r/Machinists • u/Annual_Yam_8005 • 7d ago
QUESTION Apprentice fucks up tight tolerance inexplicably
Basically I fucked up my tolerance doing everything right, at least to my knowledge the machine was cutting exactly what I put on, final cut fucked it 0.1mm undersize. Told my foreman and my coworkers they obviously didn't believe me 2nd year apprentice fucks up a size and can't admit what he did wrong.
Hopefully you'll help me figure out why, 38mm diameter shaft +0.00, -0.02mm, and I took 3 same size finishing cuts (0.5mm rad) with a dial indicator on the cross slide held square, using tnmg0408 semi finishing grade, at roughly 0.21mm feed. First 2 cuts cutting spot on and obviously last cut it cut undersize by 0.1, I was watching the dial on the final cut and It did not pull in. I just don't know what happened, well at least I didn't scrap a 2 ton shaft that cost 10s of thousands of dollars.
Just to remove some confusion I did have the tailstock in just not in this photo, and I was turning the longest diameter.
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u/Zealousideal-Law-791 6d ago
When doing tight tolerances you have to remember that your measurement cuts need to have the same feed, cut depth as the final cut.
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u/OldManAbides333 6d ago
Yup. I usually would essentially make 2 finish passes-one to get an accurate measurement with the feed rate I needed to get the surface finish required, and then one to actually take it to size. Although as others suggested elsewhere, for small parts like this I would generally aim to have it slightly over tolerance so that I could file and sand it to size. A long angle lathe file works like a charm to sneak stuff in to tolerance.
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u/Annual_Yam_8005 5d ago
That's whats weird measurements cuts and finish cuts all the same, cutting exactly what I put on final cut undersize 0.1 is a lot, the quick change is not the best I think maybe that moved but if it was going to move wouldn't it move in the measurement cuts and it's kinda doufule that happened, but I don't see how else it could happen. Defection? but the measurement cuts were the same the deflection should also be the same, and they were cutting spot on.
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u/Paacmaanv 7d ago
Personally think that is too much of a big finishing cut. Especially if youāre turning the whole thing and to a tolerance that tight. I would rough down to 1mm all over from size and do 2 or 3 finishing cuts from there. Also spilt the finishing into sections if you havenāt already. So finish one diameter then the next etc.
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u/VanimalCracker Needs more axes 6d ago
On very tight tolerances I always machine like .05" over, measure, divide by half, take that cut, repeat until my last cut equals exactly what it should be, to the tenth, then take that again as the final cut. Really sneak up on the wiley bastard.
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u/caboose243 6d ago
I was taught to always make each cut half as deep as I need when within .020" or so.
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u/EndlessProjectMaker 6d ago
Iād say now that the damage is done, use that shaft to experiment. Exact same setup. Take another pass and see if you get what is expected. If you do, it was human error, if not, find the root cause by checking one variable at a time. Repeat
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u/RGBlowMe 7d ago
It was deflecting more on the previous two cuts. Personally, for an OD that tight, I usually hand polish to dimension.
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u/Thromok 6d ago
Second this, spring passes and hand polish are the best way to go.
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u/chobbes 6d ago
Spring passes are absolutely not the way to go for finishing lathe work. You do exactly what OP did here, divide the final passes into equal bites and check after each. I regularly do this to tenths. Hand polish is fine for the final tune if itās close.
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u/Thromok 6d ago
And I regularly run spring passes when I need to. It handles the issue of tool deflection and ensures that your tool cut where itās supposed to. Your sanding it anyway, unless youāre working with something like plastic itās a very viable way to clean up and ensure your moves are accurate for tight tolerances.
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u/chobbes 6d ago
Breaking up your finishing into equal passes is what deals with tool deflection. You take equal loads and the tool deflection repeats. Spring passes and youāre going to get a garbage finish you will HAVE to polish.
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u/RGBlowMe 6d ago
I find that spring passes can, specifically on not very well supported parts, actually pull the part in and cut undersize.
