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Ian S C05/02/2011 11:24:38
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The main reason for 8.8 bolts could be the greater resistance to damage of the threads over a period of time compared to softer botls. The studs in my clamping set appear fairly hard. When I use bolts on the mill, its into propper T nuts, not the head in the slot with a nut on top. Ian S C
mgj05/02/2011 18:04:13
1017 forum posts
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Judas Iscariot. The lad originally asked about using some bolts of some unspecified material. The advice he was given in general was yes, crack on, but keep bolt/ nut heads thin, so that the head gives before the x/slide/rotary table/milling table.
 
Then someone came up and said IIRC that mild strectched too much -erroneusly, and that was addressed, the explanation being that mild and HT stretch about the same, (in %dimension though the forces required to provoke that stretch will be greater - ie HT is a stiffer spring - surprise surprise) before yield but not breakage, because mild is more ductile, and will stretch after yield.
 
Not that that matters because once the stuff is behaving like toffee, its not actually very useful. and once it has started to strech, stress levels may be very high, because it is thinning, but it still can't carry any useful load in our sense.
 
Then someone comes back and says pedantically that mild and HT will stretch the same - and that is factually wrong. However we knew that in principle - and the reasons why - over a year ago.
 
 
Do I calcute these things - yes I do, quite often, not because its some amazing exercise to prove that I'm better than anyone else. Its much simpler than that. I don't like wasting time, or having to repair things because of some dumbarse engineering failure. Most of these calculations are not very difficult, and don't take very long, and its nice not having to frig about sorting irritating problems. But, like anything else, there comes a point where the best becomes the enemy of the good.
 
At which time its probalby time to stop playing - both with ones kit, or the english language.


Like my little coupling rod bolts - they'll last a while. At some stage they'll get changed - sooner rather than later, because they will break sooner rather than later. The arithmetic says so. I prefer to replace a few bolts than cope with bent conrods and dinged gunmtal bearings. And if 5 minutes on a calulator saves me that, I think its 5 minutes well spent.
 

 
 
 
.

Edited By mgj on 05/02/2011 18:06:27

Nicholas Farr05/02/2011 20:06:36
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Hi, just another extract from my college days, which should help understand this streching business, and will back up what mgj is saying somewhat. First is a tensometer graph which I did in my lab work, it shows different carbon content steels streched to breaking point. Stress is shown on the vertical and extension is shown on the horizontal. It can be seen that the higher carbon content steels do extend more before the yield point, allbeit by just a small amount.
 

Proof Stress
 
Mild and medium carbon steels have a definite yield point, followed immidiatly by a plastic stage, during which the material extends considerably. As the carbon content increases, this plastic stage becomes smaller and the steel enters the ductile stage soon after the yield point. In very high carbon steels both the yield point and the plastic stage disappear, and as the stress in the steel is increased it passes directly from the elastic state into the ductile state.
In materials which have a yield point, it is important that the yield stress is known, for if this stress is exceeded, then parts made of the material will have large permanant extensions. This permanant set is often specified at 0.1%, i.e. one thousandth of a gauge length. Thus on a 2" gauge length, the permanant set would be
 
2" X 1 over 1000 = 0.002"
 
The stress providing this permanant deformation is discribed as Proof Stress
 
Hope this is of interest to someone.
 
Regards Nick.
mgj06/02/2011 09:37:19
1017 forum posts
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Bless you Nick - that makes it all crystal clear. Happy days in labs at the College of Knowledge! Should have kept my notes and handout s. One never knows when htey might be useful.
 
Perhaps one should make the point that strength (HT) and hardness generally = more carbon. (as a principle)
 
You are a wiser man than I.

Edited By mgj on 06/02/2011 09:38:55

Nicholas Farr07/02/2011 00:07:24
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3988 forum posts
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Hi mgj, thanks for the compliments. I have found a few more notes on this lab test, and have put a couple more extracts in my Nicks alubum for any one who is interested. The first one shows the object of the test, and the secound one shows a labled graph.
 
You'll see I only got nine & a half out of ten for this one. Might have lost half a point for not putting the ductile range lable on the graph, or maybe it was just my scruffy writing.
 
Regards Nick.

Edited By Nicholas Farr on 07/02/2011 00:29:20

Ian S C07/02/2011 01:55:59
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Nick, or a tutor who did'nt think anyones perfect, and proberbly never gave 10 out of 10 how ever right you are. Ian S C
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