(Re)winding and building motors, tips & tricks, checks & tests - Page 8 - RC Groups
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Nov 21, 2004, 05:17 AM
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Re: olmod Post Jun 15, 2004, 11:31 AM #3, "Want a Tight Fit?"
FOR SAFETY...Do this sizing BEFORE you grind the bit.

If you use fine sandpaper while rotating the shaft in the Dremel, you can reduce the diameter of this DIY bit by a VERY slight amount. This will provide a tighter press-fit, and gives you more room for error.
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Nov 21, 2004, 06:45 AM
4-D traveller
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Cheap GB motor mount

DC power socket cut; GB cf bearing tube glued in; Strengthened with tape.
Nov 21, 2004, 06:50 AM
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Lightened GB cans

I've lightened the GB cans, and thus made <20g main motor and <10g tail motor for my submicro heli.
Nov 21, 2004, 07:14 AM
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Interchangable can

Can with stanless tube inserted: 3mmOD and 2mm ID.
2mm shaft taken from a 250 brushed motor which has a brass stopper (can be pushed for space adjustment).
Rubber washer on top of the can, pinion with grub screw used as shaft collar.
The can spins freely on the shaft. The shaft collar and the brass stopper force the can to turn the shaft.
Last edited by glxy; Nov 21, 2004 at 08:58 AM.
Nov 22, 2004, 11:54 PM
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Measuring the gap

some hobby shops have a K&S stand wich stock brass shim material ,when cut with scissors to 1/8"or smaller(tapered is good) width and removing any burr, make great non magnetic mini feeler guages to check the gap between stator poles and magnets.
Nov 23, 2004, 04:47 AM
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Re: Power post of Jul 20, 2004, 04:21 PM #54

You might try modifying the nuts so the result is a cone-shape. Possible design advantages...The narrower end of the cones will slip slightly into the stator hole, further reducing the chance for any damage to the top/bottom of the stator, and they can be used on stators of different sizes and sized holes (up to a point...don't angle the cone shape too much) without having to worry about differences in protection overlay design variations. I'm uncertain if this has potential to encourage undesirable enlargement of the hole because of the soft laminate material of the stator core. Only way to see is to try it and let us know if it works.
Nov 24, 2004, 01:56 AM
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Scavenging Neodymium Magnets from CD/DVD/Disc Drives

I'm repeating my earlier finding here because I've not seen it anywhere else...but when you disassemble the CD/DVD/Disc drive, you can scavenge the rare-earth magnets that are hidden in the optical assembly. They are plated and come in a variety of sizes and strengths. (for example, I've found 2.9x2.95x1.12mm and 4.78x5.90x2.44mm (approx) magnets seem typical. The larger size mags were stronger than N45) Remember that you can stack magnets to increase field strength.

To match magnets for use in a given motor, you should compare them. To compare relative strength against themselves or known commercial magnets I've bought, I've tried 2 methods. Both seem adequate to the task. However, I'm also not all that satisfied with the consistency of strength ratings of commercial magnets, either, and I compare and match even the ones I buy.

A hand-held version of my tester in Method 1 can also be used to verify the polarity of your magnets in the bell/can before glueing them in place. This method doesn't risk dislodging a magnet by using another magnet to "test" the polarity. (one of my test tools consists of the Hall sensor mounted at the tip of an old pen tube. I just slide it next to each magnet. As I proceed around the ring the LED alternates On and Off verifying the magnet poles are installed properly)

Test jigs easily can be setup for either method.

Method 1:
Crude, fast, but adequate for magnets of similar size and geometry. I just set up a Hall-effect sensor to sense the magnetic field. In my case, this is a 9V battery, a resistor, an LED, and the Hall sensor. Comparable magnets provide the same response at the same distance. (since the distance from the sensor to the magnet is less in this method, this provides a bit less resolution than method 2, but still seems to yield adequate results. I solved a quality problem in a production environment when we needed an ultrafast and easy to use tester to verify magnet polarity and strength. Remember that rare-earth magnets (even plated ones) can rather easily weaken or lose their magnetism due to age, heat, moisture, and exposure to adverse EM fields.

Method 2:
Set a compass at a given distance from a known power magnet (ie; N45), and note the deflection as the magnet is moved toward and away from the compass. An unknown magnet of similar strength will result in the same deflection pattern. (I'm assuming the the magnets are of the same geometry and the size of the magnets is small compared to the distances involved, so field shape variations are minimal). (Note: if you measure the distances and deflections, you could come up with a way to quantify the strengths, but I'm satisfied with comparing.)

