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Morris Chair build ala. Wood Mag. plans #2: bending jig my way

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Blog entry by Mainiac Matt posted 02-08-2019 09:20 PM 578 reads 0 times favorited 3 comments Add to Favorites Watch
« Part 1: And so it begins Part 2 of Morris Chair build ala. Wood Mag. plans series Part 3: Mortises cut in legs »

The Wood Mag. plan makes the bent arms by laminating thin strips together while bending them over a form.

If you buy the paper copy of the plans, you get a full size template, but if you download them, you get a sheet that details the geometry.

The dimensioining is pretty good, but this type of faired curve is very difficult to layout and match exactly. (O.K., if your not anal retentive like me, you can just swag a curve, cut it on the band saw and call it good enough). For others with the same desease as me, I’ll outline my method.

.pdf files are actually quite complex and can have other file types embedded in them. That is why you can look at a .pdf copy of a print, and if it was prepared correctly, you can zoom in A LOT and the graphic will not pixulate. If your file passes this test, the .pdf file was generated in the CAD software and has vector geometry (mathamatical formulas for every line, arc, etc…) embedded into the file. You can “lock” your .pdf to prevent others from extracting this, but not many people care, or know how to do this. So often you can “crack” the .pdf file and pull the vector geometry back out of it. This enables you to import and edit the geometry in your CAD software and use it to plot to scale or to crunch cutting lines in your CAM software and program your CNC with it. Some of the more “artsy” CAM packages will do this automatically. But if you don’t have access to those higher end packages, here’s my trick.

I open the .pdf in a shareware package called Inkscape and then I save it as a .dxf file (the unniversal 2D CAD format). If you can open that .dxf file in your CAD suite, then you have sucessfully cracked the .pdf.

Note that not all .pdf prints can be cracked. .pdf files created from scanned images or photos do not contain the vector geometry (and will usually pixulate when you zoom in). As mentioned, .pdf files can be locked. Or you may just have an odd duck that won’t cooperate. My success rate doing this (on files I believe to be generated in CAD) is about 75%.

BUT… there’s one little detail you MUST account for. The vector geometry in the .pdf file has been scaled (usually down) to fit the sheet size. So you must have at least one accurate reference dimension listed in the dim text, and then you must scale your cracked geometry back up to full size (1:1).

I’m going to cut the form out of some scrap 3/4” MDF, thus I need to cut 8 pieces to make up my form. After scaling my geometry, I added some rectangular cutouts to facilitate clamping and added to 3/4” circles so I could register and assemble the layers with some 3/4” threaded rod cutoffs. Then I array the geometry 8 times and I’me ready to import it into the CAM software.

We like to use 3/8” solid carbide spiral down bits as they put downward pressure on the stock and assist the vaccum table in making the sheet stay put. Once I pick the desired tool from the tool library and select the appropriate feed speed, plunge speed, plunge pattern and spindle rpm, I make the decision whether to climb-cut or normal-cut. After this, I decide how many passes are required (I chose two) and I pick the sequence of operations (usually it’s best to make all of the interior cuts and then do the outer perimeter last. Here’s a close up of the CAM file, which now shows the cutting lines, offset from the template geometry lines by the radius of the router bit (called the cutter comp).

The CAM software then generates the G-code, which are step by step instructions to move the machine. This particular program had just over 700 lines of code. Here’s sheet one of 14,

Then it’s off to the races and the machine does the rest of the work. As long as the parts don’t shift on the table or you brake a bit, the CNC will give you what you asked for…. which is purely a garbage-in, garbage-out dynamic.

I forgot my phone down on the shop floor, but here’s an old shot of the CNC I ran these on. It’s a CNT Motion Systems 900 sereis, which is a professional duty Aluminum framed machine and is perfect for this type of panel cutting.

And here’s the results bolted up. I’ll glue them up and touch up the surface with a sander, and then I’ll be ready to bend some OAK.

Next up is cutting the mortices in the 4 legs.

TGIF Baby!

-- Matt -- I yam what I yam and that's all what I yam



3 comments so far

View NormG's profile

NormG

6415 posts in 3274 days


#1 posted 02-08-2019 11:40 PM

very impressive jig, these are great chairs, on my bucket list

-- Norman - I never never make a mistake, I just change the design.

View stefang's profile

stefang

16391 posts in 3605 days


#2 posted 02-09-2019 12:17 AM

I salute your tenacity towards achieving perfection and at the same time I’m glad I am too far away to become infected by the same bug!

-- Mike, an American living in Norway.

View harum's profile

harum

330 posts in 1913 days


#3 posted 02-12-2019 02:37 PM

Great blog! Appreciate all the details. After having sat in a chair with a recliner mechanism built in, aways wanted to build one. IIRC, there is a manufacturer of these mechanisms somewhere on the East coast.

-- "If you're not counting the ripples when throwing pebbles in the water, you're wasting your time."

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