turning small square stock

Making the big end for Stuart Models oscillating engine I need to turn a short length (1/2 inch) of 3/16 square stock. My 4 jaw is too big, and I don't have any square collets. I am thinking of milling a slot just under 3/16 deep in round bar and holding that in the 4 jaw. Is that a sensible option, or are there any other methods I could employ?

That seems a viable method provided the one jaw contacting the stock grips it enough to stop it slipping back along the channel.

Another, well-tried, dodge is to use a tubular sleeve of bore equal to the diagonal of the square, split for its full length by a single cut, as a collet that can be gripped in a 3-jaw chuck (or 4-jaw for more accurate centering).

If you have a means to mill then put the supplied stock to one side and make it from round bar, milling it square and doing any turning before parting off. I often do this as it gives a crisper cornered square than a lot of supplied small sq section

3/16" square across corners is theoretically 0.2652" , and 17/64" diameter drill is 0.2656".

So what I've done in circumstances like these is made a slit bush out of, say 8mm or 5/16" brass, drilled through with the A/C size or as near as I can get. Check your square stock across corners - it may have break-edges and you might get away with a 6,5mm through hole. Use a thin-bladed junior hacksaw to make the slit narrow.

Unless you're doing harsh machining (and on 3/16" stock it's hard to see how you could be) a decent across-corners grip in soft brass should be quite adequate.

Edited By Mick B1 on 09/07/2022 19:02:10

Thanks all for suggesting alternative methods, I shall experiment!

Jason -

Expanding on that, I have started a practice where feasible of cutting grooves with the parting-tool in the round bar, down to slightly under what will be the flats or other features, before milling.

If the finished section is symmetrical, it's easy enough to return the milled bar to the lathe and pick up the grooves again to complete the parting-off.

It avoids that horrible interrupted cutting, especially when using a rather fragile carbide insert-tool.

Don't tend to get the problem and often use the 1mm wide inserts when I'm parting small square nuts etc

The collets most commonly used* - E and the size in mm - generally have slots for contraction which are multiples of four. It is likely that you will find an example in which your 3/16 stock will fit, lodged in four of these slots. As long as the stock is accurately square it should locate reliably in such a collet.

Hope this helps - cheers, Tim

* OK, a guess.

It is not uncommon to use a large chuck to hold a smaller one directly to hold small stock. 80 or 100mm four jaws are not expensive to buy, and the loss of bed lenght is not often a problem with small work. If the large four jaw you have is mounted to a backplate, that plate could also accomodate a smaller chuck with only some extra drilled holes.

Small three jaw self centring chucks can be even less costly. At a push, the supplied 125mm chucks may suffice?

Steve,

It just so happens that I wrote a short article for MEW - issue 285, page 67 - not available in the archive, yet, on this very subject. In the article, I stressed that it was strictly a get you out of pickle solution and not suitable for mass production. The snip below is a screenshot of the drawing which accompanied the article. I hope it's self-explanatory:

It's in metric, but the same principle applies to Imperial.

John

Hi, I did this with a 2" chuck and backplate in my three jaw on my mini lathe a couple of years ago.

Hi, I made a couple of bespoke nuts from plastic with a 6mm thread, these will be fitted into a round wooden block and the flat part will sit in a recess to prevent them turning, the blocks will then be glued on the inside of a wooden cabinet.

I wondering if anybody thinks I'm cheating with my set-up for machining.

Truth is, I was too lazy to change the 3 jaw for a 4 jaw, just to do two items and then having to change them back again.

Regards Nick.

I have used a sleeve to hold square stock. I had square brass and held it with an sleeve made from free turning 2011 aluminium. I use them like a distortion sleeve. Being round, any shape change or distortion will create a tightening effect on the bar. Being small parts, the loads were very light. But the gripping power is alot. I have since purchased a self centering 4 jaw chuck. I find that I haven not used it as much as I thought that I would, but I do use and make alot of plain sleeves , that rely on the distortion to hold the parts. Small amounts like 0.04mm or so is easily accommodated with them. The downside of a slot is it to check runout, the interruption is annoying.

Being new to ME it never fails to amaze me how many different solutions there are for a given problem, or how knowledgeable and helpful the folk on this forum are - this is the 4th time this year the forum have helped me solve a problem, and with multiple solutions to each. I decided to start with the slit bush method using the square stock provided with the Stuart kit, and got pretty good results as per pic. The accuracy achieved was such that there seemed no point switching to the 4 jaw. I now intend to try the other methods suggested as part of my learning process, and also to purchase a smaller diameter 4 jaw as I intend to focus on small modes initially and the need will clearly arise again. John, I subscribe to MEW print and online so will lookout for your article, though as you say the screenshot is self explanatory. Thanks again, will no doubt be back soon with another question!

Steve - quite late in my model engineering days I bought myself a self centering 4 Jaw chuck more at the time as 'something to buy' at a show rather than an actual direct need as I had both 4" and 6" 4 jaw ind. chucks.

It very quickly became my first choice chuck and has replaced the years old but still serviceable 3 jaw Myford as permanent chuck in use. Round, square and hex it's something that makes the kind of op you are doing a doddle and, had I realised it's versatility and usefulness, is something I would have bought, many years before I did, .

Though not an alternative for an independent jaw it's a very useful additional bit of kit that will see much use. Maybe worth considering?

Tug