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Member postings for mgj

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

Thread: Silver solder or copper rod?
12/11/2011 22:02:08
Ithink most of the answers are in Tubal Cains Soldering and Brazing pp37/38 which list several of these alloys.
 
Basically it should NOT be used for anything other than copper, or tungsten and molybdenum.
 
Aslo the finished joints should not be subjected to hot suphur bearing gases, or where the service environment is oxidising over 200C.
 
And details of failures on a test boiler are included.
 
So can it be used on coal fired boilers - No.
Thread: Effect of red heat on 316 and 321 stainless tube.
11/11/2011 17:54:25
Not necessarily an because in a jet engine , the air is split into two streams. Some, a relativeley small amout is mixed with fuel in a stoichometric mix. The rest passes around the combustors and keeps the hot gases off the walls, so the combustion chamber walls are not quite as hot as one might suppose.
 
the place where the special materials such as single crystal nimonics appear are in the turbine blades, which are taking direct heat, and continuous heat (unlike an I/c engine) but even they are cooled by air bled along the length of the blade though internal drillings (often either cast in or spark eroded I think)
 
One quick point - i haven't been told how it(this boiler bit) fails. I have simply been told that this bit breaks. I want to know HOW - in what MODE - it broke. Then you will start to know why.
 
Pushing something to the limit? Well yes- obviously, butr what limit? Tempreature, fatigue, stress?
 
Its generally quite difficult to fix a problem if you don't define it?
10/11/2011 19:43:49
Richard - vibration is a very good point- but the indication should be the beach mark of fatigue.
 
If you pick up that beach mark in a magnifying glass, you know that it has been flexed and a crack has propagated. It could be stress, but it could equally be vibration induced (or both).
 
BUT - you have to know how it has failed.
 

10/11/2011 15:14:09
Windy - seriously - look at the joint and determine HOW it failed.
 
You will get nowhere until you know that. . It may well be that there is absolutely nothing wrong with the materials - just a matter of stress and wall thickness. But it could also be brittle failure due to the chromium content of the weld playing games, allowing transformation to a martensitic structure at a lower temp than expected. (Brittle failure) You just don't know, and there is no way that we can tell by post as it were.
 
So, what was the mechanism of failure. Then you get a pointer as to where to look.
10/11/2011 10:18:23
The thing to do might be to look at the fractured face.
 
If it has a beach mark then its fatigue (at high temps)
If it has just bust its probalby brittle failure, and the likelihood is either poor choice of welding rod, or chlorine/hydrogen.
If its bulged, then probalby its just got too hot stretched and burst in an ordinary tensite failure under pressure.
 
(What pressure you runng at, what is the wall htickness so what sort of stress (hoop0 loading are you running at and have you looked at a stress strain plot for your SS, to determine if you are operating in safe limits, for that temperature, because if not you should before going further )
 
(Waht about the welds- any undercuts on the edges, because if so, there is the probable culprit before going too deep)
 
You could also be seeing a phenomenon known in the gas turbine world as creep. Under a constant load and elevated temps the metal slowly stretches (and in your case will thin), but in a gas turbine in tight clearances causes fouling. Either way disaster follows.
 
To answer your question directly - no it shouldn't,depending - big point - on the temperature, but yes it most certainly can.(or it may be a latent consequence of welding)
 
I would suggest though that you are getting into a realm where you are beyond the advice of the average interested amateur. You need proper metallurgical advice, once you have determined the nature of the failure. Start at the beginning - how/why it broke, and take it from there.

Edited By mgj on 10/11/2011 10:18:51

Thread: Help
06/11/2011 09:13:33
Taps - there are 2 tricks.
 
