Same But Different

Along with several posts written about the concepts and philosophy behind my design process and workshop layout, this blog has covered the construction of two rather simple projects this year. The first of these was an iPad stand, and the second is a set of candle holders I just completed this past weekend. The two projects pictured here appear different, but yet are almost the same.

Both designs have four tenoned legs that are glued into the slot mortises of a central block. That block in turn supports a platform that supports an item that needs holding. In one design that item is a candle. In the other that item is an iPad. Both designs appear almost identical from the perspective of the workshop, and so were therefore conceptually supported during their respective build processes quite successfully by the adaptive workcell arrangement around which I built my Minnesota shop.

There is much more to learn about manufacturing under the principles of Group Technology and the relationship that the broader concept of Design for Manufacturing has on producing a product family. I talked about common design architecture in an earlier post where I described a set of different lamp stands I built that were based on a common product platform architecture. The candle holders and iPad stand are just as much a product family based though on a common build architecture.

Update: I just completed building a set of objects to act as prototype fixtures for tungsten filament light bulbs. These are described in a later entry. You can see in the one whose photo I've added here that the concept of a central block with four tenoned legs is used yet again.

My walk in to work each morning usually takes me through the Mechanical Engineering building at the University of Minnesota. Two useful books on this topic that I found on the free book table there a couple of summers ago are listed below.

Meyer, M., & Lehnerd, A. (1997). The Power of Product Platforms. New York: The Free Press.
Sanderson, S., & Uzumeri, M. (1997). Managing Product Families. Chicago: Irwin.

I also found the following white paper a good source of recent information on this topic.

Parametric Technologies Corporation. (2012). Achieving Product Diversity with Scale.

The metal candle dishes for the candle holder project were provided by Marlaine Cox Metalworks.

And Then There Were Eight

Recall from my previous post that I am making a set of blocks for a Christmas project that each have four slot mortises routed on one face, and a circular extruded cut feature routed on the other.

The slot mortises were easy to rout using the CNC router, and since they will accept a matching tenon, they only needed to be accurate.

The circle features on the other hand need to be free of visual defect which I found more difficult to accomplish given the nature of the two woods I am using.

Padauk and zebrawood easily splinter, and the router bit also tends to burn both woods more easily than others. I found that slowing the speed of the router to somewhere around 12,000 rpm and increasing the feed rate of the cut removed any tendency of the router bit to burn the wood. It was actually surprising that this combination of speed and feed rate also produced an almost splinter free cut.

What a relief.

There were originally nine blocks. The problem of speed and feed rate did not appear on the initial test run using a scrap of medium density fiberboard. It only became apparent on the first block of zebrawood.

This unfortunately happens sometimes.

That first block of zebrawood is now a block of sacrificial scrap. It was used to perfect the cnc routing process for the circles, and will be useful to test a future process that might need verification before committing that process to the other eight blocks.

Accurate Repeatability

I began a small project this week that I hope to have completed by Christmas. The design is based on a square block of wood with four slot mortises routed at each corner. A larger circular feature is routed on the opposite face of the block. There are nine blocks in all.

In total then there are thirty-six slot mortises to route. Repetitive accuracy is a requirement so the use of the CNC router was a natural choice for machining the mortises.

The design solid model served as a starting point for geometry on which to base the slot mortise tool paths. The following segment of machine code represents a single pass at depth. There are multiple passes for each slot, and each pass plunges into the block by a fixed distance until the final depth is achieved.

F14.0
G00 X0.5938 Y0.5313 Z0.2000
F11.0
G1 X0.5938 Y0.5313 Z-0.0620
F14.0
G01 X1.7813 Y0.5313 Z-0.0620
F14.0
G00 X1.7813 Y0.5313 Z0.2000

The two left mortises both top and bottom are routed together, and since the block is square and its features symmetric, it was simply rotated to produce the remaining mortises on the opposite side of the block. Doing it this way allowed the block to be clamped to the table on one side while routing took place on the other.

The routed mortises will accept tenons from four legs that support the block as a platform of sorts. It should begin to be apparent what this project is all about.