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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: Milling Machine
27/07/2011 08:25:48
Henry? I don't see how inductance is going to help?
 
Andrew
Thread: Right grade aluminium for the job
26/07/2011 22:42:50
I agree that M10x0.75 is not that common, but it is not non-standard. For UK ISO metric threads M10 coarse series is 1.5mm pitch. Fine series is 1.25mm pitch and constant pitch series are 1mm and 0.75mm.
 
A simple Google search easily finds taps and dies for M10x0.75, and M10x0.5 for that matter.
 
Regards,
 
Andrew
26/07/2011 16:51:48
I'm sure that 2011 will do the job, but it is prone to corrosion. A counsel of perfection would be to use 7075, as this is specifically used in aircraft and has good corrosion and fatigue resistance, but is more difficult to machine.
 
Regards,
 
Andrew
Thread: College Workshop
26/07/2011 09:45:26
Actually it's Qinetiq; it's RAE as was plus a few other goverment research establishments. And a load of sharp suits who think they can sell when all they can actually do is bullshit.
 
We did a 30kW AC-DC bi-directional power converter for them a couple of years ago. There are still some pretty bright guys there, but also some real dodos. We visited a few times and the place has changed out of all recognition. All the old buildings on the north side gone, including the main canteen, and replaced by new flashy science park style buildings. The training centre where I was based was on the south side, next to the IAM. No doubt that has long gone too.
 
Regards,
 
Andrew
Thread: Tee-slotted cross-slide query
22/07/2011 20:34:19
Surely if the OP has a mill available there'd be less need to have a slotted T-slide?
 
Regards,
 
Andrew

PS: We need a tongue-in-cheek smiley
Thread: Extending X capability of KX1?
21/07/2011 22:35:35
Just did some more experimenting yesterday with feeds abd stepovers cutting 1mm styrene sheet. The maximum stepover I could apply was 50% - that's all that Cut2D will let me apply. I worked a 5mm cutter up to 480mm/min with 50% step over and started getting tool marking - not too much but still enough to require finishing with sanding. I am assuming that the reason for this is that the styrene is flexible and the Multimat holding it down to the table can also be compressed so that the work might be deflecting under the tool when the pressure of cutting increases. I've started backing off to get the good surface finish.

Hi Jim,
 
Hmmmm, 50% maximum step over is a bit of a limitation. It won't make any difference in plastic, but in metal it is a good idea to avoid step overs around 50%, as it hammers the cutter teeth and shortens tool life. Either less or more than 50% is fine.
 
If you're increasing the stepover, and keeping the feed rate constant, I suspect the difference in finish may be due to the effective chip thickness varying. When cutting at low radial engagement (small stepovers) the effective chip thickness is much less than that implied by the feedrate, which will give a better finish.
 
Regards,
 
Andrew
Thread: College Workshop
21/07/2011 22:17:52
Posted by KWIL on 21/07/2011 09:33:12:
No it is Farnborough College 2010, part of £6 million new build.
 
Blimey, they've smartened the place up since my ignominious year of day release there!
 
Regards,
 
Andrew
Thread: Hex shank drills
20/07/2011 09:09:50
I suspect the terms 'quality' and 'hex drills' are mutually exclusive. I don't recall ever seeing hex shanked drills in any professional tooling catalogue.
 
Were the original drills of quality make? If they were then may be the problem is with the drill chuck?
 
Regards,
 
Andrew
Thread: Slot drills
20/07/2011 08:39:28
I'm well aware of when the original article was published, long before I was born.
 
Rationing was still in force, and I expect that slot drills, end mills and the like were difficult and expensive to obtain. So making a cutter at home would make sense. That does not necessarily mean that the home made cutter was more efficient at removing metal, which was the point queried by the OP.
 
Regards,
 
Andrew
19/07/2011 17:37:04
I doubt it was true when it was written, and I doubt it is true now. I assume that any home made cutter would use silver steel, and there's no way it is going to compete with HSS or carbide in the metal removal stakes.
 
Regards,
 
Andrew
Thread: A Little Light Relief - Mine's Bigger Than Yours...............
19/07/2011 16:04:53
Yes, but as I'm getting older the trousers need to be flared at the waist line to allow for expansion, rather than at the trouser end.
 
Andrew
19/07/2011 15:47:36
Posted by NJH on 19/07/2011 12:21:05:
You Guys need to get out more!
Cheers
Norman
 
Quite the reverse actually! I seem to be out and about non-stop, so very little workshop time available, hence the rush that resulted in this thread.
 
Dave: Actually the loooong swarf was the exception rather than the rule. Most of it came off in lengths between 1" and 10". It seemed to alternate, one cut long, the next short. Presumably some subtle changes in the cutting parameters determine the length, but I'm blowed if I know what. It was one of the short purple bits of swarf that got into my shoe, and resulted in the ejection of said shoe about a millisecond later. Left a nice hole in the sock and a pretty pattern burnt into my foot.
 
According to my calculations I was removing 2.9 cubic inches per minute. Allowing 1hp per cubic inch per minute, I was on the limit, as I have a 3hp motor on my lathe. You could certainly hear the headstock gears taking the strain and slowing down slightly.
 
