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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: Lathe boring tool - top rake?
19/11/2019 11:15:21

Having read up a bit more I think SoD is wrong. Negative rake inserts create higher cutting forces, so for a given cutting speed they consume more power. However, as SoD says, negative rake inserts are stronger than positive rake inserts. So they allow one to increase DOC and feedrate. Of course that requires the machine to be rigid and have plenty of power available. But if one has such a machine then negative rake inserts allow one to utilise the machine to its maximum capacity and thus maximise volume of metal removed per unit time. That's advantageous for industry.

I won't be changing to negative rake inserts as I'm quite capable of stalling my lathe (3hp) using positive rake inserts.


19/11/2019 10:45:11
Posted by Michael Gilligan on 19/11/2019 09:47:06:

I would be grateful if you could explain that, Dave [my emboldening]

I think SoD is getting confused with units. Power per unit volume removed would be a better metric. Intuitively I'd think that negative rake would consume more power per unit volume, but I'm not sure without looking it up. However, it makes sense that less power per unit volume is used as cutting speed increases. That's due to increased heat in the cutting zone, so the metal is hotter, and softer, and shears with less force.


Thread: Aircraft General Discussion
18/11/2019 20:40:39
Posted by martin perman on 18/11/2019 17:09:21:
.........but aircraft recognition is as you know an interest of mine.

Quite so. I knew it was a Shorts, but I had to search to find it was a Skyvan, which is why you beat me to the reply.


Thread: To pin, or not to pin
18/11/2019 20:32:00
Posted by old mart on 14/11/2019 19:03:14: should try it some time....................

Well I did try it.

I don't have any 638 so I used 648, a similar high temperature Loctite. The pin was a short length of 5/8" low carbon steel and the block a scrap piece with a 16mm hole drilled in it.:

Loctite test before.jpg

The hole measured 15.96mm and the shaft 15.86mm, so a gap of 0.05mm. Before assembly the parts were abraded with emery cloth and cleaned with acetone. Assembly was as per the datasheet with a twisting action to ensure complete coverage. After assembly the parts were left in the airing cupboard for 72 hours to cure.

After curing, with the body in the vice and a Mole wrench on the pin, nothing moved no matter how hard I pulled on the wrench. I then put the parts in the electric furnace set at 300°C. After an hour or so I extracted the parts and put the body back in the vice and the Mole wrench on the pin. The pin moved under the weight of the wrench and came straight out . The Loctite had been converted to a light brown powder which brushed off easily leaving the original surface:

Loctite test after.jpg

Pretty much what one would expect from reading the datasheet with a bit of engineering interpretation. Datasheets are a marketing tool to help sell the product, so they're hardly likely to downplay the product while not telling actual lies.

Any questions? smile


Thread: Aircraft General Discussion
18/11/2019 16:48:29

Good grief, you need to be poised over the keyboard ready to post these days, or you get gazumped!

The Skyvan morphed into stretched versions, the Shorts 330 and 360. The wing always looked far too small, especially for an aircraft that could operate from relatively short runways.


Edited By Andrew Johnston on 18/11/2019 16:49:03

Thread: Bridgeport micrometer scale screws
17/11/2019 19:58:04

I've just looked at my screws. The top screw is about 5/32" long and is longer than the lower screw by about 15 thou. However i don't think these screws are the original. The upper hole is about 7/16" deep while the lower hole is 3/16" deep. The lower screw has been cut down from a longer screw, while the upper screw looks unaltered. My head is a 2JB.

The screws could definitely be different lengths, in contradiction to my manual. But it wouldn't be the first time the manual has been wrong.


16/11/2019 08:01:59

If you scroll down the page on this link:

Quill Assembly

The screws are indeed different lengths, although the thread looks the same. I'm surprised the bean counters didn't smack the designer round the head for using different length screws. It doesn't alter the fact that in my original Bridgeport manual only one part number for the screw is shown. Once the weekend is out of the way I'll take the screws out of my scale and have a look.


Thread: New member in Northumberland
15/11/2019 22:01:52

Welcome to the forum!

I was in Northumberland last week, gliding at Milfield near Wooler. Well, actually watching it rain instead of flying. I'm a regular visitor to the area.


Thread: Bridgeport micrometer scale screws
15/11/2019 21:14:43

I'd be very surprised if they were different threads. In my manual they're not listed as different screws. It's not uncommon for a thread to be slightly damaged or full of gunk such that a screw appears not to go in. I'd carefully run a 6-32 tap (ideally plug) into the hole to clean the thread.


