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How to drill a square hole?

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Alexander Edblad21/07/2010 13:04:49
6 forum posts
Seriously  now.
The buffer beams for my Invicta should have 1/8" cutouts for the coupling hook. What is the easiest way to do this? I guess I could drill four or so small holes and then file to get it square. Are there other options?
Peter G. Shaw21/07/2010 13:23:03
1359 forum posts
44 photos
I do remember reading about this on a number of occasions. Yes it is apparently possible - see MEW 14 etc and MEW112.
As I understand it, the idea is to have (for a square hole) a triangular cutter which is allowed to move around whilst being rotated. Sorry, I know that doesn't make sense, but it's the best I can do at the moment. What happens is that the cutter starts to dig in at the corners, rotates, and the next cutter point swivels round to produce the next corner. and so on.
But what about using a broach. Here you would have a piece of tapered hardened steel (silver steel) with transverse cutting edges on all four sides. Pressing the device into a circular hole then cuts the corners. For a better description try Len Mason's book Using the Small Lathe. I suppose it's like reaming, but in an axial dimension rather radial.
Good luck,
Peter G. Shaw
NJH21/07/2010 13:30:58
2314 forum posts
139 photos
Hi Alexander
I guess for 1/8" drill and file is the best option......however it IS possible to drill square holes!
You need the Henry Watt  square drill bit!
For how it works see here :
Other than the animation on this page I've never  seen it in action however my current project is Anthony Mounts Trapezium Connecting Rod Engine - a design attributed to Professor Reuleaux.
No -- in your case get out the files!
Good luck

Edited By NJH on 21/07/2010 13:34:55

Richard Parsons21/07/2010 16:55:32
645 forum posts
33 photos

There are four ways of doing this. Three of these have been written up in the Model Engineer/ Model Engineer’s Workshop.  For your 1/8” square holes in the buffer the best method to make up a punch broach. You make a tool which slowly graduates from a 1/8” round to a 1/8” square over length (in this size) of about 25mm. You then machine a series of cutting faces about 1 to 1.5 mm apart on it from the round end to the square end. Harden it and tonk it through the 1/8” hole you have drilled. This was written up in the mid 90s in Model Engineer.

The next method is to make and use a Rotating Broach.  This was written up recently in (I think) MEWS but I just cannot drop my hand on it. Hungarians cannot leave anything alone so God knows where it is. There was this morning also a picture of the tool somewhere on this website. It will graunch out almost any shape you can make the tool for, but it takes a rigid and powerful lathe to drive it at any speed.

Third way, which was written up in MEWS about 3 years is to use a drill based on a Reuleaux curve. This uses a Warrington coupling but will not drill a pure square, the corners are rounded. I made two 1 for me and 1 for a company, which had a problem. They had a problem with a 12mm square hole in a casting about 25mm deep. It should have been cast in but it weren’t. just after they had shipped the first dozen or so Suzuki decided to send the castings back (the model was not selling).

In the whole as you only have 4 to make I would drill the hole and file it square – a lot less fuss.
P.S. this shows I did not read Mr Shaws posting above!

Edited By Richard Parsons on 21/07/2010 16:57:15

Steve Garnett21/07/2010 22:58:59
837 forum posts
27 photos
I only make that three ways!
Small EDM? That looks like a neat way of making just about any shaped hole you can think of...
Rob Manley21/07/2010 23:28:39
71 forum posts
14 photos
1/8" square is easy enough to file with a small needle file.  I wouldn't even consider any other method.  But as has been said another method is to half grind a bit of 1/8 tool steel into a D shaped broach and take out the corners of an 1/8 hole by planing.
One day I will get around to playing with this: 
Sam Stones22/07/2010 02:34:12
851 forum posts
321 photos

Hi Alex,

I’ve always been intrigued about drilling square holes when, as an apprentice, one of the toolmakers told me it was possible using a triangular drill which went around in an orbit. That was the limit of his explanation.

Norman (NJH) has hit upon a good web-site htttp:// which is sufficiently animated to understand the principle.

Isn’t it a version of our old friend, the epitrochoidal? Better known from its use in the Wankel (NSU and Mazda) rotary engines. There’s a book (somewhere) which describes a multiplicity of rotary engines. It features a range of epitrochoidal orbits from two lobe onwards.

Rob Manley’s directive to the `square hole drilling’ video, is well worth watching, but I haven’t worked out how the drill can be fully inserted in the round hole immediately. But re-running and closer inspection of the video shows that the workpiece already has a square hole in it, and the drill chuck is designed to slide freely in any direction at right angles to the drill axis. The drill also appears to be driven by the workpiece. I have to presume that the size limit for the square hole is a direct function of the triangular drill dimensions. Could it be that to get things started, there has to be a slightly recessed pilot hole equivalent to the diagonal of the square minus the corner fillets?

While it would be a nice exercise to build the drill mechanism, with only four small holes to square and corner out, I’d stick with filing too Alex. Some of the needle files might even be 1/8" square anyway, so there’s the makings of your broach.



