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Member postings for Andrew Johnston

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

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Thread: Anyone used an Inverter with a Colchester Student
15/12/2010 20:40:16
Steve,
 
That's interesting; I also had in mind that the lowest recorded electric shock fatality was 32V. I also thought it was a DC supply, as used on farms in the USA which were not connected to the grid.
 
So, either it's true, or it's a well established myth!
 
Regards,
 
Andrew
14/12/2010 11:02:57
Technically, at these relatively low voltages, it's the current that causes a problem, not the voltage per se. To get a shock you also need a reasonably conductive path to ground, or another phase; no path, no shock.
 
Regards,
 
Andrew
Thread: You are going to the Model Engineer Exhibition aren't you?
11/12/2010 19:10:50
Hi Nick,
 
Sadly nobody at Warwick came close to having the involute cutter I wanted. The 6DP was not necessarily a problem, but the 20° PA seemed to be. One company even said 'nah, we didn't bother bringing any involute cutters, give us a call when the show's over'. I didn't bother, as despite advertising all DP and module sizes, they've never had what I wanted in the past. In the end I bought a import cutter from Victor Machinery in New York.
 
I did have a slight hiccup with H&WM when I bought my horizontal mill. It was supposed to come with 1" and 1-1/4" arbors, but actually came with 1" and 32mm arbors. When I queried this the guy on the 'phone told me I was an idiot, after all 32mm is well within the tolerance for a 1-1/4" arbor, so he said. He did 'phone back later and apologise, having been told in no uncertain terms by one of the proprietors that 31.75mm and 32mm are not the same when it comes to milling arbors! I bought a 1-1/4" arbor from them for the cost of postage.
 
On a slightly different tack, but still on costs, I had an acquaintance who sold odd bits and pieces for Myford lathes that he didn't need on Ebay. Some of them made more than the new price from Myfords, ; people are strange!
 
Regards,
 
Andrew
11/12/2010 13:57:25
Hi Nick,
 
Funny you should mention that; exactly my experience! The last time I went to Sandown, several years ago, I bought a travelling steady for my lathe from H&WM, for what seemed a reasonable price (£60). I also bought a large horizontal mill from them for £150 some years ago. I guess they'd cleared a college and just wanted to get rid of it, as it's rather bigger than most model engineers would want.
 
On the other hand I looked at their stand at the Warwick ME exhibition and thought that a lot of the INT40 taper tooling was way over what I was prepared to pay for it. They didn't have the involute gear cutter I wanted either .

Ultimately the machinery suppliers are in business to make a profit and will price according to what they think the market will stand. Conversely the purchaser has to make a decision on what he is prepared to pay for a given item, based on what it is worth to him. If the two do not coincide, then no sale. If there are a lot of 'no sales' then the supplier might think that he is overpricing.

To be fair, H&WM are by no means the only stand to have secondhand tooling at way over what I'm prepared to pay.
 
Regards,

Andrew
11/12/2010 11:36:12
Posted by Steve Garnett on 11/12/2010 10:59:5
 
 Oh, there's plenty left to buy - almost everything on the Home and Workshop Machinery stand, for instance...
 
 
Why is that then?
 
Regards,
 
Andrew
Thread: Lignum vitae under the saddle?
09/12/2010 23:22:27
Sam,
 
When I wrote it I had Uriah Heep in mind (Dickens, not the rock band) so I suppose the accent would be Cockney!
 
Regards,
 
Andrew
09/12/2010 21:23:15
May I ever so 'umbly suggest that this website has no humour, humility, or any other emotion; but as for the people posting on it, well that's a whole different story!
 
Your 'umble servant,
 
Andrew
Thread: You are going to the Model Engineer Exhibition aren't you?
07/12/2010 22:53:21
Looks like I'm the only one not going on Friday, or any other day for that matter. May be next year.
 
Andrew
Thread: dividing tuition
05/12/2010 22:34:11
I would agree that using a DRO to locate each X-Y position is fairly time consuming. However, if the DRO has a bolt circle facility then it is dead easy. Enter the data, press the button and zero the display for the first hole. Go to the next hole and zero the display for the second hole and so on. Must be quicker than making two index plates.
 
There are some interesting notes on making a precision index plate (without the use of a DRO or CNC!) in 'The Machinist's Second Bedside Reader' by Guy Lautard.
 
Regards,
 
Andrew
Thread: How to determine the pressure angle of gears
05/12/2010 22:25:56
Hi Tony,
 
Indeed I haven't seen the spreadsheet, and if it contains extra information, then I am sure it is possible to determine the PA. My point was that it is isn't possible to determine the PA given only the OD and number of teeth. I'm not sure what the 'tooth length' refers to, but I would be interested in seeing a copy of the spreadsheet. I've sent you an email request for same.
 
