Best way to cut/turn a 75mm Disc from a piece of Aluminium Plate

[GreensandsParticipant @greensands]

Hi – Can anyone suggest what would be the best way to cut a 75mm diameter disc from a piece of 100x100x12mm thick alumimium plate. The object of the exercise is to produce an anular ring measuring approximately 100mm O/D by 75mm I/D and so the centre piece will effectively be an off-cut but being mean, I would rather like to retain as much of the work piece as possible rather than simply turn it all away in the process.

[GreensandsParticipant @greensands]

[DC31kParticipant @dc31k]

https://www.zoro.co.uk/shop/power-tools/holesaws/fch0234-70mm-2-3-4inch-hole-saw-bi-metal-fast-cut-constant-pitch/p/ZT1147698X

Next size up is 73mm so unlikely to leave enough for finishing.

I think the arbor requires a 5/8"-18 UNF thread but Google will confirm.

Superglue on a thick sacrificial disk and use a shortened pilot drill if you do not want a hole in the offcut.

[Anonymous]

Trepan 6mm deep on one side, turn over and repeat. Tool can simply be freehand ground from a piece of HSS.

Andrew

[JasonBModerator @jasonb]

Same as Andrew, I like to leave the second side couple of thou short of breaking through and then do that last bit off the lathe which saves the possibility of teh central disc catching the tool

Stitch drilling on the rotary table or using the DRO are other options

Edited By JasonB on 23/04/2021 15:48:52

[GreensandsParticipant @greensands]

If opting for the trepanning route, what would be the recommended spindle speed? Presumably a tool ground as for a parting off operation but perhaps provided with a bit more side clearance would be the tool of choice?

[egaParticipant @ega]

Posted by DC31k on 23/04/2021 15:31:25:…

Superglue on a thick sacrificial disk and use a shortened pilot drill if you do not want a hole in the offcut.

Good idea but, to my surprise, following a tip on the forum I find that hole saws work OK without the pilot.

[JourneymanParticipant @journeyman]

Posted by Greensands on 23/04/2021 16:31:55:

If opting for the trepanning route, what would be the recommended spindle speed? Presumably a tool ground as for a parting off operation but perhaps provided with a bit more side clearance would be the tool of choice?

Yes, something along the lines of:-

I have highlighted the edge in red. Speed as per parting. Make sure inners and outer bits are secured so that they don't come loose at breakthrough. Probably safer to stick to Jason's recommendation and do the last little bit off the lathe.

John

[William ChithamParticipant @williamchitham75949]

Plus one for trepanning. I have just done my first one in 6mm steel aout 3" diameter. I screwed a square piece of stock to a piece of mdf and turned it in a 4 jaw. Went right through from one side, centre and ring popped out without drama.

William.

[Nigel Graham 2Participant @nigelgraham2]

I have performed similar by using appropriate hole-saws readily available from the builders'-merchants, with their normal arbor held in the bench-drill, to cut discs and rings with sufficient if not sometimes rather generous turning allowances for finishing.

This does make a quarter-inch hole in the centre which may or may not matter.

However you don't need worry about the arbor thread, nor the pilot-drill, because those are supplied.

You do need to clear the chips frequently, and lubricate the tool well. (Use an old paint- or tooth- brush.)

'

For trepanning in the lathe I'd suggest a circular-section tool ground so the top of the cutting edge is on or slightly below the centre line, then a small clearance along each flank.

'

If the work-piece is fairly hefty, then if possible I bolt the plate to the faceplate both inside and outside the annulus – the "if possible" is if holes in the central disc are either needed or permissible. I try to use tapped holes and set-screws from behind the face-plate, to minimise projections.so allow minimum tool overhang.

This approach allowed me to machine a thin-walled annular channel about 8" and 7" dias, from half-inch thick hot-rolled steel plate. I'd backed the plate with coloured plastic shims, (more building-site parts) so pretty-coloured swarf indicated starting to break through. That rigid mounting method and very light cuts at the end, allowing the springing to work out, meant a gentle break-through with no fraught tool-snagging or great lumps of steel crashing about. (The work was rotating at roughly 70rpm.)

'

Beware – all trepanning / hole-drilling is liable to leave razor-sharp edges, even blades.

'

Chain-drilling is something I see as a last resort but have used it a few times. It's best to file or grind down the worst of the teeth this process leaves, to limit the horrible hammering action they give to the tool and lathe.

[Mick B1Participant @mickb1]

Another one for trepanning. Whilst using backtaper similar to a parting tool, and greater side clearance on the outer flank as above, it's also generally beneficial to grind a slight curve on the leading face, so as to direct the swarf inwards towards the middle of the cut and minimise risk of jamming. This also promoted rippling through rather than sudden and complete breakthrough, giving better opportunity to stop the cut and separate the trepanned piece by tapping.

[Clive FosterParticipant @clivefoster55965]

If using a hole saw is best to add a couple or three holes intersecting the cut to help chips clear the saw teeth. Makes for a much faster job.

I have a mister for lubrication and chip clearance air blast which speeds things up even more. Mister turned right down for the merest hint of lubrication. Serious lubrication makes for a very messy job and chip adhesion everywhere.

Although a hole saw works fine without a pilot if used in a nicely rigid drill or mill I prefer to use one because the centre hole lets me flip the part over halfway through. Better chip evacuation and a cleaner break.

Ex kitchen cabinet side panels make nice sacrificial bases to go under the job. Woodscrews to hold it. Either fit holes or bodge up crude hold-downs.

