This post is meant as a general indicator of the three things I see cause 90% of the problems even with very experienced machinists. I am guilty of getting bit squarely on the ass by not following these principals, as is every other machinist I have ever known. This is posted in very broad brush strokes and not meant to be a detailed analysis. however like everything else they are basics.

If you know and use the basics, the harder stuff is the same thing, but more of it.

Rigidity
Rigidity in your fixturing, your tooling, and every aspect of your set up is a must. Let's say you need to cut a key way .25" wide x .25" deep. If you have the option of using an endmill that has 5/16 of flute length or one that has 3/4" of flute length use the shorter one. YOu are cutting a straight (hopefully) slot of a constant depth. The shorter length of the flutes allow you to choke up more on the overall length of the tool thereby allowing for less defelction of the cutter. Remember, shorter in this instance is stronger.
Use the knee to attain a z depth whenever possible, not the quill. The quill is best reserved for drilling and tapping, not setting a milling depth.

Cleanliness
Keep your fixtures, your machine, and your tools clean. All those chips that peel off your work piece and build up in the vise? They are fixturing problems waiting to happen. Eventually, they will build up and cause the vise to bind before it's tight, or get up under the carrier and collect friends to slowly but surely begin to move the carrier away from the vise bed.
Use a fine stone and a scotch brite pad with some WD40 or other light oil to clean the table on your mill. Remember, stone too much in one spot and you risk hand grinding a bowl into your table. Use smooth and even sweeps to find high spots. Stone those spots, overrunning the area evenly to bring them in flat with the table. The scotch brite pad is used before and after stoning to clean the table of chips and other debris that may be stuck and will load the stone or interfere with a true mount of anything clamped to the table. Like wise use the stone and scotch brite pad when you clean the bottom of your vise or other fixturing.
Easy does it! Do not hand stone a perfectly good table, vise or angle plate into uselessness! You are cleaning it, not grinding wheat.
Grit and gunk in precision instuments (Micrometers, Calipers, Indicators, and I would argue 1-2-3 blocks and parallels, etc.) will be your undoing.

Attention to Detail
Tram your fixturing in X and/or Y as needed. For some things, you will also indicate Z, all three of them, just one, maybe two. Plan it out and know where you want to go so you can think about how to get there.
If you're cutting softjaws, tram the vise in first! This way you can use them again later without essentially having to recut them all over again.

Pay attention to how your carrier on your vise closes.
If it's too loose, you will see it raise up in front when you close it on a work piece held near the top on parallels. This means you aren't holding your work rigidly and your results may be less than optimal.
If it's too tight, it will likely not close squarely against the solid jaw. This will likely result in less than ideal results as you are, again, not clamping your work rigidly.
Tram the head on your mill, indicate the tailstock on your lathe! This gets overlooked by everyone, even yours truly at least once in a while. Remember, you are dealing in measurements smaller than a human hair so the MkI Eyeball is not even close to good enough. Don't "think" it's true, Verify it.



If you know and use the basics, the harder stuff is the same thing, but more of it.

sharp bits,  correct feed, correct speed.



from what i have learned, being a good machinist requires bordering on ocd.





Soylent, we'll make this a tips and tricks thread if you and our other resident machinists are willing to share.

That's okay by me.

Another thing to keep in mind is the correct tool for the job. That $85.00 Hanita Varimill does great on steels, but it won't perform correctly in aluminum or other softer materials. Cutter selection is more than just sharpness and size. There is rake angle, Positive, Negative, Nuetral, Carbide, HSS, Cobalt, all sorts of things and that's not even covering coatings.

I am not by any means a one stop source for all things Right about machining. However, if some folks can learn from what I've done wrong...

*ETA*

While we're on the subject of cutters...

Drills

Know your chip! Watch to see you don't have an uneven chip load coming out of the hole. A drill chip that is consistantly coming out longer on one side means your drill point is ground incorrectly. You've got the chisel point oriented wrong, or the drill point is ground off center, or you may not have enough relief on the back rake of the point. The last one usually manifests itself by taking an abnormally high ammount of power to push the tool through the work. The chips look...corrugated, or rippled and come out in little pie slices and often have little holes in them. A drill chip in most metals should look like those little spiral noodles you get in a pasta salad.

