Let's Make Robots!

Workshop basics - the mechanical side of robots - post 1

Given the fact that I'm stuck in the middle of 3 months of 75 hour weeks, and Mrs Steve is *still* grumpy becuase there are no tiles on the kitchen wall, it looks like I'm not actually going to be able to build robots for a while, so I thought I would write about some area's instead.

I have found LMR's blog roll one of the most useful places to learn about robots and robot building, and so I thought I would add something myself.

 

The bit I like about robots is the mechanical construction - building frames and components. The electronics and the coding definately come second. I *think* i'm a bit unusually in this - I get the feeling most of you guys like the electronics and coding the most.

Seeing as this is where most of my background is, I thought I would focus on the actual workshop skills for robot building. You don't have to listen or regard anything I say here - some of it might be useful to someone.

I'm a bit spoiled - I have a nicely fitted out workshop - but I guess many people don't - so I'll focus on basic stuff first and then scale it up a bit in terms of types of tools and techniques used. 

Why am I bothering to do any of this? A couple of reasons:

The first is that a well built robot is a lot easier to live with. Tight tolerances and sharp angles make it much more likely that your bot will do what the code tells it to do ( which still might be very different to what you WANT it to do. )

The second is that - at least in the UK - a lot of this type of stuff isn't taught in schools any more, and it's easy get into bad habits, or just not know things, if you teach yourself everything.

 

Setting up a basic Workshop - what to buy and when:

In the world we live in, you can build entire robots without lifting more than a scredriver - you can either buy off the shelf or send away a design to a CNC cutter and get back a package of components. I like making things, rather than buying them, and I also have a wife who has a fiendish shoe habit, so I don't often go down either of those roads. 

If your just setting up there is a world of really cool looking stuff that you can get - but you don't need all of it, and you certainly don't need all of it RIGHT NOW.  So here are some of the things to get early on.

A workbench.

Workbenches always get full of junk, so as large an area as you can get is always going to be too small. If space is unlimited a big stand alone bench in a workshop is great to have. If money is also unlimited, get a carpenter to make you one out of Beech - so you can hit it with a sledgehammer and watch the hammer bounce off....

If you don't have space and lots of spare cash, a good workbench can be made out of a piece of kitchen worktop put on the top of a table. That way you can move the entire thing when you need to get at the table. Worktop will also withstand the odd touch with a blow torch, hammer or scalpel, unlike a thin sheet of plywood.

A hacksaw

Don't bother with the small baby hacksaws - with the bent wire frames - they flex and are uncomfortable to hold. Get a full sized hacksaw. You only need a cheap one - you can get excellent cheap saws, and terrible saws which are very expensive, but the key things to look for is that they are comfortable to hold and that the frame is really stiff. If you can put it over your knee, try to bend the frame with all your might and it won't move, then it's stiff enough. ( If you do bend it, hide it and check a different brand....)    The saw I use came from a garden centre as a tree saw and only cost a few pounds ( less than $10 )

For robot making, a hacksaw is likely to be the only saw you need. - unless your doing a lot of internal cuts, a hacksaw and file is just fine.

Hacksaw blades

With blades, you get what you pay for. The more you pay, the better they are, and the better they cut. The best by a *long* way are Eclipse blades. They last for ever and don't wander when your cutting.

Blades will be marked up in TPI - Teeth Per Inch. Ideally, you want some 18TPI blades and some 24TPI blades.

18TPI blades are your general blades - use them for wood, most plastics and most metals

24TPI blades are for Steel, bronze and hard plastics like arcylic. 

When the blades start to blunt, throw them away. They only cost  few pence each, and blunt blades make bad cuts. They only ever seem to make bad cuts when your cutting something that HAS to be cut right....

 

Tips for sawing:

00 Go slow. Go really slow.  Cut as slowly as you think is right, and then go even slower.  Each stroke should take 1 second in each direction at least. 

00 Use as much of the saw as you can - if your cutting something in a vise, use the entire length of the blade

00 Cut the widest surface you can. For example, if your cutting a piece of sheet 50mm wide by 5mm deep, cut accross the 50mm wide section. The blade wants to drift as you cut downwards so always cut accross the widest face you can the saw accross.

00 Set aside a blade just for brass. If you cut any other metal and then try to cut brass, the blade will bouce off - put a piece of tape on a blade end, or colour it in with a red pen, and set it aside for brass cutting.

00 Never cut along the lines...  Remember that a blade is perhaps 1.5mm thick. If you mark a line to cut out, and then cut actually along the line itself, everything will be 0.75mm too small. Cut everything fractically over sized - 1mm or so, and then take it back with a file or a sanding block.

00 Practise: Using a saw is a skill just like coding an arduino - the more you practise, the easier it gets and the more accurate you get.

 

Dremel and Dremel chuck.

