Hi, I am looking for advice from members with more experience of using a metal lathe. I am inducted on the lathe but have only used it once.
I need to make a couple of lens cells from aluminium for telescope lenses.
I have two 6 inch diameter aluminium discs, 3/4 of an inch thick.
The lenses are 4.25 inches in diameter but I will only be using 4 inches of the lens.
I need to cut a 4.25 diameter hole most of the way through the disc. Then stop and cut a 4 inch hole all the way through, leaving a 0.25 inch lip for the lens to rest on.
So basically I am turning the aluminium disc in to a ring of aluminium.
What is the best way to do this? Is it possible to use a 4.25 inch hole saw on the lathe?
The hole actually needs to be very slightly bigger than 4.25 inches, to allow for expansion and contraction of the glass lens and aluminium at different temperatures.
Would a 4.25 inch hole saw actually produce too big of a hole?
Drill a big hole, use a boring bar to widen the hole
I am afraid a boring bar would take a very long time, as the biggest drill bit I could use would leave a lot of metal to remove. If I could use a 4 inch hole saw and use a boring bar to remove the rest?
Well I trained on a lathe at school. Not sure of some of the language used in the other replies.
But I made a reversing ring for a camera lens, which is similar to what you describe.
Starting with a drill is ok. Then you just machine away the material as its aluminium it will not take long.
The problem I would expect to have is distortion because it is just a ring.
Hi David,
It was nice to meet you in the shop the other day.
As it is often the case, there are multiple ways to skin a cat!
The most conventional way is to cut the biggest hole you can with a drill in the tailstock, and then enlarge the hole with a boring bar. You are correct, this process is long, but contrary to what we all see on YouTube, machining is not a speedy process.
Your idea of a hole saw is perfectly viable, but they are very low-precision tools, so I would strongly recommend making a 3.75-inch hole or even possibly a 3.25-inch hole and then enlarging it to the size you need.
Also, DO NOT USE a whole saw on the lathe, do your hole on the pillar drill first, with adequate support/clamping.
Using a single-point cutter like a boring bar or a turning insert ensures you perfect concentricity of the cut. Any other tools like drills, hole saws and the sorts, tend to wander off or to cut holes that are not round.
Another thing to take into consideration is the thickness of the finished ring, if it’s too thin there is a risk of it deforming and coming off the chuck with potentially horrible consequences. It may be necessary to machine a mandrill or a support from a piece of scrap to use as a chucking aid.
To conclude, it is quite difficult to answer without seeing it, or at least having a look at some drawings.
But worry not, the project is well within the capability of the machine, with a bit of forward planning (and just a few scrapped parts 
) you will get there!
Lastly, do not take my words as gospel, everyone has his methods, @VintageSteel is very experienced too so it’s worth having a chat with him too!
Aluminium (and brass) can also be done on the CNC. Not sure if the geometry works for that, though 2 sided CNC is possible.
Trepanning might be the operation you are looking for? You’d need to make / buy a specific tool for it though:
Thanks for all the advice. Trepanning looks like a good idea. However, making the treppaning tool seems quite difficult. As for buying, they are expensive and there are a lot of different ones, so ti would be difficult to know which to buy.
I will probably use a hole saw on the drill press and then a boring bar on the lathe.
As I have two discs I could bolt them together to add rigidity on the lathe
Trapaning is the least wasteful in terms of stock, but it is also quite a tricky operation, usually not suggested for beginners.
You are welcome to try, but I would recommend extra caution, maybe do it when a tech is present to have another set of eyes on the workpiece.
Also, we don’t have a trapaning tool in our arsenal.
Re bolting the 2 pieces together could be an idea, but as for everything else, it carries risks, as the bolts add unbalanced loads, vibrations, and possible projectiles strapped to the jaw.
If done right is viable, if not it could be very dangerous.
I realise that I sound a bit fatalistic and that it looks like I am discouraging you from doing your project, but I can assure you that’s not the case, it is kind of my task to make sure you have all the info to stay safe.
Ultimately if this was my project, I would go for the boring Boring bar 
But of course it is your choice after all.
A 3/4 inch thick 1 inch ring of aluminium should not suffer distortion, unless it is chucked up stupidly tight. Chuck the blank disk adjusted so it runs true. Then drill and bore it. We have the necessary tools and there is no exotic workholding to go wrong…. IMHO