taper troubles

[Brent Finn]

I am having to make 2.5" dia pins 19" long out of 4340 and the lathe I am using is cutting a .004 taper over that length. My tolerance is only .002" and I have no way to leave these pins over to polish them down other than by hand. I tried one by moving the cutter in at equal increments as it approaches the chuck to try and compensate with limited success. I was wondering if there was some way to shim the tailstock over or any other tricks that would make this less fustrating. I am turing the pins from 2.75" stock held in a three jaw chuck with a revolving center in the tailstock end.

 

[macona]

You need to do this operation by turning between centers and maybe a follow rest. Your tailstock should have adjustment screws to set it over.

But really, these should be finished on a cylindrical grinder.

 

[Ray Behner]

Macona's right. A taper that slight and accurate is best done on a grinder. Plus, when you're finished grinding, it has a good finish. Additionally, you can easily get down to tenths.

 

[athack]

I believe you are trying to say you want to turn the pins straight but are getting a .004 taper over the length. If so the tail stock is usually adjustable for taper by loosening a locking nut and then there are screws that move the tail stock in and out on the X axis.

Deflection maybe the problem if so turn about 6 inches of length at a time feeding the part out of the chuck after each cut. If the spindle bore is too small use a steady rest and turn 1/2 at a time.

Athack

 

[number 2]

.002" tolerance, which is .05mm where i am from should be a piece of cake. A .1mm (4 thou) taper, is not too bad. Its a parrallel pin so that makes life easy. if it had a few changes in diameter it would suck. What I do to remove taper, which is spot on within .01mm usually, is measure from the tailstock end and mark along the shaft every time the diameter changes (hopefully up) towards the chuck .01 of a mm. If its going smaller at the chuck its a bit more tricky. Anyway, then with the machine off, move the saddle until the tool is over the first mark, mark somewhere on the bed where you can see, i use the leadscrew or the vertical flat of the bed underneath the vee way, so you know at what point the diameter changes respective of the saddle position, not the tool. Obviousley you cant move the top slide otherwise your markings will be out. Repeat for all marks. When turning the tailstock end diameter is your proper diameter you use for tool adjustments. Everything else is bigger or smaller than that and is affected by your taper correction only. So when your turning along every time the saddle approaches the mark you tap the tool in .005mm. This works pretty well, will suffice many times over for you. Then when your done run a bit of emery tape over the lot just to smooth it up. If its needed. A .005mm step isnt easy to feel but if your finish is fine, it will be more noticeable. You need to mark the bed in this case so you know how the lathe cuts. If you mark the shaft only you will cut your markings off and wont be able to repeat the cut again. This way you can. Over and over.

If the diameter gets smaller at the chuck, ideally you want to reverse it and cut from the chuck out. Otherwise, cut from the tailstock like normal, and pull the tool out. Allways use a dial indicator for measuring your adjustments as the dials even with the backlash taken out are rubbish. Pulling the tool out has its issues. You will need a fairly stiff cross slide to prevent the tool pushing the slide out as the screw is not pushing against it. Being 4340, might be a bit tricky. Can be done but. Might have to tighten something there.

Other factors that will be affecting your taper:

If the taper is constant from one end to the other adjust the tailstock either in or out. If its bigger at the chuck end, tailstock adjusts away from the operator. If the taper is constant, but its the bed wear only, adjust the tailstock anyway. Will get the job done . Not good, but it works.

Live center: make sure the live center is at operating temperture. A good lathe can cut spot on first thing in the morning. Horrid 10 minutes later, and spot on 30 minutes later when the center warms up and hasnt bent off center due to expansion etc. Does happen.

Tool pressure. Thats a decent size shaft id be putting a 1/2" center in it if its possible. The tool may be pushing the shaft away at the tailstock end. Try using a smaller nose radius with the bigger center. Shorten your tailstock quill right up as much as possible. Rigid is the word here.

Anyway taper corrections work well if you work out where its coming from in the first place, im sure with a bit of work it will come easy for you. I hope the diameter doesnt get bigger than smaller as that gets really tricky to adjust . Backlash - do i need to explain lol.

Anway im not keen on using imperial for this. IMO get a metric dial, they are way more sensitive, or a finer imperial one. You can easily split a .01 division on a metric dial.

 

[Brent Finn]

yes you were correct I don't want any taper on the pin. I didn't use a very big center drill only a #4 so I will see if a larger center hole helps out and the live center is very rough looking needs to be reground.
Also should I try and keep my depth of cuts the same for my two finishing passes do you think that has an impact on how a tool behaves or different cutting speeds . I have been going about .015 per side for my finishing cuts.
The diameter does increase towards the chuck and I do have a .0001 increment dial indicator that I will use.

 

[HuFlungDung]

Did you ever perform a preliminary adjustment of your lathe when you installed it, or did you just park it and turn it on? Tailstock adjustment is the final adjustment but should wait to be done after you get the initial levelling completed and tested by taking cuts on parts held only by the chuck.

