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Rewinding notes

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wixwacing
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Posts: 1871
Joined: Thu Jul 10, 2008 9:22 am

Rewinding notes

Post by wixwacing »

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The hidden mysteries of the ancient art of

Motor rewinding

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By Phil Wicks


More and more slotcar hobbyists are testing their skills and adding an extra dimension or two to their slotcar experience. Scratchbuilding, even in its simplest form, is very rewarding. To be able to take a body and fit another chassis and motor to it can see models that don’t normally grace the slot shop shelves appear in personal collections and become keen favourites, to put on the track at every available opportunity, even if they aren’t as fast or don’t handle as well as the RTR models. The sense of achievement is reward in itself and once having done one (and let’s not pretend the first one is going to be perfect!), the next one and the next one come quickly after with the application of lessons learned from mistakes made and inevitably, ideas both mainstream and way out.

My idea of scratchbuilt models is two fold. One, a model, as mentioned before, that is unobtainable as a ready to race and secondly, a model which is being built to do the business. That is, to contend competition. The art of fitting a chassis into a body is only part of the solution. The more serious decision is to replace the motor. These days there is heaps of choice and it is nothing to go and buy a 40,000 rpm screamer off the shelf for a reasonable amount. In my youth there was very little RTR racing and it was nearly all exclusively 1/32 scale scratch built models. Vacuum formed bodies and brass rod chassis with floppy body mounts and ball axle bearings. My first serious racer was a Lola T70 with a motor that I paid a week’s wages for as an apprentice!! The motor revved to 100,000 rpm allegedly and had a fibreglass end bell and ball races and a bullet proof com. Needless to say; I only contended one class competitively as the cost to contend more was prohibitive. Especially when I had to pay ‘keep’ to my parents and keep a two stroke ‘Ariel’ motorbike on the road!

A few older guys in the club bought some of the cutting edge equipment too but the underlying trend was for people to rewind potentially fast motors. Among the favourites were ‘milliperm’ and ‘microperm’ round case motors. I tried once and abandoned the idea of rewinding as I neither had the technical know-how or the tools and equipment to be successful. When returning to the hobby eight or so years ago, a colleague and neighbour was and still is in to restoring the old sixties Scalextric models and it came as a surprise to me to see his shelves full of restored models waiting for a motor. Maybe a donor model or a single motor spotted in a scrapyard lot on eBay. Motors were still commanding a good price, $40.00 or more and that was without brushes!!! Brushes could occasionally be purchased for $12 or $14 a pair!!

Amazed at the almost ‘black market’ in these open framed motors, I took a couple home and stripped them down. Measuring them up I ascertained the wire gauge and went about rewinding these potential gold mines. After a period of learning and adjustment I had rewound twenty or so motors over several months and had found a local guy with an amazing remagnetiser or ‘zapper!’ With the advent of the SCX vintage series, the a***e fell out of the RX recon market and for $35 or so, not only could you buy a model, but it had an RX open framed motor with brushes!! So the rewinding aspect of my hobby was limited to the odd SCX RX 41 which had prematurely destroyed its armature.

With the price of replacement motors what they are, there is still no real justification in rewinding these days for mainstream slotcars. It would be purely an achievement to add to the list and the end result would still only be as tractable as ‘off the shelf’ spares, but, to me it is a valuable part of the hobby and apart from teaching you the anatomy of an electric motor, it also helps you understand all the little nuances and fine adjustments that make all the difference.

So, as promised a while back, I have rewound an old burnt out SCX Pro Turbo motor and hopped its state of tune up to give the next model it goes in a bit more oomph! No adapter brackets or fiddling to fit. This is an SCX rewind to perk up an SCX model! Tools needed for this job are the usual selection of hobbyist’s pliers and screwdrivers and knives. The most important tool is going to be the soldering iron. I use a forty watt iron but for some this could be dicing with disaster so if this is your first rewind I recommend you start with a pointed tip 25 watt iron and self fluxing cored solder.


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The patient!



