Because if you don’t know how it lays out, you don’t know how you’re utilizing your material.

Stock strip and how it is utilized is basically everything a toolmaker does in his or her day. The pursuit of efficiently utilizing the material given to quickly produce as many accurate parts with as little wasted material and time.

Sometimes the words unit stock may be used to describe a piece of stock material and this can be confused with piece part. If a part is to be made out of the unit stock, the part is referred to as a piece part, but in large parts where the stock strip is fully utilized the piece part may be called the unit stock. I’ll try to make it as easy as possible to follow.

Stock Strip

Before a part is made there are a few factors to be aware of that will affect the quality of the piece part being made or how the stock strip feeds through the die. These factors relate directly to the stock strip and its conditions.

Coil stock is normally considered in terms of weight then length, meaning normally coil would be referred to as a 200lb coil, not a 200ft coil.

Their factors are:

• Edgewise curvature: Also called camber, this is where the edge of the stock strip isn’t straight. it’s wavy or they call it snakey.
• Longitudinal bow: This is where the stock strip has a curve on the thickest side. This is often caused from coming off a coil and the straighteners on the feeder sometimes handle that.
• Crown: Crowning is where the stock strip isn’t square but almost round or has a valley in the middle.

Grain Direction in Stock Strip

All steel will have a grain direction. This is instilled in the steel when it is being manufactured. The grain of the material will always be longitudinal to the strip. This is true for a stip of steel or a long coil. If necessary, sheet stock can be cut across the grain and there are rare times where it would be advantageous.

Material Conservation

Material conservations are really what strip layout or material utilization is all about. How important this is, is directly related to the volume of the run of parts. The larger the production run the more important it will be to have less scrap. One key principle I think is worth mentioning is saving on stock material is always secondary to producing a quality part.

Grain direction is the most important factor to consider when deciding on the relationship of a blank or a piece part. One has to understand the piece part and all its factors such as burr side, the number of parts, and quality so that it is oriented to the stock strip correctly. If the piece part is being made from a blank, blanking is normally done across the grain.

When blanking, normally it is best to cut across the grain which is the opposite if you were making your piece part out of the stock strip. 

There is 3 reasons why cutting across the grain is better:

1. Shorter distance to cut. 
2. With wide runs across the grain, you will get more blanks because it is less travelled so a faster run.
3. Blank piece part out of wide-cut part will produce less scrap because there is normally less front on the back web. 

Projecting Contours

A simple rule of thumb is that, if a piece part has any projecting contours they should trail the feeding direction.  This is a good practice because the projection can be an area to hang up while feeding through the die.  If it is impossible to do this the toolmaker may have to install stock lifters to avoid a hang up in the die.

Interlaying Blanking Orientation

There are occasions where interlaying the part in the stock strip will have considerable savings. An interlaid blanking orientation can be used with a two-pass method. This is done by interlaying the part on the stock strip where on the first pass one part is blanked out, then the partially skeletonized strip is rotated from the end and the passed through the same die again. 

There are three factors to keep in mind using a two-pass method:

1. In the two-pass method, the stock strip channel will be wider to accommodate the wide stock strip.
2. One should always incorporate a stop using one of the openings from the first pass. This will help keep the accuracy and quality of the parts.
3. Two-pass methods should be kept to simple parts and are rarely possible in a progressive die with complex contours.

Interposed Blank Orientation

Interposed orientation is a good concept or layout to keep in mind. This is when a part of the blank extends into what would have been the scrap area behind the forward part. I am sure just by looking at the part and blanks you can see why this would be useful.

Scrap Web Allowance

There are several factors that will cause a scrap size to change:

1. The thickness of the stock material: The thicker the stock material the thicker the stock web. With a thin material, it may not be practical to have such a small scrap web, in this case, normally a minimum value is assigned.
2. Length of the scrap web: The larger the scrap web the wider it must be. This is necessary to maintain web strength.
3. Shape: The shape of the scrap web will affect the web strength so, with that being said, one needs to consider the contour of the scrap web in order to widen it if necessary.
4. Type of operation: Some operations will require a wider scrap web simply for strength. Normally progressive dies will require wider scrap webs because of the many stations in the progressive die.

Width of Stock Strip

First, the overall length is needed. then add the front and back scrap and that is the minimum width of a stock strip needed. If a strip exactly as measured isn’t available, you have two options, first get a wider strip and cut it to shape or the more logical answer is to order the closest available dimension to the length of the part + back scrap+ front scrap.

Strip Layout

When the piece part requires more than blanking, which may be obvious to layout on the strip. The text has a 4 step procedure that is recommended:

1. Complete all necessary calculations including bend allowance; determine flat blanks size and contour.
2. Make actual cardboard cut out as a template of the flat blank. This is called a “paper doll” in the trade.
3. Using the cardboard template, draw the cut lines on a piece of paper.
4. Proceed by trial and error to rotate the cardboard cut out until you have the desired layout, then trace that again so you can see how each part relates to each other on the strip.

Once the position is decided on, check for the following: 1. Grain direction 2. Burr side 3. Relationship to a particular die of the stock ( polished side or plated) 4. Die operation 5. Possible construction issues due to blank orientation.

Conclusion

There are many variations on the stock strip layout, for example, scrapless layout, angled blank orientation, etc. I just don’t think they will be part of the test that I am preparing for. If you want to know more information I  encourage you to get the text and subscribe to our email list as I will continually add more to this. If you liked this content please share it.