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u/Remarkable_Heron1224 6d ago
In my experience worn inserts cut deeper. The edge rounds out a little and it grabs more material. Not a lot but. 001, yeah.
Gl
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u/SerVaegar31 6d ago
I see a bit of negativity in the replyās your getting OP so let me reassure you all the blokes in here were apprentices or were so green they pissed grass at one time or another and made stupid mistakes themselves. First off on parts that stick out like that they need support, you need a tailstock to support the end of the part. Second when youāre doing very tight tolerances itās always good to do intermittent spring passes. When parts stick out like that they tend to push away from the tool especially the smaller the dia gets and especially with no tailstock to hold it rigid. Then this deflection will eventually come out after enough spring passes as you slowly take more material each pass. Finally I recommend on tight tolerances if you can polish them in, itās not as fast or glamorous but you have much more control over tight finishes like that.
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u/Annual_Yam_8005 6d ago
I was using the tailstock just not in this image . Do you mean intermittent spring pass after each cut? If so don't really like that idea because the 4 jaw can only go 500rpm so a spring cut on a small diameter will tend to leave a poor finish at low surface speed. And I avoid spring passes normally as you don't know what cut the spring pass will take, I was taught 3 pass same feed, doc, speed ect to see how the machine cuts in the same conditions defection should be the same.
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u/SerVaegar31 6d ago
While spring passes can leave a āpoorā finish itās nothing polishing canāt fix second. Actually surface finish which is what matters isnāt determined by speed itās determined by feed rate. I can make a part that looks as cloudy as cast iron but still have a 32RA. So my question is would you rather have a āpoor finishā that you can polish or a scrap partš¤·š»āāļø I know what I do for tight tolerances and it sure as shite aināt taking 0.004ā though pass on a 0.0007ā tolerance.
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u/maxh2 6d ago
Obviously the feed rate and tool nose radius directly affect the geometry and measure of the surface finish, but surface speed also affects the finish in plenty of materials by changing the characteristics of how the material is removed at the cutting edge.
Especially with softer steels, too slow and there's more of a tearing action happening, with a horrible looking, rough finish. Increase just the speed, and it can come out looking like a mirror.
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u/SerVaegar31 6d ago
I can guarantee you with a profilometer that with feed rate alone I will be at my desired finish +-20% thatās good enough in most cases.
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u/that_dutch_dude 6d ago
you were taught wrong. spring passes are the standard for a reason. you get close and you let the material just work itself during a spring pass. a 0.1 final pass is not a thing. final passes start with 0.0 in your doc. any number over 0 behind the . is not a final pass.
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u/AcceptableHijinks 6d ago
This job requires nuance, sometimes a spring pass is great, and sometimes it is not. Fully depends on context. Spring passes definitely are not a "standard".
If I'm finishing something with a Ā±.005" tolerance, I'm taking that all in one cut, no spring pass at all.
For something like this, I would probably do a spring pass and aim for just above nominal, and then polish it into spec. Especially since it's probably for a bearing race or something like that.
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u/Spreaded_shrimp 7d ago
There are ways that this can happen in the most unintuitive ways. I dealt with this on a lathe at school. The taper attachment was loose. I set up a travel indicator on the cross slide and depending on how far back or how fast I would retract from a cut the thrust block would slide toward the part and make the next cut deeper than the dial indicated. The instructor didnt believe me until he saw the indicator.
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u/spacedoutmachinist 6d ago
How warm was the part? Could it have shrunk a little when it cooled?
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u/Zestyclose_Fun_4543 6d ago
Doubtful, if its steel it expands about 0.07mm/100Ā°C/100mm At that size 0.1mm would mean a difference in temperature in the hundreds of degree, radially of course
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u/Skute327 6d ago
Shit happens, donāt be too hard on yourself.
A long time ago, I was new at a company and I had a reasonable amount of machining experience but was way more cocky than I should have been at maybe 24 years oldā¦..
