Author Topic: Seeking advice regarding replacement of spindle bearings in a HF 90939 Knee Mill  (Read 264 times)

Offline t2krookie

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So I had the misfortune of watching my beloved half-pint knee Mill totter and then fall on the sidewalk to come t rest on the left side of the table. I "THINK" the handle absorbed a significant amount of the fall by folding nearly flush with the wheel.  At the time, the mill was not opp operating due to disassembly of the stock starter and direction switch in my bid to remount all controls at the top in a box similar to the newer grizzly 6x26 model for convenience and to add a tach and eventual VFD control. Thusly It was a couple of months until I got around to rewiring the starter switch. At first it made a horrible sound that made me fear the worst. However, after a little common sense kicked in, I found it was the top of the lead screw rubbing on the underside of the belt cover.   No problem and now I have a good reference point to start working on the pneumatic lead-screw project.

Unfortunately, 2 problems emerged as I started setting up to mill down a piece of mild steel for a lathe project (another repair).

1st and most boggling, I tried doing a rough tram using a dial setup removed from its mag base. I dont have a precision ring, but I wasn't doing precision work so I figured Id just get it within a thou over 12" and go from there.  Problem is, regardless of how many times i cut the difference in half on either side of the table, i could never get them to even come close to agreeing.

So as I have my dial in contact with the VERY clean and stoned table, I'm traveling the table towards the machines left and the indicator starts climbing and climbing, to where I ended up with a nearly 6 thou deference between the right side of the table and the left.  Could the impact have done that to such a thick piece?

Anyways, I was a bit desperate to at least get my lathe running again, so I shimmed up the damn vise under the right side of the table to where I got a decent consistent indication across the x axis  (nod was fine and remained <0.0005 across the entire 6" of Y travel) .

Was cruising along on a 1/4 Carbide roughing mill but it was taking a long time (really need to figure out mounting that darn power-feed), so I jump up to a 1/2 inch HSS end mill  and go to town.
The machine is hating it and starts chattering and scorching the work.

I had hogged out serious depths before with this same setup (minus the shim) so am now worried that the bearings (which are not the best I hear in the first place) may be damaged.

I ran the same indicator (cheap 1 thou indicator) rod with the dial attached to it and snugged it up in a collet. With my thumb i heavily pushing about an inch down from the collet, I'm reading up to 2 thou deflection but when i check run-out on a nice quality rod, im still at <  tenths.

Thoughts?

Bearings bad ? If so, does anyone know a person who had replaced them? Can you point me in their direction?  HF is useless regarding this. They are not answering emails or phone calls.

As for the bed, I had disengaged the left lead-screw collar to check the origen of the bed rising to no effect. Bent lead-screw maybe?  I plan on disassembling the bed but looking for clues on what t look for from the collective wisdom of this board.

Lastly, if all you have to add is suggestions for other machines or how yours is soo great and HF buyers are so misled, please keep it to yourself. I like this machine and it had served my amateur needs well. I just want to get her back into good shape.

Here is the skimpy manual. I had thought of using parts from a grizzly 6x26 but as most of us already know, just because these 6x26s look similar, doesn't always mean they are same specs.

https://manuals.harborfreight.com/manuals/40000-40999/40939.pdf

Offline AdeV

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Doesn't sound like a bearing fault - you've already done the deflection test, the other two things that would point to bearings are excessive heat generation followed by spindle seizure (don't ask me how I know this); and/or rumbling when running the spindle.

My Bridgeport has a dog clutch up top (which disengages when using back gear). There's tons of wear in that, resulting in a nasty rattle under certain cutting conditions. That doesn't seem to affect surface finish though (unless the vibrations are strong enough to back off the drawbar causing the cutter to descend into the work - again, don't ask me how I know that!)

The "rising rising rising" table sounds a lot like the problem I've got with my "Y" axis. I'm guessing the fall could have caused something in the knee to go out of alignment, which is causing the table to slope enough to cause what you're seeing; but without knowing more about the mill, that's just speculation.

From the description of the problem you're having with your big cutter - the only suggestions I have (and apologies if you've already ruled them out), are a blunt cutter, or a particularly nasty piece of steel...
Cheers!
Ade.
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Location: Wallasey, Merseyside. A long way from anywhere.
Or: Zhengzhou, China. An even longer way from anywhere...
Skype: adev73

Offline t2krookie

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Thanks for the answer from across the pond!! My Wife is British and she said "see, you Americans are less helpful and more rude".

 :doh: Well yes I did have a dull cutter certainly after I plowed into the bed due to the rising table lol. I wont ask you how you know because I have been many places I would not admit to myself.

Do you know by chance what is an acceptable temperature range for the chinese B grade  spindles/bearings  < 2000 RPM ?

I took the table apart and at least have ruled out some other issues and it is stable again. Something in the disassembly and reassembly procedure must have corrected the table rise error. I do think I need to replace the lead screw though in the X axis. Tips are always welcome on this subject but my first thought is to change to true inch or perhaps just a finer quality screw. DROs are going on today or tomorrow but someday I may make some new dials for the axis.

Has anyone any experience with these 6x26 knee mills or know of someone on these boards that has? The few threads I came up with in searching where fairly shallow.



Offline AdeV

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Thanks for the answer from across the pond!! My Wife is British and she said "see, you Americans are less helpful and more rude".

No problem! As you know, our default position is to be polite to everyone, and everything - including inanimate objects...! Although I am losing patience with this laptop, which shuts itself down whenever I use the keyboard for more than 30 seconds on battery power.....

Do you know by chance what is an acceptable temperature range for the chinese B grade  spindles/bearings  < 2000 RPM ?

I've only got experience of a Bridgeport I'm afraid..... but when I got it the spindle bearings were shot. After a few minutes at 2600rpm, the head casting was noticeably warm to the touch. If I carried on at that speed, especially taking heavy cuts, it would get distinctly hot, then the machine would start to labour. The only solution to that was to shut down for a while to let them cool off; at least until I got the bearings replaced. It still gets a little warm in use, but I think that's the motor rather than the bearings.

Hope that helps a little!
Cheers!
Ade.
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Location: Wallasey, Merseyside. A long way from anywhere.
Or: Zhengzhou, China. An even longer way from anywhere...
Skype: adev73