(An introduction to common home workshop materials)
In the previous article I had a look at ferrous metals. Typically, they are off-the-shelf bar stock items, and relatively cheap. Unfortunately, a large portion of the non-ferrous metals, specifically the copper alloys, are expensive, and I can honestly say I seldom buy bar stock. As a rule, all sections above 20mm are cast for purpose or I use the runners from my castings as machining stock. With this in mind the copper alloy section will deal more with first principle alloying elements. Any good metals sales person will be able to supply the closest equivalent (at a premium!).
A few years ago I bought a drawn section of phosphor bronze for use in a rather rough bearing application. It lasted six months before it had worn to a point that it was causing mechanical issues. I promptly removed the bearings and replaced them with risers from a tin bronze casting I had done and the bearings are still going two years on.
The most well-known and common non-ferrous metal is aluminum and its respective alloys. The weight to strength ratio is superb, they melt easily and machine beautifully. In fact, it is easy to obtain a mirror finish if the cutting tools are sharpened correctly and you can keep the swarf from rubbing on the machined surface. Aluminium is also highly conductive and works well as a heat sink or electrical conductor.
Figure 1: Small aluminium heat sink
The copper alloys are numerous, both in chemistry and application. I’ll have to limit this section to the four alloys I commonly use. The cheapest is brass, which I typically mix to 60% copper, 2.5% lead and the remainder zinc. The lead improves the machinability. This is a general rustproof metal for any non-bearing application, typically for ornamental fittings or airtight castings.
For standard bearings that don’t see excessive wear or pounding I make a standard bearing bronze (7% tin, 7% lead, 5% zinc and the remainder copper). These work well for valves and steam chests.
Figure 3: Standard bearing bronze valves
For high wear bearings that take a pounding, you need something a little stronger, especially if you don’t want to strip and replace too often. I have had good success with tin bonze (10% tin, and the remainder copper). This results in a tough material that shows little wear, both to the shafts and bearings.
Figure 4: Tough bearing bronze
If you’re looking for an incredibly tough material that machines like stainless steel and has a tensile strength comparable to steel then you need alumina bronze. This alloy doesn’t make good bearings, the shafts will tell you why! But I have used it for a gauge I made that needed an incredibly thin wall. I had no issues with machining the casing wall to below 0.5mm thick and it was still rigid. Incidentally, this was also a casting.
Figure 5: Alumina bronze casing
Typically, the copper alloys are soldered either with silver solder or tin-lead solder. Brazing should be avoided as the melting points are a little too close for comfort and you might end up melting the lot. These alloys are typically not welded. The welding temperatures are too high and will result in the zinc fuming. This is dangerous and is a one-way ticket to the hospital!
Most of the alloying elements for the copper alloys I obtain from commonly discarded items. I gave up going to scrap yards to get materials, in recent times they seem to be hesitant to sell to the public and when they do, they sell at prices akin to buying new materials. Having said that, you’ll be surprised by how much material you can collect just by watching what you throw away and stripping the metals out that can be reused. By keeping an eye open and knowing where these metals are commonly used I seldom need to buy alloying elements. Any scrap motors, cable or plumbing pipe is a good source of copper. Brass is a good source of copper and zinc. Tin is the only element I source commercially from a metal supply company, but for smaller amounts, lead free solder can be used at a cost! Lead can be scavenged from batteries or fishing sinkers. I also keep all the non-ferrous chips from the lathe for reuse. This is true recycling!
The two most common stainless steels are SS304L and SS316L. I prefer SS316L. It is slightly easier to machine, and safer in small boiler applications. Machining stainless steels require lots of cutting fluid and generally, you’d machine at half the cutting speed typically used for mild steel. Stainless steel rubbing on stainless steel should be avoided at all costs; it tends to cold weld. This can be avoided if dissimilar materials are used. Even stainless nuts and bolts will seize if heated or over tightened. This cold welding occurs when the extremely thin oxide layer, which gives it its corrosion resistance, rubs off resulting in metal to metal bonding.
Stainless steels can be readily TIG welded, but care needs to be taken to prevent sensitization of the welded area. This can result in corrosion or sensitivity to chlorides which would ultimately cause cracking. If the correct filler rod and correct welding amperage (to minimize overheating) are adopted you shouldn’t have any issues. A quick check for sensitization is to see if the weld has become magnetic. If it has, it’s probably been sensitized.
I once found a couple of discarded coffee bodums (the plunger type). The centre shafts made a few steam valve spindles (being of a good quality free machining stainless steel) and the strainers made a nice filter for a tender I was busy with. If you keep an eye open you’ll be surprised by how many materials are just thrown away.
Figure 6: Stainless steel steam valve spindle.
Differentiating between materials
Often you come across materials where you’re not entirely sure of the grade. I recently machined a piece of mild steel hollow bar that was anything but mild. If you do a search for differentiating steels you would come up with the “spark” test. Basically using a bench grinder you can determine the type of material by the characteristics of the sparks. I have personally never used this method but I’ve added the chart for reference. My methods are a little more pragmatic.
Figure 7: Spark test chart (internet ref.)
The first test is the general appearance of the steel. Stainless looks different to the carbon steels and the cast irons. I always have a known marked sample for comparison. A magnet will differentiate between the carbon steels and the stainless steels. A simple corrosion test like leaving the steel partially submerged in water for a couple of days will tell you if it’s stainless or carbon steel; if you’re in a rush a solution of copper sulphate will also work.
To further separate the carbon steels you can heat a test piece and quench it to see if it hardens. Then my final check is how it machines. The free machining steels make small chips that break off; they also don’t come off blue at average speeds and feeds. Generally speaking, if you see long curling chips turning blue it’s probably a high carbon steel.
The copper alloys are a little more difficult. With my bearing alloys, I start by comparing the color of the surface. The redder color typically tells me there are less zinc and lead. The next step would be to see how it machines, and the type of chips. Curly chips that sometimes discolor would indicate the harder bearing alloys, with smaller chips indicating small amounts of zinc or lead and very fine chips that shoot everywhere the brasses (lots of zinc).
Figure 8: Tin bronze left, commercial gunmetal right
So in general, if I need a stainless steel I check for corrosion resistance. If I need a tool steel I see if it hardens. Tests fit for application!
This concludes the series on workshop materials. I hope you found it useful and a good starting point when deciding what material to use. The materials discussed were rather limited but I did try keeping it generic. If anyone wants more information or further elaboration on specific materials you can drop a comment and, if I can, I’ll answer. If I can’t, I’m sure Prof Meticulous or Mr Wiki Engineer will give us a hand…
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