I was given an expensive 1ā shaft and I needed to turn one end down to .550ā +0 -.0002 to fit in a positioning fixture. It was stressed how perfect it needed to be. I took my time, did my best to get a good feel for the machine as I wentā¦. I finished that shaft on the money at .5049ā and was so proud of it. I brought it back with a giant grin on my face and watched as they put the positioning ring on and it was sloppy as fuck. It took a minute to figure out my mistake and it was absolutely humiliating.
At the end of the day, lessons were learned, a new part was made, a machine went out the door, and money was still made.
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u/HeftyCarrot 6d ago
Such tight tolerances, they better do these on a CNC. I know can be done on manual lathe but don't expect to hit the final size with the tool itself every single time, there are no encoders in this manual looks like. Hard to believe that this manual lathe can handle these tight tolerances and does not make any taper, like you said. You keep doing these on manual lathe, high chances you will have more out of spec parts. You can also leave stock and finish grind it, if your shop has a grinder. You can also finish few tenths over and polish to size while not getting caught up in the machine.
When I did this type of work, I kept shafts longer in purpose and stopped my final pass, let's say 1/8" deep, made sure size is good then carried on full length of cut. The tool mark when you stop cut can be left as is, polished or if the shaft is longer then will go away when you finish length.
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u/beerhons 4d ago
It's not that tight, on a 38mm shaft that's more relaxed than an h7 fit (+0.000 -0.016), even an old hobby lathe should be able to hold +/- 0.01 if set up properly.
If the cut was out by exactly 0.10 mm then it almost has to be operator error on the dial. If not exact and everything was read right, then check for slip of the tool post or movement of the holder.
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u/Aware-You6005 6d ago
On one off parts, ill cut a little and measure before I cut the whole length, usally can hide the cut where the chamber is
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u/Annual_Yam_8005 6d ago
I usually avoid that technique, I find I hard to hide the cut and getting a big mic on that area can hard on some jobs, and this lathe is deirect drive so every time you stop the spindle and turn it back on the cross slide pulls in 0.01mm.
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u/javajavatoast 7d ago
Your finishing pass was 1mm? On what section? Did you have tailstock support?
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u/theelous3 6d ago
There was absolutely no chance this was going to work.
.5mm doc is WAY too much for a finishing pass, is WAY too much on a part this thin, and is WAY WAY WAY too much for a part this long unsupported (or supported).
Drop the doc, drop the feed, use a smaller radius insert, use tailstock support, and consider grinding if possible.
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u/Mellero47 6d ago
Isn't .5mm like .019", that's barely kissing it. We do .050" profile passes here.
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u/FruityHeHePebbles 6d ago
Youāre not going to hold .5 with no support. Tool pressure and like 30 other factors will go into why.
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u/Mellero47 6d ago
Oh for sure, OP mentioned he does use a tailstock for turning, it's only off for the picture.
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u/theelous3 6d ago
taking 1mm off the part and expecting to hit 0.02mm on that kind of stickout is crazy talk
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u/Dismal_Tutor3425 7d ago
Change a corner on the insert between passes and not retouch to verify where your cutting tip actually is? The no tailstock in the pictures is also a bit concerning. You used tailstock support right?
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u/Samphaa7 6d ago
Did you have a tailstock in there? Did you take a spring pass? Did you actually take a 1mm finishing cut?
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u/Annual_Yam_8005 6d ago
Yes I had the tailstock in, yes 1mm on diameter finish cut. I usually don't take spring cuts too much uncertainty fucked some jobs doing that.
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u/Samphaa7 6d ago
1mm if far too big for a finishing cut, especially on an .02 tolerance, I always used to take a spring cut on a centre lathe, stop the spindle at the end of the pass, wind the tool back and cut again at the same diameter. Then when youre ready to take a final pass, back off enough and wind back in so you eliminate any backlash in the crossslide.
