5mm square hole

I have been asked to make some tap levers for BSA air rifles. The parts will be mild steel and have a 5mm square hole passing through the item. I have spent several hours using files to just about complete one hole. Is there a quicker way to produce the hole other than filing ?

What thickness is the metal?

Would a rotary broach be the answer?

Typically a broach would be used for this job.

Robert.

The metal is about 1/2" thick but the square section is about 1/4" deep as there is a larger diameter hole which takes up the other 1/4"

In that case as Robert says a broach would be the simple answer, if the material were thinner you don't get such good tooth engagement and punching would have been a possibility.

They can be made or bought, really depends on funds and how many are needed. MSC do the DuMont range

MSC one here:

https://www.mscdirect.co.uk/DMB-66631B/SEARCH:CATEGORY/product.html

Alternative via Australia:

https://www.ebay.co.uk/itm/225149641607

Note that a 5mm square broach needs a pilot hole larger than 5mm diameter (eBay one says 5.2mm; DuMont's technical data will confirm the size they need). This may or may not be important to the part you are making.

At what stage does the square hole need to be made? I wonder if you could drill through 5mm, heat it up red hot and drive or (fly)press onto a piece of 5mm HSS, with the end tapered. Once you have the 5mm square hole, use it as a datum to make the rest of the part.

A proper blacksmith would not even drill the hole but would hot punch a through hole and then forge to the correct shape.

Spark eroder EDM ! do the job in a matter of mins. Noel.

You can make your own broach with some 5 mm hss

Grind it across the flats using the round edge of the grinding wheel and turn the corners into teeth

First drill a round hole of 5mm to get rid of most of the metal

then use an arbour press or a good vice to shove it through, makes a bang when it clears

If you take this route test it on a bit of scrap first, you may get the cut slug tearing out of the bottom of the hole and messing up the last bit

Don't forget to put a support at the bottom with a bigger hole to let the broach pass right through

Edited By Ady1 on 11/05/2023 17:33:35

I dont think that I would mess about with it, especially if only a few needed. Enquire about price/feasibility from small spark erosion business, locally. Suppose I might have a go as a challenge to see how good a job that I could make of it. Thinking in terms of initial 5mm drilled hole right through, to guide a specially made silver steel 'punch', 5mm dia. to fit the hole, followed by very gradually increasing square corners.Mild steel being relatively soft, if the tool teeth increased in size very slowly, perhaps it could be squeezed into the work by bench vice?? Not certain how much force required.

John

With what you have drawn, how does it remain centralised on the hole initially?

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As an aside, any method that makes the square hole other than as the first operation on the part has to consider the clocking of the square in relation to the rest of the part

What ADY shows is more of a punch than a broach, if making your own broach start with sliver steel turn a series of steps and then square them off, similar to this one I did for a D shaped hole

Clickspring has a youtube video on making a square broach. Have a gander at that?

Would need to start with bar at root two times 5mm.

Edited By not done it yet on 11/05/2023 18:49:13

Click to make larger

Joe Pieczynski has a video on Youtube in which he creates square holes with a punch similar to that shown by Ady1, but with only one cutting 'tooth'. He uses the quill on his mill to nibble away the corners.

Probably this video.

Rob

Links to https://www.youtube.com/watch?v=ASz-Ayo6-xM

If you've got a slotting attachment on your lathe and a means of indexing the mandrel you can do one corner at a time. Slow but a lot faster than filing. Depends on how many you've got to do. If it's a lot either buy a broach or find someone who can do it for you at reasonable cost.

Depending on how meticulous you are I'd start with a 5.3 hole

All good ideas.

If only a few pieces are needed, an alternative plan would be to mill a 5 mm slot from one of the boundaries of the workpiece and then silver solder in a 5 mm wide piece of mild steel leaving a 5 mm square hole, as George Thomas did when making a square hole to align number punches on his staking tool. Do the hole first then clean up the outside of the workpiece to get an almost invisible finish.

Rod

Hi there Peter

I've tried Joe Pie's method of making a square broach, it works a treat but it is fiddly. I made it to make a plastic part in the innards of a Lister sheep shear which has a 5 mm square hole in the centre to drive the flexy spring drive shaft. I made the part out of acetyl plastic. Hence I didn't harden my home made broach but it worked just fine..

To cut steel the raw broach will need hardening, which may warp it and certainly affects the surface finish. I guess one can heat treat it while in a boric acid bath a la Clickspring which will help reduce the surface finish problem. However, having it warp would make it unusable, as it will be brittle from hardening (even it it's tempered back a bit it still needs to be pretty hard) and it's absolutely vital to be able to push straight on a push broach else it will just shatter. The "necks" of the gullets are awful small (= fragile) at this size.

Which leads me to comment three. I have had no luck at all trying a 1/4 inch sq broach in a normal bench vice - you can't get the force you need which is more than ever you'd imagine, and a vice won't push straight. You also need to take several bites at it to make sure the broach is setting square against the push face. I don't have an arbour press, I do it with hydraulics, but I've followed some on Youtube who have tried it with an arbour press and been surprised at the force you need. It's also only too easy not to push the broach through at right angles to the part, so the hole is skew-wiff. It's very much easier to get everything in line and acting straight if gravity is helping you rather than hindering. I recognise that 5 mm must be easier to push than 1/4 inch, but I still think you're on a hiding to nowhere trying to do this job without a hydraulic press.

I recently bought a second hand 5 mm square broach on ebay. - to supersede my home made (unhardened) broach for making the spares for the sheep shearing rig. New they are about £120 a pop, I paid about half this for a secondhand one which visually looks the business but I haven't tried it yet. It looks awfully fragile! From memory the pilot hole is 5.3 mm, so you end up with a square in a circle - or a circle with corners if you like. If this is of use to you PM me and we'll work out something. No promises of success - if I break the broach then I can't replace it. Did I mention it looks awfully fragile?

But it's gotta be better than filing each one individually. What tolerance on orientation of the square have you got to keep? The broach cuts the square corners dead to size, but there is the question of the slightly oversize pilot hole which may be problematical.

If you'd like to take this further leave a message here to say there is a pm to find - I check the forum most days but only log on if I need to, so I may miss the little black envelope icon.

HTH Simon

Is it daft to suggest making the section with the hole as 2 milled parts and silver soldering together? Obviously depends on the amount of meat each side.

pgk

Something like this