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View Full Version : Mikey broke his little miller



madwilliamflint
09-29-2011, 03:19 PM
Well this was weird. I figure I'd better check with y'all before I go tearing the thing down for a good poke around.

The miller in question is the Griz mini-mill.

I clamped up a soft bar of steel (cylinder, on it's side, 4" in diameter. It made the vice groan a bit, but I got it in there.) All I was looking to do was a simple parting off. Not a precision cut. Hell I wasn't looking for it to be square.

I put in a large 4-flute HSS endmill (maybe 1/2"? I figured it made more sense to have a thicker tool and take more material than have a smaller one and try to do something beyond it's capabilities.) Snugged it tight in the collet (tighter than I expected actually) and drew it in. It spun freely all happy nice nice.

I was aiming for about 5 thou maximum depth cut for the first one as I wasn't sure how it'd behave. (My miller's been largely neglected in favor of the lathe.)

I dialed the piece back into the tool (cylinder on it's side, left to right) and the tool bit into it a little bit but soon after made this horrid BANG and stopped turning.

The motor of the mill, however, spun quite merrily.

It sounded and felt quite like what happens when I've got a part-off bit too low on the lathe, where the piece climbs the tool, rocks the carriage forward and creates enough strain to shut down the lathe.

But this was different in that whatever was connecting the motor to the spindle was severed. I checked what I could check from the outside but reasoned that there's probably some kind of plastic safety gear (he says optimistically) that snapped.

Thoughts? I'll probably be tearing in to it this weekend. But I figured I'd ask you guys first.

Thanks o/

wheeltapper
09-29-2011, 03:33 PM
If its anything like my minimill there is a sacrificial plastic gear under the plate the motor fixes to.
whip the 4 bolts out and lift the plate, the gear is held in by a screw.

I've broken 2 so far:mad:

If its not that one youve broken one in the gearbox and thats a different kettle of fish altogether.


Roy

Bruce Griffing
09-29-2011, 03:33 PM
Look up replacement gear sets on Little Machine Ship.com.

madwilliamflint
09-29-2011, 03:36 PM
Cool, that's what I figured it had to be. It wasn't like I let the head freefall into the workpiece or something.

I'll go with that ;)

Thanks again guys o/

- M

Just thought of something: Does the belt-drive conversion kit LMS sells make sense? At $12 a piece I really don't want to just eat through these things.

madmec
09-29-2011, 04:28 PM
Sure that you have broken something?

I own a very similar mill and this happened to me dozends of times. It was always the gear change lever wich flipped to neutral due to the vibration, usually happens in low gear.

Greets from Germany

Thomas

madwilliamflint
09-29-2011, 04:33 PM
Sure that you have broken something?

I own a very similar mill and this happened to me dozends of times. It was always the gear change lever wich flipped to neutral due to the vibration, usually happens in low gear.

Greets from Germany

Thomas

Ya know...

That would be awfully nice. I'm "absolutely positive" (read: reasonably sure) that I checked that.

Deus Machina
09-29-2011, 04:37 PM
I have the same mill, the Harbor Freight version.

The lever can slip out of adjustment, but I've broken half a dozen gears over the time I've had it.

If you're lucky, it's one of the top two, under the motor mount. Four bolts out, swap gear, replace bolts. Easy peasy.

If you're unlucky (like me), it tends to be either the shift gear or one of the spindle gears. And while neither of these will completely ruin your weekend, they certainly try.

If it's one of those--or you just have the time--it's well worth it to replace it with LittleMachineShop's metal gear kit. (http://littlemachineshop.com/products/product_view.php?ProductID=3449&category=879658189) That kit is a godsend. I haven't broken a gear since installing mine, and if I do, the only sacrificial plastic gear left is the easily-replaced intermediate. Just make sure you get the right one, R8 or MT3.

saltmine
09-29-2011, 04:40 PM
I never had a problem with gears on my mini mill, but just to be safe, I bought the belt-drive conversion kit, anyway. Once the belt drive was set up properly, I had a much smoother running machine, and under heavy cutting I lost the herringbone pattern I used to get with the plastic gears.

I've owned my mill for quite a while, and have used it roughly a few times, but, knowing the limitations of the machine, I've never had too many problems with it. It's lasted long enough to need new spindle bearings. Of course, the biggest problem I had with it was accuracy, which improved markedly when I started adjusting the gibs correctly and replaced the lead screw nuts. The ugly handles eventually came off and were replaced with "ball & bat" handles which made using the machine more comfortable.
For convenience, I also rewired the variable speed control to provide forward and reverse on the spindle. After all these years, I'm on my third belt.

Most of the guys are right about the sacrificial plastic gears. They usually fail long before the motor can do any serious damage.

I also have a lathe made by the same people, it's older than the mill, and I've been into it doing upgrades and modifications through it's whole service life. One thing that got tossed right away, on the lathe, was the plastic change gears. The replacement steel gears run so much more quietly and hold their accuracy much better than plastic.

isaac338
09-29-2011, 05:19 PM
Cool, that's what I figured it had to be. It wasn't like I let the head freefall into the workpiece or something.

I'll go with that ;)

Thanks again guys o/

- M

Just thought of something: Does the belt-drive conversion kit LMS sells make sense? At $12 a piece I really don't want to just eat through these things.

The plastic replacements are in the $6 range.. you'd have to break a lot of them to pay for a $150 belt kit. If you spring for the metal gear (the $12 one), next time you'll just break the plastic gears inside the head (unless you also buy the metal transmission gears), and they're a lot harder to get to.

I just broke that plastic gear on my X2 and am having the exact same thought process as you.. I can't afford the belt drive right now though so I'm going to spring for the replacement gear and maybe work on making my own belt conversion.

lugnut
09-29-2011, 06:24 PM
If you don't spring for the belt conversion, at least when you order replacement gears order several. This won't be the last time you bust a gear.:eek:

wierdscience
09-29-2011, 09:01 PM
The belt kit is worth the money just because it's quiet.

madwilliamflint
09-30-2011, 10:55 AM
Sure that you have broken something?

I own a very similar mill and this happened to me dozends of times. It was always the gear change lever wich flipped to neutral due to the vibration, usually happens in low gear.

Greets from Germany

Thomas

Good grief am I a n00b.

Yeah I went down there just now and while the gear lever sure seemed solidly in place, I switched it a couple times then turned it on.

Spun just fine.

I'll put the belt drive conversion kit on the short list, after the planer and after the IRS stops bleeding me out.

Thanks guys o/

Deus Machina
09-30-2011, 01:17 PM
Try turning the spindle around by hand, on both settings, before cutting anything.

It's possible you broke just a tooth or two off. While it's usually obvious when it's running (it tends to make a whirring noise) it's better to check than to have the spindle suddenly stop spinning in the middle of a cut.