Next, it's time to go ahead and tap these holes. After countersinking them slightly, I clamp them up in a machinist vise. If you don't have a stationary vise you can simply clamp the bar to the side of your work bench or some heavy table. You just want the piece immobile. You need both hands to tap, so you cant just lay this on a table or try and hold it in one hand.
As I mentioned previously, I'm going to be using 1/4"-20 taper, and bottom taps. There are generally three types of taps; taper, plug, and bottoming. The taper tap, as the name implies, has the most taper, which makes starting the tap easiest, but the length of full threads is much shorter relative to the tap length. For through holes, this may not be an issue, as in many cases you can simply run the tap all the way through, however, with a hole that terminates, you'll reach a point where the end of the tap hits the bottom of the hole, leaving a section at the end that's not threaded, and too narrow to accept the bolt (since we drill undersized, to leave material to form the threads) a plug tap, has less taper, and subsequently less area of marginal or non threading, and a bottom tap has basically no taper.
In hard metals, or certain circumstances, it may be necessary to use all three taps on a non-through hole, to get full length of threading. However, in this case, we're working with aluminum, and we've got our holes bored 1" long or more, and we only need ~3/4" of full thread. A plug tap, would likely be sufficiently tapered to start, and even finish this tapping job, however, a taper tap alone, would bottom out before giving us enough length of full tap to work. So I'll be using a taper tap to start the tap, run it till it bottoms out, and then finish up with a bottoming tap.
Here is a photo of the taps I'll be using, with the bottoming tap on the left, and the taper on the right:
And here is a pic of the tap wrench, which is a very nice large sized wrench I got from Travers Tool:
At this point, with these premium quality taps, and the heavy angle of the taper tap, I'm able to easily do the first part of the tapping op without any cutting fluid. The trick is not to use heavy pressure, let the wrench, the taper of the tap, and the angle of the countersunk hole find level, and then use steady twisting. With hardware store taps, I recommend using plenty of fluid regardless of the tap.
If you run into excessive tension, don't force it. This aluminum should be very easy to tap, and excessive or jerky force is what will break a tap, and ruin your afternoon. You should be able to easily turn this with both hands and light force, smoothly. Once you encounter a sudden need for more force, you've probably hit the bottom of your hole with the point of your taper tap, and pushing further will only compound the pressures on the tap, encouraging a break, or atleast damaging your threads. At this point, gently reverse your motion to a counter-clockwise turn. Be careful with the first turn, it will likely need more force than it did going in, as you're breaking through chips you've created on the way down. Just go easy and steady, and it will ease up significantly the rest of the way out.
After the first heavy reverse turn, it should practically float out. Resist the urge to spin the wrench out like a merry-go-round, and once it's super easy to turn, take the tap wrench off the tap, and twist it out the rest of the way by hand, to avoid damaging any of the top threads by your wrench falling over with the tap in it still.
Blow the chips off your tap, and go to the next hole. Rinse, repeat, until you've got them all with the first tap.
Now, we've got nice threads at the top of each hole, so all we have to do is thread in the bottom tap by hand, to get it started, but first, for this tap, we'll apply tapping fluid. For most normal tapping, we won't need a ton, I apply it directly to the tap before starting it:
You'll start this tap by threading in gently by hand to the previously established threads, as mentioned, and you'll continue tapping with the wrench just as with the last operation, however, you'll likely encounter more tension once you get to the area where the tapering tap stopped cutting. Here you can see on the tap where the cutting action was taking place:
As with the previous op, if you feel a sudden change in force needed to turn the tap, stop, you've likely reached the bottom of your hole, this is of course, after the initial pressure change where you transition into tapping further down. Do all the holes, this way, then do your best to clean out the chips.
Slap the bar holes down gently on a flat even surface, etc. You may even want to lightly run the bottoming tap in and out a few times to help clear chips and refine the threads of the holes. At this point, get the bolts you're using to secure the bottom plate, and make sure you can run them all the way in:
The steel bolts will help to refine the holes, so don't be afraid if you have to apply *light* pressure to get them to screw in. However, make sure you're threading correctly, if the pressure gets heavy, you could be stripping or cutting new threads with the bolt as opposed to refining the existing ones. It should be pretty obvious, but if you're having to really torque it, there's something wrong.
Now at this point, I departed from the instructions a bit. Before marking the through-holes in the aluminum bar for the side plates, I wanted to make sure I got the bottom bar set up correctly to mount to the bottom plate. So I went ahead and countersunk the corresponding holes for the bar, on the bottom plate.
With these bigger plates, you're not going to be able to use a drill press vise unfortunately. Those of you with smaller drills, you can use a C-clamp, or a quick clamp to clamp these down to your drill table, after you've indexed the quill to center-hole of your drill table, lining up the hole you're going to be countersinking with that same hole. With my drill however, I've got a t-slot table, so I decided to use a riser, to elevate the plate. In this case I just used 4 same thickness ceramic magnets, which worked ok, but not great, but it's hard to clamp things down to my table quickly:
This is one of the few occasions where I'll just hold the work piece. Since this is a big plate, and countersinking isn't as likely to grab a chip, I just eyeballed it quickly, and let the countersink find center. This doesn't have to be super precise, since I'm using an over-sized countersink, I'm just eyeballing the depth. A little more is better than less in this case, but I'll stop and check it against one of the bolts. You want them all to sit at or below the face of the plate, since this is the side of the bottom plate that will be against your bench or stand:
I countersunk them all, then bolted down the plate.
Once I did that, instead of putting the side plate against the bar and transfer-punching the holes, I decided instead of measure the height to the center of the holes (3/4"), and scribed on the aluminum bar, the same as we did to lay out the bottom holes. Then I lined up the side plate, and used the line to mark each of the hole locations. Since these are drilled through, I only needed to mark one side:
Then I repeated the same process of marking these holes with my #5 combined drill and countersink:
And then drilled these out with a 5/16th bit, according to the instructions. This is over-size of the holes and bolts we'll be using, but this gives you some room to shift the plates around a bit if needed. The anodized spacers that will go between the two side plates higher up, have smaller holes, but are drilled very precisely, so that will help to tighten up the whole assembly. This is another example of a situation where Chris and crew thought about how to mitigate the potential pit-falls associated with putting these together with home shop equipment, and limited experience.
Here's the bar drilled out, and countersunk to deburr:
That's it for today.. It takes 20x as long to write the explanations and upload photos than it does to do the actual work guys, so I apologize for the slow progress. I'm also having some problems with google not organizing my photos properly, so sorry if I got some out of order. =\
I'll probably work on this some more tonight but, I doubt I'll have time to post again until tomorrow or so. Thanks for being patient, and if anybody has any questions, don't hesitate to ask.