Cheers for the input, I'll try go through all the points made. I should have mentioned I do a lot more work with small wood working than with car parts so that will be the main functions of the benches.
GOD wrote:With the base mostly enclosed as in your design, you won't be able to sweep or hose out any crap from underneath, so the castors are going to get jammed as soon as you try to move them.
The bottom of the bench will be fully enclosed. By nature of the design nothing (except maybe fine Jarrah dust) will get under there. They will have rubber tread the full edges of the base to add to the sealing-non-slip qualities to a the bench. The fact the castors fully retract I think protects them more than average castors.
GOD wrote:The lower shelf that the jack pushes up against will probably have to be more substantial than the drawing indicates.
The shelf is planned to be 5mm thick solid plate that will be fully welded to all four sides of the bench. Totalling 16 welds all up, about 40-50mm long each. I had thought I was over-engineering it. :\
GOD wrote:
Enclosed sides make it difficult to get at whatever you have stored under the bench and make better spider homes than open sides.
I'll be putting doors on the front of these so they seal completely. Because of my wood-working, open shelving in a small shed space ensures more cleaning than making things. Or, suffer really filthy tooling all the time. So, walled and doored benches will be a must.
GOD wrote:
I'd reconsider the dimensions - 600mm is bare minimum depth. My main workbench is 850 deep, which gives plenty of space for the job and to scatter tools and parts around it while you work. And unless you're a real little fella, they need to be higher. My benches are 1100mm high, which is perfect working height but makes the top of the big vice a smidge too high.
My current bench width is 550 so I see this as a size upgrade really and it has done so far. Though since posting I have modified the design to have an additional 100mm projecting from the front and rear of each bench so as to allow vices to grip the edges of the benches if needed. As well as give more space with the benches now totaling 800 deep.
As for bench height - I wouldn't say short, but I'm not tall either. 5'11"
... But my current benches are 870 High and I always thought they were ideal. I use my vice a lot so I spend a lot of time trying to work on/over that, additionally I have various cutting/gringing/drilling tools that will be mounted on these new benches so they can't be too high.
GOD wrote:
A box frame like that will be rock solid even with only 3mm angle. Back walls don't need to be framed with that much material.
www.autospeed.com has some good articles on setting up a shed, including workbenches.
Dane.
I want the box-frame back walls to be over-engineered for a few reasons, I have an obsession for over-engineering, I want them to be free-standing so if I need to remove them and use the bench in question as just a regular all-flat bench I can. (such as cutting or working with long lengths of wood) Or if I mount them on the walls at a later date, Also, I love the way angle iron looks so I thought I would keep the theme throughout the build.
I haven't had a chance to look at the site yet but I will peruse it over the next few days and hopefully pull my ideas from that.
Cheers for the in-depth response. I want these benches to last well beyond my years in use.
) 
Not a bad idea, but not planned right now... plus I'd still want a mechanical manual solution in case I had no available electricity. 
