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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: gundrilling, and old ME articles sought
14/09/2010 16:56:54
Here's an article on how to use a gun drill, it's by no means simple:
 
 
I hope the link works, first time I've tried posting a link!
 
Regards,
 
Andrew
Thread: How do you use indexable cutting tools.
13/09/2010 22:58:44
Hi Ady,
 
I'd be interested to know more about your experiences with negative rake tooling. Is this on a lathe? I know that industry uses negative rake indexable inserts for milling, as they are often stronger, but I understood they needed a lot of horsepower. It hadn't occured to me to look at negative rake tooling for a lathe.
 
Regards,
 
Andrew
Thread: advice on purchasing a small mill
13/09/2010 22:27:01
I would agree that metal removal rates are of most importance in industry. However, the modeller who doesn't bother to learn a little about metal removal, cutting speeds and chip loads is doomed to waste time, get poor finishes and damage cutters.
 
Milling cutters do not  care if they are running on a 0.3hp hobby machine or a 30hp machining centre; if the cutting parameters are wrong they will not produce good work.
 
Regards,
 
Andrew
Thread: Dangerous Practices
13/09/2010 22:21:34
I agree with many of the previous posters; if the technique is dangerous then take the trouble to explain why. Otherwise we don't have the opportunity to learn. If the OP can't, or won't, explain and suggest a better way, then there was little point in the original post.
 
Looks to me like a case of 'rant today, gone tomorrow'.
 
Regards,
 
Andrew
Thread: How do you use indexable cutting tools.
13/09/2010 20:23:57
Bob - Glad my random ramblings were of some help!
 
To elaborate further on the useful comments by KWIL:
 
1) It is perfectly possible to take small cuts with inserts, but you do need a fairly rigid setup, and it does seem to depend on the material. Brass, aluminium and cast iron are fine (sorry about the pun), but steel, particularly stainless, can be more of an issue.
 
2) All the above depends on using inserts with small nose radii, say 0.2mm. My experience with roughing inserts, with nose radii on the order of 1mm, is that they are less inclined to take fine cuts.
 
3) I totally agree that if you're working the machine and inserts properly the swarf should be coming off blue; or hotter, I have seen dull red. As I understand it, for steel at least, about 70-80% of the heat from the cutting process should be dissipated in the swarf.
 
Regards,
 
Andrew
13/09/2010 13:15:16
Hi George,
 
Here are some random thoughts on using indexible inserts.
 
1) In general inserts are not as sharp as HSS tooling and need a decent depth of cut (minimum several thou) and/or a rigid lathe/workpiece combination. I've never used a C3 but I suspect it comes into the category of not particularly rigid?
 
2) To get a good finish on steel with inserts you need to run at high surface speeds, normally at least 400fpm, so 1500rpm plus for a 1" diameter bar
 
3) Turning aluminium is best done with polished inserts, or you end up with BUE, which ruins the finish. BUE = built up edge, ie, swarf building up on the cutting edge of the insert
 
4) Brass should be pretty forgiving. My experience of imported indexible tooling is that the holders are ok, but the inserts are rubbish. Throw them away and buy some decent ones. I use Greenwood among others for indexible inserts.
 
5) If the material you are turning is small diameter it may be flexing away from the tool, try tailstock support?
 
Hope this helps.
 
Regards,
 
Andrew
Thread: C3 Purchased
12/09/2010 21:27:31
Hi Bob,
 
You certainly seem to have an intimate knowledge of Tiger Moths; one might almost suspect that you flew the full size aircraft?
 
Accrding to my pilots notes the slats make exactly 3mph difference in stalling speed; 48mph down to 45mph. Also according to the notes the slats should be locked during taxying and before aerobatics and spinning. Presumably at other times they were left free. I can't remember exactly what we used to do with them. I suspect they didn't change much either way.
 
Regards,
 
Andrew
10/09/2010 23:39:23
Hi Ian,
 
Thanks for posting the Mustang picture. Where I live, quite close to Cambridge, we see quite a lot of WW2 fighters, particularly Spitfires. We're pretty close to the Imperial War Museum at Duxford, in clear airspace, and there's an ex-WW2 airfield just beyond the back gardens of the houses opposite me in case things go pear-shaped, so we get quite a lot of practise and test flights overhead.
 
I'd certainly pay a thousand pounds to have a go in a P51D. It's definitely on my list of things to do. However, I think it's rather difficult in the UK due to some daft CAA rules.
 
Regards,
 
Andrew
10/09/2010 23:24:57
Hi Bob,
 
 Wow, you're Tiger Moth certainly has all the details! I hope it has working slats too. Ours (G-ANOO) also had a sticky impulse mag. If it didn't start in a couple of pulls, you opened the cowling, tapped the mag, which freed up the impulse mechanism, and then it would usually start.
 
I did have a look at your aeromodelling website; interesting. I haven't been to the exact area, but for a number of years I visited Aboyne (due west of Aberdeen) with various gliders to experience the wave. It's where I first achieved my Diamond height. Twice in one week, 21000 feet on the Tuesday and 22000 feet on the Thursday.
 
Thanks also for the RC link. It all sounds pretty sophisticated these days. Certainly spread spectrum techniques should be pretty immune to multipath interference problems and what I assume will be block codes for forward error correction should limit the effects of burst interference.
 
Regards,
 
Andrew
10/09/2010 20:22:18
Hi Ian,
 
Your gentle reminder that I don't have a profile has shamed me into writing one; as will be apparent I'm not an expert modeller.
 