The main problem with SCX motors to date is that they usually develop a commutator area fault of some kind. The noise suppression ring breaks and wrecks the windings, or the comm. itself is a bit out of round or a segment comes loose. Other less fatal problems include wires being thrown off the pole pieces. So the first task is to dismantle the motor and determine its reusable status. If the little ring around the wide part of the com has broken then this can be carefully unsoldered using desoldering wick and the pieces can be discarded and the comm. tags cleaned up. Remember, not too much heat! Two seconds in and out!! This is apparently an electrical noise suppressor of some kind and its absence will not affect the running of the motor even if it does white out the TV! If the commutator has irregular or uneven segments then it is a write off! The core has overheated and it has melted and changed shape. Unless you can source another comm. you will be minus one! But, if the problem is a stray wire then this is no problem.


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Ready to rock!


Next up is to strip the motor. Firstly, remove the pinion. The armature won’t come out of the can with it on! Carefully levering back the tags which hold the end bell on will allow the end bell to be removed. Make sure you keep a check on the carbon brushes as these may well fall out. The end bell is set aside and the armature is removed. At this stage I test the polarity of the case magnets with a hikers compass and fit a button magnet on either side respecting correct polarity. You have to be careful here as neodymium magnets are very strong and will stick on the WRONG way round if you put them on so! Remember, unlike poles attract and that’s how the magnets should go on, plus to minus.

Next I remove a piece of wire and measure it with a micrometer. SCX motors are fairly predictable. Standard SCX RX motors have between 180 and 220 turns of 0.13 m.m. wire on them depending on their age. Pro Turbos usually have about 160 turns of 0.17m.m. On this job, as I’m looking for that bit extra performance, I will be winding 120 turns of 0.20 m.m. This, along with shimmed magnets, should give me an extra two to three thousand rpm at least. There is a small brass ring at each end of the armature. This sets the end float and comm. position for the arm when assembled. One of them will need to be removed so the one at the comm. end is gripped firmly with some small pliers and twisting the arm back and forward, the ring and its attendant nylon washers are coaxed off and placed somewhere safe.

The commutator can now be detached from the windings if it is still attached and the comm. can be slid quite easily off the end of the armature shaft. Lastly, the rest of the windings can be removed. You can either unwind these or if you are careful, you can slice through them at one end using a sharp modeller’s knife. Be careful as there is a nylon endplate each end of the armature whose sole purpose is to minimise the risk of the windings shorting to earth and we don’t want these damaged. Once stripped it’s time to take stock. The armature should be fine to re-use. The commutator should not have any loose or out of align segments and if the ring is broken, remove it carefully using a soldering iron. The commutator has a nylon type hub and too much heat will see the segments move on this. Rule of thumb is that you don’t apply heat for any longer than two seconds at a time and if possible, clip a small crocodile clip onto the segments whilst soldering to act as a heat sink. Once you are happy with everything we can now start thinking about putting it all back together.



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Ready to roll!


If you have small fingers, you may well be able to hold the armature during the winding process. Because the motor trade has left me with a bunch of saveloys to work with, I have made a nifty little jig to hold a variety of armatures. Firstly a dummy commutator is slid over the comm. end of the armature. This will space the wire away from the shaft whilst winding and eventually when the time comes, the commutator will slide easily into its lug sockets. Mine is made from telescoping a couple of pieces of polystyrene tube over each other and the top of the winding jig holds it in place. If you are hand winding, a small spot of superglue will hold it in place until you are ready to remove it. Make sure you don’t get the glue in the lug sockets in the top nylon plate.

The next task is to start winding. The winding process needs to be fairly neat and spread evenly over the pole while winding. The main reason for this is that you may not get all the windings on if it is too messy and there is a risk that the wires may move during operation if they are not held firmly, this could lead to the enamel coating wearing through and then eventually, a short circuit. The loose end of the wire is attached to a small tube or peg on my winder and if you are hand winding, wrap the loose end around the top of the armature shaft. Next, to wind. It doesn’t matter if you wind clockwise or anticlockwise, but whichever way you go, all pole pieces should be wound in the same direction. This example is wound clockwise and whilst winding it is necessary to ensure the wire goes on moderately tightly as the longer side runs of wire will start to swell outwards because the choice of wire is thick. Thinner wire normally doesn’t suffer from this and it is easier to apply a smooth flat wind. 120 winds has been chosen because for this gauge wire, as we get close to the end of winding one pole piece, the wire will sit flush with the side gap on the next pole for the next winding. Too much wire on the first pole piece will see the gap close for the next and you will not be able to get enough winds on the second pole.


