Eric Hill, Plataine
In the previous article, we discussed the critical elements of designing a frame that is both structurally sound and takes advantage of the latest CNC technology for maximum production efficiencies. However, even the most proficient design will go to waste if we fail to follow the same efficiency-minded approach in the transition from design to production.
a. Avoid over-cutting
The scenario is familiar to every person that has spent any time in furniture production. It starts with an order for one sofa. More than likely, that sofa will require over one sheet of plywood but not quite two, leaving you with a portion of a sheet of scrap. To avoid plywood waste, the common solution is to nest two sofas over three sheets so there is no scrap left.
The problem starts once we have the extra cut frame waiting for a new order to come in. First, the frame parts have to be stocked in inventory, with the hope that none of the parts get lost in the meantime. Assuming we got lucky, a month later an order comes in for the same sofa; but this time it is for three sofas, so we are going to use up our inventoried parts and cut two additional new frames.
However, in the fast-paced world in which we operate, there is a good chance that in the month that elapsed between the two orders there was a design change, and all of a sudden the frame that was on the shelf is no longer up-to-date. Not knowing there is anything wrong with it, the frame is pulled from the shelf and sent to production. Only when the frame-builder starts assembling the frame, off goes the alarm and over come the supervisors and find out this is an old design. So the old frame gets trashed and a new frame has to be cut, and our story starts all over again…
Not only was plywood wasted, but significant labor cost was incurred in the process of first cutting the extra frame, then moving it to a storage shelf, taking it off the shelf and sending it to production, and finally discarding it.
This common scenario shows how a practice that was intended to avoid waste ended up throwing a monkey wrench into your production process, as the entire upholstery line flow was disrupted. This is just a simple scenario with a small quantity of frames; just imagine what happens in a large factory producing hundreds of frames each day.
How do we avoid over-cutting? What we want to do is cut that one-sofa order and instead of cutting another identical frame for inventory, use whatever is left of the second sheet to start the next order (for a different frame). The method is called Rolling Nest - it is like putting the plywood on a roll but keeping all parts for different frames separate.
By only cutting what we need, we not only save on material and labor cost, but we also free up space that was occupied by inventory bins with frames that collect dust.
b. Streamline the coding of your CNC machine
Once the nesting of the frames is done, a toolpath has to be created and the G-code has to be entered into the CNC machine in order to execute the cut. Done manually, each step in the process can be both time-consuming and a source for damaging errors.
Using a program that automates toolpath generation and the creation of G-code for your CNC machine can save valuable production time and eliminate potential failure points. It also frees up your programmers and designers to produce new fresh designs rather than spend their time on tedious tasks.
To make things even easier for your CNC operators, the program can generate printouts for each nest, letting the operator know in advance exactly what the machine is going to cut. With the addition of a barcode reader, all the operator has to do is scan the barcode, press the cycle start button, and let the machine do the rest.
c. Choose the CNC machines that best fit your product
CNC choices are abound, with options ranging from vacuum, roller, vacuum and roller to twin tables, single table, tool changer, drill banks, and the list can go on and on. The most important factor in making your selection is matching the machine features to your product characteristics.
For example, if your product requires special operations such as roundovers or degree cutters, you might need a CNC with an automatic tool changer; or if your product requires a lot of drilling, you might choose a drill bank that can hold multiple bits.
Consider the material you plan on using. If the material you are going to cut is a Grade A material, a vacuum might be all that you need; but if it is of a lower grade with some warping, you would probably need both a roller and vacuum.
Since the purchase of a CNC machine is a long-term investment decision, it is important that you consider these factors not just for today, but also for the future as you adjust your product design to streamline production.
d. Control the feed and speed of your CNC machines
Every so often, I go into an upholstery factory and a machine will almost pierce my ears with a squalling sound. When I look at the machine, I usually see the spindle Rpm is wide open and the machine is cutting at full speed. Yes, we all want the most productivity from the machine, but this doesn’t always equate to maximum spindle Rpm.
Operating a cutter is part science part art. I usually get started by using a chip load calculator, then tune it by ear from there. The calculation is rather simple:
Chip load [in.] = feed rate [inch per min.] / (RPM / # of flutes)
I have found that a good chip load is somewhere in the range from .02 to .06 inch, as this load helps dissipate the heat from the tool and lengthens the tool life. So if you start, for example, with a feed rate of 1200 inch per minute and 22,000 Rpm, you will end up with a chip load of .02 inch using a 2-flute bit.
From here on, you have to use your ears to fine-tune the sound of the bit cutting by adjusting the spindle speed up or down. You know you are there when it sounds like tearing a piece of paper. That’s the art part.
e. Select the right tools
Speaking of tools, there seems to be an overflowing selection of tooling to choose from these days, and a new tooling manufacturer around every corner. When choosing a tool, make sure the vendor can guarantee the quality of the tool.
Use the right tool for the speed and finish you plan to achieve, keeping in mind the material and the machine characteristics. For example, if your machine will only cut 600 inch per minute and only turn 18000 Rpm, you don’t want to use a 3-flute tool which will make the chip load too low (<0.02) and cause it to start burning. In this case, you may need a 2-flute tool instead.
Remember that every tool is going to be different. When you reorder the next batch, while the new tools may look visually identical, they may function slightly differently. You may need to make feed and speed adjustments to accommodate such differences.
Summary
While many of the challenges faced by upholstery manufacturers are the result of global trends outside our control, there are things that every manufacturer can do to combat these challenges. Staying competitive in this day and age requires all stakeholders - managers, designers, programmers and operators included – to scrutinize every step and find new ways to better serve the customer, increase process efficiency, and make the most out of new technology innovations. Frame design and production are important parts of the process; I hope this article provided you with some ideas you can implement in your factory. I would be happy to hear your thoughts and any ideas you might have that could help our industry move forward.
