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.

First lathe casting pattern - the carriage

So today I started making the first casting pattern for my lathe. I'm starting with carriage, because it makes up the base of the tool-holding system, which I will need to use when I get to the headstock.

There are two ways to go about this (assuming you want to make the pattern from wood): cut it out of a single block, or glue it up from separate parts. I chose the former, because it seemed easier to me after taking stock of my selection of stock. You can do it the other way, it would be the easier of the two options, providing you had stock of the right dimensions handy.

Anyway, here are some photos of it, and here is a link to a pdf with the cutting sequence I used. I hope it makes sense, if not, let me know.

The top of the carriage

And the bottom

 

Obviously the pattern isn't completely done, I need to finish plugging little holes in the surface, give it a good sand, and paint it with something glossy. But it has all its tapers and rounded edges.

I cannot stress this enough

Support the steel you're using for your ways!

I realised this afternoon that I had made the same mistake again. Frustrating! This time it was on the narrower piece of steel that will become the ways on my cross slide and topslide. I had stored it unsupported, and it developed a 0.8mm bow. This would be acceptable in the slideways, as it would be a much smaller deviation across their length, but I want to use it as a scraping reference for the front bedways. A 0.8mm bow is certainly not acceptable in the front ways. I've clamped it to my workbench under the bedways to straighten out, but still, bloody annoying.

Getting into bed

So, I can't remember if I've said this or not yet, but I scrapped the steel I-beam as a bed, it was too bowed, and wasn't going to deal with the torsional strain. I've changed to a metre-long piece of 75mm box-steel. This will deal much better with the torsional forces, and also I can fill it with concrete to add to the lathe's mass, always a good thing.

The problem with it is that it's not very flat. The biggest milling machine I could get my hands on had a maximum pass of 700mm, not long enough. So I have to do it by hand. I filed it back to get the longitudinal bow so common in box-section out, but it's still way off flat. I started scraping as per Gingery's instructions, but soon realised that it would take years to do it that way because of the amount of material that needed removing.

Then a thought occurred to me: why not use an angle-grinder? Just for roughing-in of course. I had a lot of material to remove, and wanted to get it close to flat, as close as possible. So, here's my method of roughing in. It's not finished yet, but I've done enough to see that the method has legs.

Lay your surface plate out (mine is the cold rolled steel that will one day be my ways). spread a thick layer of oil paint onto it, keeping it as even as possible.

Clean your steel and remove any burrs. Place the steel upside-down, very carefully, on the painted surface plate.

Do not apply pressure! It's cheating. Instead, gently push the steel forward a few centimetres, then pull is back again. Remove the steel and place it on your workbench. (Your workbench should be a good distance away from the surface plate to prevent grit landing on it.

Get out your angle grinder and make sure it has a grinding wheel in it. Now gently grind the spots with paint on them.

Rinse and repeat, taking a break every three or four passes.

A few important warnings:

Make sure your surface plate is well supported - mine wasn't, so I spent a good amount of time diligently ruining my own hard work. I fixed the problem, but am still trying fix the results of the problem.

Make sure you take those breaks - they're not for you, they're for the steel. You don't want the steel heating up too much, otherwise it will deform. Then you will be wasting effort on a bent piece of steel that will cool back to a different shape.

Make sure you clean the steel thoroughly before you put it on the painted surface plate - if there is any grit or burrs, it will render your markings meaningless.

Repeat this process until you have a pretty even spread of dots on the steel, reasonably close together. Once you have that, you can start scraping and it should be a lot faster.

My theoretical pet metal

People who work with metal tend to have a favourite; under normal circumstances, I would say my favourites are steel and copper. When talking about casting, however, my (theoretical) pet is aluminium bronze. I say 'theoretical' because I haven't yet cast with it. It is, therefore, my favourite only by virtue of what I have seen and read about it.

Aluminium bronze is an alloy of copper (~90%) and aluminium (~10%). It takes on a vaguely brassy look, and I believe that Australian gold coins are made from it. Here are the reasons I like the idea of aluminium bronze so much:

great material properties - it conducts heat well, is highly resistant to corrosion, and is very strong and hard. It's used to make bearings in aeroplane landing gear, boat propellers, engine parts, and many other things.

Great aesthetic properties - it looks a bit like brass, but not as 'bright'. It's also antimicrobial , apparently, as a result of the copper content.

And finally, great castability - aluminium bronze is a 'short freeze' alloy, meaning that it turns from liquid into solid almost instantly. The solidification happens from the outside face and progresses inwards. This means, according to this paper, that, assuming you have enough risers and feeders etc., you can obtain almost maximum density when casting aluminium bronze. Virtually pore free!

These reasons have all combined together to make me want to cast my lathe parts out of aluminium bronze instead of plain old aluminium. I may cast them in aluminium first, to make sure everything goes all right, and then re-cast them in aluminium bronze. But we'll see...

Lathe bench

I'm thinking ahead now. I'm going to need somewhere to put my lathe when it's done, and given that it's going to be bigger and heavier than Gingery's lathe, moving it around will not be an option. I've drawn this bench up, it's made of 25mm box section painted RHS, welded together. The small box shaped cavity in the top left corner will house the motor at the back, and have a little cupboard or drawer or something at the front. It's a metre long, so I mah have to shorten my lathe a bit, I'm planning on a 1m bed at the moment... we'll see.

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.