Tuesday, June 19, 2012

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.

Sunday, June 17, 2012

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.

Tuesday, June 5, 2012

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.

A message to Gingery lovers/loathers

If you've arrived at my blog by googling 'Gingery' then you probably know that there's a small war going on between people who like Gingery and people who don't. Usually, the argument from those who don't like him is that his machines are inferior, and that it would be quicker and easier to buy the machine instead. I just thought I'd put my two bob in. I find myself agreeing with both sides of the Gingery war: true, his machines will never match commercial machines in terms of accuracy or longevity. At the same time, I completely sympathise with the idea of building a machine as a learning process. I find myself, however, coming out with more respect for the pro-Gingery party - not because they are correct about everything, not by a long shot - but because I feel the anti-Gingery mob are missing the point. People who build Gingery lathes do so for different reasons than people who buy lathes. Some of the reasons include: they can't afford to spend much money on a lathe, they relish the challenge, they need something to fill their time, they want to learn to cast. The list goes on. Some of the arguments against building these lathes include: it will never be as accurate as a commercial lathe, aluminium is a poor material to make a machine tool from, it will take much longer to end up with a lathe, etc.

To these arguments I would respond thus:

If you are careful, you can get decent accuracy, but at the same time, how accurate do you actually NEED your lathe to be? If you're building it yourself, you're probably a hobbyist, someone who can't justify buying a lathe, but wants one anyway. For this sort of person, accuracy to a tenth of a millimeter might be just fine. You have to take into consideration what you actually want your machine to be able to DO. If, like me, you tend to work by eye, then absolute accuracy is probably not important. If, also like me, you tend to make things on a lathe that aren't part of fiddly little engines, or of critical, life-saving devices, then likewise, how much accuracy do you actually need? Personally, I would be happy with 10th of a millimeter accuracy. Better is better, but I'd survive with that.

As for aluminium, yes, it's not a very good material for machine tools. So what? The builder can choose a different material, like ZA-12, or iron. These things are possible in the backyard. Another option open to the  maker is to cast only the parts he can't fabricate, and fabricate the rest out of steel. Personally, I think Aluminium is probably fine for small work, and the fact that with a furnace and the pattern handy I can whip up a new cross-slide pretty easily means that breaking parts is a hassle, but not much more. The other option open to the maker is to start with aluminium, and upgrade over time. I for one plan to complete it in aluminium, then slowly build up to iron casting and replace the pieces bit by bit.

And yes, it certainly will take longer. But if you're not a professional machinist, who cares? For hobbyists, it's all part of the journey. Some people are time-rich but money poor, this is perfect for them. Others aren't in any particular hurry, and would rather use the money for rent/food/grog.

My response to the pro-Gingery mob who are up in arms about these issues would be to think about what you need and modify Gingery's plans to suit. I feel certain he wasn't being prescriptive when he wrote his guides; feel free to change them! I am, my bed is made from steel box-section and will be filled with concrete, it's also a metre long! Do your own thing, using Gingery as inspiration and for basic methodology.

So there's my two-bob.