my lathe dont make straight cut

[larsinist]

The id is bigger than the outer diameter,it sucks, cause i make alot of snowmobile axles,and i allways have problems to mount the bearings on them,cause of the od is bigger near the chuck.i got a copy of the sieg c6,and what can i do to make it more accurate???

Lars

 

[Swifty]

Are you turning between centres? If so, you need to adjust the tailstock so the cut is parallel. Google lathe setup and you are bound to find some videos showing how to do this.

Paul.

 

[larsinist]

Are you turning between centres? If so, you need to adjust the tailstock so the cut is parallel. Google lathe setup and you are bound to find some videos showing how to do this.

Paul.

Thank you for reply, i am turning without the "dog", i place the axles in my chuck and then start turning, so i think i need to purchase some "lathedogs"
can i try adjust the tailstock,for to get a more straight cut?,or are there other parts i can adjust

 

[BaronJ]

Hello Lars,

You need to spend some time learning how to set up your lathe properly. As was said by Paul the tail stock has to be exactly in line with the spindle. However you can't do that if the lathe bed is slightly twisted. So you need to make sure that its straight and true first, then align the tailstock.

There are a number of good references on how to do these things on line.

 

[larsinist]

Hello Lars,

You need to spend some time learning how to set up your lathe properly. As was said by Paul the tail stock has to be exactly in line with the spindle. However you can't do that if the lathe bed is slightly twisted. So you need to make sure that its straight and true first, then align the tailstock.

There are a number of good references on how to do these things on line.

sorry for beiing noob, but how do i check if the lathe bed is twisted??

 

[BaronJ]

Hello Lars,

There are several methods ! The easy way is to Google the two collar method described below, or look up
<http://www.practicalmachinist.com/vb/redirect-to/?redirect=http%3A%2F%2Fneme-s.org%2FRollie%27s_Dad%27s_Method.pdf>

Essentially you compare the size of two collars say 6" apart on the same shaft, machined at the same crosslide setting and adjust the bed twist by shimming under the back or front mounts at the tailstock end so that the collars are the same diameter. I use a piece of 1" dia aluminum bar relived by a few mm between the collars.

 

[xpylonracer]

Hi Lars

What you describe is turning your axle in the chuck and the finished work is larger at the chuck end, this means the tool is moving away from the lathe centre line as it approaches the chuck.
This may be difficult to put right and if the headstock is not adjustable will involve packing under the mounting points to "twist" the bed as described in other posts.
Until the lathe turns parallel when using chuck mounted work without centre support there's no point in making adjustments to the tailstock as that will have no benefit to your current issue.

Emgee

 

[BaronJ]

Hi Lars,

Just a note about adjusting the headstock. Don't even think about it. If you move the headstock you will find it extremely difficult to re-align it. Make any adjustments by shimming under the tailstock end mounting feet.

I have jack screws on the feet of my Myford and it takes only a partial turn to get it true over a 6" test bar.

 

[xpylonracer]

Hi Baron

If the headstock is adjustable on the Sieg copy then it is already out of alignment with the lathe shears otherwise it would not be turning a taper, so why not adjust the headstock ?
Your Myford I believe has 2 wider spaced fixing points at both ends, headstock and tailstock which allows you to shim as described, the Sieg appears to be much narrower with 1 central fixing down screw so not so easy to gain the same effect with shimming as it won't overcome the stiffness of the bed.
Emgee

 

[BaronJ]

Hi Emgee,

Thankyou for your comments:

I must admit I've not taken too much notice of the Sieg lathe. I am under the impression that it is made from folded sheet metal and that the headstock is not designed to be adjustable. However I take your point that if it has only a single central fastening at the tailstock end it could be quite difficult to adjust the bed for twist.

You are quite right in that the Myford lathe has quite substantial fixings, two at each end of the bed and the jack screws make it easy to true up. I still wouldn't like to have to start re-aligning a headstock !

 

[Nick Hulme]

this means the tool is moving away from the lathe centre line as it approaches the chuck.
Emgee

Or is the work flexing away from the tool (material flex or bearing slack, the result is the same) where the work has less support?

Working between centres would reap benefits.

- Nick

 

[larsinist]

Thank you all for comments, i got a quick change tool post, and all seems better, but it dont cut totaly straight,i will take some pictures of the mounting holes in the headstock,it has 2 allen screws,and i im sure i can move the headstock a little sideways, if i unscrew the allen keys a few turns. i got the dixon quick change toolpost, and all is better, i got better/nicer cuts,and a little more straight.i also have mounted some ballbearings in the cross slide, and it also became more accurate,i can take some pictures if you want to see!!

will it help if i take some pictures of the mounting holes to the headstock?
and last question,will the lathe cut straight if i turn between center?

Thanks again

Regard:Lars
Sorry my bad English,i talk and write North-Norwegian!!

