935 harmonic hum

[petertha]

I have a (not PM) but likely identical Taiwan 935-VS mill. I'm not sure if I just never noticed a sound that has always been there, or maybe hearing it different recently with the speed set at similar RPM on recent machining op. When I first turn it on, its the silky smooth motor running sound for maybe 5-7 secs. Then it kind of makes a low frequency in & out, low hum, maybe a second in duration, spaced 3 seconds apart at say 1000 rpm. Its not a problematic vibration or anything. If I put my finger on the underside of the cover plate & squint my eyes, it seems to corresponding to where the noise is coming from. If I switch to low gear noise is either gone or barely perceptible (hence my guess rpm/harmonic?).

From what I can see of the parts, schematic, that bottom cover is below where the sliding cone discs & a spring is located. I could readily visualize the spring doing a bit of shimmy like this, although it could be any combination of sliding/moving part.

Any comments? Any advise before removing the cover plate? Will parts come flying out or is it just sealing off the assembly inside? I undid the cap screws, it didn't drop off so thought I'd ask first (but could also just be new machine paint in the crack).There is kind of a circular boss in the casting although I don't think it holds a bearing attached to the drive shaft assembly or anything. I see a (45) spring flange but assume that just retains the spring etc. & not connected to the plate?

 

[davidpbest]

You have a mill with a Reeves drive, which is somewhat different from my own PM935, but these links may help you know a bit more about what you're getting into.

My challenges link

Joe Pie's Reeves drive overhaul link.

Hope this helps. Good luck.

 

[petertha]

Thanks David. I remember your posts (with both admiration & apprehension LOL). My sound is more like a low frequency hum. Nothing i would call 'grindy or clackety'. But seeing your clear videos again makes me think that's exactly what I should do because hearing is different than attempting to describe with words. Let me embark on that. I taught myself enough with the camera & editing software to make a hack job shorty clip. I'll figure out how to post it to my cloud account or some place I can share.

And maybe I screen grabbed the wrong assembly drawing? My manual is just brutal, that's why I referenced the PM docs & came here.

 

[davidpbest]

See if the sound changes if you move the select levers for the quill feed.

• Disengage/engage
• Distance per revolution
• Forward-neutral-reverse

If that doesn't make the sound change, then it's probably in the spindle drive mechanism rather than the power downfeed system. Does the sound change if the back gear is engaged/disengaged? Your spindle drive system looks different from my mill head.

 

[C-Bag]

What if it’s just that big toothed belt coming off the disks and making the belt vibrate?

 

[petertha]

I'll have some pics for tomorrow & fingers crossed a video. I removed the bottom cover & indeed there is a recess to contain a bearing. That's why it didn't just slide off. I gingerly opened the gap equally around the sides until it came off. So I'm pretty sure the upper schematic is not my machine but also quite certain the lower one matches exactly. I need to noodle upon this. My bearing looks & feels great. There is a bit of belt dust in there but I would say as expected for a low hrs machine. I noticed the one toothed belt was riding a bit low on the toothed wheel, like adjacent to the upper housing floor.

I removed the side cooling grill covers to shine a light in there while running. Nothing out of ordinary stood out. I see a few linkage levers with pivots, shaft within a hole type mechanisms that could be hum culprits. The spring seems very stiff to be humming, but its all relative to the power & motion. Maybe it is mild belt hum related to tension. I pushed the end of a Popsicle stick lightly on the side while running & don't think I cancelled anything, but there are other belts in there.

Well, let me organize some things & provide more visual to what I'm seeing (and hopefully what I'm hearing).