Regards,
 
Andrew
Thread: dividing tuition
05/12/2010 10:48:17
To expand slightly on the excellent introduction to dividing using index plates as described by John. To deal with numbers that are not possible using the standard index plates there are three possibilities:
 
1) Compound Indexing - In this method the crank is moved the appropriate amount and then the index plate itself is moved, forwards or backwards, by another amount. In essence this method consists of two simple indexing movements in sequence; hence the name
 
2) Differential Indexing - As described by John, gears are interposed between the main spindle and the index plate such that the index plate is moved as the spindle moves, to give a differential movement
 
3) Make a custom index plate - pretty straightforward these days with DROs with bolt pattern functions, or with CNC mills
 
As it happens I'm in the process of making 63 and 69 tooth gears, neither of which are possible on my 40:1 universal dividing head with the standard indexing plates. From a mathematical point of view this is because there are no common divisors of 40 and 63, or 40 and 69, ie, they are mutually prime. While I have the gears and quadrant arm for my dividing head I do not have the extension shaft that fits in the back of the main spindle. Therefore it is going to be much quicker to make a custom indexing plate using the DRO on the vertical mill. It's all designed, I just need to make it! I also need to make some riser blocks for the dividing head as well, but that's by the bye.
 
Regards,
 
Andrew
Thread: How to determine the pressure angle of gears
04/12/2010 22:42:04
Errr I don't think so, the OD of the gear and the number of teeth will enable you to determine the DP of the gear, but they'll tell you absolutely nothing about the PA.
 
Regards,
 
Andrew
Thread: Scale model Economy hit & miss engine builders wanted
02/12/2010 10:53:33
Hi Charles,
 
Great stuff, thanks. I think I understand what is going on now. I'm not sure what the percentage of fuel to air is, but I do remember vaguely from my thermodynamics courses that the stoichiometric air/fuel mixture for petrol is approximately 14.7, by mass, for complete combustion.

I think the best thing to do is make and fit the carb and then design the fuel tank to suit. I'm planning to mount the engine and ancillaries on a square piece of jig plate, so I also have some flexibility on the height at which I mount the engine.

Regards,

Andrew
02/12/2010 10:14:20
Hi Charles,
 
Thanks for the updated carb drawing. I'm not sure I understand the purpose of the non-return valve. I assume that air drawn thru the carb causes fuel to be sucked from the tank? Does the non-return valve stop it running back? At what level do you think the fuel tank should be relative to the carb? I'm planning to TIG weld an external stainless steel fuel tank, so I'll have freedom to set it at what height I want.
 
Regards,
 
Andrew
Thread: Endmill, Slot Drill
01/12/2010 19:07:37
Errr, Neil, dare I suggest that that's not a good analogy? The neutral current in a three phase system is only zero in a balanced system. Of course, if there is no neutral connection the sum of the currents at that point, for a star connection, will be zero, but the currents will not be balanced.
 
Clearly a three flute cutter which is not cutting equally on all flutes will be unbalanced. Now there's a question; is there a machining operation wherein a three flute slot drill will be cutting equally on all flutes? I can't offhand think of one. I discount the case of a plunge in Z, as we're talking about side flutes, and the end flutes are not equal anyway.
 
Regards,
 
Andrew
Thread: reversing a milling machine
01/12/2010 16:57:54
Errr, assuming that the motor in question is 1hp, you'll still have 1hp at the spindle, whatever the speed reduction? It'd be a neat trick otherwise, and would have the Nobel prize committee winging it's way to your door post haste!
 
Power is torque times angular velocity, so if the speed is reduced by 12 then, as you say, the torque goes up by 12, but the power stays the same.
 
Regards,
 
Andrew
Thread: dividing tuition
29/11/2010 10:56:27
Hi Toby,
 
I'm probably too far away in Cambridge, although you're welcome to visit if you are ever in this neck of the woods. To whet your appetite I've added a few photos showing some gear cutting in progress.
 
Regards,
 
Andrew
Thread: Scale model Economy hit & miss engine builders wanted
27/11/2010 20:46:33
Hi Charles,
 
Congratulations on getting the engine to run! Although I have already part machined the carb to the published drawings, if you don't mind I'll half inch your design instead. It's a lot simpler. Thank you.
 
Regards,
 
Andrew
Thread: Mills
27/11/2010 20:31:03
Terry,
 
I agree that end mills are perfectly capable of cutting on the end. However, unless they are moving in Z, then all the cutting takes place with side flutes. I guess what I was trying to say is that they are somewhat mis-named. While they will cut on the end, that's not what you generally use them for. You'll have to be patient, I are an engineer, so naturally I struggle a bit with the English language, like what it is wrote.
 
I rather suspect that industry is moving away from end mills per se. Most of the 'end mills' I've bought recently are effectively three flute slot drills, even the roughers. I guess this has been driven by CNC requirements, where you need to enter the material at an angle, or a helical path in Z, in order to pocket out an enclosed area. Once at the correct depth you are essentially endmilling, but it helps to have a centre cut facility on the way in.
 
Regards,
 
Andrew
26/11/2010 08:25:30
In the odd way of the world an endmill, despite it's name, is intended to cut on the side, not on the end. What an endmill will not do is cut straight down. That's where slot drills come in. They can be plunged straight down into the material, and so can be used to machine closed slots or profiles.
 
Regards,
 
Andrew
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