 

[Brent Finn]

I am just two weeks in to my new job as an apprentice. They gave me a drawing and some material and now I am trying to fight the taper that the lathe makes on the cut. At the shop I used to work at the guys complained about machine cutting tapers but I never did any long cuts during my time as an apprentice there for it to be a problem. I have wondered how it could be fixed or if it was possible?

 

[AlanBeckett]

Did you ever perform a preliminary adjustment of your lathe when you installed it ...

On my Chipmaster there's a specific way to check that the Headstock is in line. In essence you turn a length (0.005" cut I think) and check the diameter at various distances from the chuck. If it's out you adjust the Headstock alignment.
Alan

 

[juergenwt]

19" long, 2.5" dia. needs to be turned between centers. If your live center is bad, use a dead center. Adjust your tail stock by 0.002"!!! If both ends are the same dia. and the center of your pin is larger you need a traveling steady rest. If the center of your part is smaller in dia. your bed is worn or your lathe is not level. A lot depends on the condition of the lathe. If it is a piece of junk - you are not going to make it and you will have to grind it.

 

[Brent Finn]

is it not possible to adjust the tailstock while holding something in the chuck to adjust out the taper?? I have never done it on a lathe but I do on the cylindrical grinder if it is held in a chuck because it can't be held between centers. I cann't turn it between centers so if nothing else i will have to infeed as I go.

 

[number 2]

Yes keep depth of cuts the same. Successfull fault finding requires elimination of possible sources of error. Changing depth of cuts changes tool pressure and will also affect taper in the job (smaller at the tailstock end).

 

[juergenwt]

0.002" you should be able to adjust your Tail stock with your part in a chuck.

 

[tdmidget]

Oh please!!!! Juergenwit is right. this is first day stuff at any schooling. Center your tailstock and get to work. if tolerance were larger you could do it with a cold chisel and a file.

 

[Mike C.]

You are going to burn soem time on one of thse pins, but it will be well spent time. Put a dial indicator on the tailstock ram and then loosen the hold down nut to where it just barely has tension on it. Zero the indicator. Now loosen the front side of the tailstock adjusting screw and watch the indicator. (50/50 chance which direction it will move, and if your luck runs like mine, it will go the wrong way.) Once you deduce which way to turn the screws (there will be on on front and back that work opposing each other, loosen one, tighten the other), adjust until your indicator reads .002 to the left of zero, that means you have moved the tailstock away from the tool .002 and your error should be eliminated.

That probably won't do it, though. Set a new pin up and take a .010 pass the entire length. Mic it and see if that fixed the problem, if not, do it again, and again, until you get it as close to zero taper as you can.

If you get a football or dogbone shape of a thou or so, you'll just ave to live with that (bed wear or warpage), but you should be able to get it within tolerance this way.

If you can't get it within tolerance, use the marks and vary your cut along the length, but that's a last resort. I use that technique on the crappy Summit at work that cuts a .008 football in 4ft. Have I mentioned this week how much I hate that machine?

 

[HuFlungDung]

If the lathe is not properly levelled, it will turn minor tapers when boring holes or turning pieces that are chucked and not supported by the tail center. No amount of tailstock adjustment is going to fix this condition, obviously, so you should do whatever you need to do to get the lathe to bore straight holes (and turn straight ODs on short pieces). This can often be done with simple tweaking of the levelling screws at the tail end of the bed.

Then, when you put a longer piece between centers, or chuck and center, if it turns a taper, you know it is not a problem with the lathe bed levelling, which means it is due to tailstock alignment.

If the lathe is not levelled (putting the spindle centerline exactly parallel to the ways), and you adjust the tailstock for taper at one position, it will require further adjustments at every other position along the bed. This gets highly frustrating for the lathe operator to make good use of his machine. It can even result in people blaming the machine for being no good, when it was never set up correctly by someone who understands what steps need to be taken.

 

[Mike C.]

True, hu. That said, this guy is an apprentice right now. He's liable to catch grief for spending time messing with the tailstock, even worse would be trying to level a lathe for the first time.

 

[amdinc]

YOu need to level your lanthe first and then set the tailstock. you cannot adjust the tailstock properly unless the lathe bed has no twist in it

John

 

[Brent Finn]

Yes bed leveling is out of the question I have only been at this new job for one week it would not go over well if I tried to level a lathe for the first time ever. I am looking to make these 12 pins without a lot of headache or just hoping that things cut well. I will try moving the tailstock over the .002 and see it that works because I won't have to move it till all 12 are done. If that doesn't work I will just have to use and indicator and move the cutter in at set intervals.
Thanks for all the input

 

[HuFlungDung]

Yes, you don't want to get in trouble. But without any monkeying around, you can tell if the lathe is set up straight when you bore holes of any significant length (like over an inch long). If it bores taper, then the lathe is not level. And since fussing around with emery cloth (because that is about all you can do inside of a hole) to polish out any significant amount of taper can open another can of worms with polishing technique, to me, straight boring is the primary concern with lathe setup. And miraculously, with a tweak of a levelling screw, you can correct minor taper boring cuts and save yourself a lot of fussing around. Do the adjustments while you've got plenty of material left to bore so you can find out which way you should be adjusting. Ask for permission first