Concrete and dowel

This evening I filled my lathe bed with concrete. I said I was going to buy some 10mm dowels and grease them up before putting them into the bolt holes, but that wasn't possible. Because Magnet Mart are so good at stocking usefully sized lumber, I had a choice between 9.5mm and 12.5mm dowel...

Obviously, I chose 9.5mm. The process I used was only marginally more complex than the grease option. I wrapped each dowel (which was cut to just a little longer than the height of the bed) with a small piece of standard printing paper, fastening it with a piece of tape. Then I wrapped each piece with thin plastic packing tape, hopefully preventing the conrete from adhering to it.

The goal of this approach was two-fold: to make up the missing 0.5mm, and to facilitate the removal of the dowels with minimum fuss, ideally without leaving anything but air and concrete behind.

The concrete is curing now, so I'll let you know how it went in a while.

Furnacey fun

I made the rest of my casting patterns the other day, and after painting them with glossy paint, set to finishing the furnace.

I lined it with the clay mix from before after grinding it down to a very fine powder. Because I knew I was short on volume I added all the sawdust I could find, most of a bag of perlite, some more sand, and the last of my bag of bentonite. I also filled the bottom of the body with dirt to fill up some of the extra space.

When lining furnaces it's important that your clay not be too wet. That was my problem the first time, and the loss of moisture caused a lot of shrinkage, and thus, cracks. This time I made the clay much drier. I spread the dry clay out as thinly as I could on a plastic ground sheet and sprayed a very fine mist of water over it, then mixed thoroughly with a rake. I repeated this until the clay was uniformly damp. It looked a bit like breadcrumbs and clumped together when squeezed, but did not feel wet. After that, I covered it for a while so it could percolate a bit, and to give me time to get the body ready to be lined.

The inner form was made by wrapping the metal from an old canola oil tin around a pair of wooden discs. Before doing this I rolled it up as tightly as possible and tied it like that for a few days. This meant that the metal was springy and would hold the discs, but also that it would pop in on itself when the discs were removed. I packed about an inch of clay down straight on top of the dirt, making sure there was a former for a drain hole. That done, I set the inner form on the clay bottom and began to ram clay in around it. When I got to the level of the burner I put the pipe in and continued to ram. You should build the lining up in very small layers, working around the furnace as you go.

To line the lid I just placed it on a piece of plastic sheet on the ground, inserted a piece of PVC in the middle as a former, and packed the clay in. I rand wires all through the lid to add stability. After using the concrete here it occurred to me that I should have used concrete in the bottom instead of dirt too...

Unfortunately, I was STILL short of clay, so the lid was only 2 thirds full, and the body lining stopped short about an inch from the top of the steel wall. To compensate for this I ground the steel on the body back to the level of the clay, I also filled the top of the lid in with quick set concrete. This is not a hotface, so concrete was fine. It was just to add stability.

I also began work on a lifting mechanism for the lid, but haven't finished it yet.

I cast my first four lathe parts the other day, and they came out reasonably well. Some had little pockets caused by loose sand, but the pockets are in non-critical places. It took all day, but I got the carriage, cross slide, compound base and compound slide castings done, and turned all my scrap into ingots. I'll upload some photos soon, but at the moment I'm busy machining the castings on a friend's milling machine.

Disasters and discoveries in the alloy aisle, plus a new furnace

So, given my somewhat less-than-satisfactory results with the aluminium bronze, I started looking into other options. I looked at buying pre-alloyed aluminium bronze, but shipping will bite me. My next thought was Za-12, a zinc/aluminium alloy with excellent castability and hardness (google it).

While at a scrap yard looking for the piece of steel that makes up the front leg of my new furnace (read on for more), I discovered that the man who owns the scrapyard also sells ingots of zinc, 5kg each. AU50c/kg! At that rate, I'll be going with za-12, I hear it's easy to make at home given the low melting point of zinc.

Now, on to the furnace...

While 'making' the aluminium bronze, I overheated my furnace, driving the Portland cement reaction backwards, and making my refractory more like 'refractury'. A short while later, I found an abandoned water heater, one of the little ones. I also found a stock trolley in a skip. After putting my thinking cap on, I decided to build a completely new furnace, rather than relining the existing one.

I cut the water heater down, cut a ring out of it for a lid, and welded it onto the trolley, about 20cm above ground level. Then I welded a piece of 40mm box section steel -- the one that the nice guy at the scrapyard gave me -- onto the front, to provide support for the weight of the furnace.

The recipe for the refractory that I picked cannot be found here, because something has gone wrong with my link. Instead, I will just list the ingredients.

40% bentonite
25% 200-mesh silica sand
20% fire clay
10% alumina
5% borax


It has no cement in it, so is essentially a ceramic reaction, which I won't be able to drive backwards, no matter how hard I try. Hopefully.

I haven't done the lid yet, because I ran of of clay... But I won't be able to cast again for a while anyway, so that's fine.

[EDIT] I made the clay too wet, so it cracked and buckled all over the place. As a result, I had to let it dry and powder it right back down. Given how little extra volume I actually need for the lid, I've decided to add stuff to the mix willy-nilly to make up the volume. I have some perlite, which will go in, and I'll put in some sawdust as well, which will burn out, leaving a stronger, honeycomb-like structure. I might add some more clay and sand too, depending on how I'm feeling. More news later.

A new project: ambitious, but possible

OK, I have a new major project. I'm going to build myself a Gingery-inspired lathe. If you don't know who Gingery is, google him. In short, he was a man who needed a machine shop, but lacked the funds for one. He had plenty of time though, so he decided to build one. He made many machines out of cast aluminium, which he melted in a furnace in his back yard. I admire his ingenuity, but feel that his lathe design is somewhat lacking in some areas. Those areas are:

size - his lathe is very small

fittings - his lathe has no standard tooling fittings

engineering - there are numerous engineering points that I disagree with, I'll cover these as they appear though

So, the first part that needs doing is the bed and ways. (Engineering flaw #1) Gingery cast his bed out of aluminium, making a 60cm hollow ribbed block for rigidity. Being aluminium, though, it is not particularly rigid. I imagine this lightweight casting introducing many inaccuracies, mostly through flexing and sagging. The other major downside to the bed is that it's a very big casting, especially for novices, at whom the series is mostly targeted.

To counter both of these problems, I have decided to skip casting the bed, and opt instead for a steel I-beam. This should give me the extra weight and rigidity that I yearn for, while also allowing me to scale the whole lathe up a bit (this addresses flaw #1, size). I plan on scaling it up to a 75mm swing, with around 60cm between centres.

Here is my basic idea for the bed: The black I-shaped section is, strangely enough, the I-beam. The thin grey piece is an aluminium match plate that I will cast up, and the red is a piece of bright flat bar.

The purpose of the match plate is simple. I will first scrape one side of it to fit the I-beam perfectly, then I will scrape the other side to match the bright bar perfectly. This will mean that the inaccuracies on the surface of the I-beam won't matter, as they will be averaged out by the aluminium plate. Then, given the very close tolerance of the bright flat bar, the matched aluminium surface will be almost perfectly flat. This will mean that I can bolt the bright bar onto it and not have to worry about it deforming.

Now, the only problem I can see with using I-beam is that it is not designed to withstand torsional force, which is exactly what I will be putting on it. I have two thoughts on this:

1) it may be so grossly over sized that it doesn't matter

2) I could weld straps into it, between the two parallel surfaces, and perpendicular to them. This would change the side view from being a long, wide channel, to a series of boxes. I feel that this should strengthen it sufficiently against the torsional forces.

So, those are my current plans. I'll update you as I build. But for now, google Gingery and see what he's all about.