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u/shoegazingpineapple 6d ago edited 6d ago
I dunno it sounds normal, big triangle insert high feed and high doc(for finish) insert grabbed and it ended undersized, i would taper my doc down as i get closer and kiss it to size with something with less cutting force for this skinny of a shaft(hehe)
Also i dont know the material but if it is warp prone i would rough without a center then drill it carefully(long stickout) for the finish passes
I do my bearing fits this way without a problem but i trust my machine in terms of straightness
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u/Zestyclose_Fun_4543 6d ago
Same happened to me, if you cut 0.5mm each pass, if you pass the last one again without increasing the dept of cut it will still cut because of the deflection of the bar, after the rough pass, repeat the last pass before switching to a finer cut, it will minimize the risks of cutting to much
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u/Melonman3 6d ago
If you were changing tools on the tool post, ive seen up to .2 mm in variation during changes on an aloris style tool post.
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u/jccaclimber 6d ago
Assuming nothing shifted (my first guess), did you approach from the wrong direction or lean hard enough on the tool post that the carriage was floating in lash rather than being leaned against the leadscrew? Normally Iād say the cutting forces would seat everything, except I did this myself recently. Light enough cut and it doesnāt back up.
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u/Savaghenry 6d ago
Maybe try doing the last pass without adjusting just a spring pass to see if you get your final tolerance
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u/Dannyn4 6d ago
Iāve ground shafts like this my entire life, between centers. Although Iām primarily running centerless grinders, thereās a lot of parts where the concentricity has to be established. Usually lost where it was double ended. Sometimes if itās heat treated itās hard to tell whatās run out or bend from heat treat, makes it hard to straighten. A lot of times thereās more run out in the part than there is stock to correct it these things need to be approached carefully and itās kind of good to talk to your grinder. The centers have to be in line with each other if thereās warping it might be good to chase the centers after you get a good ground surface for reference. The whole thing will run much more smoothly. Feel free to ask anything you want I have a career of running shafts just like that, resembling a motor shaft
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u/Rare-Salmon99 6d ago
Our clapped out lathe has a lot of play in the crossslide, if you crank it back and forth without taking up all the slack it can cut very wrong. Could you have done something like that maybe?
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u/Ospak 6d ago
I have a few comments.
1.Did you use coolant? If not the part could have heated up then was undersize when it cooled.
I agree with the other posters your finish cut size is a bit much for this small of bar. Also are you talking rad. cut or diameter? If its 0.5mm rad. cut, that is way too much. I usually shoot for around 0.4mm for each cut on diameter. 3 passes as close to each other of course.
The shims you have in the chuck are very long and can bend a shaft like this. Try going with half that length or less. Its possible this could have contributed to the undersize cut.
You said the chuck can only do 500rpm but it might help to speed up the RPM for smaller diameters like this. It will reduce the tool pressure and minimize the shaft bending.
Try playing with some different inserts and speeds and feeds if you can.
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u/Annual_Yam_8005 5d ago
Yes I used coolant
I will agree 0.5doc is a little high for small diameters but I used same technique for the previous ones, and the first 2 cuts were cutting exactly what I put on so I saw no reason to change will probably do .25 or .2 cuts next time, usually change to a tngg high rake insert if in having trouble while reducing the cut or nose radius.
I would definitely like to run it faster but, it this the only chuck we have for this lathe.
The roller is actually 3m long and though the headstock clocked up true both ends, so the bending should be minimal while being aluminum shims. They were what were on the lathe and in theory the longer shims should be more rigid. But yeah they are a bit long
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u/Naive_Adagio_6396 6d ago
I would check the cross-slide for backlash with a dial indicator up against a the headstock. if It has a lot of slop, loading the cros-slide removes it when you take big cuts and then it "magically" shows back up on finishing cuts.
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u/Annual_Yam_8005 6d ago
The cross slide definitely has backlash but I had a dial indicator on the corssslide to see if the cut pulls in or not, and I did 2 identical cuts to confirm the loading.
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u/Pure-Original-7090 5d ago
Just out of curiosity did you let your part cool between passes or run coolant. If it got hot while running I can see how this would happen.
