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Dore small boring bars

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Nige15/08/2017 12:38:34
370 forum posts
65 photos

ega: Yes enjoying the book once I got past all the technical stuff that takes up the first 116 pages. Not a complaint as I can see info there that will be useful in the not too distant future. Glad I bought it and I am re reading sections that are currently of use to me. Martin Cleeve does seem obsessed with the time it might take to do something but his background seems to be a production one with lots of items having to be made economically; again not a complaint as that environment generates good ideas which he has the talent and experience to exploit. Hurry is not what I am about

KWIL15/08/2017 14:09:58
3550 forum posts
70 photos

I think one thing could be that the resonance of the bar would be altered.

John Haine15/08/2017 14:13:58
4639 forum posts
273 photos
Posted by ega on 15/08/2017 09:31:49:

Has anyone mentioned that the push rod avoids the need for a grub screw at the business end of the bar?

Nige: I trust you are enjoying MC's excellent book.

I think that's the real point, it means that the bar will be shorter and the tool closer to the bottom of the hole as it were.

jimmy b15/08/2017 14:29:46
780 forum posts
42 photos
Posted by duncan webster on 14/08/2017 22:01:54:
Posted by Andrew Tinsley on 14/08/2017 20:14:20:


I have just done some mathematical scribbling and surprisingly, the bar will bend less for a given deflecting force if the central rod is under compression. I am a CGS man and looking up constants in MKS and even worse, modern units, gave me a headache.

So I don't know if the stiffening effect is appreciable at the compressive force applied by a grubscrew, but I suspect it is!


You'll have to post your scribbling, I can't see how putting the centre under compression and the outside in tension changes the second moment of area, which is the important factor

You can get tunable boring bars, damps out the vibrations. **LINK**

Some bars are filled with fluid/springs. I've used some at work 25mm dia bars working 300mm deep and no chatter!

Rainbows15/08/2017 15:14:29
647 forum posts
225 photos

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

KWIL15/08/2017 15:57:59
3550 forum posts
70 photos

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

Rainbows15/08/2017 16:11:22
647 forum posts
225 photos

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 115/08/2017 16:28:39
807 forum posts
39 photos

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

ega15/08/2017 16:44:21
2500 forum posts
200 photos

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.

ega15/08/2017 16:49:15
2500 forum posts
200 photos


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.

SillyOldDuffer15/08/2017 17:28:46
8513 forum posts
1914 photos

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.


Roderick Jenkins15/08/2017 17:56:11
2176 forum posts
608 photos

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.



colin hawes15/08/2017 18:24:50
557 forum posts
18 photos

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

Clive Brown 115/08/2017 18:24:51
807 forum posts
39 photos

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

JasonB15/08/2017 18:29:04
22604 forum posts
2643 photos
1 articles

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 Gilligan15/08/2017 18:37:22
20108 forum posts
1043 photos

Well done, Dave yes

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.


SillyOldDuffer15/08/2017 20:18:24
8513 forum posts
1914 photos
Posted by Michael Gilligan on 15/08/2017 18:37:22:

Well done, Dave yes

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.


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.



Sam Longley 115/08/2017 20:22:57
939 forum posts
34 photos

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 webster15/08/2017 21:52:50
3945 forum posts
63 photos

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 Gilligan16/08/2017 07:40:22
20108 forum posts
1043 photos

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


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.






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

 ... 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

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