Posted by andrew lyner on 13/05/2019 19:44:52:
I keep finding the need to cut discs out of 5mm steel (I bought a lot of 100mm square offcuts for pence and I am using them where I can).
I tried the circular cutters that come in a set. One was fine and I managed a 65mm diameter hole in some 6mm sheet (also from the offcuts place). I used plenty of cutting fluid and put my toy pillar drill on the minimum speed. It worked fine because I took ages and ages over the job. The 85mm version just wouldn't touch the steel, despite the oil and slow speed; annoying.
So I tried a 2mm HSS parting tool and it actually worked but it was at the limit of the mini lathe capabilities. The belt kept slipping and graunching and I imagine the spare will soon be needed. Not the best technique although it gave a result eventually.
I was wondering about trying one of those Adjustable Circle Cutters. Some have on blade and some seem to have two blades. But they are sooo cheap that I cannot believe they would do anything substantial.
Apart from the "I wouldn't;t start from here" answer, is there a way forward?
A number of good answers already, especially from Jason, but I'd comment this is a variant of the classic and often very difficult hobby problem 'asking too much of the machine available'. In this example, although the job can be fitted physically, both pillar drill and mini-lathe lack the power and/or rigidity needed to hack through that much metal. The larger circular cutters are a particularly big ask.
The way to approach it is to find ways of reducing the amount of metal removed. Chain drilling works by making lots of small holes over a longer time, trepanning by allowing a single point tool to attack a small area of metal. Or you can use a brutal tool like an angle grinder to chop off as much as possible before tidying up on the lathe.
Faced with the problem of making discs from plate, I'd chop the corners of the plate to an octagon with the angle grinder, then super glue the octagon to a mandrel in the mini-lathe, and turn what's left to a disk. This reduces the work done by the lathe into its comfort zone.
But! I think using scrap to make discs is more trouble than it's worth. As you've spotted, it really isn't the best place to start. A better solution is to buy rod that will fit in the lathe without needing loads of tedious pre-processing. And buying new metal avoids another potential booby trap; a surprising number of alloys do not machine well. After several painful experiences I now avoid random scrap, instead buying alloys intended to be machined. It saves a lot of time and trouble.
Last comment, my single most useful workshop purchase was a cheap bandsaw. Hand sawing is good for small items, but otherwise nothing but boring time wasting hard work. Metalwork is much more fun when you can get on with it. Don't despair. I started small and gradually built up the machines and metal over several years. Not flinging money at my workshop was interesting in its own way and taught me many tricks. But I made a lot of mistakes, got frustrated, and the floor is still bloodstained…
Dave