Lathe Accuracy

[Linstrum]

About two years ago I got a new Jet 13-inch swing by 40-inch gap bed lathe to replace the old Atlas lathe I had just shy of 50 years and wore out. My new lathe is a South Bend clone, so there are some pretty major differences between the two. When I got my new lathe out of the crate I checked it over for loose bolts, being lubed, plus other things like that, and I was really annoyed when :confused: I found that both the headstock and tailstock had never been tightened down sufficiently at the factory and the accuracy that had been painstakingly set by the factory technicians had all been lost!
:violin:
My first test cut produced a 2/1000” taper on a cut one inch long, which translates to 24/1000” per foot, pretty darned bad! So I got a 1.5-inch diameter bar of hot rolled steel about three feet long and chucked it up to start the alignment procedure. It is a must to sharpen the tool bit before beginning! Basically all it takes is common sense to adjust a lathe, a cut is taken up close to the headstock and the cross feed zeroed for that cut. Then the carriage is run down the bed about 10 inches and another cut made, with the final pass done with the cross feed at the same zero mark so the two cuts will hopefully be the same diameter. It takes a bit of finesse to make the two cuts so they are equal as the bar cannot be supported with the tailstock during this procedure since the alignment of the headstock with the bed is what is being checked and using the tailstock will prevent that. The natural flex of 10-inches of unsupported bar, lathe rpm, and the sharpness of the tool bit will all affect the depths of the two test cuts. Usually with the cross feed set at the zero point, three or four passes with a sharp tool bit at very slow rpm to avoid chattering at ten inches out from the chuck is good enough to make these test cuts on mild steel. If the two cuts are the same diameter then the lathe bed is on the same axis as the head stock axis. When they aren’t, the headstock is adjusted a tiny amount and then re-tightened down HARD, and then a light test cut is made at the same two points. When the two diameters match, that job is done. The next part of the job is to center the tailstock since the headstock was just moved, albeit just slightly. A shorter bar of scrap steel is mounted up between centers (don’t forget to take a truing-up cut on the center in the lathe chuck) and then a test cut is made and the bar checked for taper. Adjust the tailstock until there is no taper on the test bar, and when that is accomplished you are done. Don’t forget like the guy did who adjusted my lathe and forget to tighten things down! In fact, the test cuts MUST be done with everything as tight as they can get since tightening things up will often change the settings!
I had to adjust my lathe headstock twice, not long after doing it the first time I was making a very heavy cut that began to chatter violently and it knocked the headstock right back out of alignment. After the second time I adjusted it I also tightened down the headstock adjustment stop bolts as well to prevent sliding to the side and it has behaved itself since.
My latest job is machining a barrel for a .50 rifle :castmine: :cbpour: and in preparation for milling 26 inches of the 30-inch long barrel into an octagon measuring 1-1/2” across the flats, I trued up the barrel diameter between centers so it would be straight and parallel. After making the final 26-inch long cut I checked the barrel diameter and found that it measured 1.6004” at one end and 1.6000” at the other with an even taper down it of 4/10,000 of an inch! I’m not going to even think about fussing with that since getting a part machined to within 4/10,000” is quite often difficult at 12 inches length due to uneven heat treat hardness of many steel bars. :twisted:

It is nice to have a really good lathe! :drinks:

[Dale53]

I have made it a point over the years to never "covet" anything of someone else's. However, I am just a TAD green eyed:mrgreen: .

Congratulations! You will enjoy that for a good long time! good things, to good people!

Dale53

[redneckdan]

One of the parts I make at work has a tolerace of -0 +5 microns. I check the chuck alignment everyday. After awhile it doesn take long at all.

[Buckshot]

One of the parts I make at work has a tolerace of -0 +5 microns. I check the chuck alignment everyday. After awhile it doesn take long at all.

............Must be in a temp controlled shop :-)!

It's ALL made of rubber, and you have to watch it. While my 11" Logan is a good machine and will do (most) anything I'd like to do on it, it ain't no 24" swing American Pacemaker :-). I've had the machine about 3 years now, and it was about a year ago I realized there was no reason to "Push the Envelope" with it.

I'm not in production, nor do I have to hog down a 5" piece of anything to a 1/2" OD. If I did I'd start with something smaller to begin with! But any machine will try and do what you want, so long as you can hump the metal up on it. That is, till something breaks. I was doing some fairly heavy metal removal and what was peeling off wasn't matching what I was dialing in on the crossfeed.

Also the surface finish wasn't what it should have been. I really had the QC box growling. I had it setup right, SPM and tool geometry wise so I couldn't figure it out. Come to find out the compound was actually flexing forward on the cross slide due to the pressure on the toolbit. This because of the DOC I had dialed in.

Speaking of big jobs, there is a place on the web that shows a guy who turned a 8" OD cannon barrel on a 10" Logan [smilie=w: He had to use a hoist to get the blank up on the lathe and machine half, then swap it wround to do the other half. Luckily he had a freind with a big Monarch with a 60" bed so he could bore it out!

...................Buckshot

[uscra112]

Nice job, Linstrum. And that's coming from a geezer who built and rebuilt machine tools for a living for nigh on 15 years. That's exactly how we used to do that task. This ought to be posted as a sticky.

