Dore small boring bars

[RainbowsParticipant @rainbows]

Out of curiosity, would a carbide push rod increase rigidity or would it just snap as the steel outer flexes?

[KWILParticipant @kwil]

Rainbows, just use a carbide boring bar, marvellous things but a tad expensive.

[RainbowsParticipant @rainbows]

I had them in my mind as £100+, still expensive but more reasonable at £50 from cutwel. Maybe one day. Still more than £8 for a theoretical blank which would give push rod and cutting blade though.

[Clive Brown 1Participant @clivebrown1]

To answer your original question, a carbide push rod of near maximum diameter for the drillway, would increase stiffness, but probably by an insignificant amount. Whether it would snap depends on the load that the cut imposes on the assembly.

Edited By Clive Brown 1 on 15/08/2017 16:29:52

[egaParticipant @ega]

On Rainbows' value for money point, I followed up a recommendation by richardandtracy for boring bars direct from China and now have a 7mm dia bar which must be about as small as is possible with a CCMT06 insert. The cost complete with ten insert was under £8 and on test the bar works quite well. It is long for its diameter at 125 mm but, of course, is used with minimum projection from the holder.

My preference remains for a nicely-tuned bar with HSS bit.

[egaParticipant @ega]

Nige:

Thanks for your response.

I don't remember whether you have GHT's Model Engineers Workshop Manual which has a truly excellent chapter on boring tools.

I think it would be fair to say that hurry was not what he was about either.

[SillyOldDufferModerator @sillyoldduffer]

The experiment and results you've all been waiting for! I hope it's worth it because I broke a NCIH parting blade making a thick washer.

I took a 750mm length of 1/2" O/D thick walled steel gas pipe and passed a metre of M8 DIY store studding through it. Each end was terminated with a 6mm thick steel washer and an M8 nut.

One end was held in a vice and about a kilogram of scrap was loaded on the other.

A bamboo pointer was stuck on the far end of the pipe.

The picture above is of the pointer position with the nuts cranked tight, ie with the pipe compressed. A pencilled mark records the position.

Tightening the nut caused the end of my pipe to move downwards. That's not what's expected if the pipe becomes more rigid when compressed due to tightening the studding nuts. Loosening the nuts causes the pipe to rise back to the start position. Again this isn't what I expected if pipe rigidity was being improved by compression.

I think the pointer falling as the pipe is compressed is due to the pipe bending. It follows the line of least resistance, and reinforces the pre-existing bend due to gravity. The pipe behaves like a spring, not a stiffened rod.

I put enough force on the nut to damage the thread on the studding.

Twanging the pipe to make it vibrate wasn't any more encouraging. The amount of tension on the studding doesn't seem to effect the amplitude and duration of the pipe's vibration one way or the other.

Unless there are experimental flaws (feel free to point them out), it looks very much as if compressing the pipe has no useful effect. In other words my experiment suggests 'bunkum' is more likely than 'rigidity improved'.

I can't claim the experiment is conclusive because the M8 studding I used isn't a tight fit inside the gas pipe. I suppose it's just possible that a much tighter fit would stop the pipe bending.

Dave

[Roderick JenkinsParticipant @roderickjenkins93242]

Nice experiment. I think the studding needs to be truly axial. By shifting the studding in the tube you can probably get it bend the other way – similar to a truss rod in a steel strung guitar, which is purposly non-axial to counteract the pull of the strings and bring the fingerboard back to only a slight concavity.

cheers,

Rod

[colin hawesParticipant @colinhawes85982]

A taper along the bar's length would make resonance less likely. Colin

[Clive Brown 1Participant @clivebrown1]

The tube might not have been quite straight and / or the end faces not truly square.

[JasonBModerator @jasonb]

The weld down the length of the pipe will more than likely have an effect on which way it bends too.

As Rod says a top hat washer to keep the studing central in the bore would also reduce the tendency to compress one side more than the other.

[Michael GilliganParticipant @michaelgilligan61133]

Well done, Dave

Just one [maybe significant ?] point from me … You are stressing the tube in the opposite sense to that boring bar.

i.e. you are tensioning the studding; which compresses the tube,

but they put pressure on the rod; which stretches the tube.

MichaelG.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Michael Gilligan on 15/08/2017 18:37:22:

Well done, Dave

Just one [maybe significant ?] point from me … You are stressing the tube in the opposite sense to that boring bar.

i.e. you are tensioning the studding; which compresses the tube,

but they put pressure on the rod; which stretches the tube.

MichaelG.

Not so well done I fear – you're right , I need to push, not pull. Now we know why peer review is so important. Ah well, back to the drawing board.

Well spotted Michael.

Thanks,

Dave

[Sam Longley 1Participant @samlongley1]

You are compressing. the tube but due to missalignment of the studding within the tube you are applying the force asymetricaly.

I would suggest that you turn some nuts round to make a sliding fit within the tube. These should be placed at close evenly spaced intervals along the studding which can then be inserted inside the tube. Hopefully this woul keep the studding central whilst pre stressing takes place.

