Banding on finish when using powerfeed on G0602

[Lew Hartswick]

Are the bands always the same pitch?
Have you tried it with a center in the tailstock supporting the work?
You should be able to see the tool or compound move when that happens it's big enough.

That is a perplexing problem.
...lew...

 

[90LX_Notch]

Something has to be causing a change in the velocity of the carriage in relationship to the spindle rpms.

I would check for excessive endplay on the lead screw as well as excessive backlash in the halfnuts. Another thing to look at is wheather or not the halfnuts stay fully closed as the carriage travels along the bed. If there is slop in the lock cam for the halfnuts, they will open slightly as the carriage encounters heavier forces. This would explain the collars. As the carriage stalls the tool dwells and cuts deeper. Then, as the carriage moves it creates a high spot. My Atlas 618 MKII had such bad slop that the carriage wouldn't move. I was going to make new halfnuts, but when I really looked at the problem I found wear in the cam lock for the halfnuts. I made a new one and changed the centerline distance to increase the travel of the halfnuts when closed.

Also, it could be something as simple as the belt slipping. That or there is flex somewhere, but I would think that would give chatter or a bad surface finish (pulls, tears, etc.).

 

[wannasteam]

Thanks a lot for such interest in this problem. I am very impressed by the assistance you have given.

Ken: That is EXACTLY what I'm getting. Does it go away if you manually feed with the saddle wheel? How about turning the leadscrew by means other than the gears?

Marcello: I will try your suggestions on the weekend. I will rig something up on the gears end of the leadscrew and drive it with a battery drill and see what happens.

Dave: Another idea I have is to mount 3 lasers on the toolpost pointing in X Y and Z axis and shine the lasers on the walls and ceiling. That will amplify any movement and maybe that will give me a clue. I'm not sure what this will tell me but it would be interesting to play around with this.

Lew: Yes they are the same pitch as the leadscrew. I agree...this is certainly a head-scratcher. :-(

Jason: I'll take the saddle/leadscrew assembly apart on the weekend and report back. Notch's idea about the camlock is interesting, thanks.

I bought this unit from an ebay store based in Adelaide, South Aust and returning the unit is not an option as the freight costs involved would far outweigh the benefit. I want to try to fix this problem with everyone's help and maybe help someone else with the same prob.

I am sure that by taking a systematic process of elimination, we will find the solution...As Dave said "We will leave no stone unturned"

(I wonder if I will get the same banding if I turn a stone) :big: :big:

I'll try to do a few things this evening but I'll leave most of the disassembly stuff until Friday and hopefully get some positive result.

In the meantime, keep the letters coming folks!

Cheers and thanks again.

Joe

 

[wannasteam]

I hope I'm not violating forum rules by placing this link to another BBS forum but it might be of interest. If I am, mea culpa, mea culpa, mea maxima culpa.

http://bbs.homeshopmachinist.net/showthread.php?t=36425

Consensus seems to point to a bent leadscrew. I'll take mine off this evening and check it carefully.

Cheers

Joe

 

[Ken I]

Joe, it goes away if I feed manually - but only on finishing cuts, wierdly I am also getting banding on face cuts.

I was also incorrect when I stated that its definitely leadscrew pitch - its not !

Since I am trying to diagnose this I have started to consider everything.

I now realise that my lathe is a lot noisier than it was when new and has a cyclic vibration to it - the periodicity of that vibration matches (I think) the banding.

I am begining to suspect spindle bearings.

Since yours is a new lathe I would not think this to be the case - but then again I have seen brand new bearings with brinelled or corrosion pitting of the raceway.

The amount of noise this can kick up is amazing, I once changed my car's gearbox mainbearings in an attempt to cure a serious noise/vibration problem - the wear/pitting on the bearings was almost nothing but it turned out to be the culprit.
I had a similar experience on a brand new industrial fan unit - the supplier (tried to say it was normal) would not change them - I said I would pay if there was nothing wrong - this thing was screaming like a banshee - again it fixed the problem and again the pitting was so microscopic it was hard to believe it could have caused so much noise.

New bearings can be brinelled (dented) by rough handling / installation and corrosion pitted by temperature swings causing internal condensation or by induction due to rotation in a magnetic field (as occurs in unshielded electric motor bearings).

I'll let you know what I find.

Regards,
Ken

 

[wannasteam]

Hello Ken

Joe, it goes away if I feed manually - but only on finishing cuts, wierdly I am also getting banding on face cuts.

Yours is even weirder than mine is!

I take it that you have powerfeed to the y axis? You might be on the right track with that cyclic vibration you describe as it seems to affect both axes.

My lathe seems to run pretty smoothly but I'll disengage the drive to the spindle and see if I can feel any roughness there.

Cheers

Joe

 

[Omnimill]

I apologise in advance. But not related to your problem Joe, your lathe looks like it's NOT variable speed - maybe belt change? Anyone know if this type of Lathe is available in the UK?

Sorry for butting in ... :-\

Vic.

 

[Ken I]

Joe, I have powerfeed only in the Y axis - not crossfeed - thats what made me think its not powerfeed related and made me look at the pitching of the bands which appears (hard to tell for sure) related to the up/down harmonic vibration.

