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| Oval bores help |
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| Jasonb:
25mm OD and 20mm ID is not 8mm wall thickness, so Bogs could well be on the right track, at 2.5mm wall thickness you could well distort the tube in the chuck. Jason |
| ALB:
Sorry Jason my bad you spotted the deliberate mistake lol 35mm dia bright not 25mm |
| snoopdog:
--- Quote from: sbwhart on April 18, 2011, 01:52:38 PM ---ames What the material is it your turning, is it a bit of drawn tube that you are opening out. Some types if drawn tube will distort its all to do with the directionality of the grain structure as you thin the bore out it will want to move more and more. Just a thought Stew --- End quote --- Im with sbwhart, some alloys are not stress relived and will walk all over after boring and turning. Bronze and stainless are real good for that. You can bore some brass or bronze to 6.5" od and bore it to 6" ID , part it off throw it on the bench and it will be an egg no matter what. if you want to test it out turn and bore a chub of bronze or brass and dont part it of after the roughing operation. let it sit all night in the lathe then put an indicator on it in the morning. You will be shocked. |
| picclock:
Hi JasonB >>the bore is oval measuring 20.00mm < 20.08mm in dia I don't know how your measuring the deviation, best way IMHO, is to use a dti (dial test indicator - not a dial indicator) on the inside of the bore. If you do it this way observing where the high and low places you may find they coincide with the chuck jaw positions or it may lead to additional clues as to the cause of the problem. Using the dti mounted on the crosslide will allow you to measure how parallel the bore is. A standard dti will just fit inside a 20mm bore for 30mm. To prevent workholding distortion machine up a plug that's a good internal fit to the tube, or if you boring out a solid bar don't bore it all the way through and use the solid end for holding, then just part off the tube when finished. Carbide indexable inserts are hopeless for getting to the last thou IMHO. They will happily take deep cuts but anything less than 5 thou is wishful thinking in my experience. With a sharp HSS tool and the work set up to avoid flexing <1 thou is possible. Your issue could be related to headstock bearing play - easily checked by chucking a bar and using an indicator on the side of the chuck whilst applying pressure to the bar. The amount of 'ovality' your talking about here is quite large - around 3 thou in old money - so I'm inclined to think it should be easy to sort out. John (Bogstandard) suggests in his paddleduck engine guide that you turn up a close fitting softer metal part and use an abrasive compound to lap the inside of the cylinders (check out his paddleduck instruction guide) . This may work to get rid of the 'ovality' if you can't find the root cause of the problem. Good Luck picclock |
| ALB:
Thanks lads for all your useful info I tried another bore last night with cast iron round 45m dia x 25mm bore x 40mm long I have not finished yet got to 23mm id and it seems to be round as a pound :scratch: i used my solid brased tip boring bar and tried some finishing cuts just to see whats going on the result a reasonably tru bore in summery Allowed extra to hold in the chuck used a brazed tip boring tool and used very very light finishing cuts back and forth a few times (6) so it could have been a combination of all or just one process but in the end it's all good thank's to everyone that posted :nrocks: |
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