I started out going gentle and easy on the bearing as suggested...but it just didn't seem to be leaving much of a mark even...so I started using more pressure and more time. Eventually did get a scuffed surface but not a groove like in David's photos.
Anyway...I have to do it again. Yesterday was one of those days when I apparently should not have even attempted anything
I started prepping my new flywheel. It did not go well. Even though the support material came out easily The M2 screws didn't want to go in...so I had to drill them out a little...and then it turned out that they weren't quite long enough. Then all my 1.5mm allen wrenches started spinning in them with almost no pressure - those tiny screws are finicky. Finally go the M2 screws in and playing nice and then it didn't want to seat all the way on the motor. I didn't modify that part of the print so no clue why it didn't want to fit this time. Bit of work with a blade and got it to go on eventually but never fit quite as well as the previous flywheel. Then tried to mount up the bearings and the new snugger holes are just a bit too snug. Grrr. So...I went back to the flywheel I had mocked up previously but felt the bearing holes were just a hair too loose on. Figured it would be ok at least for a quick test.
I need to find some non-allen head m2 screws....looks like Amazon has a nice set
https://www.amazon.com/dp/B01FTI8TM8/ref=cm_sw_r_cp_dp_T2_TxICzbH15V8YW But the heads look kind of big....hmm...those are IBM notebook screws...I think I have an old thinkpad stashed away somehwere...if not I know I've got an old Dell and HP that aren't usable as computers anymore...maybe I can scrounge some suitable screws off them tonight...
Whipped up a quick guide mount and all was looking pretty good:
Closer inspection shows that my bumps for the guide bearings are indeed just a hair too tall. I had assumed the needle would come off the center of the flywheel bearing...but it actually sits somewhat off due to the coils wrapped around the bearing - so removing the washers from behind the lower bearings would help...but then it wouldn't line up with the guide as well. So I think a second washer behind the flywheel bearing or making the flywheel just a tiny bit thicker would be a better way to go:
But turning it by hand action felt and looked great! I was getting hyped!
So...bit of work to balance the flywheel. I don't have a grains scale...so I just went for the prop balancer style method. Grabbed an M3 screw that was a bit shorter than the one that holds the bearing and put it in the furthest hole on the basis of "That looks about right" - put the flywheel on a shaft and gave it a few spins...and it sure seems to be pretty well balanced - though without the actual needle so maybe a hair more on the counterweight in practice will be needed.
Put power to it (without the needle) to see how it felt...and...motor just juddered
Crud. Apparently in all my messing around the tonight one of the motor wires got damaged. It will run if I hold the wires "just right" though and I have a couple more of these motors just have to dig one out. For now got it to spin up...and yeah that feels pretty darn smooth.
So mounted it in the machine and tried to get it going. These motors have a nice shiny chrome bell so I had just put some black electrical tape around most of it leaving one patch of shiny - which I figured would make the optical tach happy. Nope. Got all kinds of crazy nonsensical readings. I know this thing is doing more than 22RPM and highly doubt it's doing 10k at less than 1/4 "throttle"
Ok, piece of the reflective tape that came with the tach on the shiny spot...and...we're getting readings that seem ok again.
Oddly enough...much like my old one it runs really quiet and smooth at around 5k, and at about 8k. But at 6k where I usually run it there's some kind of harmonic vibration that isn't great. 5k doesn't make a good cut though, and the old one would blow the crank out and cause the needle to come off at 8k. I'm hoping this one will be happier at those higher RPM's mostly so it will run smoother but also because I'm hoping it will let me run a slightly faster feedrate on cuts.
Anyway...got a feel for where my speed needs to be...things seem good...let's put the needle back on and fire it up!
Ok...so far so good...all looks good to my eyeballs. Sounds pretty good. Let's try cutting a bit of foam! I didn't actually do any cuts, just grabbed a scrap of foam and held it under the guide and moved the foam instead of moving the tool. It was working...then...the needle came off. Grrr. And before I could reach the servo tester to turn it off...is that...is that PLA I smell? BOOM! Crud.
Apparently even at about 6k somehow the flywheel bearing was running fast enough to build up enough heat to heat up the mounting screw enough to melt the PLA. No clue where the screw, washer or my bearing are - I looked for them but they're MIA so can't fully post-mortem the bearing. I'm guessing my attempt to groove the bearing resulted in some damage to it that caused it to run hot. Either that or when the needle came off it got stuck behind the bearing and was rubbing creating heat...but it sure looks like it was the screw that got hot.
I'm half tempted to order up one of these - it's 10mm OD instead of 8 but is pre-grooved:
http://www.vxb.com/623VV-V-Groove-Guide-Miniature-V-Bearing-p/623vv.htm
Just don't want to spend $5 on one silly little bearing...and they only offer UPS shipping so no way I'm spending $14 to ship a $5 bearing
They are on Amazon for $6.55 with free shipping though:
https://www.amazon.com/dp/B00TKYW7MG/ref=cm_sw_r_cp_dp_T2_0tICzbNEBBS80 Kind of tempting.....
4x13x6 v-groove bearings are apparently very common and cheap (I can get 10 for $3 with free shipping from China) but seem a bit big for this. (that's 4mm ID, 13mm OD, 6mm deep) though as deep as the groove on them is the od at the bottom of the groove is probably pretty close to 6mm. (apparently it's 9.6mm at the base of the groove according to one spec sheet I found.)
I was actually looking for flanged 3x8x4 bearings but found those grooved ones instead...a 8mm version of the 623vv would be perfect...but doesn't seem to exist. And VXB's photo of the 623vv is somewhat different than the others I'm seeing - VXB's is grooved - other are a more pronounced V shape....hmm.
Looks like F693-ZZ is a 3mm ID 8mm OD 4mm thick flanged bearing...two of those together could make a nice needle retainer that doesnt' need manual grooving....but that's two bearings so more weight. They are available 10 for <$5 on ebay though....I have a few spare flanged bearings from my 3d printer upgrades...I need to check what size they are....
But the big question is...why did that bearing run so hot that it melted the flywheel. It was only running for a minute maybe two at most. And shouldn't have been running too fast or anything, even if the tach wasn't quite right the motor kv and voltage I was at should have had me very conservative on speed at the "throttle" setting I was at - About a 2 on my servo tester... 850kv motor on 13.8v power supply should = 11,730RPM at full throttle. And without the flywheel I was seeing about 11k at full throttle and 6k at the 2 setting I was using. (note this was just hooked to my cheap radio shack supply for testing - eventually I plan on powering it off the same 12v PC supply I use to power the MPCNC so will actually be running on 12v not 13.8 so theoretical max RPM will be 10,200 and I'll hopefully move the sweet spot closer to mid-throttle.)