In theory under normal load the tube will want to resist deflection if placed as a cantilever. As load is increased it will react wiithin itself to resist this deflection.tension will immediately occur in the top sector

If one prestresses it the initial stages of tension are counteracted by compression & until the 2 forces balance bending will be significantly reduced( subject to what happens in the bottom sector) if the stud had been placed in the top sector the tube would have bent upwards but we want a demonstration of an axially placed rod so compression applies on upper & lower sectors equally. This should not affect the increased performance of the prestressed unit.

However, one has to consider how much compression can be applied by a single set screw & if that is adequate to make a difference

It is 50 years since i did theory of structures so cannot prove mathematically anymore—-apologies

 

Edited By Sam Longley 1 on 15/08/2017 20:33:17

[duncan webster 1Participant @duncanwebster1]

Dave, what you have is akin to a bow (as in Robin Hood). The tube is bent because of the side load, you are then applying a pull along a straight line between the two ends, not co-axially down the bar, so it bends more as it would if you tightened a bow string. As well as being in the wrong direction, it is not the same as a close fitting rod down the centre which would be forced to bend with the tube, and so would apply any force co-axially. If the push rod in the Dore bars is not a close fit all the way down, I might be prepard to amend my previous opinion!

Edited By duncan webster on 15/08/2017 21:53:31

[Michael GilliganParticipant @michaelgilligan61133]

I've just found this observation; which may inform the discussion:

[quote]

Some time ago I was doing a lot of boring, and got through a number of carbide tips for my boring bar. The bar was none too ridged, particularly under load as the edge of the carbibe insert went dull, so I decided to find some other boring bar. Hemingways seemed to offer the ideal solution with their boring tools designed by Dore. These items work rather well as they have a bar held in compression along the whole length of the toolholder. When making the actual tool holders it is best to drill them 3.3mm (tapping size for M4) and use an M4 to provide the compressing force. I tried several times to use the recommended 1/8th inch bore, and found the compression bars became stuck in their bores. To get these cutting without deflecting the bars it is crucial to grind the cutting tools correctly, and to set them correctly. The bodies are from cast iron, and work well if made to plan. I rarely use the bodies these days as I adapted the design to be used with the quick change tool post (below) which allows a more accurate setting for the cutting tips.

[/quote]

**LINK**

http://www.maths.lancs.ac.uk/~lucy/hobbies/tools/tools.html

MichaelG.

.

_____

P.S. … The design in this patent is interesting [albeit not directly relevant]:

https://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=4998851A&KC=A&FT=D&ND=&date=19910312&DB=&locale=

 … BUT its claim is for damping, not stiffening

… AND it is much more complex than the Dore design.

Edited By Michael Gilligan on 16/08/2017 08:01:02

[JasonBModerator @jasonb]

I think the "blunt" nature of an insert will always tend to push it off the surface more than a sharp HSS tool particularly if sneaking up to a finished size. Just changing to HSS in a solid bar would more than likely shown similar results.

Where bores are important eg engine cylinders I will rough out with insert tooling and then switch to HSS for the last 1/32" or so. Its easy to see that the HSS bar is flexing less as the tool rubs far less when withdrawn than the insert.

J

[jimmy bParticipant @jimmyb]

towards the bottom of the page is more info on tuneable/anti-vibration bars

**LINK**

[Michael GilliganParticipant @michaelgilligan61133]

Very interesting, Jimmy … thanks

You may also like the patent that I've just added to my previous post.

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by JasonB on 16/08/2017 07:50:05:

I think the "blunt" nature of an insert will always tend to push it off the surface more than a sharp HSS tool particularly if sneaking up to a finished size. Just changing to HSS in a solid bar would more than likely shown similar results.

.

Agreed, Jason

The reason I quoted that extract was that it includes reference to dimensions that people have been wondering about.

MichaelG.

[JasonBModerator @jasonb]

They obviously did not watch the video I linked to

He suggests a 3.0mm drilled hole to give a loose sliding fit on the rod rather than reaming 3.0mm which is too tight on the 3.0mm silver steel rod, about 9mins into the video.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by JasonB on 16/08/2017 08:36:49:

They obviously did not watch the video I linked to

He suggests a 3.0mm drilled hole to give a loose sliding fit on the rod rather than reaming 3.0mm which is too tight on the 3.0mm silver steel rod, about 9mins into the video.

.

But we're digressing a little from the Dore/Hemingway design that is the subject of this thread.

The Hemingway site is not specific about the dimensions of the long series drill that's supplied in the kit, and makes no reference to reaming, or to the 'fit' of the pushrod …

http://www.hemingwaykits.com/acatalog/_Dore__Small_Boring_Tools.html

I was simply gathering information to inform our analysis.

MichaelG.

[jimmy bParticipant @jimmyb]

Related to this, I've seen anti-vibration boring bars with springs and weights inside. Sometimes vibration and be removed from a job by REDUCING the clamping, its all about altering the frequency of the vibration…

Related to the liquid filled anti-vibration boring bars, we had an apprentice chop on down once, when the magic liquid leaked out, he realised he'd made a bit of a mistake…………

[Author]

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