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Drill Hacking -101

Welcome to the RoboWars Australia guide on the easiest way to make a simple, cheap and reliable drive system for your feather or lightweight robot from the cheapest of Battery Powered Drills. Using this guide, parts that can nearly all be obtained from your local Bunning’s Warehouse and a bit of work, and you can have a lightweight very powerful drive system for around $25 per driven wheel.

There are many other ways to setup your drive system, but we don’t know of any that are as simple, easy to get going, and powerful for your dollar than this technique, so in the interest of helping people get their first robot rolling, we are presenting this step-by-step guide and collection of tips to show you what we have learned after building many such drive systems..

To start with, you are going to need at least 2 of the XU1 Battery Drills (or similar, most cheap battery drills are nearly identical internally). They usually sell for under $20 each, You might even want to get 3 or more to either have spares, or even keep one intact to use as a drill (how novel !)

Heres what they look like
(you can click on any of the thumbnails to get the high-res full picture for more detail)
(right click and "open in new window" to have them and the text at the same time)

image 1 - Bunning’s XU 1 drill in box
image 2 - Bunning’s XU 1 drill out of box

BEFORE disassembling the drill (and voiding your warranty), Fully charge the battery and run the drill in either direction until its completely flat (you can put a cable tie or tape around the trigger to hold it going). This will “Run-In” the gearbox (and give it a reliability test). When new, the gearboxes are quite stiff and doing this will loosen it up for maximum performance. Also, if it doesn’t make it all the way through this test, you can take it back to exchange for another one without having pulled it apart. Once you have done this, then we can move onto taking it to pieces.

First, remove the battery pack


The standard battery packs are made with very cheap NiCad Cells (what do you expect when the whole drill is less than $20 ?) which are not capable of supplying much current, and so aren’t much use for powering a serious 12kg Robot Drill-Drive system,. They also drop quite a lot of volts under heavy load which will reduce the Robots pushing power significantly, and they will be slow to charge 3-5 hours), which will be annoying between battle rounds.

Then remove the Chuck locking screw.


Removing the chuck and locking screw before you disassemble the drill will be easier, since you have power from the motor to push against the unscrewing motion. Open the chuck-jaws up all the way and use a Phillips-head screwdriver that fits the screw well, or you might damage the screw-head. Put the gearbox in reverse, hold the trigger and (hopefully) wind the screw out. They usually come out easily, but sometimes they are quite tight and will take some effort to remove..

Don’t forget the screw is a left-hand thread and undoes in the opposite direction to what you would normally expect ! Don’t lose this screw as it is very hard to find replacements and you will need it later to hold your wheels on

Now you should be able to wind the chuck off leaving just a threaded output shaft. Once this is done, you can get on with disassembling the drill case.
Disassemble the drill
Disassembling the drill is easy, just a few screws hold the two halves of the case together.

Once disassembled, you will see the Battery Clip (1), The trigger mechanism and reverse switch (2), the motor (3) and gearbox (4) and the Mosfet transistor (Speed Control) (5) mounted on a heatsink.
Don't bother trying to use the standard drill speed controller in your robot, it just wont cut it. The Mosfet can barely cope with the power needed for the drill, and fitting any more powerful batteries will immediately result in it smoking, plus, the reverse is a mechanical switch, and you would have to rig up something to try and mechanically move the trigger and the reverse switch. Trust us, plenty have tried to use them, no one has succeeded. Just bin it.

Battery Pack with Charger and Battery Clip

If you want to use the standard batteries for now then keep the battery clip that engages with the top of the pack so you have an easy way of connecting to the sliding contacts on the battery packs.

They are OK for testing your Robot to start with, and have the advantage that they come with a basic Battery charger which means you don’t have to buy one up front, but don’t plan on using them for serious combat, they just wont go the distance.. Sooner or later, you will need to replace them with either Good Quality NiCad’s, or Sealed-Lead-Acid batteries and a quality fast charger.

Remove the clutch shell mechanism with the two screws on the end, There are a couple of different spring styles under the clutch shell, but don’t worry if yours looks different, we wont be using it anyway..

Turn the motor upside down and let all the ball-bearings fall out of the holes (hopefully into something rather than all over the floor).. There are usually two ball bearings in each hole.. make sure they are all out, and don’t worry about keeping them, we wont be using them either..
where the ball bearings came from, we are going to screw in grub-screws (or set-screws) to lock the clutch up (since we don’t want any clutch slippage unlike the drill application) The set-screws are M5 (Metric 5mm) thread, and are 8mm long, with an Allen-key drive, you can get them from any bolt shop. for about 16 cents each,..

