Thread: Using a Between-Centres Boring Bar

LOL

Dave

Originally Posted by franco

Because the full length of the item being bored has to pass over the tool mounted in the middle of the boring bar, the boring bar must be at least twice as long as the item being bored. This can cause problems if the boring bar is too light for the job because the bar can flex more as it gets further from the support of the centres and produce an hourglass shaped bore.

can you explain more of this??? the cutter doesn't move so how is the support from the centers going to change during the operation could you mean it can flex in and out (dig in and cutting deeper in the middle or ends) producing erratic sizes???

Originally Posted by nadroj

An interesting point is that an advantage of boring between centres ensures a hole that isn't tapered. Any misalignment of the headstock results in a taper, if the job is rotating with the lathe's spindle.
But, if the centres aren't in alignment, it could end up elliptical!

Jordan

if boring between centers the bore will be parallel to the bed regardless of headstock and tailstock alignment

Yes he is talking about flex, that's why you should use the largest bar possible or available to you. Franco said he only took light cuts because of the size of his bar.
With your last message I agree as it is only a single point cutter it wont cut a taper. It will only cut a taper if the job is not set up in line with the bed ways both ways.

To go along with the Bob's picture of the Moore and Wright boring bar micrometer here is another Rob Wilson invention of a simple home made one that has the thumb screw at the bottom.

And here is a right up of a guy building one
Boring Bar Setting Device - for between centres

Dave

Originally Posted by new_guy90

if boring between centers the bore will be parallel to the bed regardless of headstock and tailstock alignment

That's what I understand, but if a line between the centres isn't parallel to the bed, then the hole will be an ellipse rather than a circle, even if it's parallel and not tapered.
To demonstrate the principle, if you shine a torch directly perpendicular to a wall, you'll see a circle. But if it isn't perpendicular, you get an ellipse.

Jordan

Originally Posted by new_guy90

can you explain more of this??? the cutter doesn't move so how is the support from the centers going to change during the operation could you mean it can flex in and out (dig in and cutting deeper in the middle or ends) producing erratic sizes???

Patrick,

Yes, I did mean flex, but you are correct (I think!) - flex can certainly result in erratic sizes, but not necessarily a consistent error like an hourglass or barrel shaped profile.

Frank.

Dave,

The concern I have with the device Rob made is that you have to assemble then dismantle the thing each time you set the cutter height. Some sort of quick release would be handy.

The dial gauge would provide a more sensitive "feel" than the micrometer.

Bob.

Found the reference:
In the Model Engineer's Workshop #168, Oct10, Ted Barclay has a description with photos.
Ted used a solid knuckle joint on the boring head instead of a centre and dog to increase the rigidity.

Presumably Ted loosens the joint to adjust the head, then nips it up before cutting.
The point is made that you get half the shift on the tool that is indicated on the head due to the tool being halfway along the bar.
Another aspect is the use of a "R" centre drill when machining the tailstock end of the boring bar to provide a curved surface presented to the tailstock centre.

John.

Originally Posted by nadroj

That's what I understand, but if a line between the centres isn't parallel to the bed, then the hole will be an ellipse rather than a circle, even if it's parallel and not tapered.
To demonstrate the principle, if you shine a torch directly perpendicular to a wall, you'll see a circle. But if it isn't perpendicular, you get an ellipse.

Jordan

ok i get you but its a bit over board i cant even think about doing the maths to work out the "ovality" of the bore my guess is it would be well under 0.01mm and so why bother ok yes if i ever do this ill make sure my centers are true

Originally Posted by electrosteam

Found the reference:
In the Model Engineer's Workshop #168, Oct10, Ted Barclay has a description with photos.
Ted used a solid knuckle joint on the boring head instead of a centre and dog to increase the rigidity.

Presumably Ted loosens the joint to adjust the head, then nips it up before cutting.
The point is made that you get half the shift on the tool that is indicated on the head due to the tool being halfway along the bar.
Another aspect is the use of a "R" centre drill when machining the tailstock end of the boring bar to provide a curved surface presented to the tailstock centre.

John.

i have that issue to what a good idea i would love a good facing head i suppose with that sort of kit you could easily turn tapered bores just like a horizontal borer

Originally Posted by Anorak Bob

Dave,

The concern I have with the device Rob made is that you have to assemble then dismantle the thing each time you set the cutter height. Some sort of quick release would be handy.

The dial gauge would provide a more sensitive "feel" than the micrometer.

Bob.

I read over on the other forum that they slide the unit onto the tail stock centre to save taking it off while doing the job.
It could be improved on easy enough, lets see your new and improved version Bob, then we can copy it. lol

Dave

Originally Posted by new_guy90

ok i get you but its a bit over board i cant even think about doing the maths to work out the "ovality" of the bore my guess is it would be well under 0.01mm and so why bother ok yes if i ever do this ill make sure my centers are true

You're right - it's hardly likely to be a problem in the real world.
Handy to know if ever you want to make an elliptical hole though

Jordan

So I take it then that a dead centre is used in the tailstock also when boring between centres. You would need a device with the clamping holes a fair distance apart to straddle most live centres if it was to be "stored " on the tailstock barreI.

I had sketched up a couple of ideas for a device at lunchtime today, incorporating a hinged section that would swing away to facilitate removal, a bit like a fixed steady, but realized that it would only work on a single diameter boring bar. Opening the thing up to accommodate a larger bar would throw the indicator and the cutter adjusting screw centrelines out of alignment. Something has just come to me....if the hinge point was made adjustable then the devise could be used on different diameter bars...

Where's Ken? He's the industrial designer, albeit retired. I'm only a pretend one.

Anyway, I'll draw something and scan it for appraisal. I still use a pencil.

BT

I had thought about a hinge like a fixed steady as well. I think something like Rob has built only with the hinge and a slot, and also machine in a recess for extra nut on top of the bottom part of non hinge side to hold it in solid and position when it is undone.

Dave

Originally Posted by Dave J

I had thought about a hinge like a fixed steady as well. I think something like Rob has built only with the hinge and a slot, and also machine in a recess for extra nut on top of the bottom part of non hinge side to hold it in solid and position when it is undone.

Dave

O.K. Dave, Here's my first rough sketch.