1st is to only advance a litlte at a time before reversing and clearing. This I ALWAYS do, and I have never broken a tap. (or pack hole full of grease, which squirts out with the swarf)
 
Second is not to believe many of the tapping drill guides. Many that I have seen quote a 80%+ amount of engagement, which is unnecessary and makes tapping far too forceful an excercise. 75% engagement is plenty, and makes all the difference in the world, without affecting strength. (I suspect this has more do to with my not breaking taps than the blatant lie above, especially when I run most of my taps in under power with a small Bosch cordless - has a clutch.. Not my idea - Tubal Cain again) The guides for many of the ME threads are particulary bad for this.
 
HP50g. Thanks. I have the manuals and the guides - its just that you have to guess which mode you are in vids-a-vis hte book of words! But its very good and far better than the Ti89 when you do have the hang of the small bit you are using!!
05/11/2011 17:18:12
That such things- exist. Of extended drills and centres. Or you make one drilling accurately into a bit of bar accurately clocked in a 4 jaw!
 
Perhaps we have all been spoiled recently with the advent of very adequate Chinese/Taiwanese tooling, which have rendered almost everything from small milling machines (the Dore Westbury) downwards not worth making.
 
Still every now and again one does have to do it, and if truth be known, I am glad that I am sufficiently decayed as to have come from that era. Gives one a lot of flexibilty

Which I am rather needing - having just bought an HP 50g calculator, which is so vastly expensive that a decent manual is obviously beyond them. Or perhaps, it just has so many modes that even the authors (who write very good English!) forgot where they were. Even setting its clock is an adventure.
05/11/2011 09:29:52
Indeed - but the problem is that there is always an error when you pick up an edge, or centre. Everything has a tolerance.
 
Again, it depends on the clearances you allow yourself. But if you wish to fit a rod into a reamed hole, the total error allowable is about.005" each end, because you only have about .001 clearance on the smaller reamers. So there is an eccentricity in the drill chuck holding the gauge, and an error in the mechanics of finding the edge.
 
So sure, picking up by whatever method can work, but it is not as inherently acurate, and is possibly more work setting up and swapping, than doing it all from one end in this sort of situation - particularly when there is no need to introduce the error.
 
You could use a long series drill bit, though they are not very nice to use, but they do have a properly designed fast sprial to clear swarf from deep holes.
 
Or, if you have to rotate the object, and the set up is small enough, use some kind of indexing device. The Versatile Dividing Head on a Myford (or mill) does that sort of 180 dec spin pretty well. And very quickly. too.
 
 
One must do as one sees fit of course, but it is always best to use a single datum point..
 

Edited By mgj on 05/11/2011 09:30:23

05/11/2011 00:21:23
Wolfie - the reason it broke was probaly because you were not withdrawing it on a deep hole to clear the swarf. (clicking crackling noise?) And that means the hole probably isn't straight..
 
Which, if you haven't got to the far end may well not matter. Mill it out, around the drill bit.. If you are going to hit the drill bit with the milling cutter, failing extraction by some onter means, you could easily make a mess of your cutter (depending on what it is made of) and possibly the work too.
 
Best sequence for that sort of thing I have found is to mill out the cavity. Drill the hole at the larger end - make an extension drill bit out of properly centred rod and a drill bit and loctite 2 together. Also extension centre bit. Use centre bit first. Also oil the gland hole through which the extension passes and uses as a guide.
 
That way you avoid deep hole drilling altogether.
 
I've never found drilling from both ends that wonderful - for me at least, even using properly held drills in a collet in a Quorn to produce a 4 facet bit, they never align absolutely perfectly. Adequately, yes, but rarely perfectly. Even with a DRO, you will have an error when hte job is reveresed end ofr end. Then of course, define perfect -.001" clearance.(And file flat to let the air out)
 
For my money - I'm afraid I would consign that one to the scrap bin of experience - or press a bronze plug in the bottom and restart.
Thread: Case Hardening - can't even harden a washer !
19/07/2011 10:59:42
I don't normally come here but......
 
We have got ourselves in a mighty tis was, and it might be worth reading Hardening and Heat Treatment more carefully.
 
Basically there are two separate processes.
 
