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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.

Thread: Is this 3 phase motor suitable for delta/VFD?
27/03/2022 11:06:34
Posted by choochoo_baloo on 25/03/2022 19:59:02:

...wouldn't a star connection actually run at one third* of the motor rated power...

I was thinking in terms of mechanical output power:

Power = torque x angular velocity

Assuming both voltages are at 50Hz the angular velocity is constant. The voltage reduced by the square root of 3 leads to phase currents reduced by the same. To a first approximation the torque is proportional to the phase current. So the torque, and hence power, is reduced by the same factor, ie, the square root of 3.


25/03/2022 18:49:25
Posted by choochoo_baloo on 25/03/2022 18:29:45:

...can supplying 240V 3ph (so approx half the rated 3ph voltage) actually harm an induction motor?

In a word - no. Running a 415V star connected motor with 240V phase to phase simply reduces currents, and hence torque and power, by the square root of 3.


Thread: Indicator
25/03/2022 08:02:57

The zero references are arbitrary, not absolute, so MG is correct.


Thread: Using kerosene to clean ground surfaces
24/03/2022 16:18:44

I buy my acetone 25 litres at a time from a glassfibre materials supplier, about £2.50p per litre.


Thread: Indicator
24/03/2022 16:14:50

To avoid confusion on the left is a dial indicator, on the right is a dial test indicator (DTI):

test gauges.jpg

The dial indicator is intended to make precise measurements with the plunger moving axially. Using it to indicate while applying a side load is inaccurate.

If the greater movement of a dial indicator is needed to centre work in a lathe chuck then the technique being used is wrong. One can get well within 0.5mm by aligning the jaws on the chuck with the concentric rings on the chuck body, or by eye against a lathe tool. Or just note where the DTI touches and where it doesn't and adjust according. Then fine tune position using the DTI; I normally aim for less than 0.02mm total runout. I only use a DTI for aligning parts in the lathe chuck. I find it to be fast and accurate, given the correct technique.


PS: I acknowledge MikeK beating me to it smile

Edited By Andrew Johnston on 24/03/2022 16:15:54

Thread: Using kerosene to clean ground surfaces
24/03/2022 11:43:49
Posted by John Doe 2 on 24/03/2022 11:11:58:

Does white spirit leave any residue after it has evaporated?

I think it does, using Loctite on parts that have been "cleaned" with white spirits doesn't work.

For removing superficial rust on tooling I use wire wool and a quick spray with WD40. For all other cleaning, tools and machined parts, I use acetone.


Thread: Tapping straight
24/03/2022 09:15:36

If I have a lot of holes to tap I use a reversing tapping head on the vertical mill, fast and accurate. Takes a few seconds per hole. For smaller numbers of holes I put the tap in the drill chuck, after drilling, and start the thread by rotating the chuck a few turns by hand. The thread is finished by hand on the bench. I normally use spiral flute taps for the majority of my tapping. If using hand taps, for freehand tapping, I start the taper tap a turn or so, and then fine adjust the tap to be perpendicular in two planes by eye. In all cases it's a lot quicker to do the operation than describe it.


Thread: Bevel Gear Replacement for Astra L2/L4 Mill - any ideas?
22/03/2022 12:14:25

The picture posted by JohnP is definitely a spiral bevel gear, as the teeth are curved. The gear set probably is driving the spindle, you wouldn't bother with spiral bevel gears just to move an axis. I agree with a previous post that nylon seems an odd material to choose.


22/03/2022 11:05:05

It's not really a spiral bevel gear, but the teeth do look slightly skew. That seems an odd choice, and makes manufacture more difficult. If possible a picture of the mating gear would be helpful as, presumably, it is less worn.


Thread: Advice on Cluttered Dimensions in Drawings
21/03/2022 15:04:00

Time to settle down with beer, nibbles and a dictionary, pausing only to don appropriate PPE against flying toys. smile


Thread: Todays daft question
21/03/2022 14:52:07

Possibly to distinguish them from side and face or slab cutters as used on horizontal milling machines.


