Why do plumbing fittings have cast nuts?

I bought some brass plumbing connectors today, 1/2" bsp to 1/4"bsp male thread. The main body is machined out of solid brass bar, but the nuts look like they are cast and then machined.

This got me wondering why? Why go to the trouble of casting something and then machining the threads, using different processes and probably different factories, surely it would be quicker and easier to machine them out of hexagon bar, in the same factory which makes the main body?

IanJ

For high volume production (ie millions of parts / year) every gram of metal counts and every second of machine time counts. For plumbing nuts that are only made in a few sizes it probably makes cost sense to make a dedicated cast / machine line for them in Asia. For the fittings, they could be machined in many sizes and so hex bar gives max flexibility.

Years ago I worked at the Canadian factory of a big US controls company. We had a whole section of the plant making hot water heating valve bodies and all manner of fittings and nuts. I know of which I speak. For 3/4" size valve bodies alone there were 28 variants of fittings sizes / threads to accommodate the world's plumbing needs. We also made 1/2" and 1" valve bodies with a similar number of variants. When I started there the valves were made by soldering machined brass fittings into a centre brass forging. Later we changed the valves to be made by sand casting and I designed the castings to be able to machine all needed threads and fittings in 2 or 3 way valves for the whole 1/2",3/4" and 1" families with 6 different castings. The centre machining was same for all valves the only variation was it was cut a bit deeper in the 3 way versions. Changing to the machined castings vs the soldered machined fittings / forging saved many hundreds of thousands of dollars per year. We produced between 4 and 6 million of these valves per year out of our plant.

Are they cast or are they hot pressed? Some years ago our club visited a place that was making brass fittings for LPG gas service. They were done in large power presses by heating up a suitable length of brass bar and then stamping it in the press. This gave quite complex shapes with good detailing including quite fine lettering. I think Stuart used to do some of their brass parts this way.

John

Whether cast, drop forged, pressed or sintered, you can rest assured they were done that way because it is cheapest. Production engineers spend their careers thinking about such things. I would guess some kind of pressing/drop forging process so you get the stamped numbers etc incorporated and no waste swarf.

I did most of my apprenticeship with a divison of Pegler valves (Hattersley Heaton) - at one point I was seconded to Pegler's Halifax plant for 3 weeks to assist their electricians rewire the hot press shop following a fire.

The Halifax plant made brass gate valves - the bodies & gates were sand cast in an on-site automated foundry, the bonnets & gland nuts were hot pressed from brass bar billets & the screws were cut from bar stock on Wickman 6 spindle bar auto lathes.

The hot pressing made both the bonnet and the gland nut from the same blank. I can't recall how they were subsequently machined. The cast bodies were machined on German multi-station transfer machines (Diedesheim or Vogel IIRC) - the operator removed a finished part, inserted a cast blank & the machine indexed one station and cycled all the 8 or so cutting operations. Every cycle a finished part came off. The machine shop was known as "The Golden Mile", as the floor was encrusted with brass swarf. All the brass swarf went back through the foundry - a hellish place. Hot, dark, dirty and noisy I was glad I wasn't working in there.

Peglers also had their own machine tool rebuilding company in Halifax & I later spent a couple of months there on secondment working on rebuilding a Diedesheim machine, replacing the old relay control system with a PLC as part of the rebuild.

All the Pegler facilities mentioned are long gone now.

Nigel B.

Labour costs money and scrap (as in machine swarf) costs money. If you can spread the cost of a machine over thousands or millions of parts it works out cheaper than using labour or having scrap to get rid of. Scrap metal value is well below the original material cost.

Whenever I or my colleagues wanted to buy capital equipment (assets with a resale value) we had to write a report on predicted annual saving and expected payback time when the cost versus payback broke even. This was compared to the expected asset life of the equipment and if the asset life was longer than the payback you could proceed, if it wasn't then you had failed to come up with a business justification so it was a no goer. Some of the reasoning employed was a bit nebulous at times so you had to be able to defend it if it was questioned. Think of the business case for the HS2 railway but on a smaller scale.

