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Old 08-09-2004, 03:14 PM   #76
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rcnewb2004 - Don't worry about the increments, when I cut I usually turn the depth adjustment less then 1 increment at a time anyway.
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Old 08-09-2004, 04:53 PM   #77
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icic, what's the avg thickness of an unused comm? (i am just wonder what would happen if i did take off 0.05mm of the comm)
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Old 08-09-2004, 06:17 PM   #78
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I think a fresh, non-KR comm is about .298" to .300". I could be wrong though. 0.05mm is about 0.0018 inches, which isn't a whole lot, but probably a lot more than you may want to take off in one pass.

One warning.... Do not use the markings as reference points. You might be tempted to say that a certain mark is equal to .295 or whatever reference, but this is not always true. Any lathe will have slop in it's adjustment threads, so you should always visually check how far you are cutting into the comm. If you are working on a given comm and have to stop the drive motor, take special care of the bit. If you have to back it off to make some sort of adjustment, you don't want to jam the adjustment immediately to where you left off. If something in the alignment of the armature changed, you could potentially damage the bit. Remember, slower is better, take off a small amount of copper at a time.
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Old 08-09-2004, 07:22 PM   #79
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So i should do a "half" increment by naked eye if I want to lathe this thing?

On increments of 0.05mm, i am sure there will be some slop lets just hope its not going to be enuf to piss me off.

I ordered my lathe, and just waiting for the store to get it ready for me. That's the problem w/ internet shopping...

I want it now...
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Old 08-10-2004, 01:11 AM   #80
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Rcnewb2004....If I understand you correctly, what your saying is that serrated brushes exibit their performance advantages "solely" because of having less surface contact on the comm......We'll I can assure you, and if you do some further research, or ever get a chance to try different tests on a good high end dyno, that way you can see exactly what changes occur,you'll quickly find out that the practice of putting different sized holes....slots....and serrations on brushes.....has "much" more to it than to "simply" create less brush contact ,intern causing less friction on the comm to provide more rpm......I thought everyone knew that by now
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Old 08-10-2004, 01:53 AM   #81
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Rcnewb2004....It's been a while, but Just for the heck of it I just checked out the motor theory section of Big jims book...... he mentions that the addition of serrations or holes, flow more current to the comm do to the increase of edges.......who knows, it's all theory
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Old 08-10-2004, 02:18 AM   #82
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what are everyone's opinions on cobra motor lathes???

im after a lathe at the moment, and i know hudy make great lathes, but there are other lathes that are alot cheaper... so what do i look for???

can anyone reccommend anything apart from trinity???

i'd like to get something decent for a reasonable price, but i DEFINATELY DONT wanna buy something that isnt calibrated properly or anything and that will damage my motors

your help would be appreciated
thank you

robbie
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Old 08-10-2004, 04:07 AM   #83
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well, everyone here has told me to get a cobra lathe. I just bought one from 3racing. Haven't tried it yet.

JoeB:
Actually, yes, its true. I apologize. I have some basics in physics, but I failed to realize the real world works a little differently than perfect cases.

The comm in most cases is not perfectly round. Therefore having "bumps" on ur brushes, will infact create more surface area between the comm and the brushes. It also helps reduce friction at the same time.

Slots can sometimes also help remove the carbon ash that collects on the comm for better contact.

I actually figured it out after i sent the msg, but forgot to post my apology.

(if my "theory" is wrong again, do correct me. I want to get this right, that's the reason why I am posting)
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Old 08-10-2004, 04:35 PM   #84
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rcnewb2004....your mostly right ...... But,In an Ideal situation you want the comm near as perfectly round as possible and without any taper.....this way you keep the harmonics or brush bounce to a minimum...alot can happen with something spinning even slightly out of round on motors that turn 50,000 rpm's ...........I wasn't completely sure how you felt from what you said in your post, but for serrated brushes to work better, they dont (nor should they be) mated up against a uneven comm...........the cobra lathes are pretty good "especially for the price"...I used one for many years as well as some of the other comparable priced lathes....in my experience no matter what kind, or quality, of bit was used, these lathes were able to get the comm anywhere between 1 and 2 thousanths of being perfect....I think thats about what that they advertise in their instructions also.....recently I got my hands on a lathe that can cut within 2 tenths of a thousanth and (I wouldn't have believed it before) and not that it's nesessary ,but you can actually see a definitive difference in power and effeciency with the comm being rounder by just that little bit........all this for toy cars
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Old 08-10-2004, 04:59 PM   #85
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So u want perfect round+serrations?

