Creating rectangular blind hole

I need to machine a rectangular blind hole in mild steel. It needs to be about 10mm wide by 20mm long by 30mm deep. It doesn't matter if it has slightly rounded corners. Is this even possible? What is the best way to do it? My current plan is to use an end mill (say 5mm) to make the basic shape, then use a smaller end mill (say 2mm) to square off the corners (if I can get a 2mm end mill that's long enough and stiff enough to cut 30mm deep). Will this work? Is there a better way?

Thank you - Rowan

For blind holes with sharp internal corners i mill out the waste and clean up the corners with a length of HSS steel in a slotting head.

Andrew

2 mm endmill x 30 deep will not work, slotting as per Andrew J is doable in the home work shop, what machines have you got, both a lathe and mill can be used as a make shift slotter.

Tony

Even with a 2mm  long neck rib processing cutter you are likely to get flexing and the tool pushing away from the work. Also trying to feed them manually to keep a constant small chip load will be very hard so breakage is likely.

I'd be inclinded to drill out the corners2mm first then mill out the majority of the waste with a 4mm long series carbide cutter as they have 4mm shanks so easier to reach down and then you could use your mill as a slotter or redesign for a larger internal corner radius or find  afriend with an EDM

Edited By JasonB on 15/04/2023 15:51:59

My thinking exactly. 30 mm is deep for a 2 mm drill, but it should be do-able if you 'peck' frequently to clear the swarf from the hole.

George

Rotary broaching looks fun! It drills square holes...

Dave

The Hemingway kit for a rotary broach works extremely well, a friend made allen key holes in some filler plugs for me using his , very quick and looks very professional when done. The kit is on my list of to do's but it might be too much work for a one off, unless you are like me and any excuse to make some tooling!

Edited By Dave Wootton on 15/04/2023 17:07:21

Just run me through whether the allen key holes were regular hexagons or not please.

If they were, please advise how the rotary broach would help with the RECTANGULAR hole the OP wants.

Not much spoke of EDM is another way ! Mill out the bulk then do the corners ! Noel.

Two 10 x 10 square holes side by side would do nicely to form the required 10 x 20 rectangle

Same principal as using a morticing chisel for woodwork, they cut square holes but the work is just moved along to form rectangles

Edited By JasonB on 15/04/2023 18:46:29

Without knowing the applicaiton, over-cutting into the corners is another approach. Means that you can insert a rectangular plug (or whatever) without needing to round its corners. Tidier with CNC as you get controlled cutting but I guess you could calculate coordinates to just take you into the corner. For this kind of thing, I aim at an arc in the corner that just touches the corner of the hypothetical rectangle, with generally whatever cutter I am using to rough out the rest. The resulting gaps either side of the corners can look quite neat - but it all depends on just what you are trying to achieve!

Cold Chisel

😋

If I'd designed something like that in my first DO job, I'd have had a foreman whacking the print down on my desk demanding to know how the **** am I supposed to make that? ' Can't you redesign it away?

Welding it would probbly be easiest

Mr Thorpe's article in model Engineer dated may 1983 using his method of slotting square and rectangular holes will help.If you pm me I can send it.

An alternative method for making square slots. Although the hole here isn't blind the method could be adapted.

Edited By Graham Stoppani on 16/04/2023 07:39:41

Dc31k

In response to your very agressively worded response to my post, yes they were regular hexagons. But I also know he has cut square holes with the same set up, to make a rectangular hole it should surely be possible to cut one square hole , blank it temporarily and cut another next to it and remove the blank.

I would agree with small holes in the corner then mill out as much as possible. Then modify a small cold chisel to be sharpened on one side only like a wood chisel, and use this to clean out the corners. With care, a neat result is not difficult.

Recesses like this are amenable to be made with a small hole down each corner, centred on the corner, not tangential to the walls. This gives the mating part clearance without it needing radii or chamfers; and obviates trying to use chisels or slotting-attachments.

Drill full depth by drill diameter to just a touch past the floor; and obviously before milling the recess. If the floor is too thin to accommodate the drill-cone, remove the conical portion of the hole with a slot drill, or a twist-drill ground a bit like one.

However, without knowing what you are actually recessing, this does seem a remarkably awkward thing to create. Is it in the end of a bar or thick block? Is it in a plate whose design might allow modification to a through-hole that can be milled to half-depth from both sides?

I agree that needing a blind, rectangular hole with sharp corners is very poor design. And yes machine shop foremen can eat a rookie draffie in a single mouthful. There has to be an alternative.

dc31k - you could have politely pointed out that rotary broaches can not do rectangles. It would have had more impact.

Welding/bolting/glueing a piece with a through rectangular hole onto or into the larger piece would be my favoured option.

Martin