Cutting a motor commutator & motor tweaks

[EddieO]

Good score.....the stock motors aren't very good for crawling, but they can work ok in basher trucks and such.

All the mod motors are good teardowns pretty much.....looks like a lot of Reedy based stuff....just need some crawler arms for them.

Later EddieO

[marvinbeaverdip]

looks like you made quite the score. I was happy getting my checkpoints for $5ea. What's your plan with all of them?

Speaking of cutting comms, what IS the ideal/recommended setup for powering a lathe? mine came with some 27t motor and a traxxas mechanical speed control attached. I believe I read on this thread that a 3 cell nimh was plenty, but plenty for what motor?
I'm thinking about using an old computer power supply to run 3.5v, but I'm not sure what motor would provide the correct speed for cutting comms.

[EeePee]

I use a peaked 3 cell NiMH pack and a 27 turn and it works good.

[Charlie-III]

My general rule on spinning speed is......."How fast do I want a flying arm to hit my ugly face??!!"

The spin speed relates to the feed speed. Fast spin, fast feed.....slow spin, slow feed.
Pretty basic, even an engineer can understand that.
(I'm a field guy, engineers are a little bit above sales, then politicians all above lawyers)

[EddieO]

My fantom has the stock fantom lathe motor in it, but its been reblanced and rebuilt, and I cut at 2.6 volts usually with most arms.....I find that even using the 4.8 volt pack with the fantom was too much.

I don't recommend cutting much over 5 volts. Anything over that is WAY too fast, regardless of what anyone else wants to say........its copper you are cutting, not steel....

Your 3.5 volt computer power supply should work fine, though a stock motor can get a little quick.

The best way to do it is to find an older charger with a motor run in feature(LRP, GFX, etc) or motor checker (orion, integy, much more, trinity). You can find most of them dirt cheap, especially the motor checkers...


Later EddieO

[Scattman]

I use a converted computer power supply. There are 3.3 and a 5 volt wires as well as 12 volts.

I run my Orion lathe has a 35 turn handwound and I run it on 5 volts.

[JohnRobHolmes]

The voltage used is relative to the motor running the machine. Going under 5v with a 55t running the lathe makes HORRIBLE cuts as the copper will not chip properly.

"regardless of what anyone else wants to say....." The face speed is more pertinent to getting proper cuts. Voltage is merely a secondary factor determined by comm diameter and driving motor speed.

Soft copper machining is generally around 200 feet per minute face speed. HSS bits will typically be used from 150 to 300fpm rates, and carbide and diamond are used with rates of 400 to 600fpm. Diamond also works well at slower speeds, but I have found carbide to be difficult to work with at slower rates as it is not as sharp. A 10mm diameter comm converts to .39 inches roughly. Multiply times Pi and we get 1.2246" circumference. That is close enough to .1 foot, so we can turn the 10mm comm at 2000rpm for starters. 7.5mm comms will need to spin 33% faster for the same face speed. Light cuts can be made at faster speeds, and sharper bits (diamond) allow for slower face speeds and deeper cuts. Very basic machining principles. I will always choose a faster and lighter cut. Bits last longer and each cut can be fed quicker. It also allows for better cuts with a machine that is less rigid, as many of the toy motor lathes tend to be quite flexible.


I use HSS and diamond bits depending on what is handy. Both can do a superb job. My little motor lathes have roughly a 3 to 1 geardown from motor to armature. Turning a 55t on 3s lipo gets around 9k loaded, a 27t motor would be ran at 3.7v for the same speed. This gives me an arm speed of 3000 and face speed around 300fpm, which allows me to take very light cuts of .0001" at a time on my big lathe and .0005" (radius) on my motor lathes. I can also switch to 7.5mm comms and the face speed is 225fpm, still in a good zone without having to change the voltage. If I tried to run the 55t on 5v it would take forever to complete the job, and the finish would not be satisfactory with a face speed of 135fpm on a 10mmm comm. I tried, it sucked. The copper tears and doesn't chip off properly. A laser tach aimed at the flat spot on the armature shaft being cut is an easy way to find the face speed of any setup. Divide RPM by 10 for 10mm comms and divide by 13.3 for 7.5mm comms. Keep it between 200 and 250 for easiest and most consistent work.

Because of the V blocks on the little lathes it is better to run them a bit slower if you are worried about the tool life. As long as the face speed of the copper doesn't fall below 150fpm and doesn't go above 300fpm you are in the best zone for cutting copper with either HSS or diamond tooling. With a 27t motor timed close to zero and 3:1 reduction you could go down to 1.6v with a 10mm comm or 2.2v with a 7.5mm comm and still get a fine cut. Once the face speed falls below 200fpm an inexperienced user can start to get tool grab and copper tearing, normally caused by taking too deep of a cut or a tool that is not rigid.