So, as we get close to the end of the first wind we will need to leave enough wire to form a ‘pigtail’ that can be soldered to the commutator terminal. On my jig I run the wire up to the peg and back again to the next pole in the direction of winding. I.e. If you are winding clockwise then you will move on to the next pole to the right looking at the wound pole, make sense?, I hope so, this is crucial! If you are hand winding then it is best to leave a loop about 20 m.m. long wrapped around the top of the armature shaft as before. Before continuing, as we are using thick wire, it is advisable to press down the long side of the windings with something like a narrow lolly stick, this will ensure there is enough space for the other pole windings. With my jig the armature is rotated one pole to give me access to the next. Continue with the second wind and again leave enough of a loop to allow for another ‘pigtail’ later on. Again, press the long side of the windings carefully with a non metallic object and finish with the third wind and at the end of the third wind the loose end will need to eventually be attached to the original starting wire end which was wrapped to the shaft or attached to the peg at the beginning of the first wind.



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Winding finished



We now have an armature fully wound, the next task is a little trickier. Getting the pigtails together and in a more manageable form. Starting with the first full loop we created at the end of each wind, not the start end of the wire, we should see that it comes from the top left of one pole and heads off to the top right (or thereabouts) of the next pole to the right!! So far, so good? I gave made a miniature hockey stick for want of a better description, from a piece of insulated single core wire. I insert the hook of the hockey stick through the loop on the windings, lifting it up until it pulls firmly but not to hard on the loop material, I twist it between thumb and forefinger for several revolutions which should have the effect of putting a ‘candy twist’ in the loop wire, leaving it a lot more manageable. This ‘pig tail’ as I have been calling it should come from a point between the two poles it is attached to. Remove the ‘hockey stick’ and bend the wire so that it sticks out at nearly ninety degrees. Repeat this for the second loop and bend out also.


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Hockey stick and pigtails



Now, lastly, we have the start and finish ends of the wire! These have to be joined at the very end to fashion the third loop. It’s not too critical how you do this as they will eventually be discarded but firstly they need to be firmly fixed together. The simplest method is to twist them two or three times at the very end. Next, the trusty hockey stick is applied to this new loop much the same as the previous two. You now have a wound armature with three ‘pigtails’ sticking out at ninety degrees. The last operation of this part involves a candle or a cigarette lighter or other flame source. I use a candle simply because the wife has a myriad to choose from. The aroma doesn’t make any difference but the right choice could help clear your sinuses. You just need to be able to get to the flame easily from the side. Firstly, remove the dummy comm sleeve that has been in place during winding, (this will melt if you forget!, trust me!!). Holding the armature, move it to the flame so one of the pigtail ends enters the flame. As it does so you will see the enamel burn back. We don’t want the enamel to burn back beyond its inner end so stop as the heat gets there. This should have taken about a second and a half! Repeat this for the other two. Next, get a 25 m.m. square piece of 400 wet and dry and fold it down the middle. Grip the pigtail lightly between the wet and dry with your thumb and forefinger. Pull it off the pigtail several times in several positions. This should clean the burnt enamel off the copper wire. Repeat this for the other two pigtails. If you have some lighter fuel or meths, using a stiff paintbrush, you can clean most of the remaining enamel but it isn’t crucial. Lastly, with a hot iron, tin the three pigtails well, up to their inner ends.



