Where is the looseness? Indeed!

Well I finished the modifications after getting some parts from MSC.

Here is a pic that tells the story, it is a before and after on the same part in the same setup, with the same chuck and tool. Parting off, an operation I had never been able to do at all.

Back before the whole mod issue started, I had started trying to part this piece off, but had to quit due to the horrible chatter. The machine was trying to jump off the stand, despite being bolted down.

I quit working on the part, and started trying to figure out what was wrong and what to do. Logan was no particularl help, I think they really weren't sure what to do or what the problem was.

I came up with a theory on the problem, and designed a fix which involved a small modification to the machine. After the mod, I put the part I had trouble with back in the same chuck and had another run at it with the same tool.

The pic.


The part is a 0.750 OD bushing. The outer portion, the nasty looking "gear tooth" pattern is the chatter marks from the two attempts to part off prior to the bearing modification. Two speed feed combos, hence two patterns.

The smooth inner portion is the recently parted off surface after the mod. No problems, just needed a change in speed to get it to part off quiet as a mouse. The barest hint of a "hum" was discernible if I didn't get the feed even. This was hand fed parting, with no oil or other cutting lube.

The modification involved no un-reversible work on the machine, so if one wanted, one could put it back to "stock" (if one were really a nutter).

I think it worked quite well.

[ 10-03-2004, 09:58 AM: Message edited by: J Tiers ]

So what is this mod you are talking about?

Randy

Too much to post details.
I have been sending some pics and info to interested Logan owners.

The history of the issue....some long threads from a couple weeks ago give the issue.

http://www.practicalmachinist.com/cgi-bin/ubbcgi/ultimatebb.cgi?ubb=get_topic;f=1;t=007062;p=1

and

http://www.practicalmachinist.com/cgi-bin/ubbcgi/ultimatebb.cgi?ubb=get_topic;f=1;t=007171

Basically a fix for the fact that new bearings from the manufacturer of the machine (Logan) did not work like the original ones. Specs changed since the 1940's and the original bearing company (New Departure) is long gone.

J. Tiers:

I followed both your original threads with interest and concurred with your theory about the rear bearing fit. I'm curious about the amount of preload that you've imposed on that bearing.

Some advocate that the spindle (with chuck attached) be given a brisk rotation by hand, after which the spindle should rotate a further "arbitrary" amount (usually given as 1/2 to 1 turn). This procedure, IIRC, applies to spindles with tapered roller bearings.

What was your procedure to determine the "right" amount of preload ?

Thanks for the information.

I'm no expert on this, but from the pic it sure looks like the spindle had end float. That would give a combination of axial and radial motion to the spindle, and the chatter would seem to be inevitable. Radial play only (seems to me) wouldn't account for the way those markings are.
As far as figuring the amount of preload, firstly, it has to be enough and then some to lift the weight of the chuck with workpiece in it, against the angle of the balls in their races such that even the upper balls are under some compression. That will easily be enough axial tension to keep the spindle from deflecting inwards when taking a facing cut. No way to measure this, except by the one method described, how long will the chuck freewheel on it's own, or by giving a bit of run time and checking temperature. You could just monitor the temperature, and when the bearings heat beyond warm with extended running, you've
passed the optimum setting. It's been a while, but IIRC, the instructions for the tapered wheel bearings on the 'cruiser' say to tighten, then back off 1/8 of a turn, then monitor the heat, and back off until it gets no more than warm. It seem obvious to me that unless there's heat being generated, there's no problem with taking out all the play and having some pre-load.
I suppose one could consider that if there's pre-load, but no heat being developed, you'd have the opportunity to increase pre-load to better control spindle position. In J Tiers case, the rear bearing might be the one to check for heat, since it's smaller and with the same tension as the larger bearing, it will heat more. Of course, another factor is the heat generated by the seals. You'd have to judge that difference by running with no pre-load for a while, then adding tension and gauging the difference in temperature.

I don't think there is any question of heating. That seems to be more of a tapered roller bearing issue, but I might be wrong. In any case, I didn't notice any heating particularly.

What I did was to WAG it and apply what I conveniently could, which is about 50 lb. Got that figure from an NTN document on spindle bearings.

I can, if I decide to, change the setup and get it higher. So far that does not seem to be needed, I get acceptable performance.

The rotation test gets about 1 1/2 turns, but that includes the drag of the flat belt with no tension. That number of turns is about as expected.

J. Tiers, thanks for the info. I agree with the 1-1/2 turns as being about right (as I recall, you have duplex b-b ball bearings in front and a single row in back).

It's been a frustrating experience for you but your travails will most probably benefit others with a similar problem.

Cheers

No kidding. I think JT should type up the
entire experience and submit it to HSM magazine
or such. I would be willing to bet there's
a *bunch* of folks out there with similar
machines, headscratching the same problems.

At least it might convice them to leave their
bearings alone if the machine were running
right, knowing that the ones in there were
pure unobtainium.

Jim

The nose has a double-row bearing, not duplex. As such, it is equivalent to a single bearing in either thrust direction. The opposite direction, what would be pulling the spindle out, has a lower rating.

I suspect that if I had checked it before applying preload, it would also have been about 1 1/2 turns. Even with 50 lb preload, I doubt if the friction is anything but just normal grease-stirring.

The bearing has a 1 million turns rating of 4 tons. Probably only the most minimal deflection and indentation would be caused by 50 lb.

I don't think an HSM article is appropriate. I already got yelled at plenty for posting the original questions on the Logan list. Folks, including Scott Logan, seemed to think that was "airing dirty laundry" in public, and that I should have restricted myself to asking Scott privately and directly. I don't get it, but whatever stirs their chips.

An article would probably cause serious howling. I don't want to employ a defense lawyer just for a $50 author fee and some publicity. I don't need either, nor the hassle.

I ought to mention that if anyone has a Logan model 200, and has problems similar to mine, I can forward a writeup and pics if you are interested. Just email me.

It is about a 280K file, if you happen to have a slow modem hookup. I have a 56K, and it is a little slow, but not bad.

What was that 3rd bearing you used?

Thanks,
Rick

Being new here, I don't know what I have to do to view the 2 links in post #3. I get the 403 permission error. Normally, links like this work fine. I am extremely interested in everything related to bearings, chuck mounts and chatter as I have one lathe that is unusable due to chatter, a couple others that are touchy when parting and one that is a complete joy and has never hinted at chattering either turning or parting (D1-8 spindle). Thanks...

Ohhh I see what I did. I got to this page via a link in a current discussion and now realize that this is 11 years old. I am sorry I pulled it up again. Still I would like to know what to do to see those links.

Try this.


http://www.practicalmachinist.com/vb/general-archive/where-s-looseness-81590/

Thanks! That gets me there.

Hello, I am replacing bearings in a 815 so very similar to the 200 and would love to see your write up. Thanks. I am new to forum and fumbling my way through so I hope this works.