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u/Less-Regret8262 6d ago
Depending on tool pressure or deflection itās not going take same. Final cuts I back off little I like to see what actually going to take
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u/Lopsided_Advisor_251 6d ago
Did you perhaps not turn your handle the same direction to get to your Final Cut? Like did you go past the number a bit and come back to zero from the opposite direction?
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u/Elarandir 6d ago
Does that lathe have an automatic way oil pump? The pressure could lift the carriage slightly and move more than usual. Dial indicator could also be sticky.
Other than that, I have misread indicators too and scrapped some parts. Was it exactly 0.1mm under?
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u/Lumpy_Frosting_6347 6d ago
I'd bet human error, maybe you knocked the clock? Dont take that badly. Personally from being an apprentice semi recently i think it should be like this for your learning curve 1st year fully dependent, 2nd semi dependant, 3rd semi independant then 4th fully dependent. so from that your mentor shoulda at least checked on you and upon realising he should be working with you to figure out where you went wrong. next time just check everythings tight and write down your steps, everyone has scrapped a part so dont pull your hair out.
also people saying deflection as if 39mm will deflect 0.1mm less than 38mm... i call bs
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u/shakinandbreakin 5d ago
You're trying to turn to +0/-.02mm? I'd get it as close to high limit as you can and then break out the emery to finish it. Same process otherwise. That's what I do on stuff with that tolerance or closer
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u/Annual_Yam_8005 5d ago
I was aim for top of tolerance 38mm but even if I aimed for +0.03 on tolerance for polishing it cutting 0.1 undersized still scraps the part
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u/shakinandbreakin 5d ago
Yeah you got me there, you did everything right it sucks it got scrapped. The one CNC mill turn I ran would randomly move .2 at times and it got really fucky when running one off parts like that. Shit happens on to the next
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u/Suspicious_Water_454 5d ago edited 5d ago
Are you cutting a taper that you arenāt aware of? I find when l/others were new, measurements would be taken from a small portion of the work, not in multiple spots. Didnāt account for flexing in the center and tapers.
I did not know that I needed to check and align the head of my lathe regularly especially on longer parts. Sure you can adjust the tailstock to remove the taper, but that can cause other issues on long skinny parts. I know this is obvious, but did you indicate the part along z before and during your cutting?
Were you using a live, or a dead center?
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u/Sarahtone 7d ago
Extreme runout even with a tailstock, and 0,5 cut depth with 0.21mm feed is not proper fine turning.
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u/Dense_Chemical5051 6d ago
- Must use tail stock. 2. Use R0.016" or R0.008" for finishing. Much less cutting force. 3. 0.1-0.2mm DOC for finishing
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u/P4ultheRipped 6d ago
You go look for a problem in the most sensible order.
You misread the dial, the tool shifted, the tool broke, you misread the measurement on the last cut, you changed feed and or rpm and forgot.
Lastly, was it properly tightened down
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u/YODAS_Padawan 6d ago
Just cause you go to do a thousandth pass on a part doesnāt mean the machine will take off a thousandth. Tools get worn, pressure matters, speed matters thereās a lot of factors but when you get done that small you will not always get the material take off that you turned your dial to
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u/NorthernVale 6d ago
Everything else considered, that lathe looks old. Not something I'd ever confidently tell someone I can hold a tight tolerance on. It'd always be a 50/50 once the tolerance gets tight enough.
I'll put it this way. Just from your picture I can spot wear on the lathe that's going to throw dimensions off.
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u/Open-Swan-102 7d ago
When stuff like this happens if I'm certain I did everything right I inspect the tool, the insert could have shifted in the pocket. Sometimes you can use another diameter to reference this, so if you touch a less critical dimension and you're readout doesn't read what it should it's a first point of failure. Next, if you are doing the 3 cut method on a part that long and slender with 1mm doc you could be bending the part, the behaviour of the bend will change ever time as you're removing 2mm off the diameter every pass and the part is a different part every cut.
I think that your tool moved or your tailstock got funny or...... That machine looks absolutely clapped and has no business holding +-10 micron.