 

[xpylonracer]

Hi Lars

Pictures always help, it would also be interesting to see if the work is turned parallel between centres.
As Nick says there could be a problem with the work piece flexing away from the tool, especially if you have several inches protruding from the chuck and don't use a fixed or running centre for support,
The reason I don't think this is the case is because your work is a greater diameter at the chuck, if flexing were the problem then the larger diameter would be towards the tailstock. (material flexing away from the tool so diameter is reduced less)
Emgee

 

[Scott_M]

Hi Lars
The first thing to do is determine what the real problem is.
Using a piece of steel of know straightness and consistent diameter. Like a piece of drill rod / silver steel , chuck it in the headstock and run an indicator over its length. this will tell you if the saddle is moving parallel to the axis of the headstock. If it is then the problem during a cut is some flex somewhere.
If it is not then adjust the headstock until you get the indicator to run true over the length of the drill rod.
After you get the headstock dialed in, bring up the tailstock and put the center in the end of the drill rod and check again with the indicator. if the tailstock moved it off center then the tailstock needs adjusted as well.
When you get everything squared up in this "Static" test , then you can proceed to "Cut" tests.

I hope that made sense.

Scott

 

[goldstar31]

There are too many imponderables about alignment but Baron is worth reading because he addresses the question of 'twist'. Ignoring everything else, you need a minimum of two decent spirit levels and more usually three to get the lathe ways parallel with each other and with the lathe perfectly horizontal. Once the lathe is true, you can then address the rest of the niceties of whether the headstock and the tailstock are out of alignment.
All this playing about with cutting two rings and miking them assumes that you have a brand-new lathe- from Myfords. It would have been set with just the right amount to toe-in at the headstock to compensate for the pressure imposed on the work by a normal lathe tool. Myford are no more and you haven't got a Myford anyway.

Once the lathe is truly level, you should be looking at further alignments but do remember that many cheap Chinese lathes need new gibs or the original ones scraped in- to fit. Again, there is plenty of internet information to do this- before risking alignments of headstocks.
Further, it has to be remembered that your lathe is not rigid- regardless of just how much preparatory work which I have mentioned. Again, I question whether a Quick Change Tool post on a small- cheap lathe is sufficiently rigid to cut straight anyway. Merely leaving work unsupported and expecting it to be parallel is expecting too much. It should be supported by either a fixed steady or the tailstock and a correctly ground and set lathe tool. All of these are pretty tall orders to address but I have a somewhat larger lathe- ie a Myford Super 7 with a gearbox and the whole lot has been slideways ground and handscraped to almost new again. I still have to regularly set up the lathe and ensure that I make the lathe as rigid as possible and have work supported.

There is nothing magic or difficult or even unique in this , it is what everyone has to get used to as part and parcel of their hobby. My other lathe is far more flimsy that yours and to operate it, I have to take even more care- so I do understand and sympathise

Norman

 

[BobsModels]

Lars

You have received a lot of excellent advice, the main point is learn your machine and what means are available on it to adjust for turning parallel.

Just for a point of reference. I have three lathes and let me tell you what I do twice a year ( December, May) to each one. First the machines are in an insulated heated and air conditioned building on 6" insulated concrete floor. All leveling is done with a Starret Master 199 level.

The 15" x 50 " Triumph 2000 Colchester is old, 1978, and just a little worn. I first level the lathe. I then use a 2" x 6" bar with rings and tweak the headstock adjustment until they turn to within .0003/4. I check the tail stock with a center in the headstock and a center in the tailstock. Run them close and put a my 6" rule in between the centers if it stays nice and straight I am done otherwise I adjust the tailstock. I do not do any further adjustment to the tailstock because if I am going to turn between centers I have a test bar that I use and carefully adjust it at the time of use.

The 12" x 24" Clausing 5904 was virtually brand new when I got it in 1996. Bed is real nice. I level it first. I use a 1" x 6" bar with rings and tweak this to .0001 or less depending on my patience. Same test as above for the tailstock. I use this lathe about 75% of the time. So it gets checked if I need something really close for a job. This lathe stays put pretty good.

The 7" x 19" Myford Super 7B was brand new when I got it in 1981. I level it first. I use a 1" x 6" bar with rings and get this to .0000. I use this machine with collets often and want it as close to dead nuts as possible. I use a test bar between centers to set tailstock as I want it dead on. This machine is not real robust but it holds setting OK for what I use it for ie not hogging off metal.

Now all of the above means nothing if the material is not supported, and the cuts being taken are not appropriate for the material, the machine, the feed, the speed, the cutting fluid, etc.

By the way I have two milling machines, a band saw, a grinder, and one drill press that all get similar treatment.

May sound like a lot of work but if you do it regularly it actually takes very little time. I do the whole shop in a day including milling machines, a band saw, a surface grinder, and drill press. Not to mention small machines like drill sharpener, sander, die filer etc.

I have been doing the above for over 30 years and it is a pleasure to walk up to a machine and get the result you want first time (well assuming no operator error!).

Bob

 

[goldstar31]

Lars is quite correct but he has probably a similar Myford to mine and not worn.
If the lathe is worn- ie near the chuck , it is virtually impossible to use the two rings to get alignment. He is correct in suggesting that alignment should be made between centres and - I would add at the extreme end of the lathe where wear is either nil or the least worn.

Normally, such measurements are using hollow test bars to minimise deflection owing to weight of a test bar but perhaps you go back to a earlier situation where there is an opportunity to download Georg Schlesinger's book which sets out the detail. The same article contains how to get at Connelly and overhaul procedures but it is so detailed that as to become mindboggling. Yes, I've read it- and fell asleep several times.

Do read Schlesinger, however

Regards

Norman