Nesting vs point-to-point: among others, Ken Susnjara says that reduced scrap, higher yield and the ability to machine around defects are some of the more prominent advantages nested based manufacturing has over point-to-point machining.
Custom woodworking, especially custom cabinets, has really changed in the last few years and continues to change at a pretty rapid pace. Most of this change has developed as an outgrowth of nested based manufacturing.
The first evolution in cabinet manufacturing occurred when 32mm dowel construction first moved into the United States, from Europe. This construction method lent itself to a level of automation using a panel saw and point-to-point machining center.
This approach worked to automate machining using the same basic approach as had been used traditionally. First, the large sheets of material are sawed into individual blanks and then the individual blanks are drilled and machined into their final form, one at a time, on a CNC machine. With the panel saw/point-to-point, the machining and perhaps the sawing were now automated.
Cabinet design software evolved at about the same time and offered to create the programs to drive the panel saw and the point-to-point.
At this time, I need to explain what I mean by "point-to- point".
The original CNC boring machine used for 32mm drilling were relatively low-cost machines, but were also limited in their function. To save money, [operators] did not attempt to coordinate axes movement when moving from one drill point to the next. Instead, they were only interested in the position of the machining head at the point at which drilling occurred. Since the machine was only used to locate drill points, it became known as point-to-point.
Then, axes coordination and the ability to rout were added and the manufacturers attempted to change the name to machining center. At the same time, drills were added to CNC routers in an attempt to address this same market and these were also called machining centers. Things can get quite confusing when two totally different approaches and two totally different machines are called the same thing.
Therefore, in this book, I will call a machining center that processes pre-cut blanks into final products a "point-to-point" and I will call machining centers that machine a nest of parts directly from a full sheet a "CNC router."
It is important to recognize the differences between these two approaches because the evolution from point-to-point to nested based machining on a CNC router has resulted in a major increase in productivity, especially for shops that do custom work. Let's look at the differences in actual use between the two approaches.
Start with the panel saw/point-to-point approach. The basic steps required to machine parts using a panel saw/point-to-point are to first transport the sheet material to the panel saw. These are then processed through the panel saw where the sheets are cut into blanks. The blanks must then be sorted and identified and those that require edgebanding must be separated from the rest.
These are taken to the edgebander and edgebanded.
Then all the parts are transported to the point-to-point. There, parts are processed through the point-to-point, one at a time. It may also be necessary to adjust the position of the vacuum pods as different sized blanks are machined.
Point-to-point machines generally use individual pods--some square and some round--to hold the blanks. These pods have a rubber seal around the top and use conventional vacuum at a fairly high vacuum level. This approach creates a substantial holding force and tends to hold the blanks quite well, but only works if an unbroken seal is developed between the pod and the blank being held. Hence, the reason for the rubber seal.
It is, therefore, important that the pod be located under the blank and away from any edges that will be cut, and also away from any holes that might be drilled through the part. If the pod is incorrectly positioned and the machine cuts into it, not only is the seal, and thus the hold-down force, eliminated but often the pod is damaged or destroyed.
For this reason, it is important that the location of the pods be verified and adjusted for every part, and this takes time.
Now let's compare this to nested based machining on a CNC router. The first step is to transport sheet material to the machine. These full sheets are loaded onto the table of the CNC router and parts are machined directly from the sheet. Since the sheet is held in place using a high-flow vacuum system, there is no adjustment needed to hold the parts in place. The parts then are edgebanded.
You might notice that I did not include the step of transporting the parts to the edgebander. The reason for this is that most of the time, the CNC router operator can do the edgebanding work on parts from the last sheet while the next sheet is being processed. A full machining cycle typically requires five to eight minutes, allowing sufficient free time to do edgebanding. With the point-to-point, pods must be adjusted and each part must be handled individually, keeping the operator fully occupied and not allowing any free time for other processes, such as edgebanding.
Overall, a process that requires four to five people using a panel saw/point-to-point can be accomplished with two to three people using a CNC router and nested based manufacturing. The first advantage of nested based manufacturing is that it requires less labor.
The next advantage of nested based manufacturing is that it normally results in better yield.
The yield improvement comes primarily from the fact that a panel saw can only cut straight lines while a CNC router can cut in any direction. A panel saw must cut all parts as rectangular blanks, even parts that are not rectangles. Excess material from these parts must be scrapped. A CNC router can cut the actual part shape from the sheet so parts can be intertwined in the nest. On certain designs this can result in significant material savings.
Even on rectangular parts, a panel saw requires that the edges of the rectangles be lined up in the nest along common cut lines so the saw can cut them. If the rectangles are not the same width, this requirement results in additional scrap. A CNC router allows the rectangles to be nested in the most efficient manner (it's ca[led True Shape Nesting), eliminating the need to line parts along cut lines and eliminating the extra scrap.