There is the the slow carburisation process, which involves old bits of bone, leather etc, and more "modernly" various carburising powders. The process involves (normally) sealing in some airtight box, (not always depends on the powder)raising all to the appropriate temperature and then iron being a solid solution, the carbon migrates through the material at a particular rate enriching the surface with carbon before being quenched. It is slow, but depth of case can be more or less as deep as you want, properties are very controllable and there is les risk of distortion. Much used in industry.
 
The other is the Kasenit type process, much more suited to our sort of setup and effective enough for what we want.
 
Nothing inferior about anything, nothing went wrong - just a case of the right powder for the wrong process or vice versa. Full details are in Tubal Cains book in the chapter about case hardening.
 
Brinne is a good idea if you are stopping a tempering/heat treatment process (at say light straw), or if distortion is likely to be a problem - mostly it isn't because the substrate is in our cases mostly non heat treatable mild steel. We don't really toughen or otherwise heat treat solid steels other than Silver steel.
 
It has little effect on case hardening because all one is doing is freezing the crystal structure in the martensitic condition. IOW you are stopping the carbon rich surface from transforming back into pearlite, which it will do if allowed to cool naturally. So as long as you cool it faster than it can transform, and that you will do with water, the system will work perfectly.
Thread: A Challenge - How Would You Machine This Part?
06/06/2011 22:52:57
Rarely have I had the privilege of reading so much twaddle about over-engineering a component for an unstressed rustic device.
 
And as for blinkers- thank you but I have done my share with cogs. Real ones for racing engines, and tank turret fire control systems.
 
And on that note may I bid you all a very fond farewell
06/06/2011 19:09:05
Jason - I thought they were to be cast, but it could be a mix up in terminology.
 
I agree about the milling machine - I did my 8DP gears on a Dore Westbury, the largest being about 9" in dia. 6DP will be a bit of a shaker - which is one reason why on my vertical mill I got one that would do a sensible bottom speed (54 RPM IIRC) and not the 100+ rpm that many machines will do. A horizontal would be better, but if one gets the head down and keeps the quill extension to a minimum it will be fine (but I'm not using a DW for the 4" engine!)
 
To finish really. Were I building a gunsight with zero backlash and all then 'd specify true bevels and all. When I built the DW, Quorn and GHT dividing head, then setting to .0002" was normal (and boring and painful) for certain bits. But when one is building a TE, and one is struggling to keep the hornplates more or less flat after all that riveting, and they are what carries crank and 2nd motion shafts, the gears will fit where they touch, like it or not!
 
And especially if it is a road locomotive with springs and the axle is moving around!
 

Edited By mgj on 06/06/2011 19:11:11

06/06/2011 17:58:54
Andrew- of course, if one wants to, and I did make that point. But turning these things out in ali first to make a pattern for a set of true bevels seems a very long and relatively expensive way around the houses.
 
Further, the earlier part of this thread created an impression (to me) that one needs to go to such lengths to make a set of diff gears for a TE, when actually one does not, and a very satisfactory set of gears can be knocked out with the much more common vertical mill and some kind of indexing head.
 
I just had the impression that perhaps the approach was more how to make a set of true bevels, when possibly a more holistic view - how to make a satisfactory TE diff might have advantages.
 

05/06/2011 19:04:40
Well its good thing they had CNC when they made traction engines!!
 
Andrew - you really have to ask why? Or I do, because it seems to be a very wonderful solution to a problem which never existed in the first place. Unless one simply wants to do it this way for fun - which is as good a reason as any, if one has the time and the pennies..
 
One simply doesn't need a true bevel for this application - and with the clearances in a TE diff, it wouldn't matter if you used lantern pinions.
 
