Managing sheet inventory is one of the many ongoing challenges for fabricators. They don’t want to consume their cash flow and floor space with too much inventory. Likewise, no one wants to impede production by not having what is needed readily available.
Specifically, the first challenge is to have sufficient sheet quantity on hand. The second challenge is to have the right sizes available. The right size is defined as sheets sufficient in area to meet the need, but not too large or ill shaped that there is excessive scrap.
Engineers and programmers have struggled with this problem since the dawn of fabrication. And there isn’t an easy solution to it, unless or until you turn to nesting automation to provide the answers.
The Case of the Shipbuilders
The right-sized sheet problem plays out on a very large scale for builders of ocean-going vessels. Here’s the challenge they face. The plate material they are cutting is 0.1-8” thick. Because their product is mostly steel, raw material expense is very high – so there is no room for waste. In order to reduce the waste, they order custom plate sizes directly from the steel mill. There is a six month lead time from order to delivery for their material. Their process is to first complete the design of the metal parts of the ship and nest these parts while the remainder of the design details is being completed. This allows them to customize the plate size to the nest with a special feature called plate cutback. This feature starts by providing the nesting system with hundreds of discrete plate sizes to pick from. The nesting software picks the best plate size for the parts being nested and then if there is any waste at the edges of the plate, it cuts the plate back to a perfect fit. This process helps attain the highest possible material efficiency, but at a cost that only very large volume producers can afford. First the plate sizes are custom and will cost more per pound than standard sizes. The second cost is that the plate must be tracked from receiving into production to make sure the correct parts are cut from the plate. Finally, this process requires long lead times be built into the production process; in the case of shipbuilding, this is not a problem, but most products are produced in much shorter time frames.
This planning process allows them to know what size and how many plates the parts they want to cut will need long before the torch is fired up and parts are cut.
The Sheet Size Selector
It is possible to do something similar and avoid the some of the problems of the ship building model. The extra cost for sheet and plate sizes are mostly associated with the width of the material. This is due to the fact that steel mills like to produce in quantity and offer best pricing on standard width material. Most flat steel begin as a coil of a certain width. Sheet stock is produced by a cut-to-length process that sets the length of the sheet. This cut-to-length process is often done after the coil leave the steel mill. The cost of changing a length is small compared to the cost of changing the width of a coil previously produced. This means that custom lengths are much more cost effective that width changes.
Using this fact, a strategy of allowing the nesting software to select the optimal length is a practical way to determine what lengths with standard width should be inventoried.
If you are lucky enough to have a cut-to-length coil line, it is even possible to customize the length for every nest. If not, you can select an optimal set of lengths from a large range of lengths that fit you machines and your parts optimally.
Selecting from standard widths is also included in this process. The result is a set of sheet sizes that fit your part set optimally.
One drawback of this process is that you must know the part mix you will be produce for long enough to purchase and stock the raw material.
Working with Your Steel Service Center
Another approach is to build a relationship with your steel service center where they keep the various coil widths you need in stock. If the steel service center can deliver quickly, order the optimal width and custom lengths as you need them. If delivery times are too long, select a group of sizes and keep the minimum amount of inventory of each size and reorder as you use each size. The nesting software will pick the optimal size from your available stocked sizes. As you nest on material, the sizes you use most will be replenished as you reorder; this will allow you to keep a low inventory on site that is dynamically replenished by your steel service center. The result will be an optimal mix of raw material.
There is no need to guess about something as costly as material inventory when there are tools to quickly and easily get the answers you need to make informed decisions.
How about you?
How do you forecast material needs? What tools do you use? How is it working for you?
If Optimation can be of assistance in better managing your material inventory, contact us.