Get the “cup-point” type (Rather than pointy tipped ones). You will need at least 4 per drill, to put one if every 2nd hole the ball bearings came from, but if you want, you can go the whole hog and put them in each hole (8 altogether).. so for a two wheel drive setup, you will need at least 8 (or 16 to do all the holes) set-screws.
If you want to make the job of inserting the grub screws easier, then get a M5 thread tap (preferably an intermediate type - or if you can only get a starting tap, then grind a few mm off the pointy tip using an angle grinder) and a tap-handle (or use a pair of Vice-grips to hold the tap), they're only a few $, and will make it much easier to insert the grubs screws after carefully tapping threads in the plastic.

Holding the gearbox with the holes facing down and putting a light coating of grease or similair on the tap first will prevent any swarf from the tapping process falling into the gearbox
The idea here is to just cut a few threads into the start of the hole to make the initial alignment of the grub screw easier. Don’t wind the tap in all the way to the bottom or allow the the tap out to press hard against the ring gear. if you don’t want to buy a tap, then you can force them in with the Allen-key without tapping them first, its just harder to get them started and keep them straight.
As a minimum, put a set-screw in very second hole (or all of them if you like), and carefully screw them down until you feel them *just* touch the ring gear inside, then back them off just a bit. you don’t want to put any downward pressure on the ring gear, or it will try and push the motor off the other end of the gearbox as well as loading up the gears. you should be able to turn the output shaft back and forth freely and feel the bumps on the top click against the grub screws, with a few degrees of backlash.

Do NOT disassemble the gearbox ! Here are some pictures of what it looks like inside for you so you aren’t tempted.. (its a 36:1 double planetary gearbox). There are quite a few things you can easily get wrong when reassembling it, that will cause it to run for a little while then break if you don’t get the gears lined up perfectly. We have had far less trouble with the ones we have left together, than the ones we have disassembled and reassembled no matter how careful we were (and we've done dozens of them).

You can see how the set-screws come down through the plastic housing and engage with the bumps (called castellations) on the top of the ring gear (See arrows)

A bit of black electrical tape over the vent ports in the motor will help stop the motor from getting debris into the brush-area (the magnets will tend to suck them in), and jamming things up, and for the short duration we are using them for in 12kg robots, they don’t really need the ventilation. we don’t tend to drive the robots at high speed for long anyway, its more short-run start/stop action ,which means the inbuilt fan doesn’t do much cooling anyway.

If you are going to use the standard wires attached to the drill, make sure the soldering is good, and try not to flex them back and forth too much since that will fatigue them and cause them to break off (broken wires are the most common type of failure in a combat robot).

This is a good time to add noise-suppression capacitors to the back of the motor, which will greatly reduce the interference your motors will cause to your radio and help improve your radio reception.. for detailed information on suppression - including using ferrite beads, twisting the leads and capacitors, refer to this team delta application note

For ease of replacement in battle damage, we recommend you use the two pin spade/block connector (a Narva part) available for just a dollar or two (cheaper if you buy the 10 pack) available from any Repco or Burson’s auto parts store, which will make changing motors much easier.

Once you have added the capacitors and connectors, the motor is ready for robotic use. Next we show you got to fit the drive wheels
*Fitting the Wheels*
By far, the easiest way of getting wheels onto your drill motors is to use a wheel we discovered fits the output shaft of the drills very nicely, offers pretty good traction and is very cheap ($4 each) from any Bunning’s Warehouse. Known amongst robot builders as "The Red Wheels" (because they are the only red plastic wheel from Bunning’s)
they are 95mm in diameter, 32mm wide, and have a black plastic insert

that screws nicely onto the threaded output shaft from the drills.

Grab a few spare wheels while you're there, they’re so cheap, and they're handy to have as spares in the event of damage, plus the tread tends to wear down after a bit of driving especially if you practice on bitumen

Its tempting to just screw the wheels directly onto the drill, since the black plastic hub insert will fit nicely onto the drill’s output shaft thread, and while this will work for a while, we have found the hard way, that the black insert is made of a fairly soft plastic and will quickly strip under the sudden reversing loads, so you have to do a bit of fiddling about (of course) to make them hold on reliably. If you go to the trouble of doing this, you will have no trouble with the wheels.. we have done many this way and had perfect reliability so far. Heres how you do it…

First you will need two packets of large diameter (about 30mm) “Body” washers. (available from Bunning’s, Super-Cheap-Auto, or wherever) - one packet with a 1/4" hole in the center and the other with a 5/16" hole in the center.
The shaft on the drill is actually 3/8" diameter, but if you buy the 3/8" washers, the center hole will be a little bit larger than 3/8" and they wont sit nicely on the shoulder of the shaft - it will rock about loosely.

The idea here is for the inner washer to press up against the red plastic part of the wheel, which is a harder plastic than the soft black bit, which will prevent it from stripping and help spread the load a bit.