Last edited by escudo; Nov 24, 2004 at 03:43 AM. Reason: Add another tip for polarity detection
Dec 01, 2004, 01:01 PM
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Magnet Sources and testing

I've been very happy overall in the past with "The Magnet Source" (aka Master Magnetics). UNFORTUNATELY, "The Magnet Source" does not provide anything over N35. A real downside for us in this application. When I used them, "The Magnet Source" had consistent very high quality plating and strength through hundreds of plated neos at a time with excellent shipping and customer responsiveness, and would execute small and large orders with equal consideration. Very important factors to me.

For a general list of manufactureres, see magnetassemblies.com as a start. Most of them, however, list lower strength, lower temp magnets.

I'm intrigued lately by mmcmagnetics.com for NdFeB magnets used in electric motors. They claim high field strength and high temperature resistance.

MMC is offering NdFeB's with "working temperature" ratings they specify as quite high. I'd like to get my hands on some and check them out in a really rigourous test. Something a bit more sophisticated then my previous post. MMC sells neos rated with a working temp from 80degC to 200degC (this is below the Curie point, but it apparently means the magnet can stand up to some length of prolonged heating will little or no ill effect. Something that could definitely benefit brushless motor assemblies. I'm waiting on some clarification from them now, and still wonder if this might be specmanship). Many available NdFeB's are rated at a "working temperature" of less than 100degC, and these higher ratings being available in these higher "N" ratings to the hobbyist is a new thing.

I also just found out about gobrushless.com, a small startup designed to help the CDROM motor conversion hobbyist. However, GB is not a magnet manufacturer, and they don't publish detailed specs (like plating info and mechanical tolerances) or supplier information. As you see from my prior post, plating is a big factor in Neo life-expectancy. GB resells neos appealing to the hobbyist motor builder in a limited number of sizes and strengths, and sells them in useful motor building quantities (15's, 50's, and 100's). Prices seem roughly competitive for the quantities involved. Although their web pages still show N45's they claim to carry strictly N50's from now on.

A test of GB's magnets would be very useful, however, they could change suppliers the next day and the test data would be valueless since they don't guarantee any peformance parameters.

I have no special interest in any of these companies.

Last edited by escudo; Dec 07, 2004 at 03:00 AM. Reason: qualification on availability
Dec 01, 2004, 09:33 PM
Originally Posted by FRAMEDNLVS
One problem I have is that I order from slofly. Don't get me wrong, they are great. But they don't put down what "N" there magnets are. I'm afraid that i'm going to start mixing magnets that are different. I have ordered 24 and then I ordered 100. So now with 4 or 5 different motors laying aroud i'm going to get some mixed up. I'm probably going to throw a set away if one gets lost.

The other problem I still have is how to get the can and the shaft to stay together. I have two that are straight, but they will slip under load. JB weld doesn't seem to work. Solder didn't work. Any ideas on this?

Check out the setscrew hub cans available at www.strongrcmotors.com

I also buy magnets direct from the factory with strength and temperature specifications clearly shown.
Last edited by bz1mcr; Dec 01, 2004 at 09:36 PM.
Dec 01, 2004, 10:04 PM
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I guess I was miss-informed I guess I did not pay attention when we talk

on another note I have 3,000 bells that will fit the 20mm GB stators and N50 curved magnets that were made just for this bell,
The bell has a nice fat piece of brass in the center to press the shaft through and gives great support for the shaft.
I am selling off all these parts in groups of 10 or more.
PM me if interested
Dec 02, 2004, 01:49 AM
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Impact of Manufacturer Tolerances on

I've posted a new thread, taking the discussion of the impact of tolerances on motor design/fabrication to another venue. What was a simple tip to acquire and take tolerances into account in calculations and fabrication, looks like it requires a separate thread.

"Impact of Manufacturer/Other Tolerances on Motor Design/Fabrication"


This thread kicks off by demonstrating calculations that show significant Stator/Magnet interference issues for common motor parts (22.7mm stators and cans, and 5mm Wide magnets, of 2mm and 1mm Thicknesses).

How one might calculate a perfectly viable motor using the nominal part dimensions, but discover significant problems when the part dimensions begin to change, but remain within commonly specified limits.

Last edited by escudo; Dec 02, 2004 at 02:38 AM. Reason: clarification
Dec 02, 2004, 06:53 PM
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Planning/Assembly To Reduce Chance of Assembly/Operational Failures

Planning/Assembly To Reduce Chance of Assembly/Operational Failures

Here are some practical ideas on how to help reduce the chance of stator/magnet obstruction and some other tolerance-affected problems during assembly and during operation of the motor.

