Firefly :: Alignment Table Fixtures

So we made the alignment table… but that was just the beginning.

Next we had to make fixtures to securely hold the frame while checking it’s alignment.

We started with the tower that holds the bottom bracket. We chose to hold the frame 6 inches on center off of the surface of the table. That made for a nice round number to calculate the proper height of all of the various diameters of tubing that we use.

The tower was made from a solid piece of steel that we drilled and tapped 3 holes in to secure it to the table.

Bottom brackets start at 68.5mm in width, therefore we made the tower slightly shorter than we needed it and have 2 shims that we use to measure the frame with. One shim for the 68.5mm width and one for after we face it down to 68mm (post welding). The shims sit on top of the tower and underneath the bottom bracket putting the centerline of the frame at 6 inches exactly.

We then had to locate where we wanted the tower to go on the table.

We started by drawing out the extremes of the sizes of the frames that we plan on building. We placed the drawing on the table and moved it around to find the perfect placement for the tower. We only had one chance to get the hole placement right, so we had to be sure.

To drill the holes we used our ridiculously huge Swedish rotary drill press. There were many benefits to using this piece of equipment for this task. First of all it has a really large reach that is adjustable via a large swinging arm. This made it so we could move the table right next to the drill, swing the arm over and line up the drill bit precisely where we needed to drill the holes.

Secondly, it has precision downfeed, making it a hell of a lot easier to bore through the 3/4 inch plate.

Thirdly, it is so bad-ass, so we just had to use it.

The huge Jacobs chuck is capable of accepting up to a 1 inch drill shaft, much larger than the holes we needed to make, but good to know none-the-less. We started our holes with a short countersink drill bit (above). This bit is short and stiff and makes precise pilot starter holes. After we started the hole with the countersink bit we stepped it up and made a small pilot hole that went all of the way through and then we finished off the hole with the final size drill bit.

There is always a bit of apprehension when drilling holes in a beautiful piece of metal like this. The first one was the scariest, the next 7 holes weren’t as much so. We measured way more than twice before each hole was drilled. Above you can see the first hole, next to it are two little indents. This is where the other two holes will go to mount the bottom bracket tower.

We then drilled the holes for the second fixture: the seat tube center locator. This fixture was bolted to the table in line with the bottom bracket. The tower portion of it is mounted to a slide that travels back and forth, aligned with the seat tube. Mounted to the tower is an interchangeable plunger, made to fit precisely into the seat tube, perfectly on center at 6 inches above the surface of the table.

The plunger unscrews from the tower and can be replaced with different plungers, also made by us, specifically for the different diameter seat tubes that we use.

We then bolted everything to the table and proceeded to check the alignment of the tower. We did this by making a dummy seat tube/bottom bracket and securing it to the tower.

We checked the height of the tube at various points on the table using our vernier height gauge.

At each location on the table the reading should exactly the same.

Luckily everything worked out perfectly and the height readings were the same. We had a feeling that this would be the case, but we had to make sure.

We will check the table once a month to make sure it is perfect.

Next up: How we check your frame to assure it is straight.


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