Regards,
 
Andrew
Thread: Carbide Insert Quality
10/09/2010 20:13:37
Hi Paul,
 
Thanks for the tip regarding the use of polished inserts on steel, I'll give it a go. And yes, I know there's a pun in there.
 
For ordinary steel, such as EN1A or EN3 I find you really have to run the ordinary carbide tips at high surface speeds to get a good finish, whereas other materials, including stainless, seem to be more forgiving.
 
Regards,
 
Andrew
09/09/2010 21:24:09
I have a couple of Glanze boring bars, which I am pleased with, but I found the supplied inserts to be very poor. Indifferent surface finish and easy to chip. I bought the bars some years ago, so may be they've upped their game on the inserts.
 
I buy inserts from Greenwood, J&L and from the local professional tool supplier, but I'm not tied to any particular manufacturer. I find that the special polished inserts from Greenwood are excellent on aluminium. The Greenwood general purpose inserts work well on brass, bronze and 303 and 316 stainless. On steel they also work well, BUT do need to be run fast; a surface speed of at least 400fpm.

Regards,
 
Andrew
Thread: C3 Purchased
09/09/2010 21:16:24
Hi Bob,
 
Sounds like the Tiger Moth model flies very nicely. Do you have the anti spin stakes fitted? I don't think they were ever fitted to RAF Tigers, but a lot of civilian ones had them. The Tiger Moth (G-ANOO) I flew from Thurleigh (near Bedford) was used primarily as a glider tug. Climb rate was a bit limited, but we did have the advantage of a 10500 foot runway.
 
Most of the Pawnees used as glider tugs in the UK are 235hp too. Nice smooth flat 6 cylinder engine. Actually I've been flying our Pawnee today, did three tows and then watched the day degenerate into heavy showers. Still, another 25 minutes in the logbook. I have flown a 260hp Pawnee, and I've seen, and had a tow behind, a 160hp Pawnee. I think the 160hp Pawnee has a shorter nose. Despite that you still look under the cowling and wonder where the engine is.
 
Vis-a-vis the RC interference I was curious as to how you got on with all the other users of the ISM bands. I assumed that the receivers are coded (PCM?) to the transmitters, but how do they cope with external interference?
 
Regards,
 
Andrew
Thread: Is this hobby dying?
08/09/2010 14:30:56
Well, that's easy. The crooks swapped their lead coshes (made from stuff liberated from the church roof) for iron bars. We all know lead is bad for your health, particularly in a small, high speed form. Whereas iron is of vital importance within the body to enable the blood to transport oxygen.
 
So by bashing 'em with an iron bar their 'ealth is being improved.
 
Regards,
 
Andrew
Thread: Making studs
08/09/2010 12:31:42
I can't be of direct help, as I normally make studs using a capstan unit and a Coventry die head, but when I need to reverse a stud I make up a 'tophat' with an accurately sized blind hole to locate the plain portion of the stud and a few threads of the appropriate size at the bottom of the hole to stop the stud turning.
 
It's a bit of a pain to get the stud out at the end of the operation, but it's better than ending up with no stud.
 
Regards,
 
Andrew
Thread: C3 Purchased
08/09/2010 11:51:18
Hi Bob,
 
That's the beauty of the internet, you never know what is going to pop up!
 
The Pawnee is a good glider tug, if rather thirsty. We reckon about a (UK) gallon per thousand feet. On the hand it's built like a brick outhouse and copes very well with wet and muddy airfields in the winter.
 
The Tiger Moth is definitely difficult to fly accurately, particularly in turns. Also spins well. Although it wasn't a requirement to be taught full spins when I did my PPL the CFI's viewpoint was 'this aircraft will spin, therefore you will demonstrate three turns of a spin and recovery in each direction'. But then he was 'old school' having learnt on Tiger Moths at Cambridge during the war.
 
I assume that the 2.4GHz RC sets are part of the ISM band? How do you get on with interference problems?
 
Regards,
 
Andrew
Thread: What is the ultimate lathe for model engineering
08/09/2010 11:27:41
Hi John,
 
Thanks for the info, I was pretty sure that I didn't understand due to a lack of knowledge on my part!
 
Regards,
 
Andrew
Thread: BMS angle not 90 deg
07/09/2010 23:07:52
Well, you learn something new every day. I'd always assumed the bright MS angle was made by cold drawing through a die, obviously I was wrong.
 
John, can you tell us a bit more about how BMS angle is manufactured?
 
Either way, it doesn't surprise me that the angle isn't actually square. I seem to remember this being discussed back in the early 70's when I was a small kid (as opposed to today's big kid, with bigger toys) and was a member of the Bedford MES.
 
Regards,
 
Andrew
Thread: What is the ultimate lathe for model engineering
07/09/2010 22:43:07
Hi David & John S,
 
Yes, absolutely right, I have a Myford cylindrical grinder. It's an early MG12, built in 1969 according to Myford. Bought on Ebay years ago, and is quite well equipped with an internal spindle and a swivelling workhead. More unusually I got the original manual with the grinder, tied to the machine by the serial number.
 
 
David: I'll put it on the list to write an article about cylindrical grinding on the Myford
 
John S: You'll have to explain to me the logic of assuming it might be a planer, saw or moped? Obviously something you know and I don't. By the way I think we have met, although I doubt you remember me.
 
Norman: Very good, hadn't thought of My Ford! Having been the unfortunate owner of an Austin Princess and Ambassador in the distant past even a Fiesta would be a step up.
 
Best Regards,
 
Andrew
Thread: C3 Purchased
07/09/2010 20:45:15
Hi Ian,
 
Forgot to say, one place I'd really like to fly a glider is the NZ Southern Alps; may be one year.
 
Regards,
 
Andrew
 
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