Intro: Ridiculously Heavy Table
My computer desk was a little flimsy, so I wanted to make a sort-of-industrial table to replace it. A friend of mine, Roland Lapp sketched out an industrial-looking table that I modified and modeled in Autodesk Inventor, then built out of steel and oak.
I made this at Techshop San Francisco, using a manual mill, their large Shopbot, and their powder coating setup.
Step 1: Design
I based my sketches off of Roland's sketches, but I can't sketch at all, so I threw away my sketches and made the design in Autodesk Inventor. This is way harder to change than a sketch, but I had a decent idea of what I wanted to make, which was a way too heavy table with legs made out of steel angle bar and large screws going through the top of the table.
I've included the 2d drawings (generated mostly automatically from the Autodesk designs) , just as an example of how awesome Inventor is. I used the drawings in the metal shop to actually make the pieces. The slots are actually off by 1/4 of an inch in the drawing, because I mis-calculated the width of the legs, so I'll be modeling a little harder in the future to make sure I get everything aligned.
Step 2: Parts
The top of the table is a Numerär countertop from IKEA, which I actually used the dimensions of to design the rest of the table, since it's cheap, sort of finished, and easily available.
The screws (large ones and small ones) are all black oxide coated from McMaster-Carr. That place is expensive, but often the only real source I can find for odd parts like these. The small screws are 1/2"-13 thread, 1" length, while the large ones are 1"-8 thread, 2-1/2" length.
The steel is all hot roll mild steel from OnlineMetals, which may be more expensive than your local metal shops, but it's also about a million times more convenient. The 16 leg bars are 2x2x0.125, while the 4 brackets that hold the legs on are 4x4x0.250. This isn't that much steel, but it turns out that steel is really dense. I don't live very far from Techshop, so I tried to carry some of the pieces and it was quite a workout.
In summary, if you pick sufficiently heavy projects you can save money on a gym membership and your Techshop membership will practically pay for itself.
Also I didn't take a picture of the huge cardboard tubes full of metal, so I have instead attached a picture of how totally ripped I am from carrying all this steel around.
Step 3: Machining
The steel parts were all cut on the manual mill using a 1/2" carbide end mill for everything except the 4 big holes in the tops of the brackets. The carbide is pretty tough, which is great because steel is really hard. I got a short bit (5/8") to minimize deflection and chatter while cutting. I am told that chatter will dull the end mill pretty fast.
All cuts were done with the 1/2" end mill, and even the big holes were first cut with the end mill, then drilled out with larger drill bits (Techshop SF has jumbo drill bits going up to about 1-1/16", which is the size of those holes). The work holding tended to use lots of 2-4-6 and 1-2-3 blocks (super useful for this stuff).
You can see a 2-4-6 block in the first picture holding the bracket at the right angle for the cut on the edge of the bracket. There is another one underneath the bracket, used to hold it against the table. Note: I am told that you should not extend the quill all the way when milling hard materials like this.
Each of the 16 legs had 8 holes, so I needed to cut 128 holes. For quickly cutting the holes I had this super sweet dual-vise setup that I didn't get any pictures of. But I assure you, it was super sweet.
I poured coolant all over the steel when cutting it, and still got super hot chips burning my arm. I'd recommend going really really slow or using a CNC machine. I am told you can also set up shields on the sides of the table to stop chips and coolant from going every which way.
Step 4: Grinding
I wanted to powder coat the steel black, so I had to remove all the mill scale that comes on hot-rolled steel and prevents the powder coat from adhering. My brute-force solution to this was to grind the entire surface of each metal piece using an angle grinder and some flap discs. I then sand blasted the metal and cleaned it with mineral spirits to prepare it for powder coating. The powder I used was from Harbor Freight, but I've heard good things about Prismatic Powders.
It's really nice if you can dry the steel off quickly after cleaning, because it will rust so amazingly fast when it has water on it.
Powder coating is basically applying crushed plastic to metal and then melting the plastic so it sticks to the metal and acts like a tough paint, only less toxic and messy. Techshop has a little area where you can borrow their powder coat gun and spray some metal, and an oven where you can bake the finish if you wait long enough for it to heat up (a long time).
I always underestimate the amount of time required to powder coat anything. If I had the metal all ready to go, I could probably get a single coat done in 2 hours, maybe 2 coats in 3 hours. The oven I used was really slow to heat though, which was the primary limiting factor.
Step 5: Shopbot
I cut the countersunk holes for the table top using a Shopbot (CNC wood milling machine, basically). There is probably some super easier way, but I didn't have time to learn Inventor CAM, so I exported one of the countersunk screw holes from Inventor and created the shopbot files using Cut 3D. I then did a 3D raster cut at each screw hole location (which I wrote out in a text file) by going to the x-y coordinates of each one, zeroing x and y on the shopbot, running the 3D raster cut, and then setting x and y back to their previous values. I also zeroed the z axis on each cluster of 4 screw holes, as the shopbot table I was using was not particularly flat, and the screws were countersunk pretty precisely.
This method was incredibly slow and error prone, and I would not recommend it to anyone. The correct way to do this might involve an 82-degree V bit or countersink bit, or at least making the shopbot files work without having to rezero all the time.
For the bottom of the countersunk holes that go all the way through the table, I did in a similar program called VCarve Pro. I did 8 holes at a time with that, and it was way faster. I didn't do all 16 holes at once because it can be hard to get the table top aligned precisely with the shopbot table. By doing one half of the table top at a time, the chance of the holes being far off relative to the edge of the table top was much lower.
When I finished cutting the holes, I did a test fit with the steel brackets and 1"-8 countersunk screws.
Step 6: Assembly
Assembling was pretty straightforward. I didn't leave much clearance for the screw holes (they were almost exactly the diameter of the screws), but fortunately the holes were pretty accurate and I didn't have much trouble assembling the legs. The slots on the brackets were actually cut a bit off, but I noticed this somewhere around the end of Step 3 and re-cut the slots.
I hand tightened everything together then got a socket wrench to get everything nice and tight before getting someone else to help me flip it over. I put some dinky felt pads on the bottom of the legs and I'm fairly certain the floor was unharmed except for a few gouges.