Thread: Hi guys just bought a Harrison m300 and looking for info etc
15/11/2019 21:09:56
Posted by Russ Bulley on 15/11/2019 19:47:12:

Andrew, do you have a travelling steady?

Yes, what's the question?


Thread: Workshop lighting
15/11/2019 19:26:38

I am in the process of changing the flourescent tubes in my workshop to LED tubes. I am also changing the battens to LED specific ones. I decided to change as the original tubes were becoming increasingly unreliable and one choke was getting too hot to touch. They're 20 years old, so time to change. I chose high colour temperature LED tubes, 6500K. Impressions so far are good; the tubes start instantaneously and give a bright, almost clinical, light. I should save between a half and two-thirds the power of the flourescents.

Having had LED downlights fitted in my recent main bathroom and kitchen refurbs, and fitted new wall lights with LED bulbs in the lounge, I am now changing all my house lighting to LED. For the house I chose a lower colour temperature, 4000K.

I expect there to be a significant power saving as well as better, brighter, light.


Thread: Star to Delta
15/11/2019 11:54:11

The way I read the original post the 44 ohms was between any two terminals with the motor in star. Given that the star point is buried it presumably doesn't appear at a terminal. Therefore the 44 ohms is measured across two windings in series. So each winding should be 22 ohms.


Thread: Warco WM 250 problem?
15/11/2019 11:21:40

Forget the stock and remove the chuck. If the lathe still wobbles then there is a problem. If not then the issue is with the chuck/stock. Also check to see if the problem gets worse with increasing spindle speed.

Personally I'd bolt the lathe down. Even new it won't be perfectly balanced and a small imbalance can be magnified if the lathe is free to move.


Thread: Yet another 'which mill shall I buy'
14/11/2019 22:16:36

Sometimes you just need a larger mill:


And VFDs are great but sometimes you need power at low spindle speeds to drive a 1" countersink in steel:


Can't remember where I got the countersink, but I think it was in a box of bits 'n' pieces I bought from a member of my gliding club as they were too big for his machine tools.

The steel plate is 600x500x15 and is one of the very few times when unknown material I've been offered has turned out to be useful.


Thread: To pin, or not to pin
14/11/2019 16:00:14
Posted by old mart on 13/11/2019 19:29:09:

If you can keep the temperature below 300C the Loctite will be ok.

Where did that come from? Looking at the datasheet for 638 it is down to about 35% of initial strength at 200°C. Extrapolating the curve (always a dangerous thing to do) it crosses the zero axis at about 250°C. I wouldn't trust 638 at 300°C.


Thread: Boring bar with inserts shape choice??
13/11/2019 11:52:42
Posted by Ian P on 13/11/2019 11:21:30:

There is no right (or text book) way to bore a hole, my suggestion would be to get some hands on experience and see what works for you.

I'd agree; "rules" in books, and on forums, are for guidance not blind obeyance.

While boring bars are not generally defined as thru hole or blind they are defined by the features they will machine. Tthere are many that will not bore and face a blind hole. Search for SSKCR/L, SDZCR/L or STWPR/L for instance.

This is what I meant by microboring tools:

Microbore Tools


Thread: Traction Talk Forum
13/11/2019 09:31:54

Had it once yesterday, but not today. Seen it before; I just ignore it.


Thread: EN3 bowing after machining
12/11/2019 22:56:45

I've milled a lot of hot rolled steel and never had a problem with distortion. Although there was a thread recently where someone had experienced distortion after milling hot rolled steel. If I recall it was quite a thin section. I'll see what happens when I mill the small hot rolled section to make the beading for my traction engine tender.

Cold drawn is a different animal, and not just steel. Cold drawn brass will distort as well. If I need to mill cold drawn steel I normally heat it to 800°C and let it soak for 30 minutes or so per inch of thickness. I then let it cool slowly. Of course I have the luxury of an electric furnace so soaking and slow cooling are fairly simple. I've never had a distortion problem after heat treatment.


Thread: Boring bar with inserts shape choice??
12/11/2019 15:54:39

The insert shape used is determined to some extent by the reach and whether the hole is blind or thru. It's a moot point anyway; I doubt you'll find a common boring bar and insert combination that will bore 8mm out to 10mm. Of course there are specialised microbore insert tools available, but you'll need to sit down before looking at prices. A small HSS boring bar will be just fine.


Thread: Preventing distortion.
11/11/2019 19:20:49

Depends upon the exact material spec. If it's EN24T then it's hardened and tempered so has already been stress relieved to some extent.


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