By continuing to use this site, you agree to our use of cookies. Find out more

Member postings for DrDave

Here is a list of all the postings DrDave has made in our forums. Click on a thread name to jump to the thread.

Thread: Effect of Tensioning a Boring Bar
23/02/2020 19:53:09
Posted by Graham Meek on 22/02/2020 12:38:35:

In years gone by when I was learning my trade, mechanical engineering college courses included lab experiments.

That rang a little bell in my memory. The lab experiments that we did on my Engineering course were intended, amongst other things, to be a practical demonstration of a theory. If the results did not support the theory, then either the theory was wrong, the measurements were in error or the assumptions were wrong.

A solid cantilever bar of the same geometry as those tested should require 71.6 N to deflect 0.05 mm. A pretensioned bar cannot be stiffer than a solid bar, for the same diameter. Graham’s tests showed 37.8 N (3.85 kg) to deflect this amount and the pretensioned bar 44.4N (i.e. stiffer than the solid bar).

In the bending boring bar tests that Graham did, I have no reason to believe that the measurements were wrong. Equally, we know from decades of experience that the theory of beam bending is correct. Therefore my assumptions must have been wrong. The assumption that is probably wrong is that the cantilever is (dare I say) rigidly built in at one end. As Michael said, this is a convenient assumption because it is very difficult to achieve in practice. Any real-world flexibility at the support will decrease the load required to reach the 0.05 mm deflection.

I think that we have been concentrating on the bar in isolation, rather than considering the whole system. Small changes in the support would have a noticeable effect on deflection and vibration and could be an answer to the “why do two boring bars have such a difference in chatter” question. Can I ask, Graham, was there any difference in the way that the two bars (solid and pretensioned) that you tested in bending were supported that could have affected the results?

Dave

Thread: Tanks for CO2 engines.
17/02/2020 19:10:19
Posted by Steviegtr on 17/02/2020 00:25:44:

According to my bitdefender it just blocked a Trojan virus in your posting.

Steve.

We blocked this dangerous page for your protection: https://thumbs.worthpoint.com/zoom/images1/1/0411/10/gasparin-air-tanks-co2-engines-pcs_1_618756a20836c88d22f138e9b90a3f4e.jpg Threat name: Trojan.GenericKD.33016580 Dangerous pages attempt to install software that can harm the device, gather personal information or operate without your consent.

Be careful.

Thanks for the heads-up, Steviegtr. I did the browsing & posted the link from my ipad, so didn’t have a warning. You never can be too careful nowadays.

Dave

16/02/2020 22:17:47

Or try here

16/02/2020 21:37:56

Have you tried to contact Gasparin to see if they do spares? They still have a website (gasparin.cz), but it is not clear if they are still a going concern.

Dave

Thread: Effect of Tensioning a Boring Bar
12/02/2020 19:31:54

Regarding the magnitude of the pretension (or compression) on the assembly, this will have no effect on the simple, linear model that I showed above. The FE program does not even have the ability to consider preloading. Why no effect? The response is really just the sum of the vibration modes of the system: I will have to dig into a little maths, so bear with me. The natural frequency of a system boils down to frequency = sqrt(stiffness/mass). In this case, the stiffness is effectively Young’s modulus, which is independent of load (if you don’t yield it). So preload cannot affect the fine element analysis.

But, in practice, there are all sorts of non-linear effects creeping in. Friction between the parts possibly being one. I see no point in following the analysis route any further, so I will bow out (and let the elephants loose again). I think physical testing is the best way forward, but I do not have access to the actuators and accelerometers requited to do this justice.

Lastly, Gray’s comments and findings about torsion are interesting. However, after chatter has started, any torsional frequencies will be so high that I suspect that they play no part in the process. However, I have been wrong with some of my preconceptions on this topic and I am quite happy to be proven wrong on this, too!

Dave

11/02/2020 19:45:59

I had a look round the office for a few fine white elephants and found these lurking in a corner:

fem.jpg

This purports to be a 10 mm tube with a 6 mm hole with a 5.8 mm pin down the centre, both 70 mm long. Material is "steel". A nominal 200 N shear load is applied at the RH end and the LH end is built in. I also found a solid 10 mm bar lurking in another corner.

Varying the frequency of the 200 N load from 0 Hz to 5 kHz gave the three curves below. They are not all quite what I expected...

