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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: What Collet type, 5C or ER25
15/05/2011 23:03:28
Nobody has commented on the centring indicator; do you mean a co-axial centre indicator? If so, then personally I wouldn't bother with one, it's an expensive luxury.
 
But then what do I know, as I've never found the need to use an edge finder either, so that would be bottom of my list.
 
Regards,
 
Andrew
Thread: This months MEW are 3 CNC features two too many
14/05/2011 12:50:11
Why not work towards making Model Engineers' Workshop magazine, entirely non electric in its content?
 
Quite! Let's get rid of all those pesky electric motors and revert to the treadle lathe; then parts made can honestly said to be a product of one's own blood, sweat and tears. Might mean the end of milling machines though, as I've never seen a hand-cranked mill?
 
And as for VFDs, well, who on earth in their right mind would use such new fangled technology.
 
 
Regards,
 
Andrew
14/05/2011 12:13:30
My turn to stir the pot!
 
I do have a CNC mill, as well as manual vertical and universal horizontal mills. So, obviously I'm in favour of CNC milling when appropriate. I use the CNC mill for two reasons. One, to make multiple parts, eg, the spokes for my traction engines; I have no interest, or the time, to spend hours twiddling handles on a manual mill just to make the spokes. Two, to make things that I cannot make on the manual mills, eg, the bevel gears for my traction engines.
 
There seems to be a slightly old fashioned view here of CNC as it applies to industry. Very few professional machine shops are now entirely manual, except for some niche markets. Even for ones and twos CNC milling is used. Why would I want to pay some-one to spend ages twiddling handles on a manual mill?
 
However, having said all that, my initial reaction on opening the latest edition of MEW was exactly the same as the OP. Too much CNC, and wasteful listings of G-code. For a variety of reasons I acknowledge that CNC is a specialised interest; so may be one article an issue, or every other issue? I don't believe that the answer is a separate magazine. I doubt it would be viable. It may also take subscriptions away from MEW, which would damage the viability of MEW too. I wouldn't subscribe to a CNC magazine, and if there was no CNC in MEW I probably wouldn't subscribe to that either. I did try a free trial subscription to 'Digital Machinist' from the US, but didn't take up the paid subscription, as I didn't think it was 'meaty' enough to justify the cost.
 
Finally a note to those who don't like the articles on CNC; don't whine about it, write an article on your own favourite topic!
 
Regards,
 
Andrew
Thread: Cutting a 365 tooth gear
14/05/2011 11:32:50
Ady,
 
Your arithmetic is at fault, the answer is exactly 73, no error, weensy or otherwise.
 
Regards,
 
Andrew
13/05/2011 17:14:08
I don't think you can do it with the plates you have. For 'n' divisions of the circumference, and a 60:1 worm ratio, then the crank handle movement, 'a', for each index is:
 
a=60/n
 
or in this case:
 
a=60/365
 
Simplifying gives:
 
a=12/73
 
Since 73 is prime no further factorisation is possible, and since you don't have a plate with 73 holes, it would seem that you have lost your paddle! I think you'll have to make a plate with 73 holes, and turn the crank past 12 holes per index.
 
Regards,
 
Andrew
 
PS: I did the maths in a hurry while taking a break from the TIG welder, so check the maths before using it!
Thread: My drill bits are not cutting
13/05/2011 11:12:47
Yes, but.......
 
.....the shank of HSS drills is often left soft, so unless you have a means of hardening it, they won't be of much use.
 
Regards,
 
Andrew
Thread: D bit grinder work head calculations
07/05/2011 15:38:57
Hi Lawrie,
 
The glib answer is that the difference between radians and degrees is a bit over 57.
 
The degree is an arbitrary unit, and a circle contains 360 of them. Each degree is divided into 60 minutes, and each minute is divided into 60 seconds.
 
The radian is an SI derived measure of angle. It is defined as the angle a segment of a circle of radius r subtends when the arc length is also r. A full circle contains 2pi of them. The transcendental functions (sin, cos, tan etc) are naturally expressed in radians and such things as angular velocity are normally expressed in radians per second.
 
To convert from radians to degrees: multiply by 360/2pi
 
To convert from degrees to radians: multiply by 2pi/360
 
Hope that helps.
 
Regards,
 
Andrew
 
PS: So that means that 1 radian is 360/2pi degrees, which is about 57.3

Edited By Andrew Johnston on 07/05/2011 15:41:05

05/05/2011 18:39:37
I must be missing something; I can't see a reply from a Mr Thomas?
 
Regards,
 
Andrew
05/05/2011 14:37:50
I'm probably being thick, but I don't understand what you mean by rake angle on the bit. Can you post a diagram of the D-bit showing which angle is being referred to?
 
