lathe;mounting a squarish sheet of metal to be cut into a disk?

[modmechengine1333Participant @modmechengine1333]

Working on creating a disk with a lip on its border, this will be my first real lathe project. Starting stock is flat Galvanised sheet, roughly 100x100mm, 2mm thick. I’ll be making these in small batches, always the same size

I have chosen to go with the Colchester Master lathe at my hackerspace, as it seems ideal, and I want to scale up my competence with Lathes.

The hackerspace offers training, so I got that covered, I have also watched a ton of YouTube, trying to understand Lathes, and their processes

Where am stuck on; my workpiece is just a sheet of metal and I need to find a way of securing it not just for safety but also to prevent bending, shearing, etc.

All the videos and texts I have found for lathes out there deal with workpieces that have significant depth to them, allowing them to be clamped easily.

Questions I’m chewing on:

– Are sheets of metal outside the scope of Lathes?
– Best way to mount a square flat blank for a first operation, faceplate?
– At 2mm thick, is this thin enough to need a proper backing/support setup, or is it fine as is?
– Any tips for making sure the pocket cut never breaks through into whatever’s behind it?

plan and 3d model view of the project;

https://i.ibb.co/F4mRQMvC/plan.png

https://i.ibb.co/XrbbTjzk/f3d-kllvr-D5s-L7.gif

[John HaineParticipant @johnhaine32865]

You may get away with supergluing each blank to a flat sacrificial piece of wood which will be discarded. With luck it would hold enough to withstand cutting forces if you take very light cuts very slowly with a sharp tool. Hold the backing in a 4 jaw. Heat to separate (boil) and expect to need a new backing for each one.

Why are you using 2mm sheet when the total depth is 2.5mm?

Cut the outside to a rough circle by sawing, an interrupted cut on the corners would break the glue bond.

I suspect galvanised sheet steel will be horrid to machine.

Frankly, I think turning this object from sheet steel is going to be hard. If the application permitted I would use engraving brass and mill it, profiling the outside with holding tabs. Expensive material though.

[JasonBModerator @jasonb]

There are several ways to hold thin sheet but looking at your design I think that will be the least of your problems. If I understand the drawing you want to recess the face of the disk to leave a 0.5mm thickness to the base in gummy galvanized steel sheet.

One method is to bond the sheet to a backing such as a piece of MDF which in turn can be fixed to a faceplate. However the heat and your thin final thickness is likely to see the bond break or at the very least you will get an uneven 0.5mm bottom. That can be improved uppon by drilling the corners of your square plate for some small screws which will take the drive loads but I still worry about the thin base lifting.

If you do get the base machined then to cut the OD You can trepan it from the sheet while still screwed and bonded to the backing. Or remove from backing, saw off the corners to reduce the amount of turning. Then use a pressure pad to push the work against the backing and turn the OD to size.

Spinning or pressing 0.5mm sheet would be a better option to turn up the sides if 0.5mm thickness is OK for them.

Also why use galv steel as you will only be left with thebottom galvanized once the rest has been machined?

Couple of shots of a pressure pad in use

Although this shows acrylic sheet the method works with metal. Tape is applied to the backing board and also the work. You then superglue the two taped surface stogether and when dry can turn the item. Backing board held in a chuck in this case. You don’t need to heat/boil the superglue to break the bond the tape will peel off so the same backing can be used multiple times.

Trepanning is using a thin tool to cut in from the face.

[John Haine]

On John Haine Said:

 

Why are you using 2mm sheet when the total depth is 2.5mm?

John, overall is 2mm thick

[Andrew CrowParticipant @andrewcrow91475]

I think Jason has covered most options for machining your required item, the only thing I would add is about the material you are using.

Any thin sheet steel contains many stresses in it’s surfaces caused by the manufacturing process, by cutting on only one side those stresses will be changed and the part will almost certainly not be flat when removed from whatever holding fixture you use. You may be able to alleviate this problem by prior heat treatment to stress relieve the blanks, but I’m not sure how this will effect the galvanised steel you are proposing to use.

Andy

[noel shelleyParticipant @noelshelley55608]

If you are going to use galvanised material then you need to understand the galvanising process and machining 3/4 of thickness away will leave you with the base being 3 layers, a zinc layer, zinc iron alloy and the parent metal iron all in .5mm – not ideal. The zinc/iron layer will be brittle. I would reconsider the material for this job.  Noel.

[Frances IoMParticipant @francesiom58905]

seems ideal task for printed metal – stainless steel – you haven’t stated its intended function – eg if to hold liquid then an annular ring soldered to thin sheet might work

[Nigel Graham 2Participant @nigelgraham2]

Andy –

Heat-treatment will destroy the galvanising. Though I don’t understand why you are using galvanised steel anyway unless this project is using existing “stock”, because only one end-product surface will still be galvanised anyway!

If do try it – risking considerable distortion – be aware that the zinc vapour is poisonous.

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I have successfully machine thinner steel discs than that but was able to use holes both intended in the finished itemand drilled in the off-cut areas, to screw the material to a shuttering-ply disc bolted to the faceplate. Put big washers under all the screw heads to spread the loads hence improve the grip.

 

 

 

[egaParticipant @ega]

The blank could be produced with a suitable holesaw.

[Howard LewisParticipant @howardlewis46836]

“Squarish” and suggests using shears to bring the blank closer to a circle, before tackling the problem of how to hold it and machine away a tiny bit of metal.

Being thin blank, it is not going to be easy to machine.

As said, a sacrifical wooden backing, sharp tool on centre height, VERY light cuts and the power cross feed set to bring the Boring? tool outwards at the finest possible feed per rev, (0.002″ if you can manage it)

Since you are thinkiing in terms of “batches”, you are going need a lot of backing pieces

Since the material is thin, maybe metal spinning, or pressing, might be an easier way, once the blank  has been turned to round.

Howard

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