[Linstrum]

Hi, uscra112, good idea! http://castboolits.gunloads.com/images/icons/icon14.gif Even though this is not a machine shop board I think with a bit more info than what I posted it would be a good idea to cover the real, real basic methods used for testing and adjusting a lathe because a lot of guys here do own them and make casting-related tools on them but are not aware of how to check a lathe for accuracy or know how to make the adjustments. It isn't that hard and even if somebody doesn't get his lathe up to the accuracy of redneck dan's machine, they can at least improve things to be better than they were if or when they discover that their Grizzly, Taig, or other kind of machine is out of whack. I know that there are a few really good lathe books that cover the basics, and the names of the books should be posted as well. The book that was put out by South Bend 65 years ago back during World War 2 on how to run a lathe that was meant for training lathe operators who previously only knew how to shovel manure or wash clothes is probably the best one ever written and Buckshot posted about it in another thread a little farther down this section. Yeah, a comprehensive sticky could be organized and then "stuck" up with the others here since questions covering this stuff get asked often enough by our own boolit-casting members to warrant its creation.

It always amazes me exactly what some machinists are able to coax out of their lathes and milling machines! :veryconfu Machining an 8" diameter canon barrel on a 10" lathe is a darned good example of that since the things that are normally machined on 10" lathes are objects like brake drums and rotors that you can pick up with two fingers, but you'd be surprised how heavy things can be that a lathe can handle with both safety and ease. The major thing about really heavy or awkward things in a lathe is to protect the bed ways with a big thick rubber doormat (the big black traditional kind of door mat made out of linked rubber tabs cut from old tires that you can roll up, or stand on in front of your lathe to ease your tired feet from standing on the hard concrete floor) in case the heavy object being machined gets dropped when mounting or dismounting. That includes when installing or removing the chuck, just look at any old lathe and the bed ways under the chuck are almost always full of BIG dings! Steel round bar that is 8" in diameter weighs just about exactly 14 pounds per inch of length, so a 30-inch canon barrel blank before boring and tapering weighs in at 420 pounds!

By the way, the easy way to calculate how much an iron or steel bar weighs, or even stainless steel and brass, is easy. Iron and steel weighs between 39 and 40 pounds per "board foot". It varies a bit due to the alloy. A board foot, for those of us who never had wood shop, is 144 cubic inches and is the volume of a board that measures one foot by one foot by an inch thick. Yeah, I know a 2"x4" is really 1.5"x3.5" but it is calculated as if it really were the size stated. Take and figure out the volume of the chunk of iron and then divide it by 144 cubic inches to give the "board feet" and then multiply that by either 39 or 40. For most kinds of stainless steel and brass multiply it by 42 pounds and you'll be close. I use that to check to see if I'm getting what I pay for when buying bar stock at the steel yard. :bigsmyl2:

[codarnall]

This trick is so cleaver it will blow your mind. Get every thing tight. Level as you can. Loosen a one bed footing leveling nut say where the lead screw is bushed. Place a known true 3/4" or greater drill rod between centers. Put a dial indicator in the tool post. Run the saddle back and forth watching the dial indicator and adjust the loosened foot until you're happy. You should get at most .002 over 30 inches easy in five minutes.

[uscra112]

You know your beans, Linstrum. I saw dozens of lathes of all sized with those nasty depressions. We used to have a wooden deck that was placed on the ways before we changed chucks. That absorbed the dings. I still do it with my old South Bend.

[grumpy one]

The best reason to put a wooden cradle under a toolroom lathe's chuck prior to removing it, is to keep from cutting your fingers off. My lathe is only 14" swing, and my biggest chuck is 13.8". For that one I just put a sheet of aluminium between bed and chuck, because there is virtually no distance to drop. However the 12" four-jaw has further to fall and is way too heavy to mess with. I put the cradle under, undo the lock ring, break the chuck loose on the mounting thread, turn the spindle backward until it comes free, and the chuck is sitting on the cradle. Brace yourself properly to lift it from the cradle to the die truck, or if necessary use a swing-over hoist (I'm not that old yet, but I may get there). Keep the chuck vertical throughout and propped so it can't fall over - getting a finger under a toppling chuck is another way to lose the finger.

As the chucks get smaller and lighter it becomes less important to have the cradle fit the chuck closely, and you're putting it in there mainly to protect the bed, but I'd still rather squash my fingers against flat wood than against the sharp edges of the lathe bed. I made just one cradle (for the 12" chuck) and use it with all of them - even with my smallest chuck, a 6.5", I always use the cradle. I've dropped that chuck onto the cradle, with a finger in between, often enough to have no desire to stop using the cradle.

[floodgate]

grumpy:

Yep! Smarts, don't it? But dinging the ways smarts even worse. I've been lucky with my Smithy, but those are only 5" and 6" chucks - and they still bite! You've inspired me to go out and make a cradle tomorrow a.m.; I've put it off too long, and just been lucky so far (except for catching a fingertip in the gear train last year; that one DID sting a bit!).

floodgate

[fiberoptik]

waaaaaaaaaaaay back in the old days (1982) when I was in high school, I worked on a Monarch lathe, a Bridgeport Mill (in Bridgeport!) and an old shaper, maybe a South Bend?? Also on surface grinder that I never paid any attention to the name of. That Monarch was one BIG OLD BIG OLD lathe!! Wish I still had all the stuff I made then, V-blocks, a powder trickler, etc, etc, etc.