A parallel depth bevel is, accepted, an approximation, though it does offer certain advantages in running in certain circumstances. Despite the (very small) approximation,  at 20 RPM you ain't going to notice, nor even when winching, and certainly not at the 1/2RPM differential velocity when the bevels are doing their differential bit. -
 
And we go to all this trouble for something that can be chopped out of a standard bit of cast iron or steel? The calculations take about 5 minutes on a scientific calculator, order the cutter - it will be smaller than nominal but as you say, a standard off hte shelf DP cutter - , three evenings, a vertical mill, a simple dividing head and for addition, a DTI to set the blank roll. It will take longer to make the arbors to hold these blanks than it will do do the gear cutting, and you have saved the trouble and cost of getting everything cast anyway.
 
And why do we need 4 axis CNC for something that was originally made in a blacksmiths shop?
 
Forgive me, but it seems to me, this is a wonderful way of making a very simple bit of machining very complicated and expensive, for nil gain in performance.
 
Did you hear BTW of the bid BAe put in in the 1980s for some NBC fan housings (sort of aircon units) to go in armoured vehicles. Most people used pressed steel housings. The BAe bid was 4 times the price of anyone elses- but then they intended to start with a huge chunk of steel and a milling machine?

Edited By mgj on 05/06/2011 19:05:32

Edited By mgj on 05/06/2011 19:07:59

Thread: Taunton Exhibition
04/06/2011 20:27:37
Its at Monkton Heathfield Community School on the A38 just outside Taunton.
Thread: Super glue (cyanoacrylate)
03/06/2011 17:48:12
I've always found the model aircraft people have these superglues sorted. You get them in different thicknesses and different quantities. Some are gap filling, some are so runny you need a special capilliary tube to apply them. Some are even odourless!
 
They are not that expensive either they do benefit from being kept dark and in the fridg as Coalburner sugests. .

Edited By mgj on 03/06/2011 17:48:35

Thread: A Challenge - How Would You Machine This Part?
02/06/2011 17:55:11
Depends on what it is for - but I have a couple of bevel pinions for the TE to do - a 4" Little Samson in 6DP. I'll do them in cast iron as a parallel depth bevel - see Ivan laws book. I will also do the bevel ring gear like that too. He does do a cast version which I used on the 3" LS, but I fancy a change
 
With a GHT VDH the blank  roll is easily achieved using the plate wheel.
 
No CNC at all - fully manual devised during WW1. Aint exactly difficult with a dividing head and mill.

Edited By mgj on 02/06/2011 17:56:33

Thread: Machining what am I doing wrong?
01/06/2011 18:01:47
Affected by a cut on an 8mm dia workpiece - the scenario I envisaged was a touch of clearance (maybe the bearings weren't properly degreased before assembly?) and you have a touch of clearance. In compression the load drives the taper home, in tension it pulls out.
 
I used exactly that check on my 6" Chinaman when I put the new bearings in, for exactly that reason. A .003 or so cut on a 1" bar was enough to move the shaft and a socking great 6" Pratt chuck and all. All it took was a bit of pressure with the tailstock centre and an extra bump with the hand on the C spanner, and that was that sorted. At which point I discovered just why the bearings were so darned expensive, because the precision suddenly became fantastic in both directions, and has remained so.
 
It is effectively a vibrating load, and its going to find any slack toot-sweet as they say.
Thread: Is it abuse if I...........?
01/06/2011 17:49:55
To amplify Ians point. How much to take off with a reamer - couple of thou tops, so a reaming load should be very low.
 
I've seen it sometimes recommended that one can drill to say 1/64 (.015) undersize and then ream, but that frequently resutls in an unround hole. If one doesn't want to bore ot the reaming size then a metric drill will often provide the answer.
 
A reamer shouldn't really do more than just wipe the hole to size, and as a result a tapping load should be much higher. Of course, many charts are notorious for specifying tapping dias which give 90% plus engagement, which is a good way of causing jam ups, so getting the tapping dia right reduces the load considerably too.
 
 
Thread: The Cambridge Turning Trials
31/05/2011 23:00:39
Yes thanks. It would seem that there is a lot of variation!
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