You could make a D bit or use a 2.5mm ball ended milling cutter, in both cases just plunge straight in.

Well if it has length then the ball ended milling cutter would still be the tool of choice if it's straight across the cube and a D dit could be held stationary just like a form tool of it's an O ring groove. If a dimple in the middle of the cube then plunge in as per my first reply.

I wonder why I have that 'here we go again' feeling….

Many times previously, threads have developed a life of there own with many members suggesting solutions for problems the OP never even thought existed. Without knowing what Chris wants all we can go on is guesswork. I know I mentioned O-ring but TBH it was a bit tongue in cheek (O-rings are rarely or never fitted to semicircular grooves).

To me a groove is a groove. A stationary D-bit would work as a form tool but its an unlikely tool to use in the lathe or mill because most bit are relatively long so holding it would not be easy.

Ian P

1.25mm plunge would be better

The OP has proven many times that he is expert in not explaining the problem to be solved very well, arguing with good practical advice, and not trying various things (or even googling) before wasting peoples' time.

+1 for that Jeff.

Andrew.

Exactly, my error for stating 2.5mm.

But now we know the groove is right across the part then it seems a round nose form tool/slot drill is the tool needed.

Emgee

If the problem is clearly defined, the means of solving it will be more easily found..

For those fortunate enough to have been in Engineering all their lives, this becomes second nature, but for a newbie this has to be part of the learning curve.

Naming a process or part incorrectly can cause confusion and produce answers to a question that has not been asked. A picture or drawing often clarifies matters.

So it is beginning to sound like a groove 1.25 mm deep across the face with a 2.5 mm ball nose end mill.

Personally, I would be wary of doing it all in one cut, half the diameter of the end mill, even with a very fine feed.

(Unless you have LOTS of spare cutters to hand! )

Maybe gradually take out out some of the material with a 2mm end mill, for part of the depth, and then finish off with the ball end cutter.

Howard

Jason has given you an excellent choice. Should you decide to get a 2.5mm ball ended milling cutter to do the work, you will have to be very careful milling it. You will have to do the groove in careful stages to avoid breaking the cutter. With that size, I would recommend the following stages;

First cut 0.5mm deep.

Second cut 0.3mm deep.

Third cut 0.2mm deep.

Fourth cut 0.15mm deep

Fifth cut 0.1mm deep.

Run the mill at its maximum speed. Don't rush the feed rate. You can cut in either direction as you increase the depth.

Use oil if using HSS, or dry if solid carbide.

Edited By old mart on 29/12/2020 15:02:15

Though Chris said 'groove', I read his drawing as a dimple!

As this is a classic engineering problem with 2D drawings, may be worth mentioning the answer. It's to provide two or more views of the object, which together provide all the information needed to derive the 3D shape.

Basically, draw what the object looks like from three sides, like plan, front and side:

Actually this object only needs two views to show Chris said and meant 'groove'.

Far more to Technical Drawing and Draughtsmanship than this example, but the basic idea is simple enough.

Jason removed ambiguities by developing the object in 3D CAD, which I generally prefer because the 'real' shape can be viewed from any angle. But 2D drawings are also useful, and good ones are a positive joy to look at – art!

Dave

Although the size in this case would not suit the method there is no reason you could not bore a circular groove