The solid bar gave the silver curve: a typical single degree of freedom forced response curve. Good. The "Loose" curve is for the model shown above: only the outer tube has responded to the load. The static deflection is slightly greater, because it is hollow. To my surprise, the natural frequency and the size of the peak have increased (not that the axis has a log scale). The frequency has gone up because the mass has decreased more than the tube's moment of inertia, one of the useful properties of a tube. I am not sure why the magnitude of the peak has gone up.

I then tied the two free ends together. I have called this "preloaded", to signify that it is modelling a boring bar when it has been pre-loaded. This has produced some interesting results. It has two resonance peaks: one for the tube and one for the bar. I am surprised that the first peak, and its magnitude, are nearly identical to those for the solid bar. Make from that what you will, because I certainly don't know!

Evidently the two-piece boring bar does work to reduce/eliminate chatter, but the mechanism but which it does this is still obscure. I would suggest friction between the two parts acting to damp the vibration, but the elephants had all left the office by then & were nowhere to be found...

fem response.jpg

Dave

10/02/2020 20:32:11
Posted by Michael Gilligan on 10/02/2020 20:26:22:

Thanks, Dave yes

I do note however that you omitted [inadvertently ?] my observation that:

If so, I must mention that what it “tells us” appears to actually be one of its several assumptions.

(...)

As I hinted earlier: I think we need to see some decent Finite Element Analysis.

MichaelG.

I was hoping that you would not spot that...

I tried doing the maths for a two-spring system, but it never was my strong subject so I gave that up. I will try to round up some fine white elephants at work tomorrow and see if that answers any questions, or just gives more of them!

Dave

10/02/2020 19:58:04
Posted by Michael Gilligan on 09/02/2020 23:27:12:
Posted by DrDave on 09/02/2020 21:20:32:
[…]
For a round bar, Engineer’s Theory of Bending tells us that the tensile and compressive stresses are the same.

.

I was a little nervous when I first read that assertion ... so I tried some self-help and found this; which I think is what’s known as Engineer’s Theory of Bending: **LINK**

http://www.idc-online.com/technical_references/pdfs/mechanical_engineering/Simple_Bending_Theory.pdf

Yes, that is what I had in mind with Engineer’s Theory of Bending: my apologies for using jargon. Duncan explained it rather well, above.

Dave

10/02/2020 18:58:27
Posted by Graham Meek on 10/02/2020 12:51:16:

In the first test it took an average of 3.85 kg to deflect a Plain bar 0.05 mm, (0.002" approx). While the boring bar under a pre-tension took 4.53 kg to deflect the same 0.05 mm. An improvement of 15%.

In the Torque test the plain bar took 1.13 kg to deflect the arm 0.05 mm, while the boring bar under a pre-tension took 1.59 kg. This time there was a much bigger improvement of 30%.

Graham, can you let me know whether the “plain bar” was a simple, solid bar, or whether its was the “bar within a tube” boring bar that you did the pretensioned tests on? Ta

Dave

09/02/2020 22:40:08
Posted by Michael Gilligan on 09/02/2020 21:54:07:
Posted by DrDave on 09/02/2020 21:20:32:
[…]
Engineer’s Theory of Bending tells us that the tensile and compressive stresses are the same. Preloading does not change the stiffness of the assembly.

.

Then could you please explain, in reasonably lay terms if possible, the findings in the report that I posted early this morning.

Thanks

MichaelG

A most interesting paper, MichaelG. The gist of it is to demonstrate the use of a high strength cable inside a structural steel section.. The steel cable is prestressed in tension, putting the tube into compression. Then, when an external tensile load is applied, the tube has to unload through zero and then start to pick up part of the applied load. The cable also picks up part of the applied load. The combination is much stronger than the tube on its own, when loaded to failure. I guess that the prestressed tube is lighter than just using a bigger tube without prestress.

One thing I hadn’t realised is the support provided by the cable increases the Euler buckling strength of the tube. Clever!

So, prestressing a low-strength tube is an effective method of increasing the ultimate tensile strength of the member. I cannot see, however, any correlation with vibration in a boring bar.

Dave

09/02/2020 21:32:12

Is that still the case with something having a preload? I thought general black steel stretched more for a given force than it compressed (thinking back to materials lab days) but you may well be right and I will stand corrected.

Yes, unless you exceed the elastic limit (in tension or compression) for the boring bar material. But if you do, chatter is the least of your worries!

