Turning up a set of wheels

The aim of this tutorial is to show how Robotbuilders can use the centre lathe and drilling machine to make wheels for robots. There will be more general hints and tips liberally scattered through the tutorial that are aimed at the novice machinist to help them avoid common errors and work safely.

Workshop safety is no accident.

Please take care in the workshop. Read all you can about workshop safety and understand the dangers. Think about what could go wrong and then take steps to either stop it happening or to minimise the consequences of it going wrong. This tutorial will highlight some particular hazards, but is not guaranteed to be a comprehensive treatment of workshop safety. Ultimately, you are responsible for safety in your workshop; all the practices discussed in this tutorial are used in my workshop, but may not suit your situation. Before attempting anything described in this tutorial it is down to you to make sure it is safe to proceed.

Materials

The hub is turned from free-cutting mild steel (EN1AM in the old system of identifying steel). This is a leaded steel. A small amount of lead is included in the alloy. The lead serves as a lubricant making it easier to machine and achieve a superb surface finish. Leaded steels are good for lightly loaded components, but should never be used for stressed components nor any part which will be welded at a later stage of manufacture. The actual piece of steel I used was a roller from an old photocopier.

The disks were cut from a scrap of 4 mm thick polycarbonate sheet. Perspex could have been used in its place. Other brands of acrylic sheet may have been suitable, but I would recommend drilling a test hole in a  scrap piece first as some types are very brittle and shatter easily when drilled.

The rubber O ring was objet trouv é from my workshop, but kits of assorted O rings are readily available.

General description

The 2” diameter wheels fit the 3 mm diameter shafts commonly found on cheap motor/gearbox units. The hole in the hub is drilled with a 3 mm drill, this was found to give a sliding fit on the shaft. The final assembly will use Loctite 638 high strength retaining compound to fix the wheels to the shafts. Each steel hub has a 6 mm diameter spigot turned to be a tight fit in the 6 mm holes in the centre of each wheel disk. Three M2.5 screws pass through clearance holes in the wheel disks into tapped holes in  the steel hub to secure the wheels to the hubs. A Vee groove cut in the periphery of the wheel disk locates the tyre.

Machining the hubs.

Start with a reasonable length billet of steel, 70mm long. The material used in the photographs is 16.5 mm diameter. Even though you only need a few millimetres of steel in the finished hubs, you need a good chunk to grip in the chuck. A common error is to try to grip short length of material in the tips of the chuck jaws. This imposes a great strain on the chuck and leads to a permanent loss of accuracy. Don’t spoil an expensive chuck to save a few pennies worth of steel.

Use a sharp knife tool to face off the bar. I ran my Myford 7 at top speed for this operation. Take fine cuts, say 0.005” until the end is nicely cleaned up.

Using the same tool, adjust the tool angle to ensure there is some end clearance and continue to remove metal to leave the 6 mm diameter spigot. This needs to be about 4.5 mm long. Rather than relying too much on your micrometer, turn this diameter to get a nice tight fit in the holes in the wheel disks. No need to make the disks yet, just drill a hole in a scrap of the disk material using the same drill you plan to use when making the disks. This will ensure a good fit even if your drill cuts over size.

Now centre drill the bar. Just enter the drill tip into the bar enough to give the 3 mm drill a good start, no need to go so deep as to get the countersink cutting at this time. As a general point, it is always a good idea to use a centre drill to start holes when drilling in the lathe. The centre drill is much more rigid and will always find the true centre of the work compared to a standard drill. I often put a centre hole in work, even if it is not necessary because it makes it much easier to set the work up in the 4 jaw chuck if for any reason the job needs to be returned to the lathe at a later stage, say for repair or modification.

Drill the hole, about 10 mm deep using a 3 mm drill. I checked several different, nominally 3 mm drills to find the one that gave the best fit on the actual motor shaft. I also tried a 2.9 mm drill, but the degree of interference was too much to produce a good force fit without apply undue force during assembly. I opted for a sliding fit and used retaining compound to secure the hubs on the shafts.

Use the tip of the tool to turn a small Vee groove about 2 mm back from the shoulder of the hub. You will use this a  saw guide when parting off the hubs.

Remove the bar from the chuck, reverse it and turn a second hub on the other end. Working this way helps to make sure both hubs are identical.

Now is the time to part off the hubs from the stock. My lathe is rather old, and I avoid using a parting tool as much as possible, call me a coward if you like, but I would rather saw the hubs off in the vice, than risk a snarl up with the parting tool and have to start from scratch with the hubs. So remove the work from the lathe and saw off the hubs, leave an allowance for cleaning up the faces.

Return the hubs to the lathe. Lightly grip the 6 mm spigots in the 3 jaw chuck and face off the rear faces. As these are not critical for the truth of the wheels you will get away with this. The crucial faces are the hole in the centre of the hub, the front face of the hub and the 6 mm spigot. Since these were turned without removing the work from the chuck you are sure they are all concentric. When doing this type of operation I start with the smallest component and take off just enough to clean up the face. Then, noting the compound slide reading, repeat the process with the second longer component. This ensures that both hub flanges are the same thickness.