My $0.02.

[marvinbeaverdip]

Originally Posted by JohnRobHolmes

The voltage used is relative to the motor running the machine. Going under 5v with a 55t running the lathe makes HORRIBLE cuts as the copper will not chip properly.

"regardless of what anyone else wants to say....." The face speed is more pertinent to getting proper cuts. Voltage is merely a secondary factor determined by comm diameter and driving motor speed.

Soft copper machining is generally around 200 feet per minute face speed. HSS bits will typically be used from 150 to 300fpm rates, and carbide and diamond are used with rates of 400 to 600fpm. Diamond also works well at slower speeds, but I have found carbide to be difficult to work with at slower rates as it is not as sharp. A 10mm diameter comm converts to .39 inches roughly. Multiply times Pi and we get 1.2246" circumference. That is close enough to .1 foot, so we can turn the 10mm comm at 2000rpm for starters. 7.5mm comms will need to spin 33% faster for the same face speed. Light cuts can be made at faster speeds, and sharper bits (diamond) allow for slower face speeds and deeper cuts. Very basic machining principles. I will always choose a faster and lighter cut. Bits last longer and each cut can be fed quicker. It also allows for better cuts with a machine that is less rigid, as many of the toy motor lathes tend to be quite flexible.


I use HSS and diamond bits depending on what is handy. Both can do a superb job. My little motor lathes have roughly a 3 to 1 geardown from motor to armature. Turning a 55t on 3s lipo gets around 9k loaded, a 27t motor would be ran at 3.7v for the same speed. This gives me an arm speed of 3000 and face speed around 300fpm, which allows me to take very light cuts of .0001" at a time on my big lathe and .0005" (radius) on my motor lathes. I can also switch to 7.5mm comms and the face speed is 225fpm, still in a good zone without having to change the voltage. If I tried to run the 55t on 5v it would take forever to complete the job, and the finish would not be satisfactory with a face speed of 135fpm on a 10mmm comm. I tried, it sucked. The copper tears and doesn't chip off properly. A laser tach aimed at the flat spot on the armature shaft being cut is an easy way to find the face speed of any setup. Divide RPM by 10 for 10mm comms and divide by 13.3 for 7.5mm comms. Keep it between 200 and 250 for easiest and most consistent work.

Because of the V blocks on the little lathes it is better to run them a bit slower if you are worried about the tool life. As long as the face speed of the copper doesn't fall below 150fpm and doesn't go above 300fpm you are in the best zone for cutting copper with either HSS or diamond tooling. With a 27t motor timed close to zero and 3:1 reduction you could go down to 1.6v with a 10mm comm or 2.2v with a 7.5mm comm and still get a fine cut. Once the face speed falls below 200fpm an inexperienced user can start to get tool grab and copper tearing, normally caused by taking too deep of a cut or a tool that is not rigid.

My $0.02.

Thanks John. I actually wrote a response asking similar questions about face speed and different bit materials, but for some reason it was gone after I posted. In any case, you seem to have read my mind and answered my questions I was going to ask. I think one of my first experimental winds will be a 55T lathe motor. I figure that would be the safest application for a motor that most likely wont be perfectly balanced.

Eddie- Big Jim (and maybe you too) used to balance using epoxy blobs on the windings instead of cutting material from the arm, right? were those dynamically balanced as well?

Speaking of lathes, I have another one coming. This time it's a mystery, because the guy doesn't know what it is, and is giving it to me for the cost of shipping, along with an esc (not sure what), and some old motors. For the $15 it cost, I'm going to think of it as a surprise mystery box!

[brazwewn]

I really need to get myself a lathe. What are the best economical ones out there. I just can't justify a couple hundred bucks for a lathe. If it was $50 or less that would be great.

Wes

[JohnRobHolmes]

At $50, you will need to look for a used one. Even the cheaper Xipp lathes are generally around $100, but they are flexible enough that it makes getting a good cut difficult without really high face speeds and extremely light cuts.


Hit up ebay and rctech for "com lathe"

[EddieO]

I am not gonna continue to argue about lathe speeds. Go ahead and use 12 volts, when the arm spins off the lathe, have fun....

As for Big Jim and I using epoxy.....Big Jim only did it at Trinity when he worked there and I think maybe at Maxtech too. He personally hated epoxy balancing, because it added weight to the armature and there was cases of it coming off at 50,000 RPM, which would either throw the balance off or worse yet, lock up the motor.

