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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: Centre Drill Leaves a “Pip” - Sometimes
24/07/2021 18:24:24
Posted by Dr_GMJN on 24/07/2021 18:13:43:

......on YouTube that said the spotting drill should have a 130 degree tip, when used with a 118 degree hole drill. The reason being the tip of the hole drill will hit the apex of the spotted hole first, and not get deflected by the edge. Seems legit?

It's the subject of some controversy, even on professional machining forums. Like Jason I use 90° without any apparent problem.


Thread: Is there a chart for drill hole dia vs. boring bar size?
24/07/2021 12:42:42

Posted by Rod Renshaw on 24/07/2021 12:02:51:

Stiffness is important in single ended boring bars to avoid one end of the bore being bigger than the other.

This was discussed a while back and concluded that it is a myth. The bore may not be the diameter expected, hence the need for spring passes, but it won't be tapered. If the cutting conditions remain constant why should a boring bar cut differently at one end of the bore compared to the other?

The deflection for a given boring bar with constant load is proportional to the third power of the cantilever length and inversely proportional to the fourth power of the diameter. So doubling the length increases the deflection by a factor of 8. Doubling the diameter decreases the deflection by a factor of 16.


24/07/2021 09:42:53

Posted by William Ayerst on 24/07/2021 09:16:39:

Is there some kind of formula or reference chart I'm just missing?

There is no magic formula. Insert boring bars and carbide bars from professional tool suppliers will have a minimum bore specified. Bars from other sources will be pot luck. Les is correct; although these are internal threading bars the principle is the same, the cutting area is on the centreline of the bar, not at the top:


Likewise with this home ground HSS bar:


On insert boring bars the insert area is cut away, and slopes down, for the same reason.


Thread: Centre Drill Leaves a “Pip” - Sometimes
23/07/2021 14:29:22

For many holes you simply don't need to centre, or spot, drill first. The holes in this brass fixture -


- were drilled No.70 (0.71mm) to fit 0.64mm pins soldered to a small PCB prior to press fitting into a plastic housing. Driling was done on a Bridgeport and no centre drill or spot drill was used. Even the smallest of both leaves too large an impression.

Of course, I'm lazy and I'm not going to waste time doing operations that are not necessary. smile


Edited By Andrew Johnston on 23/07/2021 14:47:34

22/07/2021 22:50:21

Either the spindle or the chuck is allowing the centre drill to move in a circle rather than rotate around it's axis.

However, I never use centre drills as a precursor to drilling holes on the vertical mill. As Steve says I only use centre drills for drilling centre holes in work for use on the lathe or cylindrical grinder. For drilling on the vertical mill i use 4-facet drills so there is usually no need, and no point (pun intended), to use a centre drill first. For accurate placement (better than a thou or two) i will spot drill before using the normal twist drill.


Thread: Digital CNC phase converter build
21/07/2021 09:36:18

Posted by Robert Laurenson on 20/07/2021 22:25:32:

If you put 3 stakes in the ground as an equilateral triangle, if you stand between two posts they are 180 apart and the other is at 90, if you stand next to a single post the otber two are at 60, and in the middle they are all at 120. Said he was an electrical engineer and had a PP system running all kinds a stuff from a cnc lathe to other things.

All it means is that the reference point is being moved, basic geometry. The angles change but also the magnitudes, so the system will not be balanced. Not really comparing like to like. Of course perpendicular phases may well work, depending upon the application.

I worked for a company where one of the electrical engineers insisted that 4000 series logic had three states, not two. He then proceded to demonstrate that soldering on a circuit that is live at 600V with a grounded tip is not a good idea. Consequently I'm a bit cynical when people state that they're X or Y, the implication being that they know what they're doing. disgust


20/07/2021 20:29:33

Posted by Bob Worsley on 20/07/2021 20:03:53:

...........A 100A 400V 3ph supply is about 24kW.........

A 100A 3-phase supply is 100A per phase, so 3x240x100=72kW.


20/07/2021 19:54:59

Super yachts have power converters that take shore power from anywhere in the world and generate clean 3-phase for distribution around the yacht. Here's an example:

Shore Power Converter

I don't own a super yacht, so don't know how much they cost.


19/07/2021 21:15:30

Best place to start is to describe what you are trying to do. Are you intending to run a 3-phase motor from a single phase supply, or create a substitute for a utility 3-phase supply.

I'm not convinced that capacitors on the AC output will help with start current.

Personally I went with having a proper 3-phase supply installed at home.


Thread: Can't get the hang of HSS!
19/07/2021 14:36:55

Posted by Jim Smith 8 on 19/07/2021 11:35:06:

Once you get the compound slop and backlash minimised you can further improve finish by using a lubricant and keeping the work piece cool or letting it cool between breaks.

I'm afraid first principles have led into a cul-de-sac. My lathe has 15 thou or more backlash on the cross slide, but I still get good finishes; a few microns Ra. Likewise I get good finishes on my repetition lathe, and that doesn't even have leadscrews, just levers. So on the push test backlash is several inches!

