Dore small boring bars

I need to make up some small boring bars in the near future. Looking at various designs I came across the Hemingway kit for the "Dore small boring bars". In the description it states that "Tension induced by the pushrod makes even the smallest tool surprisingly rigid"

Now the description infers that the boring rods are drilled out and a pushrod is inserted. Presumably these push rods must be under some serious compression to fulfil the above claim. There doesn't appear to be anything to produce this compression from the photo. What am I missing?

Andrew.

You can just see a grub screw poking out t back, that is used to apply pressure to a rod. Whether the tension makes much difference I don't know.

Good video on making similar boring bars below

Edited By JasonB on 14/08/2017 17:37:11

Sounds like bunkum to me. What material is the push rod said to be made of?? Presumably the push rod is a means of locking the bit in place from the other end of the bar.

Perhaps we should drill out all of our boring bars and insert a compression rod just to stiffen them up. I wonder why nobody has thought of that before. In fact, the applications are endless....

Merry

Thanks Jason,

I thought that the "things" poking out of the back were the rods, didn't realise they were grub screws! Memo to self, get your eyes tested!

It should be a relatively simple matter to calculate what the effect of internal compression should have on the deflection characteristics of the bar as a whole. My applied maths is getting very rusty, maybe I should try the calculation. Or maybe it is bunkum! I don't fancy trying to drill the bars out to that sort of depth. When people talk of long series drills , I tend to take cover.

Andrew.

I can't do the maths either so my opinion might well be bunkum too. Is there an expert in the house?

My guess is that the effect is akin to pre-stressing concrete or inflating a pneumatic tire. Being in compression means the inner rod of the boring bar will resist any bending force that tries to compress it further. At the same time, the outer part is in tension making it more resistant to any bending forces trying to stretch the metal. So as a result I reckon the bar will be more rigid compared to an unstressed equivalent.

As there must be a downside I suppose that a stressed bar is more likely to snap than bend?

If no-one knows the answer I might make one and test it. The joy of being retired!

Dave

Dont think I,ll worry too much about this one ! I think I,ll just "pass".

It's bunk. The comparison with concrete is not valid. Concrete has very little tensile strength, the purpose of prestressing is to put it in compression when not loaded, so that when it is loaded it doesn't go into tension. Steel is very good in tension, and even better in compression, and has the same modulus of elasticity both ways until you reach its limit of proportionality (near enough yield between friends)

Hi there, Andrew,

If you go ahead with the Dore design, you'll need some of these:

I found three of these in my 'stash' some time ago while looking for something else. I posted these pix here asking if anyone recognised them but nobody did.

Best regards,

Swarf, Mostly!

Probably cheaper to make the holders from a bit of steel or CI bar than buy castings, and then you only really need them if you want to mount direct to the topslide rather than use a toolpost.

I think SG iron would be OK but with the Dickson type the tee slot edges may be a bit weak if you used grey iron which is the usual bar you buy though SG is available.

Given the fact SG iron is not a lot different to machine than steel you may as well just use steel.

Hello,

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!

Andrew

Perhaps you could make up some very tiny boring bars to do that job.

(I'll go away).

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

I have just confirmed my recollection that the earlier (ie Neil Hemingway's) catalogue did not contain the claim that "Tension, induced by the pushrod, makes even the smallest tool surprisingly rigid" (and I have not seen this suggestion in relation to other push rod-based designs).

The primary function of the push rod is to secure the cutter, of course, but, instinctively, the claim seems valid; my understanding is that the reverse ie compressing the bar would make it stiffer.

Perhaps the OP should ask Kirk Burwell about this.

PS presumably the claim could be tested empirically.

Edited By ega on 14/08/2017 22:23:52

Edited By ega on 14/08/2017 22:42:51

Not an expert (or even close) in these things but seems to me a bit like a guitar neck, which has a steel truss-rod through the centre which is in tension, thus keeping the timber neck pre-stressed in compression, making the neck much more rigid so that lateral forces need to be a whole lot higher to impact tuning than in an un-stressed neck.

The compression rod might well make the boring bit more rigidly held in the bar. But the system must be linear so it is hard to see how applying a constant compression or tension in the bar can increase its rigidity.

By coincidence I came across this yesterday while reading "Screw Cutting In The Lathe" by Martin Cleeve. No mention of bar tensioning effects or anything other than locking the tool bit in the end of the holder. Hope it's ok to post this picture. INSERTED BIT TOOLS...para refers.

Any F360 users fancy trying out their FEA skills on this?

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.