Learn to sharpen your own drills! I'm such a hypocrite for telling people this, I could do it once upon a time but now I just usually make things that look like a nail

Chatter/harmonics

Your speed is too high, or your feed is too low. Either drop the spindle speed or increase the feed (spin slower or push harder) and see where the sound takes you.

Another possibility is your rigidity in either the tool, the work, or both sucks. Although often in cases of work rigidity being low its more of a *CRASH BANG THUMPATHUMPATHUMPA.....silence* sound than a chatter sound. Check your fixturing for anything loose and flapping around. Make sure you have the work supported and you aren't trying to cut something sticking out like a noodle (either tool or work). A thin cross section of the work piece will often casue trouble with this and honestly sometimes you just gotta go with it. But check to see if you can hold it better anyway. Put a jackscrew under it, clamp it further in the vise, do that turning job in two ops instead of one if possible and if you're having chatter problems.

Also, consider your depth of cut. If you're too shallow, you're just tickling at the work and aren't getting a good enough bite to really get physics working for you yet. The tool is deflecting because it can find no real purchase in the material.

Dull tools can and will chatter, but mostly they will have a deeper, growly, impacting sound as they are trying to bulldoze material now instead of trying to shear it off. Check your cutters. Depending on the material, the wear or damage may not be readily noticeable, use a loupe or run your fingernail over the cutting edge. The sense of feel is of immeasurable value for this and highly underestimated especially by many beginners. You can feel a chipped cutter face by starting behind the corner (or wherever you suspect the damage to be) and running your fingernail softly along the edge. It will feel smooth and uniform until you get to a built up area or a chipped face, you will feel this as it drags on your nail and causes it to catch ever so slightly. Look there with the loupe, if it's built up you can try to hone it away with a stone. If it's chipped, well you will have to get it sharpened or replaced if you are trying to maintain size. If you aren't, you can keep it and use it for a roughing tool.

Tight Enough Is Not Too Tight

When you put a chuck on the lathe, or load up a vise on the table for the love of God, do not use a three foot cheater bar!

Get a torque wrench if you have to, but overtightening causes damage, damage causes rigidity and/or accuracy problems, and those thigns cost money. One place I worked, we had a guy that used a cheater on everything. I was trying to set something up on the machine he normally ran and noticed I couldn't fit the "good" T-Nuts into the slots past the point where the G54 vise was usually mounted. Couldn't get to it from the other way either but they did fit in the slots not normally used for mounting vises.

Weird...

A little investigation showed that he had over tightened things so much and so often he had mushroomed out little spots in the T-Slots just enough that the taller ("good) T-Nuts couldn't get past those points. A quick indicator check showed that this had not translated into a raised area on the mill table...but damn.

Threads ripped out of vise solid jaws, ruined fixture plates, screws that have to be machined out of those plates, broken angle plates, distorted vises, etc.
All these things from people overtightening a simple capscrew. Don't be that guy.

That's almost as good as *whiiiiiiiiirrrrrrrrrEARTHSHATTERINGFLOORSHAKINGKABOOM* 2k IPM rapid makes bad noises sometimes.



Stopping the spindle on a BPort or similar mill DO NOT use the fwd/off/rvs switch to stop the spindle by reversing it. This destroys the keyway in the motor shaft.

Clean the spindle taper once in a while. If it has burrs in it, use fine(600+) grit sandpaper or a fine round stone to remove them. Always check...some old guy might have slipped a piece of paper in there to true up a tool.

If you have to tilt the head to work on anything...you need to first make sure it's square(ram travel) to the table travel. If you don't and there is a close tolerance on whatever you're cutting/drilling, you might be scratching your head when it comes out wrong––––you're working in different planes than normal so the tool won't *just* do it.

Son of a bitch.  I made some vee blocks for a project once and never could figure out why the vee wasn't quite perfect.  I never trammed the head in that plane prior to cutting.

Imagine trying to figure out why the holes you're drilling are walking .040"+ off location.

Bump

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In a more general viewpoint, whether it is machining, gunsmithing, or any other craft...

Always sell your best work and avoid the “I don't care what it looks like as long as it works” customer. While a quick and dirty job that skips a few details may satisfy the customer looking to save a buck and get it done now, every other person that sees the work product will assume this is an example of your usual skill level and attention to detail. Always sell your best work.

Besides, that guy's gonna bitch about the cosmetics anyway.