Thread: Engineering tuition recommendations
18/03/2022 14:26:38
Posted by SillyOldDuffer on 18/03/2022 13:54:46:

...criticising posts going off subject is fair comment!

Not really, it just reflects a closed mind and assumes that there is a right and wrong answer, which is rarely the case. Engineering doesn't work like that; it's as much an art as a science.

This thread is a case in point. It started with a general enquiry about learning manual machining. It then transpires that the OP is interested in building a large scale traction engine. That helps narrow down the advice given, and may make earlier posts seem less relevant.


Thread: Screw cutting problem
18/03/2022 12:41:02
Posted by Hopper on 18/03/2022 08:27:57:
Yes I have almost always found screwcutting to be a "fit and try" effort the final five thou or so...

Exactly, I use thread depth as a guide, but use a mating part as a gauge for final fit. I aim to achieve a shake free fit, and agree that a couple of spring passes can be the difference between go and no go. For brass and cast iron a brush to remove swarf dust also helps. I rarely use dies, many external threads are screwcut, or done using Coventry dieheads.

I screwcut a lot of 32 and 40 tpi ME threads, where thread depths are small, A few thou can be the difference between a shake free fit and a rattling poor fit. My ME taps and dies are old and carbon steel, so it's not clear that they are particularly accurate. Consequently trial fits are essential.

My original comment in response to Clive's comment was partly tongue in cheek. My lathe is ex-industrial and I have an Ainjest unit fitted. The unit was bought on a whim, just to experiment, but I've found It makes screwcutting, especially blind internal, much easier, and faster. So I don't see it as cheating. I use techniques that are appropriate to the machines that I have. I distrust people who are dogmatic about the right, or wrong, way to machine something. I recall a forum member telling me that CNC milling wasn't really true modelling, until they got a CNC mill that was.


18/03/2022 12:15:30
Posted by Ramon Wilson on 17/03/2022 16:46:28:

Inverted 'commers' Andrew....

I was picking up on the spelling rather than punctuation, but also thought the thread could do with a little levity. smile


17/03/2022 14:55:45
Posted by Ramon Wilson on 17/03/2022 14:25:58: considered a 'Medlar'...

Nothing wrong with medlars, got one in the garden. They look horrible after bletting, but taste lovely. thumbs up


17/03/2022 11:38:23

Posted by Clive Foster on 17/03/2022 09:53:16:

Hardcore manual machinists consider single tooth clutches and Ainjest attachments cheating.

More fool them, but they probably also regard the electric motor as an invention of the devil. smile


Thread: Milling - first cuts
16/03/2022 21:27:46

A few pictures for interest, milling the edge of tooling plate, 25mm thick, with a (worn) 10mm carbide endmill. First, conventional milling and cutting dry:


Climb milling, cutting dry:


And conventional milling with a squirt of WD40:



16/03/2022 13:02:14

I wouldn't start from here. The cutters look to be poorly ground on the end, although both are theoretically capable of plunge milling. I buy cutters from professional tool suppliers. At least one can then be assured that they are properly ground. On a slot drill one edge is ground for the full radius for plunge cutting, the other edge is shorter:

slot drill.jpg

Similarly 3 and 4 flute cutters intended for plunge cutting have one edge at full radius and the others are short. The only professional cutters that I have seen with two, or more, cutting edges at full radius are those with radiused corners, intended to provide an excellent surface finish on final facing cuts, like this:

centre cutting.jpg

Low power mills need all the help they can get, which is why I use professional cutters.


Thread: Converting brick outhouse to (very small) workshop...
14/03/2022 22:05:18

Anything is better than nothing. When I was a kid, and a member of the Bedford Model Engineering Society, one of the members had a small workshop in the back of his greengrocers shop. It was tiny, can't have been much more than 4 or 5 feet square. But it didn't stop him building models.


Thread: yet another angle plate
14/03/2022 20:34:52

When I was looking for an angle plate I only considered the second pattern. All round much more rigid, and easier to adjust. Can't help with precise size requrements, as mine was bought secondhand. It's knocked about a lot, and is a tad on the large side for the Bridgeport (10" x 18" ), but nothing is going to shift it while machining:



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