There is a practical way to help reduce some of the impact of manufacturing and asssembly tolerances in the magnet/stator obstruction situation. However, you need a micrometer, you will likely need more than just the minimum number of magnets to assemble a motor (a motor using 12 magnets might require 20 or more from which to measure and select, so unless you want to wait for a second order of magnets and are willing to trust in luck, order more magnets the first time around), and you may need chemicals/tools to modify your stator.

1) Measure your can/rotor Inside Diameter in several separated locations. If any measured ID is LESS than what you planned for, either correct the rotor/can or use another.
2) Measure the orthogonality of the shaft to the rotor. Verify that it is perpendicular to the rotor. (One way to do this is to measure the distance from the shaft to the inside surface of the rotor/can near the shaft/rotor press-fit joint, and again near the "back" end of the rotor/can. Take into measurement consideration any changes in ID caused by rotor design.) Take several measurements. Correct as needed or use another rotor/shaft assembly if you cannot achieve values to maintain the AirGap calculation in the MRC.
3) Measure your stator Outside Diameter in several separated locations. If any measured OD is MORE than what you planned for, either correct the stator or use another. Realize that modifying the stator can seriously impact assembly and motor operation.
4) Measure each and every magnet you plan to use (measure each magnet dimension). If a magnet dimension exceeds the value you have used in your calculations, do not use it in your current assembly. Do not attempt to modify the magnet.
5) Avoid designs in which the magnets will end up touching each other on their sides. While some people have successfully assembled motors with touching magnets, it is risky (see caution2 note below)
6) Select only magnets that according to the "Motor Rotor Calculator" (MRC) will produce Air and Magnet gaps you know you can assemble.
7) Examine your stator for any design or assembly elements that extend beyond the "face" of the stator. Modify, remove, or compensate in the MRC for any extensions as needed. Remeasure your stator if needed. Realize that modifying or removing this sleeve may have other considerations.
8) Factor into your calculations that adding glue,epoxy,paint, protective stator sleeves, or other factors into your assembly will change some dimensions after assembly.

Caution1: I've got one stator that has a plastic protector "sleeve" on the top and bottom laminates. This sleeve INCREASES the OD of the stator beyond the OD of the metal stator "arms", but this extension is NOT very wide...it's more like a tiny peg extension, 0.08mm long...I almost missed it. It is not flush with the face of the stator arm, and forms a channel into which the magnets slot. If I hadn't seen this, was able to assemble my motor, and were to use magnets that did not fit totally within the space formed by this sleeve (with adequate clearance as the stator shifts along the axis), I might eventually have a situation in which the sleeve contacted a magnet(s)...most probably while the motor was running. I'd soon have a small broken pile o' parts or a seized motor. This sleeve was clearly designed for custom magnets, in a custom manufacturing assembly, and trying to use generic magnets and a hand-made assembly with more "slop" is inviting trouble. The solution? Modify or remove the protective sleeve. If you do that however, realize you may have to deal with wiring issues and be extra careful about stator-wiring shorts..

Caution2: thermal expansion during operation will cause the stator arms to lengthen, and magnet height, width and thicknesses to all increase. If you rely on assembly variations on the order of a few hundredths of a milli-meter (0.001 inches), this may cause problems. As the stator warms up from heating caused by the wires, the Air-Gap will be reduced.* If youíve provided too little clearance between the magnets and the stator faces, you will end up with stator/magnet obstruction. The magnets also heat up and expand as the motor runs. This also reduces the Air-Gap by some degree. It adds another factor to consider during assembly. If you have assembled the motor such that the magnets are too close or press against each other when cold, then once they heat up this could contribute to adhesive failure, warping of the rotor/flux-ring (due to mechanical stress caused by the heated magnetsÖnot thermal stress), or buckling of the magnets. Any of these factors could contribute to motor failure while running.

*The rotor shell will also expand with heating, which could help reduce the severity of this problem. However, because of itís greater distance from the heat source, its greater ability to dissipate heat, and itís less significant contribution to radial movement, itís contribution is likely not as significant in affecting stator/magnet obstruction during operation.
Last edited by escudo; Dec 02, 2004 at 07:07 PM.
Dec 03, 2004, 12:14 PM
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Stator Winder Construction Details

Dec 08, 2004, 04:20 AM
homo ludens modellisticus
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DIY magnetic flux/strength meter:
Last edited by Ron van Sommeren; Dec 08, 2004 at 05:47 AM.
Dec 20, 2004, 11:02 PM
Model Bender
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Old VCR's and printers are a great source of pre-ground shafts,circlips & bearings to use in homemade cdrom motors. I have lots of 3mm shafts with circlip grooves accurately cut in them already (some with splines as well!)

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