Just a though, but would a slitting saw work, with the job mounted on an angle plate?  A little clamp shuffling might be needed.

A better approach if you can manage it is to use a suitable width of slitting saw on a horizontal mill. Saw speed should be about 50 rpm with a good steady feed, cut each run in one pass.

Regards   Brian

Sorry, I see that route has already been mentioned

Hi Jack,

Your speeds seems to be OK. There is a nice calculator here. There is another calculator that gives both speeds and feeds here.
I always machine Cast Iron dry, Bernard’s suggestion of using vacuum is good.

Thor

On 5 February 2024 at 10:23 jaCK Hobson Said:

Shaper – you need a bigger one than I have.

Is this a case of large material or small shaper?

We should be told 🙂

best

Robin

Duffers patent rule of thumb for mild-steel:

rpm = 10000 / dia in mm

So 10000/2 = 5000 rpm.

If cutter is carbide multiply by 1.5 = 7,500rpm

In this case, material is cast-iron so a further correction may be needed.   A little complicated because cast-iron is a family of alloys with different cutting properties, not a single standard metal.   Generally, I halve RPM when cutting cast iron, so:

7500/2 = 3750 rpm   Then I experiment, because faster is often better: Jason’s 5000rpm is perfectly reasonable.

Beware!  Cast-iron is frequently nasty, full of occlusions, and/or chilled super-hard by being hosed down in the foundry.  Machinability varies from wonderful to very poor depending on what you have.  Rule of thumb cutting calculations may not work out in practice because “cast-iron” is pretty random, especially if it was chilled, causing a hard skin of unknown depth.  A small diameter cutter could sail through it, or come quickly to grief.

Using a small diameter cutter on an unpredictable metal is high-risk.   The cutter might happily plough through before suddenly hitting an occlusion or hard spot, blunting the edge or suddenly increasing the side load to breaking point.   Rapid swarf clearance is important, because it adds to side-loading and causes blunting.   Only the operator can judge, and I’d expect to break a few cutters before finding a safe optimum.  A steady hand is important too – bumping is likely to break a small cutter.  (You can guess how I know!)

Jason recommends cutting full-depth in one go and he is a much more experienced and effective machinist than me.  However, another duffer rule of thumb is to cut no more than 20% of cutter diameter deep, which is 0.4mm.   Therefore I’d probably cut these grooves in two passes.   My 20% rule is conservative though, and not entirely sensible!  Although shallow cutting reduces the side-load, taking two passes blunts the cutter extra fast, which increases the side load.  The pros and cons aren’t black and white.

How long will the cutter last?   Roughly speaking, a lightly loaded HSS cutter machining mild-steel will last a couple of hours before it gets too blunt.   Driven hard, fast and deep, perhaps only 15 minutes.   Jason’s 40mm per minute feed-rate is about right, so the number of cutters needed can be estimated from the groove lengths, and assuming mid-range life of an hour.  Except 2mm diameter cutters are easily broken…

In short, a highish RPM with a slow steady feed-rate, in either one or two passes, varied as necessary by the operator depending on the nature of the cast-iron.  Could be straightforward, or tricky, or somewhere in the middle – I’d expect to break a few cutters finding out, because so much depends on the cast-iron’s properties, which might be lovely, horrible, or mixed.

Dave

On 5 February 2024 at 11:36 Robin Said:

… Is this a case of large material or small shaper?

We should be told 🙂

A decent grade carbide cutter should be able to do all three plates without replacement. I use K2 cutters from YG. These are South Korean professional, albeit bottom of the range, cutters.

For this job I would use a 2mm, 3 flute, stub cutter; 0.5mm DOC, 9500rpm and a chip load of 0.01mm/tooth, giving a feedrate of ~285mm/min. Running at 5000rpm would give a feedrate of 150mm/min.

Andrew

On 11 February 2024 at 19:45 jaCK Hobson Said:

I can then get shiny glass with cerium oxide, but it is still a bit milky so I’ve got some learning to do still. And I don’t think the glass is coming out ‘optically flat’.

Has the surface you’re polishing previously been ground flat?

Do you know what type of glass it is?

You may well be seeing ‘short finish’ where the glass between the grinding pits is shiny, but the pits remain – you should be able to see this with a loupe, etc.

There are two answers:

1) Polish some more, at the risk of losing the accuracy of the ground blank and rolling off the edges;

2) Re-grind to a finer grit. Ideally the blank should be ground finely enough that it has a waxy sheen before you start polishing.

How flat is ‘optically flat’? If you are looking for something that is better than a few fringes, then it is a bit of an art!

Good luck!