So buy a packet of 5/16" hole washers and carefully (don’t hold the washer by hand) drill the center hole out to an exact 3/8" ( or 9.5mm) diameter
which will make it a nice snug fit on the shoulder of the shaft .

Now, whether you need to do the next step will depend on whether you got the XU1 Drill (which has a slightly longer locking screw) or another brand (many of which are a bit shorter) and the thickness of your washers.

If you have the shorter locking screws (remember that screw you carefully kept earlier ?) and thick washers, then you will need to make the wheel a bit thinner so the locking screw will reach all the way through to the center shaft. Because the left-hand-thread screws are hard to find, its actually easier to thin the wheel, rather than find longer locking screws in most cases. If your screws are long enough to get a few threads engaged with the washers then you can skip this part.

To do this the wheel, you need to carefully grind the protruding red boss
(white circle) on on the *back* of the wheel down so it is level with the ribs on the back of the wheel. its only abut 4 to 5 mm thickness, but it makes all the difference in terms of having a good engagement for the screw. Try to get it as flat as possible, without grinding up rest of the wheel..
You can do this with a Dremel-style tool with a cutting disc, a Lathe (if you are lucky enough to have access to one), or a 22mm wide-spade wood drill bit.

Then pop the center black drive hub out of the wheel and grind a small amount (just 1mm or so) off so it is just a bit shorter than the outside red part it fits into. That way you can be certain that the red part of the wheel is bearing all the load (from being squished between the washers) rather than the soft black hub.

Once you have done this, you can either tap a thread into the black hub with a 3/8 UNF (to match the drill shaft) tap in a bench drill
so you are sure you get it nice and straight, or if you are careful (like fitting the grub screws) you can force-thread them on by hand directly onto the drill shaft.

When fitting the black hub back onto the drill output shaft, put some super-glue or Loctite on the thread and then carefully hold the wheel at a perfect 90 degree angle as you thread it on, or it will wobble as it spins afterwards..
Wind it all the way on until you feel it bottom out against the washer,

Don’t do it up too hard, or you will strip the threads in the black hub. remember, the main gripping force is going to come from the crush between the two washers, not the black center hub threads.
Pop the wheel back on over the black hub, and press it down hard against the inner washer
Now, the smaller 1/4" hole washer goes on the outside of the wheel (a bit of Loctite or superglue between the washer and the wheel could help here if you want to be extra sure). Put some Loctite on the end of the locking screw as well (important), and screw it in tightly which will squeeze the red part of the wheel between the two washers.


- Let the Loctite dry , and your motors are ready to be mounted !

Mounting the motors

Ok, we're nearly there ! Last of all, you need to attach these motors to your robot properly, so that it moves and the motors don’t ! There are many different ways of doing this, but the simplest, lightest and proven reliable way we have found is to use good quality stainless steel hose-clamps wrapped around a bracket at both ends of the motor. One around the gearbox and one around the motor itself.

The hose clamps are approximately 38 mm diameter for the gearbox and 50mm for the motor. Don’t get the cheap aluminium or low quality steel ones or the threads will strip easily. the “Tridon” brand ones are good.
If you look at the end of the gearbox, you will see that one side of the "D" shape has a flat face and one side has s a small bump on it (white circles) .
you want the flat side to rest flush against your bracket, with the bump side facing up (Under the hose clamp) so it sits nice and level against the bracket. This flat will help stop the motor trying to spin itself around as well..

Grind two slots in whatever you want to mount the motors to that are wide
image imageimage
enough to thread the hose clamps though easily and Pop (battle – they may take some bending) the hose clamps through the slots and around both ends and do them up moderately tightly. To keep both motors rigid and parallel to each other, two pieces of angle aluminium under the motor hose clamps works very well.
You can use a longer piece if you want them mounted further apart than this photo shows…

And there you have it ! a ready to go, fast, powerful and reliable drive system for around $50 !

image image image image
Now *that* should get your bot project underway and moving like lightning !

Basic tools necessary to build this drive system
2 x Bunning’s XU-1 Battery Drills
2 x Bunning’s Red Wheels
1 x Philips head screwdriver
1 x Allen key to suit your grub screws
16 x grub/set screws with cup-points
1 x 3/8’ or 9.5mm drill
2 x 5/16 large diameter washers
2 x 1/4 large diameter washers
6 x .1uF Disc Ceramic 50v Suppression Capacitors
1 x Loctite or Super Glue
4 x Stainless Steel Hose Clamps

Optional Components, but nice to have
Angle grinder
m5 tap
3/8 UNF tap
2 pin block connectors
22mm Wood Spade Drill
Safety Gear - gloves goggles, ear muffs, safety gear

This guide was bought to you by Team Rotwang, TechnoMagic and the number 42.

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