This is going to be one of those long boring (sorry about the pun) bits of text about how to put great big holes in a defenceless lump of metal. Putting it on a slimming regime from the inside out.
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First off, we've got to find where we are going to be putting the hole and mark it up so we can find it again when we come to put the holes in it.
This first pic shows some of the tools than can be and are used to get the centre position marked (this is where I want it, in the centre, if you want to put it in another position you can use the same sort of techniques).
First off, put something on the metal so you can see the markings as you put them on. I use marking out blue, but coloured broad tipped permanent markers do just as good a job. I use a blue marker for covering up mistakes on my marking out, rather than redoing the full face.
I tend to use either a height gauge or a centre square for this type of job, so I will concentrate on those two bits of kit.
A height gauge is one of the most important bits of kit IMHO that is needed in the shop, either digital for the lazy ones, or a manual one like this for people that won't spend the money for the other type. Both do an equally good job of putting lines on bits of metal in the right place. They can also be used as very good precision hammers (just joking of course, I use my digital verniers for that job). But if you can read a vernier, there is really no difference between the two. On this job, where all sides are perfectly equal, the side length was measured, divided by two, set the vernier to that figure and scribe a line across the blued face, turn the job thru 90 deg and repeat the marking, to double check I went round all four sides, and as you can see, they all line up perfectly.
They really need to be used on a totally flat surface. My marking out table is a genuine cast iron one that I also turned into a tapping stand, isn't it great, dual purpose machinery. But a sheet of plate glass does just as well. For many years I used the platen glass out of an old photocopier, until I dropped a lump of the heavy stuff onto it. Steel and glass don't mix.
The other bit of kit is called a centre square, another piece of essential equipment.
These are really designed for finding the centre of round bar, but if you are using perfectly square bar, it will do the job admirably. They are usually fairly cheap to buy and are a worthwhile investment if you are building up a stock of tooling.
I have shown how they are used, just scribe a line along the blade, and you can see that the centre matches up perfectly with the one done with the height gauge. So either method could be used in this case.
So we have the centre. Now that centre mark has to be made into something that can be used to locate things into it.
I will just bring up a point here. Basically there are two types of point on a centre punch. One at 60 degrees and the other is 90 degrees. We need to use the 90 deg type, no arguments on this one, the 60 deg is for marking out edges of things, like edges to be filed to, the 90 deg one is made that way so that it accepts the 90 deg points of other tooling.
That out of the way, how do we get that point exactly on the centre as marked. I will explain the method I use. Your centre punch should be sharp pointed, blunt and rounded over is no use at all. Put the tip of the punch into one of the first lines that were scribed on the face, and drag it down the line (you will find, if sharp enough, that it will follow the line with ease) towards the centre marking, as it reaches the centre you will feel it hit the later scribed lines. Lift it upright without moving the tip and give it a light tap with a hammer. Get a loupe or magnifier and look at the dimple you have just made. You can almost guarantee that it is near the crossed centre, but not exactly there. So what you do now is gently move the mark until it is spot on where it is needed. This is done by using the hammer and punch and drive the spot towards where you want it to go. Do not be heavy handed, just a gentle tap. Eventually by looking at the magnified image you will see when the dimple is exactly in the right place. Put the punch into this dimple, totally upright, and give it a good wack with a hammer. You should end up with what is in the pic.
It took all that effort to get just one little spot marked on the end of a bit of metal. If it was done wrong, then everything that follows will be out. As experience is gained that spot can be within 0.001" of where it should be, and tolerances that tight are acceptable.
I will just tell you now, a few days ago I would not have been showing you how to do this using a four jaw on the lathe, it would be being bored using the miller and a boring head. It is now I can use both hands that this is possible.
So, lets introduce the metal to the four jaw. Mount it up as shown, a thin parallel at the rear of the block (more on this later), a piece of soft shim between the jaws and the job (no drinks can again, but this is a bit of soft ali litho plate). Tweak up the jaws but not too tightly.
Unless your are very lucky and the sun shines out of you know where, the centre punch mark will be way off centre when the chuck is turned.
There are many varied ways of doing the next stage, with all sorts of weird and wonderful gizmos and dingdongs, but the one I will show is the generally accepted method of doing it here in the UK (we've been at it a bit longer than the colonials, and we don't accept new methods lightly).
The reason for this method is that there is usually one of these centres knocking about the place, so why not use it, save your money, this is just as accurate. I have tried all sorts, wigglers with points, pointed edge finders, play dough with pins in, spring loaded doodahs with bells etc. I always come back to this.
Set up as shown, and by tweaking the jaws (slackening on one, tightening the opposite) on the chuck get the runout to as low as possible. I class 1/2 thou as acceptable.
Take all the setting up gear away, remove the parallel from the rear of the job (more on this later). You are now nearly ready to start cutting metal.
Put a big centre in the end of the metal, and wack thru with the largest drill you have got. I must invest in a set of blacksmiths drills, I love putting big holes in things.
The next two pictures go hand in hand. You always use the largest boring bar you have that will do the job. So you may not have set it to correct height yet. If you use a QT toolpost it is dead easy. Get your favourite facing tool that is already set to height, lock your spindle if you can, if not, do this very gently so the spindle doesn't move. Using the facing tool, scribe a small line on the metal as shown in the first of this pair of pics.
Mount your boring bar into its tool holder and bring the height up to the line marked on the job. Perfect tool height achieved in seconds. I always have trouble with my biggie bars if doing it by normal methods, because the tips are angled downwards. doing it this way solves the problem.
Again the next pair of pics are linked.
I have a home made saddle stop fitted to my machine, the square lump with bolts sticking out, to the left of the saddle. If you have one of these, use it to stop the boring tool going too far thru the job and hitting the chuck. If you haven't got one of these fitted, be careful, and expect some time in the future to cut some nice circular bits out of your chuck. Go to the next pic.
This is a close up shot of what I was on about, with the stop set, the tooling cannot hit the chuck.
I will now mention 'more on this later'. The boring bar has to penetrate all the way thru the job, if there wasn't a space caused by the thin parallel, the boring bar would hit the chuck as soon as it got to the end of the hole. This is a safety area for the tool to run into. It is needed if the hole being bored is larger than the hole thru the chuck.
Now the best bit is here, metal to be cut.
All safety spaces have been created, everthing is set to length. Now start boring thru. As much as the machine will take at this stage, finer cuts later on. As you start to get to finished size, take finer and finer cuts. On this job it doesn't matter about a good finish until the rotor fits into the hole. On my very very expensive set of boring bars (kindly donated by a rep, for recommending them to the boss), they have holes going right thru them, so cutting fluid is fed into the back and emerges just behind the replaceable tip. A real neat idea, but in this case not used, after each boring run, I just stopped the machine, cleaned out the curly swarf and painted in some of my 'ali mix', then carried on until I was at the right size for the rotor to fit snugly into the hole.
The rotor, nice and snug in its future casing. But this is no use to me, I need about 5 or 6 thou running clearance to allow for expansion of different materials while being powered with hot steam.
The final cut to give me this clearance was duly keyed in and run back and forward thru the hole a few times at the same setting. This not only gives a nice finish, but also takes out the remaining metal left behind by 'tool springing'. So you end up with a nice shiny parallel bore.
JOB DONE.
Next will be putting the rest of the holes in the casing, and maybe making the converging nozzles.
John
PHEW!!