Regards,
 
Andrew
Thread: Facing Cuts - Requires the Carriage to be Locked?
05/05/2011 10:55:04
Hi Edmund,
 
No it is not normal. I only lock the carriage on facing cuts when I want to use the top slide to put on a specific (accurate) cut and I don't want to move the carriage by accidentally nudging the carriage handle.
 
Incidentally the lathe cross slide should be set up to face from flat to slightly concave, but never convex.
 
Apart from that, how do you like the lathe?
 
Regards,
 
Andrew
Thread: Dam Busters Channel Four tonight
04/05/2011 20:26:14
I can confirm that the Tiger Moth has a standard float carburettor.
 
Regards,
 
Andrew
Thread: My drill bits are not cutting
04/05/2011 19:11:41
There are no foolish questions; even the most knowledgable know only an insignificant fraction of what is known.
 
If the material is BMS then it will not have been hardened by the heat, as it doesn't contain enough carbon. So, first question; are you sure it is BMS? Give us some numbers; what size drill, what rpm are you using? Where did you get the drills, ie, are they quality drills or 'DIY' level?
 
Regards,
 
Andrew
Thread: D bit grinder work head calculations
04/05/2011 15:25:09
If you offset in one axis relative to the flat of the D-bit, as is implied by the end on view of the D-bit, then at the cutting edge the angle will be zero since the tangents to both circles will be parallel.
 
To get a clearance angle I think that the line representing the flat of the D-bit needs to be offset in two axes from the centre of the circle intended to form the clearance.
 
Regrettably I don't have time at the moment to sit down and do the analysis. If I get bored over the weekend I might have a play with the maths.
 
Regards,
 
Andrew
Thread: Cutting fluids
25/04/2011 19:59:07
I don't know about the stone, but using a small air pump is quite common; see this link:
 
 
Regards,
 
Andrew
Thread: Technical and engineering drawing.
25/04/2011 09:37:32
Precisely why I pointed out that the number of solutions is infinite! The curve doesn't need to be part of a circle, so centre lines wouldn't necessarily help.
 
Regards,
 
Andrew
22/04/2011 10:37:32
Posted by Terryd on 22/04/2011 00:16:40:

 
By the way I beleive that there is more than one solution to the conundrum set by Sam, only the simplest has so far been shown. I have never seen it before but due to my fluency in the language of graphics I think that I have seen a second solution, the one shown is the easiest to conceptualise.
 
 
The set of solutions is infinite, of the uncountable kind.
 
Regards,
 
Andrew
Thread: Cutting speed theory
21/04/2011 23:05:27
Clive and Sam have elegantly described the process; technically it's all to do with adiabatic shear bands. The plastic deformation that leads to shearing of the metal occurs in a very thin band. Most of the energy used for the deformation ends up as heat, so the thin band where the shear occurs gets (very) hot. As the band gets very hot it gets soft, so it shears more easily with less energy input needed. That's why the power required per unit volume of metal removed goes down as the speeds and feeds increase, up to a point anyway. The process is adiabatic because it happens quickly so that the heat doesn't have time to flow into the bulk of the metal, but is retained in the metal close to the shear band, ie, the swarf.
 
As an example, when turning steel, I aim to get the swarf coming off a very dull red colour. The only disadvantage is that it don't half hurt when you get swarf on your hands, or worse, down your shirt front.
 
Regards,
 
Andrew
Thread: Face cutter cutting width
21/04/2011 22:42:47
When using a face mill you can use any percentage engagement, except around 50%. So, say up to about 35%, and from 65% to 90% or so. The reason not to use an engagement of about 50% of the tool width is that it will hammer (literally) the inserts and reduce their life.
 
As for speeds and feeds I make a 32mm cutter and 1200rpm equate to about 120m/min surface speed. That's a good middle of the road speed for aluminium alloy. A feed of 250mm/min is a tooth load of about 0.05mm/rev, may be a little low for aluminium. Unless it's a finishing cut a depth of 0.25mm seems very low.
 
The only reason I use flood coolant for milling aluminium is to wash away the swarf. If the inserts are specifically designed for aluminium then build up on the inserts shouldn't be an issue.
 
Regards,
 
Andrew
Thread: Stainless Steel
19/04/2011 21:57:18
From the pictures this looks like a fairly gentle bend and twist, so I suspect that work hardening is unlikely to be an issue. I've just finished cutting and cold bending a whole load of 1.5mm 316 stainless sheet with no problems; if I can do it so can you!
 
Regards,
 
Andrew
Thread: Cutting fluids
18/04/2011 21:46:06
I use a soluble oil, Biokool14, from Hallett Oils, now owned by Caldo.
 
As far as I can tell it doesn't seem to smell (although I have a poor sense of smell) and lasts for ages. The coolant in my lathe and horizontal and CNC mills has been there for well over a year. I just top up with water as it evaporates.
 
Regards,
 
Andrew
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