Edited By Sam Stones on 22/07/2010 02:37:52

Ian S C22/07/2010 14:02:21
7468 forum posts
230 photos
Starting the drill needs the square guide that fits over the hole area, this is what governs the size of the hole. Ian S C
Ian Abbott08/08/2010 17:45:31
279 forum posts
21 photos
At Torbay Steam Fair today, I met a man from Brixham Model Club, who had a square drill on display.
I can't remember the correct name for the rounded triangular bit, but essentially, it is a three face end mill, guided by a square hole, which mills out a circular hole to square.
I'm still getting the photos organised, but I'll post a picture or two when I've done in a day or two.
Stub Mandrel08/08/2010 21:27:51
4311 forum posts
291 photos
1 articles
How do you drill the square hole in the guide?
Ian Abbott09/08/2010 17:05:33
279 forum posts
21 photos
Ah, you drill it on a milling machine.
(Two pieces of "L" shaped steel bolted together)
Ian Abbott09/08/2010 17:23:38
279 forum posts
21 photos
The image doesn't seem to want to load on here, but if you look on the "Latest Photos" bit, it's there.
The gentleman's  name is Nigel Whiteway (I hope I got that right), it was noisy.
Ian Abbott09/08/2010 17:46:51
279 forum posts
21 photos
I looked it up, the shape of the drill bit is called a Reuleaux Triangle, which has a constant width and can roll like a wheel.  Which Nigel also had on his table.
What did we do PW.  (Pre Wikipedia) 

jacques maurel09/08/2010 22:30:56
79 forum posts
19 photos
Look at my articles in model engineer: N°4285 page 493 for square hole drilling, I've also made un attachment for hexagon hole drilling, the process is slightly different, you can get sharp edges, the drill is not a solid of equal width. Ask the editor to contact me if you want a short artilcle about this  subject in the magazine. The main interest of the article (about square hole drilling) is the description of a floating tool holder very usefull for reamer holding. The second article is in ME N° 4310 p 432 about polygonal punching tools, this way is I think more convinient for small dimensions. I've put many videos on youtube, just tap "square hole drilling or hexagon hole drilling. Visit my website: jacques
Jim Whetren10/08/2010 13:58:57
50 forum posts
1 photos
Hello Alex,
I have been following the thread with interest and was surprised that the Rotary Broach didn't get a mention.
A version of this tool was described by Jock Miller in MEW issue No. 157. It is quite straightforward to make, and the tool bit is easily ground on the most basic of grinding set ups.
Also the chicken & egg problem with making a square hole in the guide bush to produce a tool to make square holes is eliminated.
In the case of the buffer beam, you would just drill a pilot hole the AF size of the square, countersink it to just accommodate the cross corners size of the bit, then position the square in the correct orientation and 'drill' away.
Good luck with it,
WALLACE10/08/2010 20:04:35
304 forum posts
17 photos

Don't know about a square hole but I find it all too easy to put a triangular one into thin metal  when I want it to be round . . 
jacques maurel12/08/2010 18:00:30
79 forum posts
19 photos
Corrected post

Edited By David Clark 1 on 17/08/2010 15:19:50

jacques maurel17/08/2010 15:56:22
79 forum posts
19 photos
Please use the little button above with the Word logo on it.
Third from right.
This should convert from Word to text when you paste it.
regards david

Edited By David Clark 1 on 17/08/2010 17:10:33

jacques maurel18/08/2010 15:01:26
79 forum posts
19 photos
For polygon drilling, there isn't any problem of "chicken and egg" as the guide can be made in 2 parts assembled together. Nevertheless this process needs tooling (floating toolholder, guide, drill) and it's difficult to get an accurate dimension for the hole.I've made it to amaze my students, and to have a good floating toolholder for my reamers. The rotary broaching process is not easy too (rotating toolholder) the adjustment of the toolbit is described in a video on "youtube" it's not an easy task for a one off job. While you have made the tool bit, it's easier to strike it ( see ME N°4310).
Anthony Knights26/08/2010 01:32:34
583 forum posts
235 photos
The cutter in Mr Abbott's picture appears to have a constant diameter cross section, made from 3  x 120 degree arcs centered on the opposite vertex. I seem to remember pencils with a similar cross section. Machining one in a 4 jaw chuck shouldn't be too difficult, assuming one takes care with the setting up.
Hi there Wallace. The triangular holes thing is another manifestation of the constant diameter phenomena mentioned above. I find the best way to drill holes in thin metal ( without making a special drilling jig) is to clamp it between two pieces of wood. I have recently replaced the flooring in the lounge and now have enough MDF laminate to use for this purpose until I am called to the great workshop in the sky.
Warrington coupling was mentioned in one of the posts, but putting this in Google did not supply any useful information. An Oldham coupling was also seen on my trawl for more information. Could this be a Lancastrian thing ? A Manchester screw driver (AKA Big Hammer) springs to mind.
Tried "Floating tool holder" and was directed to the U.S. patents site, where the are s**t loads of entries for this. Drawings are available for download. One of these  has been added to my future projects list, as has the recently published "Wobble Broach" tool.

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