Martin C

I can understand economy of scale manufacturing for millions of pieces, still not sure of how the nuts are made though. The surface finish looks like a brass casting to me, a sintered piece would maybe be smoother? Not sure what a hot pressed brass nut surface looks like, but anything pressed on a die would be smoother and more detailed?

IanJ

You can also pressure die-cat brass casting a number on the same runner or sprue. It is quite possible that the texture is some kind of post finishing like rumbling before machining.

Tim

Very much so, it's all about cost, which changes over time. Materials, transport, wages, tax, tariffs, and competition. What worked well in the past usually comes unstuck sooner or later. If the owner, management or workforce can't adapt, the business goes down the drain. And sometimes nothing can be done: Stoke-on-Trent once produced pottery in gigantic quantities because the town sat on suitable coal and excellent clay, both of which could be had at almost zero transport cost. This was great until all the Coal was gone and fuel had to be expensively imported. At that point, cheap and mid-range Staffordshire pottery became uncompetitive and the companies making it all closed down. They still make excellent pottery in Stoke-on-Trent but it's all top of the range stuff. Production has moved into small modern facilities and all this has gone:

Jeremy Clarkson on 'Who Do You Think You Are' provided another example. He's a member of the once highly successful Kilner Glass Making family - Kilner Jars. Kilner came off the rails in the early 20th century. Still using Victorian labour intensive and fuel wasting methods, they couldn't compete with glassware mass produced in modern factories using American methods. The firm upgraded expensively, but were caught again because the Americans almost immediately improved their methods again, exploiting economies of scale. At this point Kilner realised they had to replace all their existing plant, including the buildings, but were heavily in debt and losing money. They shutdown to cut their losses.

Today CAD/CAM, CNC, and methods like 3D printing are still challenging yesterdays production methods. Casting plumbing fittings is probably the cheapest way of making them in large numbers at the moment. However, if the market for brass plumbing fittings reduced substantially, it might become cheaper to machine them from hex stock. And this is happening: several modern plumbing systems are push-fit, either copper or plastic piping. Anyone making cast plumbing fittings today had better be on the ball, otherwise they might end up bankrupt with a closed factory...

CNC has all but eliminated traditional turning and milling production methods because it reduces the need for skilled operators, is more flexible, and much faster to set-up and run. Manual machine-tools are valuable for specials and small runs, but are otherwise hopelessly uneconomic, also failing to compete with specialist machines like thread rollers. Large factories full of steam-powered line-belt lathes have disappeared and the modern equivalent sits in an ordinary business park, almost indistinguishable from the Carpet Warehouse next door. This creates the illusion that British Industry has somehow 'gone', when it's actually as profitable as it ever was. What's gone are obvious signs of activity like shunting yards, coal tips, smokestacks, mills, factory gates and a huge labour force!

Dave

The double male fitting is probably machined from hex bar( it may even have the hole in ) but the nuts will be hot stampings/forgings. The brass is heated to 3/4s of it's melting temp it's now plastic and can be squeezed into any shape you need. The detail that can be got like this is quite incredible. As has been said, Stuarts use hot stampings for the reversing gear on the 10v and D10. Noel

Yes the Male fittings are made from solid bar, probably on a sliding head CNC. I don't think the nuts are hot stamping produced, because they lack the incredible detail, the lettering and surface finish is very poor.

Production costs of 'running line' components will be under continual review. When I was in the Industrial Engineering department of a mechanical controls company, we had a cost saving target to realise every year, so every time any of the 8 -10,000 piece parts came under our eyes for any reason, we'd examine any options there might be to shorten operations or save material. I doubt any modern repetition manufacturer, here or abroad, would be much different.

What might have determined the use of a casting for a plumbing fitting was the cost difference between a turn/drill/bore/thread/part-off operation with its small quantity of lost material and a load/bore/thread/unload operation with its casting and autoload/unload costs, calculated over a year's usage.