Grastens' Comm Lathe: Back to the 90s

[Grastens 2795]

Some time ago, I began the search for a comm lathe.

It started with the revelation that I am neither possessing a brushless RC car setup nor close to owning one. I suppose I also found the brushed motor to hold more aesthetic appeal than their brushless counterparts, perhaps in the same way that some automotive enthusiasts prefer the appearance of carburetors under the hood to fuel-injector bodies... Whatever the case, I had a surfeit of brushed motors, and no way to satisfactorily service their commutators. With a comm lathe, I would not only secure my commitment to brushed motors, but also get more use out of those which I had.

It should be noted that while I am not a millwright or machinist, I do have some experience with similarly-tedious tasks; I used to sharpen ice skates at the local sporting good store by hand. Getting the correct radius on a skate blade using a manually-adjustable holder and a grinding wheel took some practice and skill, as the shop could not sharpen every skate on its automatic machine (figure skates and ice hockey goalie skates were almost always hand-sharpened). I guess I came to embrace this skill, because the prospect of truing commutators no longer scared me after that!

Several months later, I snapped up a used Hudy comm lathe for a discounted price (yet still over what I should have paid, but I can't argue now ). It arrived a few weeks later:

This thread exists at the request of a few members on TamiyaClub who were well aware of my struggles to locate a comm lathe. Some were curious as to how one operates and functions, and so this will document my findings and thoughts.

Initially, I had read that my Tenergy TB6B charger, in addition to charging batteries, could serve as a DC power source. With that in mind, I attempted to replace the alligator clips on the comm lathe with a Tamiya battery connector, as that was what I had at the time.

What you see in the above photo is my second attempt, with the first one already removed from the cable  I could not figure out if the Tenergy TB6B actually has that particular function (my investigation says no), so instead I soldered a male Tamiya connector onto the end so I could use a standard Ni-MH battery (which I also use to this day). While I do not like the resulting lack of a speed controller, at least I have a ready and simple source of power.

For the uninitiated: a comm lathe uses another 540 motor and a high-tension rubber belt/band to spin a motor commutator, with little to no axial or lateral play. As a result, a fine blade can be used to "true" the commutator while it is spinning in this fashion. The blade is manipulated by two fine-adjustment dials which control axial and lateral location of the blade relative to the commutator, allowing precise passes of the blade against the comm. In this way, material can be removed from the comm, and a uniform finish can be achieved. The resulting clean comm surface gives the effect of a near-brand-new motor once reinstalled.

Some adjustment was required to ensure I would not overshoot the commutator or else travel too far past or into it. Hudy comm lathes use standard hex screws to secure things; I was able to use a standard Tamiya mid-size hex driver from my RC Craft Tool Kit to tighten everything down:

My research found that many who did/still true their own commutators preferred the hardened V-guides to the ball-bearing guides pictured on my comm lathe. I preferred the ball-bearing guides since I trusted bearings more than hardened guides for long-term use, although I have since found that replacing individual ball bearings in the guides is so tedious as to make purchasing new guides the better option. I do know a vendor who still sells those parts, so if I ever use this comm lathe to the point where the guides start wearing out, I can always either switch to the hardened type or stick with the ball-bearing units.

After checking that my soldering work on the battery connector was in fact functional, and testing a 7.2 V battery successfully (I now understand why high-turn motors are required on comm lathes!), it was time to see if I could truly true! 

[Grastens 2795]

By good fortune, I had a thoroughly-worn commutator which I could practice with:

Or, as the expression roughly goes: "You have to be good to be lucky, and lucky to be good." The fact that I wore out this rotor brought my need for a comm lathe to attention. This rotor belonged to a 13T motor that was once installed in my 037 4WD-H project; it ran so hot that the plastic endbell melted! Of course, having such a heavily-scratched and pitted commutator did not help, besides the usual mismatched gearing... In any case, it was an ideal subject for my first comm truing. Who knows, it may even find its way back into a motor can if it goes well 

I lubricated the guide bearings, the blade, and the commutator, and went to (very tedious) work. The result:

Well, that turned out nicely! My patience with "learning" the fine-adjustment dials paid off here, and I was rewarded with a clean-looking commutator 

The irony is that once I began searching my collection for rebuildable motors that required service, I only found one more comm that clearly needed truing. The next one is a 23T rotor that spins the tires on my WR-02C Opel Ascona:

It is not as egregiously-worn as the 13T rotor, but still needs some help. With the realization that my lubricant of choice could be harming the commutator (all I had was PTFE fluid), I used the permanent-marker tip that was so prevalent in the comm-truing community(?), and went to work again:

The result:

Even better than the first! What I did not realize until this comm was that the permanent marker also acts as a cutting lubricant, in addition to being a useful visual aid for determining trued sections of the comm. As such, I could be buying permanent markers instead of more-expensive cutting fluid, which made me relieved  It is good to know that I can still treat the blade with some care while saving money and making truing easier!

I was on a roll, but as before, I only had two comms to true... I guess I need to start running my models more often!

