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| Hellogoodbye:
Hi my first post so be gentle. After visiting the Harrogate show in may i took the plunge and got myself a mini lathe, nice day out it was too. Two weeks later my lathe arrived in one very big heavy crate (110kg) oh the joy of unpacking the thing. I'd worked in an engineering shop but not actually worked on a lathe so i knew there were lots to learn. Im now regretting buying the damn thing having problem after problem with it. Everytime i put round bar in the damn thing i get this happening, its only part cutting Chuck or jaw problem i really dont know but you can see somethings nit quite right. Anyone help here please. |
| Jasonb:
By the looks of the tool marks you only have a thou or two run out which is about par for the coarse with a 3 jaw chuck. Best thing to do is get a dial test indicator and then you can measure the TIR (total indicated runout) One of the chuck key holes should be marked, use that one to tighten the jaws, you may also be aqble to reduce the runout by adjusting the chuck on its backplate. Have a search for "TIR" and "chuck runout" its been covered plenty of times. This is why you should do all critical machinging that has to be concentric without moving the work in the chuck or use a 4 jaw to clock the work true if its a really critical part J |
| Arnak:
Hi, That could be normal behaviour if the bar is not truly round or the chuck is not true, most likely the former. Have you got a dial test indicator to check the chucks run out with a test bar? If not just turn the bar down until you have it cut all the way around then put the turned in back into the chuck and see if it now runs true. If it does then the bar was out of round. :thumbup: Martin |
| andyf:
Three-jaw chucks are usually a little inaccurate, holding the stock slightly off the lathe's spindle axis. So an initial light cut will only shave one side of the work. However, as you go in deeper so that the tool is cutting all round, the result will be circular in cross-section, and concentric with the spindle axis. That is fine if you are making a part which can be finished without reversing it in the chuck, and will simply be cut off from the parent stock once it is finished. If you do reverse it as you have done in your video and then try (for example) to turn the other end to the same diameter, the two ends won't match up. It may well be that you will end up buying a 4-jaw chuck with jaws which can be independently adjusted to get the work dead on centre, with the aid of a dial indicator or "clock" bearing on one side of the work. Another technique is to turn the work "between centres", so you can turn it along its whole length. Andy |
| Davo J:
Just like the others have said it's just because it's a cheap 3 jaw, even an expensive one will have some run out. If you machine something in a 3 jaw you do all operations without re moving and your fine. Once you remove it no matter what you wont get it to run true again. The fix for this is 2 things, one has been mentioned is to buy a 4 jaw so you can adjust it to run true after taking it out or machining something that needs to be true like the end of a motor shaft. The other way is if this is a bolt on chuck, is to machine down the chuck register on the back plate a little (0.5mm) and then the chuck can move a little and you can adjust the 3 jaw. The way adjust it, is to finger tighten the bolts, place a dial indicator on the job and turn it around and tap the chuck with a soft face hammer until it runs true, then do up the bolts tight. If your doing a few the same size and the chuck is not to bad, it should stay true at that diameter. I have done this to my 3 jaw on my 12 x 36 lathe and the chuck stays pretty true at all sizes. I have a few 4 jaws so I use them for anything precision. Dave |
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