Regards,
 
Andrew
 
PS: The first pair of socks were ruined when I dropped a shovel load of swarf on my shoes when clearing up. There were so many little bits of sharp swarf embedded in the socks it was easier to throw them away.
19/07/2011 00:02:53
Ah ha, I knew I'd be out-swarffed in short (long?) order.
 
Impressive, when I turn plastic the swarf always seems to wrap itself around the job in a big ball getting tighter and tighter, and generating heat, so I have to stop and clear it before it overheats and melts.
 
Andrew
Thread: Extending X capability of KX1?
18/07/2011 23:57:25
Hi Jim,
 
Funny you should mention Metalfast, I use them for all my aluminium requirements. Total shambles in the sales office but very pleasant, but then that seems to be the case with every professional metal stockist I've used! Although I don't live near Swindon they do have an outlet in Huntingdon, so I can collect, as delivery charges are a bit steep. They used to do some cutting at Huntingdon, so I could rummage through the scrap bin, but sadly they're just a distribution centre there now, so no scrap.
 
I find Metalfast to be useful for metal for jigs and the like, as you can ring up and enquire about offcuts. For instance I wanted a plate to act as jig for assembling my traction engine wheels. I specified at least 600mm square, anything from 8 to 20mm thick and I don't care if it has a gouge or several in it. That can be useful, as the finish sometimes gets damaged on partial sheets and so they're prepared to sell them well below normal price.
 
Ok, I understand why your depths of cut are shallow. It's interesting to note your step over values. They seem very small to me? If I'm facing a flat area or pocketing out I'll use a step over of between 70 and 90%, to make full use of the cutter. This is true for both plastics and metals. To a first approximation I'm not sure that the step over will have much effect on the finish. I think that feedrate is probably the most important factor?
 
Regards,
 
Andrew
Thread: A Little Light Relief - Mine's Bigger Than Yours...............
18/07/2011 23:41:41
Posted by John Stevenson on 18/07/2011 23:10:34:
If you were in that much of a hurry then you should have been running at 1400 revs and 4mm cut.
 
John S.
 
Steady on, I've only got a weeny lathe!
 
Andrew
18/07/2011 23:03:12
..........unless you know different; swarf that is! I was in a bit of hurry to machine a 3" bar of steel (EN3B I think) down to 1.5" diameter this evening. Set the speed to 800rpm, depth of cut 0.1" and feedrate 4 thou/rev. The swarf came off in loooooong tightly curled lengths, interspered with shorter lengths. The longest single length I could find is shown below; a bit over 11 feet. The ruler is a 600mm one.
 

 
I also managed to burn my foot, and ruin another pair of socks, while operating the lathe; it takes real 'genius' to be that creative!
 
Regards,
 
Andrew
Thread: Extending X capability of KX1?
16/07/2011 11:33:57
Hi Jim,
 
If it works for you then that's good enough! You certainly seem to get an excellent finish and apparently no burrs; very good.
 
Your depths of cut are definitely conservative. I've never run really small cutters, but as an example I run a 2mm cutter full width engagement and 0.4mm depth of cut in 316 stainless steel. So, in plastic, bigger depths should be no problem.
 
Regards,
 
Andrew
Thread: What happened to the Myford sale thread?
16/07/2011 00:48:46
Phil,
 
Unfortunately, as far as the liquidators are concerned they have done a good job. They've sold all the spares plus other bits in one go. Why should they waste their time selling one of widget x to person y, and two of widget y to person x, and then be stuck with all the widgets z, that didn't sell. Their aim is to clear the building as quickly as possible, probably within a week or two. Next week they'll be clearing a clothes shop, the week after that an IT company, the week after that.......
 
No doubt the above will be seen as harsh, and it is, but it's the reality of liquidation. There's no room for sentiment.
 
Regards,
 
Andrew
Thread: Extending X capability of KX1?
12/07/2011 22:48:06
Probably the best solution is, as has been mentioned, a larger sub-table that can be moved in the X axis and located on pegs or similar, so that each position is repeatable. However, if you are concerned with consistent Z heights, don't diddle about with mdf, use aluminium tooling plate. This is a rolled and tempered aluminium plate that is Blanchard, or similar, ground, so the it is flat and parallel to much better tolerances than normal rolled sheet or plate.
 
As an aside, I don't know about machining plasticard, but I do machine a lot of plastics, especially uPVC. I find that relatively slow speeds and high chip loads are best. The one thing you don't want to do is melt the plastic. Not only does it get messy, it smells bad too!
 
Regards,
 
Andrew
Thread: A New Dividing Plate for my Dividing Head
11/07/2011 22:21:20

Mine must be on British summer time as zero is at 3 o'clock.
 
John S.
 
Hmmm, didn't think of that; certainly another solution. Since each successive hole increments anti-clockwise I rather assumed that the starting angle from 0 was also positive anti-clockwise.
 
Then again I am left-handed, and therefore sinister, so may be my mill is backwards, in true Dennis Wheatley fashion.
 
Regards,
 
Andrew

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