As a post-script to this post, I wheeled out the Ascona for a quick session after rebuilding its motor. And yes, the feeling of a brand-new motor following a comm true is quite a rewarding one! Rolling the Ascona as a result, maybe a bit less so 

[Grastens 2795]

A few things I learned:

- Copper shavings get everywhere in a hurry! A small tent broom and dustpan help, although a vacuum cleaner is ideal, particularly around the bearing guides. I see now that hardened V-guides do not have the same issues with copper shavings as the ball-bearing guides.

- I prefer setting the blade close to the commutator while the lathe is off, and then backing off a half-turn on the lateral dial (towards/away from the comm) before starting it up. I adjust the axial stop beforehand to limit the blade to the end of the comm just before the rotor windings; to overshoot that would take a fairly deliberate crank of the axial dial (along the commutator's length) as a result.

- Starting at said rotor end of the comm reduces the possibility of having the blade meet the comm too deep and causing problems, but it does require a precise axial stop adjustment. However, if you are truing a comm to begin with, you are probably that precision-minded anyway 

- Once started up, I make adjustments to the lateral dial in small increments, i.e. 5 degrees or so, until I see the blade working (either the copper shavings or the fresh copper underneath a permanent marker-marked surface). If the lathe is in good condition, axial movement is fairly easy; I was able to turn the dial rather swiftly without issue once the blade met the comm. However, slow is better, as well as consistency - the comm trues more evenly when the blade moves more steadily across the comm while rotating.

- PTFE lubricant (Tri-Flow, for example) works well on the guides (V-shaped or ball bearing), but I learned to leave it off the blade and the comm - a permanent marker works that much better, anyway.

- As such, diamond bits are so expensive because when properly dressed, they do last a long time. Two comms is a short time to make such an assessment, but I can say the bit showed no discernible wear after the fact.

- A 4.8 V battery may be the better option than a 7.2 V battery for a non-speed-controlled comm lathe, but of course higher-turn motors are also available. I may experiment with up to 80-turn motors if I stick with 7.2 V. Or what if I went brushless...? 

Until next comm!

[El Gecko 1344]

43 minutes ago, Grastens said:

Or what if I went brushless...? 

Ohhhh the irony

[Willy iine 18805]

I've not cut a comm in over 2 decades!  But I still have a lathe and diamond bits.  

I didn't take a close look, but if the SuperStockBZ can be opened up, I'll cut the comm on them down the road.

[Grastens 2795]

27 minutes ago, Willy iine said:

I've not cut a comm in over 2 decades!  But I still have a lathe and diamond bits.  

I didn't take a close look, but if the SuperStockBZ can be opened up, I'll cut the comm on them down the road.

Indeed it can! I dig up this ancient thread on re-timing Super Stock TZ motors because the original poster does a far better job explaining it than I could: 

I run a few Super Stock RZs in my cars, and was able to check their comms by disassembling them. I was also able to re-time them by following along this thread 

Another thing I learned about comm lathes, or at least mine:

- A comm lathe on 7.2 V is LOUD! Hearing protection is a good idea. I suppose it is no worse than an old vacuum cleaner, and probably no louder than a regular RC car, but I definitely noticed the noise. Perhaps running at much less load compared to a motor in an RC car will do that...

[Willy iine 18805]

Thanks, good to hear I can open up the BZ

As for powering the lathe, I use a 4 cell.  

[WillyChang 1814]

Bought same Hudy with Vs plus diamond bit BiTD... cost same as brand new TA04PRO kit 

Used to run it off 3-4c from a 6c ladder pack, just croc clip to the battery bars. But today I’d probably just hookup a single 18650. 

Yeah same irony - once you’ve cut every comm you have... they don’t need recutting again when they don’t get run

oh yeah TOOL TIP #101: DO NOT EVER LEND OUT THAT LATHE & BIT TO ANYONE
 

[Grastens 2795]

12 hours ago, WillyChang said:

oh yeah TOOL TIP #101: DO NOT EVER LEND OUT THAT LATHE & BIT TO ANYONE
 

Since everyone else runs brushless now, I don't believe I will have that problem  But good to know!

As for cutting comms for non-runners: my next adventure may involve the Stock Motor Holder set - but is it true that there is no way to cut sealed-can motor comms? The accessory pack states that it can allow the lathe to cut "sealed stock motors," but I have been led to believe that this denotes open-endbell motors only. If this is not the case, I have a pair of Traxxas Stingers I can work on...

[WillyChang 1814]

On 6/22/2022 at 8:20 PM, Grastens said:

there is no way to cut sealed-can motor comms?

there is 2 models of hudy lathe afaik, the one we have is the mod motor model. The other one is bigger with an extra layshaft I think; you mount the whole unopenable sealed can motor on it

when they say sealed can they mean the tab-sealed stock motor with the removable brush hoods. Not the mabuchi/Johnson 540 chromecan.

You gotta take off the brush hood then poke the cutting bit thru the tiny aperture. You won’t get as clean a cut due to wobbly unsupported bit & also you’re relying on spinning the arm on its sloppy stock motor bushings.

I got into this lathe gig when I only have Techni motors and rebuildable stockers so I only need the “mod” model