Today I'd like to share some pictures and insights about a technique called line boring. The part we're going to make along this post looks simple, and in fact, it is! But some care is made necessary in a few aspects. To cite the two I find to be the most important: Bore accuracy and bore misalignment. The first is important because if end up with a hole too small, the shaft will be locked, and that's if you're able to put it in place. If the hole is too big, the shaft will be lose around the hole, and you probably can't call anymore your bored surface as a journal bearing. The second important factor, the bore alignment is the topic we're going to talk about here. As you can imagine, if you have a casing, and the holes in walls which the shaft is going to seat are not aligned, ....problems again.
The part I need to get done, 40x40x40mm U profile with 4mm wall thickness. The holes should be in the center of the part and are specified to 12mm diameter h7 tolerance. The holes will act as a journal bearing to a shaft passing trough.
By the way, this part is good example of a designer's dilemma. You can choose between a part simple to manufacture but using off the shelf products, or go with simple design but high precision demanding. On the First case you need to look for the products in catalogs, make the order, pay, wait for the post to deliver the parts. The second you need the craftsmanship needed for precision work, patience to set up the parts. Overall: both are time consuming, but in different ways. (If you read until the end you'll know which path I took.)
Back to the topic: The easiest way to achieve a good precision in this case is to ream the hole, with a reamer long enough to go through one side and reach the other side in the same setup. This is the spirit of the Line bore. In addition to that, if all goes well, the reamer should leave a h7 hole in place, solving at the same time the hole accuracy problem.
Luckily, by the geometry of the part, and knowing that we have at hand the necessary tools, we can proceed with the line bore idea. Enough talk, let's do some work.
Started cutting the aluminum U profile with a hacksaw. Most surfaces will stay untouched as they are already to spec. So the only care here is about the length.
Wow, when you are just rolling the mouse wheel to down the web page it seems so easy. Here due to the magic of photo time shift, we already have the part cutted, perfectly squared and deburred. Effortless .. uhhm ehrr not really.
Marking the part. Not really needed, but that helps in keeping Mr. Bozo away.
Using my favorite Dial Indicator to make the surface perpendicular and parallel to the ways of the lathe. Surprisingly this is an imperial one. The angularity error here was one thou (of an inch) over 40mm. I need to write more about this indicators someday.
The tools separated for the job. Is always a good idea to open the hole progressively, so here you see the drill bits (2x6 spot, 6 , 8 ,10, 11.8 and 12h7 reamer) and their respective collects. In the end I think I could used 8 and then 11.8 direcly, but I didn't want to take the risk as my setup was not so rigid.
Spot drilling. Here you can see my setup as well. The workholding is not ideal, but did the job. Looking at the picture now, maybe I'd try different things, but in the workshop the clock is ticking, and you or you spend your time improving things, or you make your move and get the job done.
Bigger drill bit
The 12h7 reamer in action
The reamer trough both bores.
Top view
Here is the final part
Ok, not super precision stuff, yet there's some precision involved. Definitely was much much faster than buying something on-line and wait for the arrival. Took me less than 1 hour, and it was fun to make.
Edi
