 1-to 5-inch by

Yields were obtained for all gang

1/2-inch increments. For each of these

widths, yields for each length of cutting were developed beginning with

the longest cutting for that grade as

the primary cutting length. Within

each grade successive runs followed,

each using successively shorter

lengths as the primary cutting. For

example, with No. 2 Common lumber

the first run at a rip width of 1 inch

used the 40-inch cutting length as the

primary cutting length. The second

run used 30 inches; the third, 20 inches; and the fourth, 10 inches. Then

the rip width was increased to 1.5 inches and the series repeated. This

continued until all rip widths through

5 inches had been completed. In each

run yields for all secondary cutting

lengths that were shorter than the

primary cutting were also developed.

Results

As anticipated there is a relationship between the grade of the lumber,

the length of the primary cutting, and

the best width to gang rip as shown

in tables 1 to 5. In all cases, and for

all grades, best yields for the longer

cuttings in each grade result from

narrow ripping at a 1.0- or 1.5-inch

width. Also shown are the ripping

widths at which the poorest yields

were obtained: always at 5.0 inches

for the longer length cutting within

each grade, switching to 1.0 inch at

about midrange of the cutting

lengths.

Differences between best rip width

yields and poorest rip width yields are

cent; 20-inch cuttings, 4.7 percent

In tables 6 to 10 the maximum

(table 4).

yields obtainable from gang ripping

for each cutting length are compared

to those yields obtainable from the

traditional manner as reported in

USDA Forest Service Research Report

FPL 1186 for random-width cuttings

(highest possible yield). For FAS

grade, yields from gang ripping equal

or exceed those from traditional processing for all except the 96-inch

length, although there is little difference in any of the yields for

lengths 70 inches and longer. Differences in the lengths below 60 inches become fairly significant (table

6). The yields from traditional processing exceed gang ripping in the

Selects grade for cutting lengths of

80 or more inches. As in the FAS

grade the medium and shorter

lengths are obtained in significantly

greater volume by gang ripping (table

7). When cutting yields from No. 1

Common grade are examined there is

little difference between methods,

although overall, gang ripping has a

slight advantage. The trend relative to

cutting lengths noted for FAS and

Selects is not apparent for this grade

(table 8). Differences in yield between

the two methods for No. 2 Common

grade, regardless of cutting length,

are very small with a very slight

margin in favor of gang ripping (table

9). Gang ripping yields for No. 3A

Common grade are moderately

superior in all cutting lengths with

the margin tending to increase as

length of cutting decreases (table 10).

The results of an analysis of the

Table 1.–FAS grade-best and poorest product yields and ripping widths of specified

length cuttings when gang ripping

Poorest rip

width Yield Difference

Cutting Best rip Yield length width

------------------------------------------------------------------------------------------------------------------------

In. In. Pct In. Pct Pct

96

90

1.5

1.5

44.2

46.6

5.0 35.1

38.5

9.1

8.1

80 1.5 49.3

5.0

5.0 42.4 6.9

70

60

2.0

2.0

52.3

55.8

5.0

5.0

46.7

51.6

5.6

4.2

50 2.5

40 3.0

3.5

64.9

59.9 1.0

1.0

54.5

57.5

5.4

7.4

30 70.5

20 4.0 76.7

1.0

1.0

60.8 9.7

64.3 12.4

10 4.0 83.5 1.0 68.0 15.5

Table 2.–Selects grade-best and poorest product yields and ripping widths of specified

length cuttings when gang ripping

3
Table 3.–No. 1. Common grade-best and poorest product yields and ripping

widths of specified length cuttings when gang ripping

Table 4.–No. 2 Common grade-best and poorest product yields and ripping

widths of specified length cuttings when gang ripping

Table 5.–No. 3A Common grade-best and poorest product yields and ripping

widths of specified length cuttings when gang ripping

Table 6.–FAS grade–comparison of yields of specified cutting lengths by the traditional

and gang rip methods’

1

 Cuttings are clear, two face, two edge.

maximum yields possible using the

two systems, traditional, utilizing random widths and gang rip, using best

overall rip width, are shown in table II

and figure 2 for each of the lumber

grades. In both cases best combination of lengths was assumed and

does not necessarily include the

longest cuttings used in the overall

4

analysis of the grade yields. For FAS

and Selects grades the longest cutting when gang ripped was 90 inches.

The other three lower grades all used

cutting mixes including the longest

cuttings produced from the grade.

Gang ripping produces higher

yields in all grades than does the

traditional cut up system when a

good selection of lengths is being

cut. Largest margins are in the two

top grades (7.8 and 8.9 percent) and

the lowest grade (4.5 percent).

Relatively small differences result in

No. 2 Common grade (I percent), probably because of cutting bill lengths

used. This aspect was not examined

in the study reported here.

Discussion

Gang ripping can produce higher

overall cutting yields from all grades

of lumber. Unfortunately, this

superiority, especially in the upper

two grades, is a result of a substantially higher recovery in the medium

and shorter lengths which offsets

slightly lower recoveries in the

longest length cuttings. Since the two

upper grades are normally cut for

long cuttings, the question arises

regarding the desirability of gang ripping when cutting the two higher

grades. Actually, most long furniture

cuttings are not as long as the 90-and

96-inch cuttings included in the study.

When these two lengths are ignored

yields of long cuttings are at least

equal to the traditional system and

overall yields are higher.

Two other factors, not a part of this

study, must necessarily be evaluated

when deciding whether or not to gang

rip. The first, favorable, is a reduction

in both the number of rip stations in

the rough mill and certainly in the

labor requirement for ripping, since

the initial gang ripping is mechanical.

The second, unfavorable, is the probability that more adhesive will be required than in the traditional method

where the average width of randomwidth cutting is probably wider than

gang ripped cuttings.

Yields of primary and secondary

cuttings from gang ripping standard

grades of hardwood lumber are

presented in charts within a separate

publication: “Cutting Yields from

Standard Hardwood Lumber Grades

When Gang Ripping,” USDA Forest

Service Research Paper FPL 370.

3.0-5-7/8 0
Table 7.–Selects grade-comparison of yields of specified cutting lengths by the

traditional and gang rip method1

1

 Cuttings are clear, two face, two edge.

Table 8.–No. I Common Grade-comparison of yields of specified cutting lengths by the

traditional and gang rip methods’

1

 Cuttings are clear, two face, two edge

Table 9.–No. 2 Common grade-comparison of yields of specified cutting lengths by the

traditional and gang rip methods1

1

 Cuttings are clear, two face, two edge.

Table 10.–No. 3A Common Grade-comparison of yields of specified cutting lengths by

the traditional and gang rip methods1

1

 Cuttings are clear, two face, two edge

5
6 U.S. GOVERNMENT PRINTING OFFICE: 1980-651-111/5

Table 11 .–All lumber grades-comparison of maximum cutting yields by traditional and

gang rip methods1

Lumber grade Traditional Gang rip Difference

---------------------------------------------------------------------------------------------------------------------------

Pct Pct Pct

FAS 77.5 85.3 7.8

Selects 73.2 82.1 8.9

No. 1 Common 71.5 75.4 3.9

No. 2 Common 63.5 64.5 1.0

No. 3A Common 48.8 53.3 4.5

1

 All cuttings 1 by 10 inches and larger clear, two face, two edge.

Figure 2.–Comparison of total cutting yields for each of the lumber grades when processed by the traditional and gang rip methods.

M 148 536