I never balanced anything with epoxy until recently......the more I thought about ways to improve a crawler arm, epoxy balancing made sense. First, since no steel is removed, you lose no torque or efficiency. Second, the extra weight is actually a good thing, as it helps slow down the initial low speed start up. The worry of the epoxy coming off is very minimal at crawler motor speeds, not to mention I think it was often overplayed by Big Jim in his hate of Trinity. So I bought some, after a painful search......the kind used by Trinity, LRP, Fantom, etc is out of business, so I had to find a new source. Glad I did, as I am not sure I liked what they used, which was a 2 part that needed to be baked afterwards, while mine cures fully in 2 hours and doesn't change weight like the baked stuff does, which would throw off balance a little. Having to wait a whole day to finish a motor kinda sucks.

I only use it as an option or on Limited Editions, because its a pain and messy.

Xipp lathes are flexible? Hard to get cutting straight sure.....but flexible?

Later EddieO

[JohnRobHolmes]

Who is arguing? 11v spins my lathe with 3:1 geardown & 55t motor at the EXACT same speed as my Fantom and Cobra on 2.6v with a 27t motor. If you would read what I posted you would see that we agree on how fast the arm should be spinning. You never took the time to ask what I run, you just assume I am spinning the arm at 100000 rpm since I am using the forbidden high voltage.

[EddieO]

2500x2.6=6500/3=2166.66667
987x12=11844/3=3948

A 3s lipo puts out a tad more than 11.1 volts...unless you are running a mostly dead pack....even then, still 3600 rpm at 11.1

Almost double the speed....


Later EddieO

[JohnRobHolmes]

Fantom lathes have 2.28:1 geardown and not 3:1. The Fantom motor spins 2700kv with zero timing, and 2900 rpm per volt with some advance.

(2900x 2.6)/ 2.28 = 3307 Measured under load on the lathe I get 3200 rpm on the arm with a tach. Pretty close to expectations.

(987 x 11)/ 3 = 3619 Measured under load on the lathe I get 2900 rpm on the arm with a tach. Higher resistance motors drop more RPM under load.


So my lathe running 11v (11.2v, half charged lipo) is actually turning slower than my lathe running 2.6v. Spinning off the blocks, right? Even a fully charged pack keeps the speed under 3300 rpm on the arm.

[JohnRobHolmes]

Originally Posted by EddieO

Xipp lathes are flexible? Hard to get cutting straight sure.....but flexible?

Later EddieO

I can change the depth of cut with light pressure to the tool block. I consider that pretty flexible. Still workable, but flexible none the less. I can measure deflection of the block at top center in .001" per ounce if you wish

[EddieO]

I love these numbers...

On your own site, you list a 25t at 2588 KV, yet on here if its a typical 27t 2700kv?

What magical lathe are you using?

And pretty much any bit lathe will have deflection if you push on the tool block with light pressure....it's a pit lathe...

Later EddieO

[JohnRobHolmes]

The 25t Kv number may be wrong, it has been years since I measured. I remember the last 27t I measured right at 2700kv, but I could be mistaken on that as well. I will be sure to remeasure the Kv and update the site if needed.

It doesn't change the fact that the stock motors included in the Fantom lathe are 2900Kv unloaded, and the measurements taken between the two lathes show that 11v can be used effectively with the right motor and setup. I assume they are 27t in the Fantom, but haven't double checked it. Maybe you know what they are? Assuming my Kv chart is correct it would probably be a 23t motor.

[EddieO]

I am pretty sure I am comfortable admitting mistakes and learning from them.....I didn't get to take a vacation from my family for nothing

The fantom uses a dog slow 27t motor.....if I get any where over 5 volts on it, the thing usually spins off the vblocks.....my xipp, which has a 55t in it runs at about 5-6 volts before it starts to freak out.....I've measured my rpm before at 3 volts on the fantom, was just over 2k...

Not sure where you are getting the kv ratings for the motor either, let alone the gear reduction number...

Might wanna update that site.....gotta keep info correct.

Still didn't answer my question, which lathe are you using?

Later EddieO

[JohnRobHolmes]

I am getting my numbers from two methods. The first is to drive the motor as a slave at a given speed and calculate the output voltage. rpm/ voltage = KV. The second method uses a laser tach. Drive the motor with a known voltage, measure the rpm. RPM/ volts = KV. Both methods gave me the same numbers for the lathe motor speeds. Measuring the armature speed is easiest with an optical tach. I used both, got the same results. Getting gear reduction is as easy as measuring the armature speed VS the motor speed with an optical tach.

Sounds like you may have a different motor than I. I get 8760 RPM on the drive motor at 3v (measured 6 times now). Indeed, it sounds like it is more important to know the motor speed more than just throwing the standard voltage at the lathe and hoping it turns out right.

What lathe am I using? Xipp, Fantom, or Logan. Just depends on which shop I am in. The Xipp is the least rigid, and is the one with 3:1 geardown.

[Rootar]

You guys dont know squawt Ive got a 70t handwound in my lathe i bought and i run it on 6-7v and its awesome!.... lol

not really, thread was getting a little tense i thought id throw in a funny.