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Soldering complete



The commutator, having been OK’d on strip down, is now ready to be refitted. Firstly, the three terminals that the pigtails are to be soldered to need to be retinned. The solder on them won’t have any flux present and will cause the joint to become messy and difficult to solder. The last thing we want is too much heat applied at this stage, that would be disastrous!! The comm. should have three little nylon lugs on the bottom. These fit in three minute sockets in the nylon end plate on the armature and sliding the commutator over the shaft, press it home firmly until you can detect the lugs having entered the sockets. The tags on the commutator should now sit directly between the poles and directly above the pigtails. The slots on the comm. should be positioned in line with the centre of the pole pieces. Gripping the outer end of the tinned pigtail with a small pair of pointed pliers or tweezers, pull the pigtail over the tinned comm. tab and down the other side, applying a light pressure. Momentarily apply a hot iron to the tag. The tinned tag and pigtail should fuse instantly. Don’t mess with it after this as you run the risk of melting the comm. core. If you are not happy with the join. Wait twenty seconds or so, re apply the iron and pull the pigtail off decisively. Re-tin the parts and try again after a suitable time interval. Remember at all times, DON’T OVERHEAT THE COMM!! It will be terminal! The secret is in the initial tinning of both the comm. tags and the pigtails. Lastly at this stage of the job we need to test the armature to ensure it is usable! With an ohm meter set low, test each adjacent comm. segment and make sure they are all very close in reading to each other. They probably won’t be exact but they should be 0.2 ohms of each other. If this checks out OK I the test for shorts. This is done by testing the resistance between any comm. segment and the armature shaft. If there is any reading at all then there is a short and you’ll have to do it all again (very rare though). The reading should be open circuit. If you are still smiling and it’s not wind then you have crossed the Rubicon!! Trust me. Everything from here on is plain sailing, pour yourself a drink!!



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The finished arm



The armature is now as good as finished. The final jobs are to coat the windings to stop them being thrown off and if you want to go that extra yard; you can balance the finished article. I coat the windings with epoxy resin and a small artist’s paint brush. This is the slow drying super strength stuff; the reason for this is to allow the resin to soak in to the windings a little before it sets. I mix up a reasonable amount to do the job then add some enamel paint. ?? This does two things. Firstly, it stops the resin setting rock hard in case we need to do it all again at a later date!! And secondly, so I can identify what I have done to the motor, I colour the windings accordingly. When the resin is dry you can apply a thin coat of the enamel paint you used to colour the resin and it should cover in one coat. There is one more optional step. That is to static balance the armature. I rarely bother with this on motors in a moderate state of tune. If you have been careful and tidy the armature shouldn’t be too far out, certainly not enough to stop it running nicely but for the record, balancing is achieved by placing two new modelling knife blades parallel and as level as you can get them in a block of wood with the sharpened edges facing up. Place them wide enough apart to be able to get the armature poles between them. Each end of the armature shaft should be able to sit on the blades. Placing the arm on the blades and releasing it slowly will see it possibly roll gently until the heaviest part of the arm is lowermost. Do this two or three times to confirm the position then mark this position at the bottom. Next, reverse the arm on the blades and it will roll in the opposite direction and once again come to rest. Mark this lowest spot also. The reason we do it in both directions is to overcome any error in blade level. You may now have two marks close to each other in one spot on the heaviest point of the armature. Hopefully, this will be on the pole piece and not the windings. Next step is to carefully start a small hole in the pole piece with a drill. (2 – 3 mm) this is done either on the mark you have made (if they are over each other) or in the centre between the two marks you have made. It is important to be very accurate here. Next with a six mm drill, carefully open up the hole a little. Place the arm back on the blades with the marks to the top and re test. If the marks still fall to the bottom then a little more metal needs to be taken out. Repeat the procedure until the arm rolls VERY slowly one way or the other. Be very careful here as if you take too much out you can’t put it back!! It’s hard to get an armature spot on with this technique but you can get a very close balance. If the marks fall at a winding gap it isn’t possible to drill the windings!! What needs to be done is to mark the pole pieces either side of the lowest point and drill two smaller holes equidistant from the mark and EQUAL IN DEPTH!! This should overcome the problem. It is important to pinpoint the heaviest (lowest) point on the armature accurately. Failure to do this will see the balance point move round the arm after each drilling and we can’t chase it round drilling holes as we go!! I normally reserve balancing for high revving motors only.


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Next up is the motor case.