I love my Dremel. I have an old basic one with two speeds. I've never really found a time when I needed anything else. As long as they still run, I don't think you get any major benefit a new one, or one of the fancy ones. Find a cheap one on e-Bay.

<<The only time I CAN see a benefit in variable speeds is if you need to drill a lot of brass or hardened steel - for brass because every different grade of brass has a different optimal drilling speed, and for hardened steel because you want the drill turning at perhaps 200 rpm or less - . But most of the robots I see on this site don't use either of these two metals, so a normal dremel seems OK.

Apart from the cutting discs and the drum sander, the other tools that come with a dremel, like the grind stones, polishing mops etc aren't really very much use at all. Save your money when your buying a dremel - don't get one of those "500 piece kit" things - just get the discs and sanders from eBay.

Dremel chuck.

This is the "drill chuck" attachment for the Dremel. This is probably the best money I have ever spent on a tool. A dremel comes with collets, which is no real use for drills, polishing mops etc.  Really good investment.

<<In general, the rule with collets is "If it's sharp, and it's not a drill, use a collet. For anything else use a chuck"

If your using Router bits, or milling bits you MUST use a collet - they can climb out of a chuck and fly off rather painfully and a lot of people have lost their eyes, and even their lives, to flying mill bits..

 

Drill bits

You want a selection of drills between say 2mm and 6mm. Buy cheap metal drills ( marked as HSS ). Some of the expensive drills have minor additional value - the titanium covered drills - the gold covered ones - are a little bit better for drilling very deep holes in brass and bronze, but unless your doing that, just get cheap ones.

Remember that thin drills snap a lot easier than wider drills, and go blunt quicker as well. Buy them off e-bay as bundles.

 

Tips for drilling

00  Match the speed to the material if the hole is important. A good guide is here:

http://store.curiousinventor.com/guides/drill_speed/

00  I would say though that wood doesn't especially matter, aluminium can be drilled pretty quick, and steel slowly. Most plastics can be drilled quite quick, hard brittle plastics like acrylic slowly.

00  For everything apart from Steel "peck" at the hole. Don't just keep pushing - every few seconds retreat a bit, let the drill clear itself, and then start again. Don't do this if your drilling steel.

00 Let the drill do the work - if you snap a drill, it's an utter fiend to get out of the hole. Let the drill make it's own progress. 

I00 If the hole is important - if it needs to be smooth such as to carry an axle - and you don't have the tools to ream a hole such as a lathe or a mill, use lubricant when drilling the hole. For pretty much everything full fat milk is an exceptional lubricant. Just clean it of afterwards othewrwise it smells bad.

00 Always pre-mark holes. Ideally, use a centre drill to make a small starter hole. For everything apart from really hard plastic, use a punch, a scribe or even just a hammer and nail to make a small dimple to stop the drill wandering.

00 For hard plastic, put a piece of masking tape on the plastic, and put a dimple into the masking tape. It's not perfect, but it's better than nothing.

 

 

Marking Out Equipment

As a minimum, you want the following to mark out - drawing lines and showing where to make cuts and holes:

- Propelling pencil/engineers pencil. One of those that you push the top and the lead comes out. 0.5mm as a preference. A cheap one is fine - get lots of leads.

- Masking tape - if your marking out on plastic, put some masking tape down first, and draw on this.

- Rule. Ideally, you want to use 2 - a 6" ruler and a 12" ruler. Both should be marked with mm and inches.

- If you can afford them, pay the little bit extra and get steel rules over aluminium - they don't bend when you sit on them.

- Even better, look for a combination ruler:

http://www.hiwtc.com/photo/products/26/00/58/5841.jpg

- Tri-sqaure.

- Get a very cheap large one if money is tight, and then a second one which is as small as you can find. I use my 2" tri-sqaure more than any of the others.

- Compass. Only needs to be a very cheap one - as long as you can tighten it up hard so that it's hard to move when you use it.

Tips on marking out.

00 paper and cardboard is your best friend. Unless your working from exact 1:1 sized plans, you will save yourselves a lot of money, a lot of time and a lot of annoyance by making mock-ups with card first. For my Hexapod, I made the entire thing in cardboard from cereal packets first. You can cut it out with scissors really quickly, make all your makings, see if everything fits together. That way, you KNOW when your using plastic/metal/wood, all of which take more effort, that hat your making will work.

00 Measure twice. I know this sounds really trite, but personally I am bad at measuring and I don't trust myself, so I check everything again and again

00 Rub out your pencil marks. If you mark something out, and realise you've made a mistake, rub out at least some of the incorrect line before you re-draw. It's amazing how often the wrong line gets cut down or drilled on.

00 Always work on a flat surface. 

 

Files

I find a good set of files to be hugely useful for robot building. Pretty much anything that you have seen made using CNC - especially cut plywood or cut plastic - can be made faster and just as accurately with a file and a vise. Only very complex 3d shapes don't really get covered by this.

