Oval bores help

[ALB]

Hi all,

can anyone tell me what's wrong with my lathe(or me )
I am trying to bore a 20mm od bore 30mm long for a wee project i am working on, But the bore is oval measuring 20.00mm < 20.08mm in dia
i am not going in to heavy final cuts are around 0.10mm and i am finnishing with a few passes to get the spring out of the boring bar which has a 8mm shank
i have tried honing the bore but no joy
it's a hobbymat md65 lathe I am using with a glanze 8mm boring bar and new tips.

any ideas?

James

[Jasonb]

Once you get an oval an indexable tip with fine cuts will just rub and follow the oval.

Change for as rigid a bar as you can get in your toolpost and use a HSS bit with fine depth of cut and keep the edge sharp

Jason

PS whats the material?

[Bogstandard]

James,

How thick are the walls of the cylinder?

The reason I ask this is that if the part you are boring is held within the vice jaws, and they are on the thinnish size, then the walls can be pushed inwards by the chucks jaws. Then after boring, when the part is taken out, the walls then go back to their original positions and you will find the bore can be all sorts of shapes and sizes, out of round.

If you are boring anything smallish, then it should really be done on material that is not being held within the jaws, but actually outside of the jaws, and then parted off afterwards.

The same can happen in a collet chuck if tightened up too much, you bore to size, then when pressure is released, the hole is slightly oversized.


Just something you might need to check on.


Bogs

[ALB]

Thanks guys

Jason, The material is just mild steel EN1A i have a old brazed tip boring bar but again it's just 8mm i can't use anything larger but i will give it a good sharpen and try it 

Bogs, The wall is 8mm thk i usualy just nip up the jaws and try not to over tighten as it is just a 80mm chuck 
I never thought about using a longer part to hold in the chuck Thanks

Thanks again lads

James 

[BillTodd]

I think Bogs is spot-on with the chuck distortion diagnosis 

You might try making yourself a simple pot-chuck from a thicker piece of steel: bore it to the OD of your cylinder. (Mark it so that it'll go back in the chuck the same place) then slit it along its length to allow it to grip your cylinder.

Bill

[Jasonb]

I can't see you distorting a tube with 8mm wall that much in a small chuck particularly if you say you are just nipping it up.

There are always ways to use a thicker boring bar than will fit in your toolpost like this

Jason

[Bogstandard]

Jason,

I am of the same opinion, they are fairly substantial walls to distort. Say 4mm and below, yes, but not that sort of thickness.

I have never had a bored hole go oval, only tapered, and I've used boring bars made from 1/16" drills. It's got me a bit stumped.

All I can think of are the final cuts are a bit heavy, normally 0.025 is about right for a fine finish, and in/out at least half a dozen times.


Bogs

[sbwhart]

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

[lordedmond]

ALB

have you tried to do a bore in a different material e.g. ali  or cast iron?

it may have been sold as EN1a but is it it may be GCQ ( good commercial quality ) or just junk bar end.

it is just possible that it has a hard spot a 8 mm bar may deflect at the hard spot.

is the bore oval or triangular ( like a three lobe cam ) they can measure like an oval


its just possible that the bearings in the head stock are shot

my 0.02 cent worth

Stuart

[ALB]

Jason that's one mean looking boring bar it would scare my little machine btu I know what you are getting at thanks.

Bogs I will try another piece tonight and  go easy on the finishing cuts.

Stew it was a solid piece of EN1A 25mm in diameter it drilled ok and seemed to bore fine also nice spiral chips

Stuart I have bored brass before for little stirling engine no probs
and it was BS970 080A15 material
I run a steel stockholding firm and I get to keep the "scrap offcuts Hmm"
as for the headstock everything seems to be tight but I am not sure how to check this?

Again thanks guys for your help
James

[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]

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

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
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