Depending on what you want to achieve will determine what happens with the case. SCX motor cases are very basic and the magnets’ position in them are quite crude. Magnets vary in distance from the armature and even a single magnet will have a wider air gap one side to the other! You can measure the gaps with an auto feeler gauge or the trick I use is done after the case has been stripped. Firstly, I determine the polarity of the magnets using a hiker’s compass (from all good camping stores!). The compass is moved towards the case and depending on which side you are testing, the opposite bearing will swing towards it. (North will swing towards an outward facing south – opposites attract – remember) I then get a couple of spare slotcar button magnets and attach these to each side of the case. Test carefully before attaching these because, as the neodymiums are so strong they will over ride the motor magnet polarity and you run the risk of joining like poles! The reason for attaching these is to retain the residual magnetism in the case that was there during manufacture. This helps hold the field round the motor with the aid of the two motor magnets. If the motor magnets are removed without these, the case looses a lot of its residual magnetism and the overall motor ‘field’ is reduced. The only way to restore it is to have the motor assembly rezapped. Loosing this field won’t stop the motor running but it may cause the motor to under perform and possibly run hotter. The next step is to cut some .002” or .003” shim metal (available from engineering suppliers) into squares about the same size as the magnets. This will be used to shim the magnets closer to the armature. The simplest method with the SCX motor is to put about .008/9” behind each magnet. The magnets are marked and removing them one at a time will minimise the risk of putting them back in the wrong sides. If you do put them back the wrong way, the motor will run backwards! There are is a single or a couple of spring clips holding the magnets in place. These can be removed at this stage by levering them up through the open end of the motor can, leaving the magnets in situe. With a magnet removed, place the curved shims on the back of the magnet and slide the assembly back to its original position. Remove the opposite magnet and repeat the procedure. Refit both the spring clips and insert the armature and bell end and test to see that the arm is not fouling either magnet. Once this is OK then the bell end can be reinstalled and the tabs carefully bent back into place. If you want a more accurate shim job, there is a simple but lengthy technique to get them spot on.

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One magnet at a time!



When removing the first magnet, over shim it so you are confident that the armature will rub on the magnets, then, one shim at a time, remove the armature, remove the spring and magnet and remove shims until you have clearance. Then remove one more shim. Repeat this for the other side until both magnets are pretty well equally spaced off the arm and refit. We use ferrous metal shims because when the motor is reassembled, the metal shims will assist in retaining the field magnetism around the motor. We want to avoid getting an air gap between the magnets and the casing as might be caused by non ferrous material. This will reduce the field and cause loss of residual magnetism.



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Shim material



Once the magnets have been shimmed the motor can be reassembled and the bell end can be carefully clipped back in place. Don’t over crimp here as it will distort the nylon end bell and cause other problems. The motor now needs a drop of oil and once the brushes and springs are fitted it can be tested. All going well you should now have a motor that is at least as good as the old one and hopefully have a bit more zip and a few more revs. The last operation would be to fit it to the model of your choice and off you go.

This article is meant to be an aid to those who might have though of rewinding a motor and were unsure of where to start. I have to stress at this point that model motors vary from maker to maker. I have used the SCX motor as an example for two reasons. One is that the SCX motor seems to be the most unreliable and therefore I suspect most people have a dead one in a junk box somewhere and the other reason is that the SCX motor is probably the easiest to rewind. Wire is obtainable from all good electronics stores. Here in Australia it is available from DSE or Jaycar. There is usually enough wire for several rewinds on one spool of wire. Steel shim should be available from engineering supplies stores and all the rest is normal hobby stuff. If you don’t have a multi meter, these are available too from local electronics stores and if you are a keen slotcar racer then you ought really to have one of these! Epoxy resin bis available most places and applying it with a paintbrush is easiest. The paintbrush can later be cleaned with either acetone, (nail polish remover) or good old auto brake fluid.



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The finished article!!



So that’s it. Give it a shot on one of those long winter evenings and very soon, like me, every time you win a race with a box standard model, you’ll be accused of rewinding it!! Praise indeed??!!
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When I'm not racing slotcars,
I'm out in the back yard, burning food!!

When I win, it's because of my talent, not my car or my controller!
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