Carbide is probably less forgiving than HSS. Often, but not always, carbide requires high surface speeds to get a good finish. Similarly it is possible to take fine depths of cut with carbide and still get a good finish, but it is dependent on material. I run carbide dry, but mostly use flood coolant with HSS. Coolant can improve finish, but conversely it can make things worse, especially with carbide.

Like SoD says ribbons are bad. With carbide tooling it's a myth than small chips lead to a poor finish. Much time and money has been spent designing inserts to break the swarf as it comes off the work. In other words the cut is continuous, leading to a good finish, but the swarf ribbon is broken after leaving the work. That might give the impression of non-continuous cutting. Of course some materials, such as brass and cast iron, are inherently short chipping.

Peruse this: Turning Trials


Thread: Best soldering iron for electronics
19/07/2021 14:15:56

I had a 15W Antex iron when i was a kid. Still got it but never use it. I now use Weller dual 80W irons set at 360°C. With a range of tips that suffices for all my electronics needs, thru hole and surface mount. I've been soldering and de-soldering 0402 Rs and Cs this morning. That's about as small as I'd want to go with manual soldering. If only because i can't see anything smaller without magnification. Even with 0402s several disappeared for no adequately explained reason.

At the last company where I worked they had some irons that I hated. Might have been Metcal, but either way they never felt right and got in the way of the work in hand. The same company also had a hot air desolderer. Useful on occasion, but by no means essential. Surface mount is divided into those compenents that can be manually soldered and those that can't, hot air or not. I've also got a large 100W Weller iron for workshop use, primarily for sheet metal and screening cans.

Of course I fall into SoDs category of those who like good tools and are prepared to pay for them. Probably makes me a dilettante on this forum.


Thread: Aluminium
19/07/2021 13:59:53

The link from SoD doesn't work. Cutting and pasting leads to links for controlling 12V DC motors; which I suspect is a different thread.

Agreed that 1050A is almost pure aluminium. But I'd disagree with it only being useful for foil and labels. I've used it for other applicationsw, such as c ustom dashboards for modified hybrid cars when testing new batteries.

The professionals seem to think it has other uses:

1050A Overview


Thread: Can auto darkening welding helmet capsule be restored.
16/07/2021 20:52:13

Flat batteries?


Thread: General mill engine design
15/07/2021 09:51:27

Oooops, looks like I was getting confused between engineering and modelling regarding the steam chest. embarrassed


14/07/2021 22:13:29

Posted by AStroud on 14/07/2021 16:41:41:

Am I also right in saying as long as the steam ports 'match' the eccentric throw then the steam chest size and dimensions do not really matter as long as the ports are large enough for the estimated air flow ?

If you do the maths you'll find that the size of the steam chest does matter. It acts as reservoir to minimise pressure drop when the inlet ports are open. Of course if the supply pipe from the source to the valve has a much larger area than the inlet port then the size of chest is less important as the supply pipe takes over the reservoir function. But the pipe might look a bit non-scale.


Thread: Lathe cover
14/07/2021 11:41:48
Posted by Steve Millward on 14/07/2021 11:10:12:

i was just wondering if many people use a cover for lathe...........

I started off with covers on the lathe and vertical mill, but stopped using them fairly quickly. Unless there is a heater under the cover they just accentuate the rust problem by stopping air from circulating. I spray all the machines with WD40 now and again, and the slideways are oily anyway. I don't generally have a problem with rust. My workshop is fairly well ventilated as there's a gap top and bottom on the garage door due to cowboy installation.


Thread: Can't get the hang of HSS!
13/07/2021 15:58:54
Posted by Jon Lawes on 13/07/2021 15:50:56:

Can you show us the HSS tool..............


Almost certainly something wrong with the tool or setup. The only caveat is what type of brass? The most common type, CZ121, should be easy with carbide or HSS, but some alloys can be problematic.


Thread: Shock at low pay for high skill
13/07/2021 13:41:45

It's sad that yet again I have to wonder why a new member thinks it's necessary to insult forum members. The OP has broken the old adage - it's better to remain silent and be thought a fool than speak and remove all doubt.


13/07/2021 11:02:20
Posted by brian jones 11 on 12/07/2021 23:39:28:uq

Does 200lbs sound a lot to you?

Yes, it's a bit of an overstatement; the theoretically correct value is 153lbs.

I suspect the question doesn't do what you expect. If the interviewee doesn't know the density of air then any value given will be a pure guess. After the values are given it is an exercise in simple arithmetic. Some of the better interview questions involve the design of something. It allows the interviewer to see how the problem is approached, what extra information is asked for and what ideas arise.

The question on power output is incomplete, for how long is the output to be sustained? For an hour or so I'd expect 75-100W, longer then nearer 50. But it will be highly dependent upon the individual.


Thread: Man management
12/07/2021 22:02:47
Posted by DiodeDick on 12/07/2021 21:52:52:

...............not a model engineer ( in either sense)...............

Me neither, far too many vices.


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