Creating rectangular blind hole

[bernard towersParticipant @bernardtowers37738]

Is it possible to two 15mm slots from either end then silver or weld a cap on?

[HuubParticipant @huub]

Posted by Rowan Sylvester-Bradley on 15/04/2023 14:52:02:

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

Rotary broaching a rectangular blind hole is not a problem. You can't get to the bottom all the way. The long cutting edges generates high cutting forces. You could mill the hole and rotary broach the round corners square. I expect you get to 1 mm off the bottom.

You can do the same with a square broach if you do the corners one by one.

[Ronald MorrisonParticipant @ronaldmorrison29248]

Posted by Rowan Sylvester-Bradley on 16/04/2023 16:17:58:

Posted by duncan webster on 15/04/2023 20:37:47:

Can't you redesign it away?#

I can't see how. It is a tool that has to fit an existing piece of machinery, so I can't change any of the dimensions. I made it a blind rather than a through hole because I think I need the additional strength that this will provide.

Rowan

Instead of making this a difficult project with the blind hole for strength, can you change the material to gain the extra strength with an open hole?

[DustyParticipant @dusty]

Rowan

It would be helpful if you were to explain exactly what you are trying to do and why. In a previous life I was a toolmaker and worked in a special purpose machine dept for a while, there were several occasions when drawings came down from the drawing office and I needed to explain to the draftsman why a component could not be made in the way it was drawn. There're many on this forum who have a lot of experience and with all the information will be able to help, but all the information is needed.

[HOWARDTParticipant @howardt]

As has been said all the information of what is intended use is really required. I used to drill or slot holes in the corners, actual placement of the holes depends on which sides your are trying to make a tolerances fit. By positioning the hole centres on the corners you shorten the contact lengths dependant on cutter size. If you align the radius of the cutter outside the rectangle you can produce a size slot on all sides. Clearing the corners like this allows you to use say a 8mm diameter cutter to finish all sides and give you a chance with the depth.

[SillyOldDufferModerator @sillyoldduffer]

Posted by HOWARDT on 17/04/2023 12:48:57:

As has been said all the information of what is intended use is really required. I used to drill or slot holes in the corners, actual placement of the holes depends …

+1 The difficulty increases with the sharpness of the corners, and these may not matter at all functionally. I've seen a real-world example like this, where there are no corners!

Most likely done this way to remove the need for tricky machining, but perhaps also to provide stress relief.

Dave

[Rowan Sylvester-BradleyParticipant @rowansylvester-bradley37244]

Yes, but I still need to drill small holes 30mm deep, and the problem of the drill wandering and not ending up in the correct position remains. I am gravitating towards milling the basic hole, and squaring up the corners with a specially ground cold chisel.

Rowan

[peak4Participant @peak4]

Posted by JasonB on 16/04/2023 13:37:12:

Though Hurco say you can do a rectangular hole

So do Polygon Solutions
https://www.polygonsolutions.com/rotary-broaches/special-form-rotary-broaches/rectangle-rotary-broach/

I've not used it yet for various reasons, but I did recently buy a rotary broaching tool holder off ebay a little while ago.
I'll not link to it, as it was dispatched direct from the far east, and the price has risen dramatically now they only have one left.

Bill

[not done it yetParticipant @notdoneityet]

I usually keep away from difficult-to-do ideas and make a work-around.

Here is yet another option: Buy (or probably make) a square broach and make a through hole, filling the unwanted portion afterwards.

[Tony Pratt 1Participant @tonypratt1]

Posted by Rowan Sylvester-Bradley on 17/04/2023 15:10:53:

Yes, but I still need to drill small holes 30mm deep, and the problem of the drill wandering and not ending up in the correct position remains. I am gravitating towards milling the basic hole, and squaring up the corners with a specially ground cold chisel.

Rowan

Who says you have to drill small holes, 4/5 mm drilled hole should be fairly doable?

Tony

[SillyOldDufferModerator @sillyoldduffer]

Posted by Tony Pratt 1 on 17/04/2023 16:56:17:

Posted by Rowan Sylvester-Bradley on 17/04/2023 15:10:53:

Yes, but I still need to drill small holes 30mm deep, and the problem of the drill wandering and not ending up in the correct position remains. I am gravitating towards milling the basic hole, and squaring up the corners with a specially ground cold chisel.

Rowan

Who says you have to drill small holes, 4/5 mm drilled hole should be fairly doable?

Tony

Yes, indeed. Rule of thumb, a twist drill will stay straight for 5x diameter, so 30mm deep calls for a 6mm drill. Actual size (10 x 20 with 6mm corners, the result looks like this:

An advantage of oversized corner holes is they only have to be straight enough to accommodate the male corners, that is not critical at all. So safe enough to drill 5mm corner holes 6x deep, or even 4mm 7.5x deep. Beyond that, keeping a twist drill straight becomes ever more difficult.

As to cutting the hole, I would remove most of the metal by chain drilling, probably with a 9 mm drill, then mill a 10mm wide slot, before finishing the corners with a 6mm two-flute. (same diameter as the corner holes). Chain drilling saves wear on my precious end-mills. As the slot is deep, 6 and 10mm dia end-mills should be used for maximum stiffness – small diameter end-mills are likely to bend, even if long enough. (There's much to be said for chiselling, though it's not very accurate.)

All this assumes big corner holes are acceptable! If not, truly rectangular holes are hard to machine. Through holes are usually broached, but a precision blind hole calls for EDM or lasers.

I do my best to wimp out when faced with difficult machining jobs: with luck a compromise is just as good provided it's strong enough and not plug-ugly in public. Often fixed with paint and putty…

Dave

Edited By SillyOldDuffer on 17/04/2023 17:31:46

[Rowan Sylvester-BradleyParticipant @rowansylvester-bradley37244]

Thank you for your suggestions. Some of these sound as if they would work for my project.

As someone previously pointed out, the holes don't have to as far apart as you have shown them. They can be moved in so that they almost touch the corner of the rectangular piece that fits into this hole, plus a bit to allow for drill wander.

Rowan

[John Doe 2Participant @johndoe2]

Slight thread drift but just out of interest, how are the square holes in 1/4, 3/8, 1/2 " drive sockets made? Are they drop forged around a square former or do they have a square former pushed through them when they are hot and soft blanks?

Ditto allen bolt heads?

[JasonBModerator @jasonb]

The small (2mm) holes were suggested as an alternative to the 1mm radius that the OP's thoughts on using a 2mm milling cutter would leave.

If you do decide to drill 2mm then spot drill first, then use a stub drill which will get the hole 10-12mm deep and is less likely to wander and should then help guide a standard length drill to complete the hole. Chances are a 2mm drilled hole will be straighter than the deflection from a 2mm milling cutter at those sort of depths.

[bernard towersParticipant @bernardtowers37738]

Most cheap Allen bolts are forged or pressed but if you really pay the money they will be broached especially for traceable kit.

[vic francisParticipant @vicfrancis]

Hi Rowan the article by Mr Thorpe is in my pics folder here…!

[HuubParticipant @huub]

I would mill the rectangle blind hole using an 8 mm end mil, cleanup the corners using a 6 mm end mill and finally broach the 4 corners using a ground HSS square tool bit.

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