Canards – Do These Ducks Fly?

[Roderick JenkinsParticipant @roderickjenkins93242]

I've been intrigued by a couple of bits of received wisdom that have been bandied about on the forum lately.

The first concerns a flexible boring bar producing a tapered hole. Is this really the case? It seems to me that the forces that push the cutting edge towards the hole centre would very quickly reach equilibrium and that the rest of the hole would be parallel, albeit smaller than required. Possibly I don't understand the whole system.

The second problem is with soft hammers. Does using a soft hammer really affect the forces on the bearings that are required to loosen a sticking Morse taper? I can understand that the hammer will do less damage to the head of the draw bar – but the bearings? Surely one has to hit the draw bar harder with a soft hammer than a hard one to provide sufficient force to loosen the taper, some of the energy having been absorbed by the deformation of the hammer head.

Just a couple of Saturday afternoon thoughts.

Cheers,

Rod

[Roderick JenkinsParticipant @roderickjenkins93242]

Some Engineering Myths (maybe)

[Neil WyattModerator @neilwyatt]

I agree on the boring bar. I have never got a steady taper, but have had issues with 'bell mouthing'.

Thinking of soft and hard hammers of similar weights, the energy from the soft hammer would be the same, but spread over a longer period for a lower shock load. But it's the 'shock' that unlocks the taper so perhaps a hard hammer needs less force?

Interesting questions… who's next?

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Roderick Jenkins on 24/11/2017 15:29:26:

Just a couple of Saturday afternoon thoughts.

Cheers,

Rod

.

Then I shall let them lie for the next 20.5 hours or so, Rod

MichaelG.

[HOWARDTParticipant @howardt]

Regarding hammer or soft mallet, I always use a hammer on my 3MT drawbar. One tap and it is released. I agree the force required will be the same what ever you hit it with to release it. In my apprentice days we only had copper or lead mallet to release the drawbar.

And I don't know were you are but it is Friday here.

[Tony Pratt 1Participant @tonypratt1]

I don't think a flexible boring bar makes a tapered hole, bell mouthed as it pushes off then parallel, spring cuts will normally help with the pushing off if the tool is sharp.

Tony

[Roderick JenkinsParticipant @roderickjenkins93242]

Doh! I've been convinced it's Saturday all day. Just another one of the joys of retirement

Rod

[mechman48Participant @mechman48]

Hmmm interesting; but there again I use a self ejecting top hat fixture on my spindle / draw bar so no need for a hammer of any description… so far, fingers x'd

George.

[Clive FosterParticipant @clivefoster55965]

A badly shaped tip on a boring bar will produce a taper. Requirement seems to be a combination of excessive tip radius and lack of heel clearance a short distance behind the actual cutting point. Most likely due to the tool being too deep for the hole.

The tool cuts properly on entering the hole as the front clearance angle stops the heel hitting the side of the bore. As the tool moves forward the heel contacts the side of the bore pushing the tip out reducing the depth of cut so the bore becomes smaller creating a greater deflection when the heel moves into contact with the smaller bore. Process can carry on until the actual cutting tip is clear of the bore leaving the heel scraping along doing a very ruff cut.

Easiest way to demonstrate this effect is to use a boring bar thats really too deep for the size of hole. Most likely to happen with the mutant golf club style. Especially if you habitually put a curve on the front so the tool blends cleanly into the shank as its easy to end up with a radius rather larger than that of the hole being produced so contact is almost inevitable.

Can also occur with the small insert type if the starting hole is too small. I have one that demands a 2 mm or so larger hole than the manufacturer suggests if its not to produce such tapers. Theoretically it will work into the specified smallest hole but in practice any cut sufficient to let the tool go in properly is too great and the boring tool deflects.

Concerning hammers on drawbars a soft hammer usually needs a lot more hitting than a hard one. If I use the largest Thor deadblow on my Bridgeport drawbar it needs some serious hitting, usually more than once, to loosen it. Best method is a strangled (held right up under the head) club hammer. Using the side of the head a sharp 60° twist of the wrist shifts things every time.

Cant see hammers are a great worry to the bearings on any properly specified machine. Industrial users have been wellying on drawbars for years yet the machines have coped just fine.

Clive.

PS Corrected statement re relative tool heel radius and hole radii

Edited By Clive Foster on 24/11/2017 16:35:17

Edited By Clive Foster on 24/11/2017 16:50:29

Edited By Clive Foster on 24/11/2017 17:05:21

[Roderick JenkinsParticipant @roderickjenkins93242]

Another canard may be that banging the bearings causes damage in the first place and the consensus seems to be that it is unlikely. The spindle on my milling machine is soft and Morse tapers can get very tight. At least with some sort of screwed extractor one uses only the minimum force required to loosen the taper whereas a blow could be well in excess of that required. Is a percussive force of the same magnitude likely to do more damage?

Rod

[FarmboyParticipant @farmboy]

In my experience, tapping it with a BIG hammer is much more effective than belting it with a small one

I imagine the softer the head the more energy it will tend to absorb. I always thought the only point of the soft head was to avoid damage to the surface being struck.

[Mick B1Participant @mickb1]

Posted by Farmboy on 24/11/2017 17:30:49:

In my experience, tapping it with a BIG hammer is much more effective than belting it with a small one

I imagine the softer the head the more energy it will tend to absorb. I always thought the only point of the soft head was to avoid damage to the surface being struck.

I think it also transfers more momentum to the target object, because it damps out bounce, which is momentum returned to the hammer by elasticity in the head and target.

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