Some proponents of the panel saw point out that the kerf of a router bit is larger than that of a saw blade, which reduces yield. This is seldom true in the real world. The only case where it might occur is if the difference in kerf width determines whether one of two parts can be nested across a standard width sheet. The common cabinet depth used today allows enough dean-up material around each part on a standard width sheet that it makes no difference.
In addition to labor savings, nested based manufacturing produces as good, and in many cases better material yield than a panel saw/point-to-point producing the same jobs. The next advantage of nested based manufacturing is that it runs parts faster than a panel saw/point-to-point. The production rate of each approach is determined by how fast parts are machined. The actual machining speed of a CNC router and a point-to-point is about the same.
Machining speed is normally determined by the tooling and material being cut rather than the machine. CNC machines, either routers or point-to-points, usually move much faster than the maximum speed at which a tool can cut typical material today.
With the point-to-point, however, the operator must unload and reload each part. The time required to do this is added to the processing time for each part, lengthening the overall machining time per part. In addition, point-to-point machines use vacuum pods to hold parts for machining. These must be adjusted for differences in part size, so this additional step adds even more time to the overall process.
Since the point-to-point must process parts one at a time, any tool changes must be clone for each part, adding extra time to the cycle. In nested based manufacturing, a tool is changed and then every part on the sheet that needs that tool is machined. This results in fewer tool changes and faster production.
Finally, nested based manufacturing requires less capital investment and less overall shop floor space. A CNC router and an equivalent point-to-point machining center cost about the same. However, with the point-to-point, you also need a panel saw. Also, two machines require more floor space and support space than one machine.
As you can tell by now, I am a real fan of nested based manufacturing. The processes around nested based have been refined so that all the little problems and techniques are handled quite well. This being said, all approaches to nested based manufacturing are not the same. So now, let's look at some of these details and differences.
The first question is: Where do you do the nesting? People naturally assume that the nest and the resulting CNC program are generated by the cabinet design software and then sent to the machine to cut. Traditionally, this is how it has been done, but this has some problems and there is a better way. To understand, let's look at a typical job.
The first thing about a typical job is that it will require two or three different sheet materials, probably in different thicknesses and seldom will you use every bit of every sheet. At the end of the job, you will have two or three or more partial sheets of material left over. Some of these may be half or three quarters of a sheet and this material has real value. It would be nice to be able to use this on the next job.
Once you have run ten jobs or so, you have 30 or 40 of these partial sheets of various materials lying around. Let's try to use these on a new job.
Most design software has some provision for using less than a full sheet. The first thing you need to do is to measure and document the actual size of the partial sheet. Now take this information to the office and input it into the design program. After doing this, you will get a nest program that incorporates these sheets along with some full-size sheets. This nested program is sent to the machine, typically over a network.
Before we are ready to run, we must sort through our pile of sheets to locate and identify the actual sheet the program wants. Depending on how many we have and how close to the same size they are, this may not be all that easy to do. It is even possible that some of these sheets have been used for other purposes or been damaged while we were programming them into our job. If that is the case, we have a difficult choice. We can either go back and re-program the job, or we can use a full sheet instead of the partial sheet. Unfortunately, the latter will generate even more partial sheets we have to deal with. As you can see, this is getting pretty complex.
In actual operation, it is probably even a little worse than that. Most shops find that the hassle and lost time that results from trying to use this extra material costs them more than any possible savings they might realize. The idea sounds good, but it just doesn't work very well in the real world. Generally, they either let these sheets pile up and eventually haul them away or they just scrap them at the end of each job.
However, there is another approach to nesting that works a lot better. What if we did the actual nesting right at the machine control rather than in the office? If we could add some additional refinements, this might address many of the problems we just discussed.
After some serious analysis, this is the approach we (Thermwood Corp.) decided to take. We were in a unique position since we not only developed the cabinet design software but also developed the software that operates the CNC control. We could modify both so that they worked together seamlessly to address these issues.
This approach not only addresses some fundamental issues but also has evolved to offer a Lot of additional capability and functionality. Instead of sending a machine program file to the CNC control, we send a description of the individual parts needed for the job. The machine control then nests these, does the CAM functions and then does the Post Processor functions, resulting in a program that can then be run. This is where some extra features come in that are essential to make this all work in the real world.
In addition to nesting and creating the CNC program, the control also prints a diagram of the parts that make up each sheet of the nest. It also prints a stick-on Label that identifies each individual part. To do this, the control itself is equipped with a set of printers.
If you took carefully, you will notice that some of these Labels have a bar code printed on them while others don't. The bar code is used to identify any part that requires machining on the flip side. Depending on the construction methods and design, some parts may need to be machined on both faces. When this occurs, we need a simple way to identify these parts and perform the flip side machining.
In practice, the label is positioned on the part in a specific corner. After the nest is run, any parts with a bar code are separated and stacked off to the side. Once all the full sheets have been machined, the flip side operations can be performed.