ur brushes hitting a bump in the comm is probably like ur little car hitting a speed bump real car speed bump. (tried it... cost me a fortune)

Then I think the theory works this way. The serrated brushes, even with less contact area, provides enough area to conduct electricity the same way as full faced brushes do. This happens because the same force applied from the springs are acting on a smaller surface area, causing greater pressure on and better contact between the comm and brushes. (Therefore you get about the same power as a full faced brush) There is no way u can increase current into the comm by giving it less surface area. you can have roughly the same, but definately not better it.

Serrrations keeps the comm cleanner, by allowing the carbon ashes to remove themselves quicker. (carbon has a high resistance in comparison to copper or silver). any build up is removed from the comm by centrifical forces between the serrations. (So here, u may get more power than regular full faced brushes)

However, I don't understand the theory with the "more edges=greater current". Maybe u can as BJ to give a little more elaboration on that? The only way "greater current" can get to the comm in a RC car is bigger surface area (assuming brush and commutator materials do not change)

2 tenths of a thousand inches right?
Is that comparable to 0.05mm? (i don't know my conversions) cause I just bought a lathe that does 0.05mm

Once again, correct me if I am wrong, but I am only looking for some right answers, and not trying to argue something. I just have to get it through my head the right way. If I can't imagine it properly,.... this will bug me for a long time to come.....

Think of it as helping a newb through some growing pains.
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Old 08-10-2004, 05:51 PM   #86
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Boy....Just when I think it's over When I get some free time I will send BJ an e-mail asking for his detailed explaination on this subject.......straight line serrated brushes do the exact opposite of keeping a comm clean.....they actually really beat the comm up with deep gouges but is worth the price (to most)for the increased power you can possibly get (depending on the type of motor).......The theroy as the way I understand it is that electricity as it applies to the relationship between brushes and the comm surface is that on a full faced brush, the electricity (as always) perfers the path of least resistance, which in this case is to travel around the outskirts or "edges" of the full faced brush as the current is transfered..........Now by putting a serrated surface or a hole or a slot on the face of the brush, what youv'e done is create for lack of a better word an (amplifier) that happens to in "this situation" give greater power transfer because of the higher amount of curves and edges, which have inturn created "many" more paths of least resistance for energy to flow and don't forgett "JUMP".......remember this is all theroy....but one thing is for certain is that they provably make more power on the dyno, and "not" just rpm due to having less surface area, and also not the same as a full faced brush, or even one that has been narrowed down to compensate for the (contact) surface area you loose with serrated brushes........man ,im ready to just go brushless after this
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Old 08-10-2004, 05:55 PM   #87
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Since my online conversion chart got deleted....Im not sure about the conversion of 2 tenths of a thousanths of an inch......
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Old 08-10-2004, 06:13 PM   #88
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Hehe Indeed, I want toget this sorted out. Thanks for the time spent on this JoeB

I will keep it to myself for now until BJ comes back w/ a complete explaination.

Ok, 0.002 in. = 0.051 mm

50 microns. quite impressive.
My 3racing lathe is suppose to give the same numbers (0.05mm) now somewhat eager to get it... hope it arrives at the same time my v2 motor does.

Btw, to run in a motor, how many cells should I attach my motor to?
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Old 08-10-2004, 06:38 PM   #89
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I believe 50microns is almost 2 full thousanths right?.......I still don't know the conversion here....... but I meant 2 tenth's of one thousanths.....let me know what you come up with if you have a conversion chart...thanks it's been fun
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Old 08-10-2004, 06:45 PM   #90
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Diddn't see you other question......most motor builders agree on 3volts for stocks, and anywhere between 2 and 3volts on mods......The amount of breakin Time obviously depends on motor and brush type
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