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: We need Pi|
+1 for the calculator.
But I must confess that, at University, I took it upon myself the learn pi to 10 significant figures, which was the precision of the calculator that I had at the time. And I can still remember it 40 years later!
|Thread: Emco F1 Mill Upgrade|
I dare not connect the controller PC to t’internet! Any software that I need to put on it (I have just upgraded to Centroid’s v4.50 software, which also required .NET Framework) I download on another computer & transfer using a USB stick.
I have done a few mods to the machine since updating the controller. I got fed up with swarf going everywhere, so a bit of bendy MDF came to the rescue. The lighting was also quite poor, so I have added a couple of led lights (they are intended to go under wall units in the kitchen) and now I can actually see what is going on!
Have you watched Matryscncgarage on youtube? I found his videos very valuable when I was setting my controller up.
|Thread: What Did You Do Today 2020|
Removed duplicate posting
Edited By DrDave on 26/06/2020 11:57:08
You got it in one!
I never know quite how much is going to come off until afterwards...
Bit of a mixed bag today. I thought that I was making good progress milling the excess off a block of aluminium for a new engine. Heisenberg, my SX2 mill, was sounding unhappy, so I put a brand new end mill in and tried again. Even worse. I got hold of the bed & gave it a wiggle: there is about 1 mm play on the Y-axis.
I wound the bed all the way out to see if I could tighten the anti-backlash nut only to find that the problem is that the nut has come loose from the bed. Ah, well, time to give it some of the TLC that it needs, and check to see what else has come loose.
|Thread: Cloth on string that hangs down on old aircraft|
|Thread: Concorde model maden flight|
I used to work just down the road from Mühldorf am Inn, so I let out an involuntary “oh!” at the start of the video!
The airfield has seen other exotic aircraft, such as the Me262. There is also the site of a WWII vintage underground hanger that was being built for Messerschmitt in the woods nearby.
|Thread: Model Turbines|
I didn’t realise that you can buy such a small centre drill “off the shelf”. Keep up the good work!
|Thread: New design of mains plug?|
I’ve just checked one of my Swiss socket strips: it says that it is rated at 10A max.
Thanks for the explanation, Bert. I suspected that it might be something along those lines.
I have never understood the British love of fused plugs: most of the world seems to be happy with unfused plugs. None of the electrical equipment that I bought when I lived in Switzerland has a fused plug, for example.
Other than the regulatory requirement here (and after all it does sound like a good idea), can anyone explain why Britain and the colonies have this different plug philosophy?
|Thread: Bending and shear force confusion....|
Michael, reading many of the posts in this thread, it does appear that there is some confusion about shears and bending. I tried to clarify my earlier comment about a pin-in-socket leading to failure of the boss. I think your comment about “double shear” and Loctite suggests reacting the moment at the end of the pins by shear in the Loctite, rather than bearing pressure as I assumed in my example. I did use both assumptions the once when I was desperate to get an acceptable answer, but that is poor practice, even if I say it of myself! I hope that you did not construe this as a slight on your good self.
Nick, can you explain what you mean by sliding forces?
PS: to clarify the terminology, for those that might not be familiar with the diagrams that I showed, shear force and bending moment as used here are internal forces in a structure, not externally applied forces. The top diagram shows the applied forces; the lower two show how the shear and moment vary along the length of the beam.
There does appear to be some confusion about shear forces and bending moments... To clarify my earlier post, I have drawn up a pair of sketches of a bar passing through the centre boss and a similar arrangement with two stub pins. They show the shear force and bending moment diagrams for the bar and for the pins. As I said before, the peak bending moment in the through bar is very nearly the same as that for the pins (for the same force, P).
The real difference is in the pressure applied to the boss. For the through bar, this is roughly uniform at a level of, say, w1. For the pins, however, the moment is transferred by very high pressures between the pin and boss. The triangular pressure distribution required to react the moment at the end of the pin gives a peak pressure (w1 + w2) of about 6 times higher than w1. This commonly leads to a bearing failure in the boss.
There is the alternative load path for the pin-in-socket, as shown by Micheal G. I think that I have only had to use a "Desperate Dan" analysis like that once in over thirty years of doing stress analysis of aerospace structures! The pin-in-socket, below, is the standard method.
Coincidentally, I have been looking at a very similar problem at work today. The difference is that the contact pressure between the "bar and the boss" is nearly 6 times higher if you have two bars in the boss, rather than a single bar passing straight through. This causes failure in the boss if two bars are used. The bars have similar strengths for either case.
|Thread: Effect of Tensioning a Boring Bar|
As an addition, to avoid cluttering my post, above, any more:
From a reference such as Roark, the maximum deflection of a cantilever loaded at the tip is delta = force x length^3/3EI.
Here, I = pi d^4/64 = 491 mm^4. E = 210 x 1000 MPa and length = 60 mm. So force to deflect 0.05 mm is 3EI delta/length^3 = 3 x 210 x 1000 x 491 x 0.05/60^3 = 71.6 N
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?
|Thread: Tanks for CO2 engines.|
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.
Or try here
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