09/02/2020 21:20:32
Posted by John Paton 1 on 09/02/2020 21:09:36:

maybe I am getting the wrong end of the stick, but ....... we have a tube to which we exert a lateral force at one end, resisted by compression of the steel on one side of it's neutral axis and stretching of the steel on the other side.

steel stretches more than it compresses so by introducing some pretension the net effect will be to reduce bending.

yes the increased stiffness wiLl alter the harmonic frequency but it is the reduction in deflection that we are really after.

To me the logic of tension is sound but the margin of improvement is probably quite small.

Edited By John Paton 1 on 09/02/2020 21:11:20

For a round bar, Engineer’s Theory of Bending tells us that the tensile and compressive stresses are the same. Preloading does not change the stiffness of the assembly. We need to look elsewhere for the reason that it can reduce chatter in a boring bar.

08/02/2020 21:11:11

My understanding of this is that the two parts of the boring bar, the tube and the rod up the centre, have different natural frequencies. Then, if one is excited near its resonant frequency and wants to “sing” the other is far from its resonant frequency and is “dead”. One part will thus act to reduce the resonant peak of its neighbour, effectively damping the system.

Thread: Aircraft General Discussion
01/12/2019 16:14:28

I experienced a few sonic booms whilst I was working in Switzerland (I think that the Swiss airforce might be a little more relaxed about speed excursions than the RAF is). First time, we were working in a Portacabin which shook like the devil. Even the floor shook! I looked around to comment about the apparent explosion, but none of my Swiss colleagues had batted an eyelid. It then dawned that it had been a sonic boom.

27/10/2019 13:30:55
Posted by martin perman on 27/10/2019 13:21:20:

I missed that one, I've been outside most of the day repairing a fence and trying to refelt my shed roof, what direction was it heading.

Martin P

Roughly south to north, up the eastern edge of Bedford. It was at only 1,00 feet or so; you should have seen it from Cople. I only noticed it because it was noisy and I thought that it was just a jet out of Luton that was still quite low, otherwise I would have missed it too.

27/10/2019 12:45:06

You don’t see many of them nowadays. A Jet Provost has just flown over Bedford. First time that I have seen one other than at an air show.

Thread: What Did You Do Today 2019
05/09/2019 12:56:24

I don’t normally listen to music in the workshop: I find that it is too distracting when using machine tools. But this morning I broke out the filing buttons to round off some clevises that I am making. Time to break out the dulcet tones of Led Zeppelin to help pass the time!

Thread: Aircraft General Discussion
19/08/2019 11:07:43

After Andrew’s comment about watching Gransden’s gliders online, I had a look at their website & found the relevant page. Watching this morning’s activity, I saw one glider had flown westwards and was circling between Cople & Great Barford: we should be able to see that from the conservatory. Staring out in that direction, lo & behold, a glider!

Don’t you just love it when all this technology just works? Well, it tickled me.

19/08/2019 09:20:12

Posted by Andrew Johnston on 19/08/2019 00:18:40:

DrDave: You are of course welcome at any time at Gransden Lodge to fly.

Andrew

A very generous offer and an interesting view into the world of gliding, thank you. I did most of my flying training (not to PPL, I didn’t progress beyond solo cross country) out of Little Gransden, but I have not been to Gransden Lodge. I will have to rectify this!

18/08/2019 22:20:22

Walking through the farms south of Bedford, this afternoon, we watched a glider pass overhead. Quite low, it circled a bit as if trying to catch a thermal. It then headed for the old RAF Cardington & descended to land. But, as it disappeared behind the trees, a motor popped out, started & they regained height.

I assume that was a bit of pilot training. They gained high quickly, going round in tight circles, and the wind carried them off towards Great Gransden. But now, MrsDave wants to try glider flying again: it must be well over 10 years since she last had a go.

Magazine Locator

Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!

Find Model Engineer & Model Engineers' Workshop

Support Our Partners
Eccentric July 5 2018
Rapid RC
Dreweatts
cowells
Eccentric Engineering
Subscription Offer

Latest "For Sale" Ads
Latest "Wanted" Ads
Get In Touch!

Do you want to contact the Model Engineer and Model Engineers' Workshop team?

You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.

Click THIS LINK for full contact details.

For subscription issues please see THIS LINK.

Digital Back Issues

Social Media online

'Like' us on Facebook
Follow us on Facebook

Follow us on Twitter
 Twitter Logo

Pin us on Pinterest