Clive

[not done it yetParticipant @notdoneityet]

First of all cut off a ~20mm strip from each side of your piece of stock!.. width will depend on your cutting ability – or hole-cut a 75+mm piece from the corner of your stock. need a bigger hole saw for that as thet are usually sized for the hole. Then remove the centre as suggested above.

That is unless you don’t think ~20mm x 12mm pieces 100mm and 80mm long are worthwhile pieces to keep…

Edited By not done it yet on 23/04/2021 19:37:22

[DC31kParticipant @dc31k]

Posted by Nigel Graham 2 on 23/04/2021 17:01:19:

However you don't need worry about the arbor thread, nor the pilot-drill, because those are supplied.

And if you do not want a 1/4" hole in your offcut?

Or you do not mind a hole, but you would like it to be no more than 4mm dia?

Or you have limited headroom and want the arbor to go directly into the Morse taper of your machine?

Or you do not like the 13mm A/F hexagon shank on a standard arbor?

Or you do not like the idea of holding a 10mm dia. SDS shank in a collet?

With holesaws, the bought-in arbors confine you to a certain path: a home brew one allows you to tailor something appropriate to your own needs. And for that, the arbor thread is about the only thing that is important.

[JasonBModerator @jasonb]

Posted by not done it yet on 23/04/2021 19:36:01:

First of all cut off a ~20mm strip from each side of your piece of stock!.. width will depend on your cutting ability – or hole-cut a 75+mm piece from the corner of your stock. need a bigger hole saw for that as thet are usually sized for the hole. Then remove the centre as suggested above.

That is unless you don’t think ~20mm x 12mm pieces 100mm and 80mm long are worthwhile pieces to keep…

Edited By not done it yet on 23/04/2021 19:37:22

How does that work, he wants a 100mm OD "washer" with 75mm ID and has 100 x 100 stock?

[Nigel Graham 2Participant @nigelgraham2]

Clive –

" Either fit holes or bodge up crude hold-downs. "

Are you sure? A task like this needs secure and rigid clamping. Woodscrews holding the metal to the backing, yes, but you still need hold that, either in a 4-jaw chuck on the lathe, or with a proper clamp-set on the bench-drill.

'

DC31K –

Why are you inventing so many problems?

I described what I have used many times, a standard hole-saw mounted on its standard arbor with its normal hexagonal not SDS shank, in a bench-drill. Or indeed the lathe. What is so complicated about that?

The arbor does not confine you to anything. It's the hole-saw diameters available that control things.

If you want a smaller pilot hole, drill that to size and turn a simple replacement with a 4mm spigot for the standard 1/4" or 6mm drill supplied with the arbor. Others have found it possible to dispense with the pilot-drill, but I am not so sure, especially on large diameters, about that.

If you want no hole at all then yes, you do need consider trepanning in the lathe.

Limited headroom? We are cutting flat plate here, not large boxes. It'd have to be a rum bench-drill that can't accommodate an ordinary hole-saw above a 12mm plate and scrap of thick chip-board.

I have used this method on a Meddings bench-drill, a Myford ML7 and a Harrison L5, lathes.

Easy!

[JonParticipant @jon]

Posted by Greensands on 23/04/2021 15:20:01:

The object of the exercise is to produce an anular ring measuring approximately 100mm O/D by 75mm I/D and so the centre piece will effectively be an off-cut but being mean, I would rather like to retain as much of the work piece as possible rather than simply turn it all away in the process.

A decent hole saw will knock you from £13 shopping around, the centre cut out approx £1.60, you work it out. The cheapy in sets wont last 1/2" depth and theyre about £8.

Centre up and drill and tap a hole in that square plate, something like M12. Literally just hold an M12 bolt in chuck, it threads on tight and turn o/d. Used to do them that way 27 years ago before i bought 100's of round blanks.
Swap jaws over and hack the inside out by any means and thru bore to size, easy done thousands worldwide.

[Clive FosterParticipant @clivefoster55965]

Nigel

Crude refers more to appearance and engineering elegance rather than function. No point in extra effort on a use once and bin it device. I'll quite cheerfully do the final finish to size actually on the job by cutting through excess clamp material. Something a bit liberating about clamp sets you don't mind chomping into!

I've usually used 4 wood screws in the corners of the (squarish) stock I started with to hold it down and another pair around each corner with the head partially overlapping the stock to prevent rotation. Works well enough.

Have also used simple strap clamps across the corners cut from more ex-kitchen cabinet sides. Woodscrews, actually chipboard screws as I have a lifetimes supply, again with suitable packing.

Obviously the baseplate gets bolted down to the drill table.

Clive

[not done it yetParticipant @notdoneityet]

How does that work, he wants a 100mm OD "washer" with 75mm ID and has 100 x 100 stock?

OOPS! In too much of a hurry! Workshop and bees were a priority.

[John ReeseParticipant @johnreese12848]

I would set the plate up in a mill then edge find and locate the center. I would use a boring head to trepan a hole slightly under 75mm. I would probably have to flip the part and trepan from the other side. After I had a through hole I would use the boring head to bring the hole to finished size. Next I would chuck the part on the ID in a 3-jaw chuck. I would use a trepanning tool to knock off most of the corners. I would probably have to flip the part and work from both sides. After that I would finish the OD to size.

That saves farting around to get the plates centered in the 4-jaw. A hole saw could be used to make the undersized hole. Drilling some through holes along the track of the hole saw will facilitate clearing chips.

Just one more way to skin the cat.

[John ReeseParticipant @johnreese12848]

I figure there are so many suggestions because each contributor has a different mix of machines and tooling. I could have done it on my mill with my Volstro rotary milling attachment. It could be done on a RT on the mill. Both of those methods involves a single setup.

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