To produce the holes in the hubs I used a drilling jig. To make the jig, find a piece of steel plate about 1.5 mm thick. Make a light centre spot using the centre punch and then, using dividers scribe the 10 mm diameter PCD circle to locate the screw holes. A useful tip when marking out, is to coat the steel in black permanent marker, much quicker and cleaner than messing about with layout blue.

The next problem is setting out the 120 ° angles to locate the holes. No need to rush off to get the protractor (or the dividing head). Instead scribe a second, larger circle. Make a centre spot somewhere on the circumference and then using the dividers, step out the radius around the circumference of the circle. I repeat the process both ways round. Inspect your work and select the best set of intersections to give the 120 ° angle. Use a straight edge to scribe lines radially from the centre to each of the selected intersections. Centre punch these where they cross the inner 10 mm diameter circle. These points mark the positions of the screw holes. Now take the work to the bench drill and drill out all 4 holes 2.1 mm diameter, this is the tapping drill size for M2.5. Open up the centre hole to 6 mm to fit the spigots on the hubs.

Notice the stud sticking up through the bed of the drilling machine. This is there to arrest the work in case the drill snatches the work as it breaks through. I know some workers frown on drilling on top of a piece of wood, but I prefer this method for this class of work. Of course I use a vice and hold downs for heavier jobs.

To use the jig, push the spigots through the centre hole and then drill the 2.1 mm tapping holes in the hubs. I put a piece of welding rod through the first hole to stop the hub rotating in the jig while the remaining holes were drilled.

At this point put your almost finished hubs aside. Don’t tap them yet as you will use them as a drilling guide to locate the holes in the wheel disks and you don’t want to spoil the threads by putting a drill though the threaded holes.

Making the wheel disks

I used a hole saw to cut the wheel blanks. Don’t go and buy one especially for this little job, but if you have got one use it. The central drill is less than 6 mm on mine so the blanks were ideal for my wheels. If you don’t have a hole saw then mark out using a centre punch and dividers and use a hack saw to cut roughly to size. I would file or use the belt sander to get a reasonably circular profile. Open up the centre holes to 6 mm using the drill you selected earlier.

Now put your bit of steel bar back in the lathe and face it off. Turn a 6 mm diameter spigot about 3.8 mm long, don’t measure it just make it a tight fit in the 6 mm holes in the wheel blanks and shorter than the blank thickness. Do not remove the bar from the lathe until you have turned both wheel disks. Push the first wheel blank onto the spigot which you have just created. Put one of your hubs, spigot facing away from the disk on the face of the disk and bring the running centre up to the hub. Use the tailstock feed screw to apply pressure to the work. This will ensure that, so long as you only take light cuts, the work will not spin on the spigot while you are turning the disks true and cutting the tyre grove.

Take light cuts at top speed across rim off the wheel. Aim to make the finished disk about the same diameter is the outside diameter of the O ring. This will ensure enough stretch to get a snug fit in the tyre grove. Note the cross slide reading and repeat for the second disk.

Set a Vee pointed tool exactly at centre height in the tool post. You could use an external thread cutting tool or do as I did and use a triangular carbide tipped tool, the sort that you can replace the insert by rotating the bit through 120°. Line the tool up on the centre of the disk and very carefully, start to plunge the tool into the work cutting the tyre groove. You will need to work the tool from side to side a bit as you cut the groove, don’t try to cut the entire groove  with both sides of the tool cutting at once. Plunge the tool in to a depth of about 3 mm. Note the cross slide setting and repeat for the second disk.

Remove the disks from the lathe and push the hubs into place. Use a centre punch to make some witness marks to identify and locate each hub and disk. In the next stage you will make the holes in the disks for the M2.5 screws to pass through and any errors in setting out the holes will mean that the two sets of parts are not interchangeable and that there may be only one correct way to assemble the wheel. I made a single punch mark on one set of parts and a double set of punch marks on the other set. Now drill 2.1 mm holes using the steel hubs as drilling jigs. Remove the hubs and open up the holes to 2.6 mm.

Final assembly

Now fit the O ring tyres to the disks.

You will need 6 short M2.5 screws. It is unlikely you will find screws the right length in stock so get some longer screws and cut them down to size. A handy tip is to find a piece of steel the same thickness as the length of thread required. Drill a clearance hole in the steel. The screws can be trapped in the hole while you saw or file them down to length. If you have lots to do, drill several holes so you can do a few at a time.

All that remains is to tap the holes M2.5. Take care, it’s quite a small tap and easy to break. Make sure you start the tap square to the work and go easy. Clean up your hubs and remove any burs. Pay attention to the witness marks and fit the hubs to the wheels. Fit the M2.5 screws and there you have it. You can check your work by fitting both wheels to the same axle, if you have made a good pair the wheels will roll in a straight line, but if one wheel is larger than the other the wheels weill veer off on a curved path.