At this point we need to do several things. First, we need to identify each part. Then, we need to retrieve the correct CNC program needed to machine the back side of the part. Then we need to properly position and orient the part for machining. This could all be a major hassle if we can't find an easy way to do it.
The bar code label solves this problem. We can scan the bar code to tell the control which part we are going to machine. This means you do not need to keep the parts in any particular order and it is all but impossible to run the wrong program on a part.
Once the control knows what part we are going to machine, it can automatically retrieve and load the correct CNC program for that part. We will use a corner of the machine table to locate the parts. There are a set of flip-up stops that can be used to position the part. Then these are flipped down and out of the way so that machining can occur around the entire perimeter of the part without hitting the locator bars.
When we placed the label on the part in the nest, we placed it in a specific corner of each part. We are now going to use this label location to help us orient the part for flip-side machining. Flip the part over and place the label against the stops in the corner. In this way, you do not have to worry about proper positioning or orientation--it is pretty much automatic. Once the start button is pressed, the flip-side machining is done.
This approach is about as simple and foolproof as you can get. Scan the label, position the part and hit the start button. The system does the clerical work, identifying the part, then finding, retrieving and loading the correct program for that part.
This is a clear example of making systems practical for real world applications. If you are going to make 100, 200 or 500 parts, a minute or so looking for and loading a program file is not a big deal. If you are going to run one part, any time required to find the program is a big deal Custom woodworking has a whole new set of requirements that are not important for more traditional applications, but are vital if we really want to automate the custom business.
The next big plus is that the control also prints a bar code label for the partial sheet or sheets remaining at the end of the job. This bar code not only identifies the size and shape of the piece but also identifies the material it is made from.
This is where another real world feature comes in. If you look at a typical piece of scrap, there is a fairly large unused surface that you would like to re-use. Protruding from this are thin sticks and spikes and all sorts of protrusions that are pretty much worthless and will make handling and storing this material a pain.
Again, technology comes to the rescue. At the end of the cycle, after all parts have been cut, the control examines the remaining material and cuts around the outer edge of the large blank, removing the protrusions and yielding a blank of a fixed, known size that is easy to handle and store.
The next time you run a job from this same material, you will be able to use this piece. When you first load the job and before nesting is performed, the system asks if you have any partial sheets or blanks you would like to include in the job. Scan any sheets lying around in the order in which you want to use them. The control nests onto these pieces first, then uses full sheets and prints labels for any material remaining.
As an added benefit, remember I said earlier that the control knows what material the partial panel is made from. If you start a job that is set to run with a specific material, but try to use panels of a different material, the system will alert you. Again, we are trying to make the process as simple and foolproof as possible for the average shop.
As you can see, nesting at the control makes using this extra material easy and straightforward, eliminating a lot of steps, fumbling and sorting sheets, but it also allows you to address another possible problem.
It is possible that one or more sheets of material may have defects that you do not want to include in your parts. When you nest in the office, this presents a real problem. When you nest at the control, however, you can identify the bad area and the control nests around it. You can now use the sheet where otherwise you would either need to discard it or spend some real effort to try to program around it.
This could be an interesting feature in the future. We have had discussions with some of the largest manufacturers of sheet material, about possible ways of using sheets with surface defects.
In their process, they use vision systems to verify the surface quality of each sheet of material they make. Some of these have surface defects and must be rejected. Depending on a lot of factors, the number of these sheets can be rather large. Sheets with surface flaws may account for as much as 5 to 6 percent of production and when you are producing tens of thousands of sheets a day, this is a lot of scrap material.
Their vision system can not only identify the defect, but can also identify the location of the defect. If this information can be supplied to the machine control, it might be possible to automatically nest around defects in each sheet of material. Thus, the material goes from something with little or no value to material that is practical to use.
The material supplier will still need to offer this stock at a deep discount, but not as deep as today. The cabinetmaker can easily use this material and perhaps enjoy a substantial discount in the process.
By now you are starting to see that some interesting technology has been developed whose primary purpose is to make machining custom nested parts practical. From this core, additional capability has been added to provide even more flexibility.
Full copies of The New Furniture: How Modern Technology is Changing the Furniture and Cabinet Industry can be purchased through Thermwood's online store at www.woodworkerswholesale.com. Thermwood also gives away free copies of the book at its trade show booths.
|Printer friendly Cite/link Email Feedback|
|Title Annotation:||NESTED BASED MANUFACTURING|
|Publication:||Wood & Wood Products|
|Date:||Jul 1, 2006|
|Previous Article:||Optimism prevails at Xylexpo: new venue, new technology set the stage for Xylexpo/Sasmil 2006.|
|Next Article:||Small shop goes CNC: Windham, ME-based Quarter Point Woodworking uses CNC technology to increase its cabinet and furniture-making capabilities.|