Plan Drawing on Sketchup using 3/4" ply

[tim walker]

Designing a shop project using 3/4" ply which is really 18 mm (0.708"). When using Sketchup for your design, do you use 0.708" or do you use 0.750"? These cabinets will span approximately 12 ft.

[Dan Barber]

I would design to the actual dimensions of my working stock. Too much chance for accumulated error(s) using the nominal size. I would also design in mm, much easier than constantly converting back and fourth.

[tim walker]

Yes I agree on the metric. The last project I did was in metric. it is SO much easier, for instance, if I need to divide 17 13/15 by three. its much easier to divide 454 mm by 3 for151.3 mm or rounding to 153. 1 mm = 4/100ths (0.04") of an inch.

[Jamie Buxton]

I usually draw the job once using nominal plywood thickness, so I know what to buy. Then I correct the drawings using the actual thickness. Frustrating, but I don’t want to be changing dimensions on the fly in the shop. That way leads to mistakes.

[Steven O Smith]

The last time I used plywood from the orange store, it wasn't even metric, a bit thinner than an even number of mm. This was on 1/4" equivalent.

[Terence McGee]

Based from using SketchUp several times for small projects, I'd say stick to the actual thickness of your plywood (0.708"). This will help you achieve the most accurate and error-free build particularly for large projects like your 12-foot cabinet. But, if you're confident that the miniscule difference won't impact your design, using the nominal thickness of 0.750" could be a simpler approach.

[Michael Burnside]

Interesting, I'm certain I'm the minority given the responses, but I would design using nominal dimensions most of the time, at least for hardwood/furniture. If I have a particular width I'm targeting, then that dimension is made critical, irrespective of material's nominal thickness and irrelevant if it's mm or inches/feet. I will plow the dado depth or cut the cross members to achieve that dimension. On a CNC I definitely don't think about it. I z to the spoilboard and target a depth z-<adjustment> and it wouldn't matter if the material was 3/4", 13/16" or 18mm. All my cabinets will come out to precise widths regardless.

I try to be perfect, go with my drawings, but generally speaking once I start building, I work with relative dimensions. If something is 1/32-1/16" off, it doesn't matter as long as it is consistent. I generally don't even think mm or inches after a certain point. I certainly wouldn't use a system I don't build with in my shop, so I personally would never design in mm (and no, I'm not interested in that debate).

[John TenEyck]

I use SketchUp for all my project designs. I stick with nominal dimensions unless I already have the materials and have measured the thickness. But that's rare because I most often design first, the purchase the materials. I adjust things during the build so that critical dimensions are correct. That often is surprisingly easy to do. I like to use loose tenon joinery because parts are cut to final dimension before the joinery is done, with no need to account for the tongue in a traditional M&T joint. So if I'm building a cabinet and the sides are not 0.75", which they never are, and I need to cut the stretchers between those sides, I set the length stop on my RAS table to the final dimension of the cabinet, put two pieces of cabinet side scrap against the stop, and then set the piece to be cut against that. Perfect length every time with no need to measure or calculate.

I use the imperial system because I can't visualize stuff in metric, even though it's far easier to use. And since most things here are sold based on the imperial system, converting those products to metric is about as bad as subtracting, etc. in the imperial system. In the end, whatever works for you is the right one to use.

John

[Mark Wedel]

On a recent sketchup I did, I set it to do decimal imperial instead of fractions - eg, 1.625, 20.75, etc. In this particular case, I don't think I was planning on anything being cut to more precise than 1/8", and I can recognize those decimals/fractions easily enough. But like John, I can't really visualize in metric - 40.2 cm doesn't mean a lot to me, and all of my tools/tape measures are also imperial, rule on table saw, etc, so going to metric is hardly simple.

I was pleasantly surprised in that some 1/2" baltic birch I bought was really 1/2 (0.5") thick - made the measurements a bit easier since I was making drawers with it, and since I knew had big the drawers had to be, just knock 1" off that to determine the size of the bottom.