I use Engineers files - metal files - for pretty much everything - even for working with wood.  They come in different grades - from Bastard cut - large teeth, to dead smooth - very fine indeed. I use bastard cut and Number 1 files ( the next step down in teeth size ) for pretty much everything unless I want a mirror finish.

They also come in single cut - a single set of grooves - or double cut - actual teeth. Single cut are only really useful for working with metal, where as double-cut can be used for anything.

Files are another thing where you get what you pay for - an expensive file is usually a LOT better than a cheap file - but that only appies if your doing a lot of work with steel. A basic set is just the job is your starting out.

Make sure that the files have 1 edge with teeth and 1 edge that are smooth. They also come with two edges of teeth, but this makes filing close to joint a lot more time consuming if you don't want to cut into a join.

Files come in a number of sizes. I like to use the smaller sizes for things like robots.

Look to get three to start off with. A flat ( flat on both sides ) bastard cut, a flat number 1 and a half round number 1. 

 

If your making a complex shape with curves - for example the leg of a hexapod - print the designs on paper, and glue them onto the material of your choice - or mark them on directly. Cut them out near enough with your hacksaw, and then set to with the file. When you've got it just right, go over the paper pattern with a damp cloth to soften the glue and remove them.

Filing is a skill that does need a bit of practise, but when you can become very accurate with them and work very quickly indeed.

A test that used to get given to engineering apprenticies in the UK was to take a block of mild steel 2" x 2" x 2", and just using a file, file it down to a block 1" x 1" x 1".

The tricky bit - all the measurements need to be within 1/100th of 1 inch ( 0.25mm ) and within 1/10th of 1 degree in the angles. And you've got 45 minutes to do it.

It can absolutely be done - just takes a bit of practise.

If you have a file, you usually don't need a CNC system.

Tips on using a file.

00 Go fairly slow - files work best when they're being made to work

00 Keep then clean. Every few minutes, give the file a whack on your leg to clean the teeth out. If it gets really clogged, use a toothbrush. Better still, you can get a file card for a few pounds.

00 Keep a seperate set if you plan to work with brass. NEVER try to use a fille to file any other metal if you also want to file brass, even if just need to brush the swarf of a piece brass tube. Either don't use brass at all, or have a completely seperate set of files just for brass. I keep my brass files in a different cupboard with some tape on the handles.

 

 

 

More to come soon - time to go to a meeting....

 

 

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Awesome post. Collected and I will also add this to my links in the LMR Primer.

Wait. I'm also Tweeting this one.

I know that talking about safety is as boring as those safety demonstrations in planes that nobody watches.

But I think that nice safety goggles are a must. 

 

 

Antonio,

Accuracy, and the desire to be accurate -  depends on your outlook, and also what your trying to do.

I make a lot of model steam engines. When I do those, I work to an accuracy of 1/1000th of an inch ( 0.02mm ) in all dimensions, which is just about good enough. If I was making a turbine, I need to work to at least 1/5000th of an inch (0.005mm) otherwise it'll pull itself apart.

For a robot, you don't need anything like that accuracy - as you can cover any mistakes in the code. You could work to 2mm +/-. if your willing to cover the slack. But... your always going to know that your bot is full of slack.

Also - remember that it takes as much work to cut something out wrongly as correctly. Not being accurate is a choice - it doesn't take any extra skill to build something to a plans dimensions.

Lastly, remember that errors compound themselves. If you have a hexapod with three servos per leg, and you make the three leg components built to +/- 1mm in length, and you use standard cheapish servos, you now have 6 legs with up to 6mm difference in length between them, and servos ( with around 3 degrees of backlash per servo ) with around 9 degrees of error in them - a total of 2.5%.

The result of this is that the hexapod will either walk very badly, and look very rough, or that you have to spend a LOT of time in the code dealing with the errors and finding walking gaits where you can control the servo errors. 

Would you build an electronic circut for your robot with 2.5% error in the PWM channels and +/- perhaps 2 volts in the power output? Unlikely.

Personally, I am building my bots at about +/- 0.1mm in all dimensions, which seems to do the job OK. 

I would say though that there is it only ever takes the same amount of effort to cut something exactly to length as it does to cut more than one part to the same wrong length, so you may as well work to the right length - it's just easier that way - you know it's not going to knock another measurement out further along your build.

I wouldn't try to cut a load of parts to the same size at the same time with a saw, as it almost never works - the saw blades drift. I'll post a couple of examples later of working with templates - one with steel and one with paper templates to show you how I do it. Then I'll jump back to workshop stuff.

Thank you for sharing your know-how SlowSteve it is really helpful stuff to know.

As you have already said, most of us are software and electronics guys. Even really basic advice can save us hours and hours of wasted work and frustration.

Please post some more!

Maybe CtC can also chip in with some tips...

(for example, I really liked the idea that you don't always need to cut things to the exact lenght, most of the time it is ok if you make sure that all the parts are of the same length by cutting them all at once or using one as a template for the others)