Strip Layout

The blank is a piece of that material cut to any desired shape.Some blanks may have round square or rectangular shapes.Some may be very irregular in shape.Many blanks are subsequently bent,formed or drawn.The term blank refers to the flat piece before any deformation has taken place on it.


Economy factor

Stock material consservation is a decisive factor in means should be tried to attain this without sacrificing the piece part.
Economy of any strip layout in percentage is fount out by the following formula.
Economy factor(E) in %=(area of the blank *number of rows* 100)/(width of the strip*pitch)
A minimum economy of 60% should be aimed at.The position of the blank in the strip decides the economy factor.
Calculate the economy factor to punch the mild steel washer in single row feeding.Outside diameter is 30mm, inside diameter 18mm and thickness 2mm.
Note:Terms used in a strip layout.
1) Scrap bridge
a) This is the portion of the material remaining after blanks operation between one edge of the strip and the cutout portion.
b) Thee portion of material remaining between the two adjacent openings after blanking is also called as the scrap bridge.
2) front Scrap:This is the scrap bridge on that edge of the strip which is towards the operator.
Back Scrap:This is the scrap bridge on that edge of the strip which is away from the operator.
3) Back Scrap:This is the scrap bridge on that edge of the strip which is away from the operator
Scrap bridge width is 1.2s
E=area of blank*100*[no of rows/(pitch*strip width)
scrap bridge width=1.2*2=2.4mm
Pitch=30+2.4=32 mm
Strip width=30+4.8 =34.8 mm
Number of rows =one
Area of blank=pie D square/4=(pie /4)*30 square= 706.65 mm square.E=706.95*100*1/(32.4*34.8)=62.69%
Calculate the economy factor to punch the same washer in double row feeding.
E=Area of blank*100*Number of rows/(pitch *strip width)
Pitch=32.4mm
Area o blank=706.95 mm square
Number of rows =2,br>Center distance between two washers are required to determine the strip width.
center distance=Cos 30 degree *pitch=0.866*32.4=28.05mm
strip width=2.4+2.4+30+28.05=62.85 mm
e=706.95*100*2/(32.4*62.85)=69.42%


strip layout for blanking tools

Choice of strip layout method
blanking tools produce blanks out of the strip or unit stock.None of the edges of the strip or unit stock forms an edge of the blank.Blanking is the most efficient and popular way of producing intricate and closely tolerated blanks.The profile and accuracy built into the toolwill be reproduced on the blank.
in the strip layout blanks can be positioned in different ways in the strip .Choice of the method depends on the following factors.
- shape of blank
- production requirement.
- grain direction
- burr side
- stock material


Shape of the blank

The contour of the blank is the main factor which decides the way in which it is to be positioned.


Production requirement

If lesser production is anticipated more emphasis should be given for material conservastion without increasing the tool cost.


Grain direction

When sheets are produced by rolling,the rolling direction orients the grains.Standard sizes of rolled sheets or strips will have the grains along its length.Bending the strip along the grain direction may result in cracks or fractures.If the blank is to be bent at a later stage the strip should take care of the grain direction.
The grain direction should be at right angles or at an angle more than 45 degree to the direction of the bend when harder varities of strips are used.


Burr side

In a blanking operation burr is formed on the face of the blank which comes in direct contact with the puch.
In piercing it appears on the face which comes in direct contact with the die.
In some piece parts the burr resulting from either blanking or piercing would be required to appear on a particular face of the blank in relation to details of the blank contour.Whiledeciding the strip layout care must be taken to see that such requirements are met.
Stock material
a comparative study of stock material conservation,tool cost and labour cost is necessary while the strip layout is made.If the stock material is precisous every means to conserve the stock material should be employed.If double pass is a one complete pass of the strip,it is reserved fed again for maximum utilisation of stock strip. This is decided by the shape of the component(two pass or double pass both mean the same)Double pass method is employed,labour cost increases.So a double pass layout should justify the cost of stock material conserved.
Based on the above factors,different striplayouts are explained in following paragraphs.
Single row one pass layout
This is the most popular way of laying out the strip.The blanks are arranged in a single row.The strip is passed through the tool only once to punch out the blanks from it.
There are two possible ways of laying out this strip.
- Narrow run
- wide run
Wide run is more desirable due to the following reasons.
Shorter advance distance of the strip promotes easy feeding.
More blanks could be prduced from a given length of strip compared to narrow run.Therefore a fewer number of strips are to be handled to produce a given number of blanks.
Narrow run is used when the grain direction of the piece part has importance.
blanks having alleast two straight parallel edges
In such cases the strip width should be equal to the distance between the parallel sides.The blanks are produced by a cut off or parting operation.If the blank has got two sets of parallel sides, a cut off operation is sufficient to produce the blanks.But if the blank has got only one set of parallel sides.These sides become the sides of the stock strip and the other non-parallel sides are produced by a parting operation.
Strip layout for cut off and parting
Cut off punch cuts with only one edge.No scrap is produced.A parting punch cuts with two opposite edges there by producing a scrap.
Nothching
Notching is a cutting operation for cutting off small prtions from the edge of a strip or a pre blanked component.
Trimming
Trimming is an operation of cutting off material to alter the shape of the strip or blank.
In nothching only a small area of the blank is cut off.In trimming a larger area of material i removed.Blank can be produced by combining notching ,trimming and piercing operations with cut off parting operations.
Blanks having irregular contour
The following factors must be considered before determaining the best method of positioning a blank in the strip.


Contour
If the blank has two parallel sides,it can be produced by cut off operation.The advantages of cut off or parting operation are
- minimum material wastage.
- less tool cost.
- no scrap strip to handle.
- speeds up production.
accuracy in strip width
Sheared strips cannot be held to an accuracy closer than +-0.2mm.If the blank must be held to closed limits on its width sides cutting off or parting cannot be employed.When the blank dimensions are to be controlled to close limits it should be produced with a blanking tool,regardless of the parallel sides it may contain.
flatness
If the blank has to be flat,a blanking tool is preferred ,a blanking tool produce considerably flatter components than other tools.
Single row two pass method
This strip lay out demands the strip to be fed twice through the tool.This is to achieve greater economy in stock material utilisation.
A two pass tool requires two stops.The stop used for the first pass should be removed or made to retract(spring loaded stoppers)from the working surface so as not to interfere with the second pass.The front and back scrap as well as the scrap bridge should be wider than those for the single pass (about 50 to 100%)
Two pass lay outs are justified only when the wastage is considerable and the stock material is costly.
Double row lay out
double row lay out
Higher economy can be attained by positioning the blanks in double rows.
Gang dies
A gang die consists of two or more similar sets of tool members so as to produce two or more number of components during a single stroke of the press ram.A gang die eliminates the cumbersome process of double pass.
the higher tool cost will be off set by higher rate of production.Gang dies are not recommended for very complex work.
Angular layout
Some piece parts require to be laid out in an angular position to make the lay out more economical.