The lathe didn't used to vibrate and nothing is bent or out of line - so I stripped my headstock this morning and am popping out to get new bearings all round.

Can't say that I feel anything wrong with them but experience has taught me that doesn't mean much.

http://www.homemodelenginemachinist.com/index.php?topic=15669.msg160811#msg160811

I busted the cast iron locknut and posted a caution.

Ken

 

[wannasteam]

Ken, what a shame about the locknut and thanks for the heads up. How would you suggest to undo it without busting it...for those that live by the big hammer ;D



Vic...variable speed drive is the next project. Having one on the mini lathe sure spoilt me. ;D

;D ;D ;D ;D

Eureka!!!

I think I found what's causing this banding thing.

I took the L/screw off to check for straightness. No prob.. .003" TIR in the middle. So put it back on and went to the gears compartment and tried to figure a way to spin the L/screw (without unscrewing the allen screw) with a battery drill when I checked the tightness of the L/S by turning the L/S gear with the halfnuts engaged.

Hmm...that felt a bit tight so I gently lifted up the halfnut lever just a bit to take some tension off the l/s and it smoothened out. :idea:

Tested it with a light cut on some steel and voila! No bands!

The halfnuts were a bit on the tight side and were binding with every turn of the L/S somehow, which would explain the pitch of the banding being the same as that of the leadscrew.

Without delving into the innards of the saddle, I can't tell whether the nuts are in fact too tight or if there is a mis-alignment between the upper and lower half nuts or something. It might be caused by the cam lock mechanism but that is to be investigated on the weekend.

I hope this helps someone else with the same prob.

Cheers

Joe woohoo1

 

[Ken I]

Joe, Glad you got to the bottom of your problem - now I have to get to the bottom of mine. I've been living with it for some time but your thread finally got me off my Butt to do something about it.

I wasn't hitting the nut to get it off - simply using it to protect the thread while hitting with a copper hammer to drift the shaft out of the bearing. Ah Well.

As regards the split nut alignment mine were out back to front which was only a case of adjustment - but they were also out up/down and to fix that I needed to machine a bit of the apron.

Let's see if new bearings cure my problem.

Ken

 

[90LX_Notch]

I was in the process of writing up a detailed explanation for this problem when I lost it. I'll make this brief for now and when I get time I'll go further into detail. My lathe started producing the dreaded bands after I crashed it due to my own stupidity. The cause is play in the halfnuts when closed. The leadscrew acts as a plane/wedge and raises the nut(s) open slightly as the carriage encounters resistance. Then, the halfnuts close up tight until the leadscrew opens them again. This causes the carriage to move with a jerky, but consistant pattern. The pattern translates into the bands as the carriage jumps forward.

-Bob

 

[steamer]

Cool!

QED!

Dave

 

[90LX_Notch]

Dave,

What's QED?

Bob

 

[around]

"Q.E.D. is an initialism of the Latin phrase quod erat demonstrandum, which translates as "which was to be demonstrated". The phrase is traditionally placed in its abbreviated form at the end of a mathematical proof or philosophical argument when what was specified in the enunciation — and in the setting-out — has been exactly restated as the conclusion of the demonstration.[1] The abbreviation thus signals the completion of the proof."

Cheers,

Adrian

 

[mklotz]

Adrian,

You really should identify the source when you quote from Wikipedia or others.

We don't want to get in trouble with the big guns.

 

[90LX_Notch]

Adrian,

Thanks for the explanation. However, in my case ironic applies better! I sure didn't want to prove my original statement by messing my own lathe up. Especially when I was .001 from the target dimension on a deep hole that I was boring. But as fate would have it....

Bob

 

[arnoldb]

Good to see you got your lathe sorted Joe Thm:


Q.E.D. = "Quite Easily Done"; easier than Latin which is all Greek to me :big:

Kind regards, Arnold

 

[steamer]

Actually Arnold...I like your translation better! :big:

Glad it worked out!

Dave

 

[wannasteam]

To the collective wisdom: Please accept my appreciation and gratitude for all your help and advice.

Now that we've discovered the cause, the hunt goes on for the solution.

I'll dismantle the saddle and have a look at the halfnut cam mechanism and see if there is anything obviously amiss.

Bob: How did you eventually uncrash your lathe? I might be able to take some more ideas to my shed for the weekend.

Ken: I'm glad I motivated you to do your bearings. If you get a minute, would you please describe what mods you did to get the halfnuts sorted on your lathe?

I'm thinking that these adjustments will be self-evident when I dismantle the apron but I'd appreciate a heads up. Thanks

As regards the split nut alignment mine were out back to front which was only a case of adjustment - but they were also out up/down and to fix that I needed to machine a bit of the apron.

Cheers

Joe

 

[90LX_Notch]

Joe,

I kept adjusting the fit of the half nuts and lead screw until I got it moving smoothly. Put an indicator on the bed and have the plunger against the carriage. Run the lathe, and as the carriage travels, impose a load against the carriage with your hand (safely) and see if a jump occurs on the indicator. If one occurs you'll have to play around with how the nuts lock up on the leadscrew.

Bob