Wednesday, September 9, 2009

Should Cut Lists Be Banned

Gel Huey over at Popular Woodworking magazine had an interesting post on using cut lists in woodworking. See it for yourself here.

His main thesis is that cut lists are hazardous to your health, due to inaccuracies, mistakes in cutting, and so on. I agree. To a point. I think every woodworker should know how to draft plans, elevations and sections from scratch to create a full set of working shop drawings. This practice will show much more valuable detail than any cutlist ever will. Luckily, I've been using CAD and 3D software in architectural firms since, oh, about forever, so I find this process to be natural. I'd hate to try to build anything without at least a basic set of plans, even if it's just a simple sketch on a napkin or a board.

Cut lists, then, are simply schedules of the parts required to build the piece. If done correctly, there should be zero difference between the data on the cut list and what is shown in the shop drawings. Ideally, the sizes would listed in gross LxWxD dimensions, and would include descriptions and remarks, such as 'TBE 3/4"' (Tenon both ends 3/4").

But they're hard to create accurately by hand. Translating even the best drafted plans and elevations into a cutlist is asking for trouble, as you mistake a "3" for an "8", or forget to add tenons to your rail dimensions, and so on.

And cutlists by themselves aren't adequate anyway. It can only tell you the absolute minimum amount of wood the project will take. You cannot use a cutlist and go blindly cutting down stock; it doesn't account for the variables in the material itself. Grain direction, sapwood, checking, knots - all of the things that make wood, well, wood - always have to be handled on a board by board basis.

So, taken as a single thing, a cutlist really is not that helpful. I don't think you can get away with a cutlist without a set of shop drawings.

This is one reason why I, like many woodworkers, use SketchUp extensively for furniture and cabinet design, and to create my shop drawings. I may also use a cutlist plugin (there are several freely available) which can lay out parts on a board to make a decent estimation when shopping for lumber. Good cutlist plugins will also take into account sheet goods, hardware and the like.

With SU I have developed a particular process which (so far) has worked really well, so I minimize the reliance on cutlist plugins. In general I spend WAY too much time in SU nailing down every detail - as well as I can given the modeling limitations of the software. I'm not going to model a ball and claw foot, for example, but I can at least block it out. of course, as most of my projects are for the shop, they are often rectangular, regular and easy. But I design and refine like crazy; it's not odd to spend an entire weekend refining a simple shop cabinet, because I always think through the intent and usage of the piece, and how it fits into the shop.

When designing projects, I get component happy. I make sure every part is a component (which can then be reported in the cutlist). I make "assembly" components composed of their nested components. A run of cabinets would have a base assembly component and a wall assembly component, each of which would have individual cabinet assembly components, each of which has the components for the side panels, shelves, door parts, and so on. Drawers are themselves assemblies of smaller components. For good measure I may also add flat 3D text to each component which labels them right on the part.

Like I said, cutlists don't tell you the best way to lay out parts for production. I do this in SU as well. I copy and explode my assemblies and by hand lay out all of my parts on top of simple rectangles that represent full sheets of plywood, MDF, 1x6 boards, etc. I space each component out by 1/8" to account for the saw kerf, and dimension each unique component right there. This way I not only enumerate how many board feet or sheets of plywood I'll need, but visualize how to best lay my parts out to maximize yield and minimize the number of saw cuts needed.

I am also very careful to account for mirrored components, which is one reason I copy and explode the assemblies into their sub-components, then move/rotate them into position on the sheet or board.

This step is pretty critical for me, because I don't (yet) have a truck and cannot (yet) purchase full 4x8 sheets of plywood without getting them cut down at the store. With a comprehensive cut plan, I can get my plywood cut down at the home center lumber yard into smaller pieces which will fit in the car. For example, I know for a fact that I can get a full 4'x8' sheet cut down into one 16"x96" strip and two 32"x48" pieces, so I plan my sheet good layouts to be close to these dimensions. Regardless, I always work hard to maximize the size of my cutoffs for future use. Having 12 1/2" wide strips of plywood isn't as useful as one 6" wide piece.

After the design work and layout is done, I then create separate Pages in SketchUp for the plans, elevations, and sections, along with part layouts for sheet goods and boards to cut. While I hate using Layers in SU (the interface is beyond awful), it is here that they come into play. I make a layer for each page, then put the parts layouts into their page layers so I can turn off all other layers in a Page. When you update the Page you then save the layer state, so each Page shows just what I want.

After this is done, I fire up Layout 2 (included in SU 7). I insert the SU model and create individual Sheets for each Page. In each Sheet I have a single viewport to show the SU Page. I can then label the Sheet with text as needed, and I now have a complete set of shop drawings.

When I get the chance I'll post my latest shop project, an outfeed table for my table saw which includes a standalone cabinet along with a fold-down torsion-box assembly table.

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