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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: Using A Taper Attachment.
25/05/2019 15:07:11

Hmmm, S99 is a nickel/chrome/moly high carbon steel intended for aerospace. I expect that the combination of slow speed, low depth of cut and tiny feedrate simply doesn't cut it. I'd suggest a feedrate of 0.1mm/min upwards, DOC of 0.25mm upwards depending upon the tool geometry, and 1500+ rpm.


Thread: Flyball Steam Governors
25/05/2019 14:40:57

Actually stiffness (presumably friction) may stop the governor working properly. At best it will lead to hunting.


Thread: Getting rid of the garage door...........
25/05/2019 13:00:38

You need to be careful, removing the garage door may be seen as a change of use by the local council. As far as I'm aware there's nothing to stop you fitting out a workshop in a garage. But removing the door and replacing it with a wall/window may be seen as an attempt to convert the garage to a habitable room, in which planning permission is required.


Thread: Drill bits
25/05/2019 11:03:55

Some production lathes normally run in reverse, like my Britan, so left hand drills are standard.


Thread: Harrison wiring
25/05/2019 09:21:52
Posted by Phil Lambley on 25/05/2019 07:51:56:
ahould ther be power to one side of the switches that the lever engages on as thers nothing ther

The microswitches operated by the saddle lever should have 110VAC on one side.

Is the emergency stop button on the headstock end enabled? It should be push to stop and then twist to unlock.


Thread: Hi guys just bought a Harrison m300 and looking for info etc
25/05/2019 08:01:34

Probably not critical but "Beginners Questions" or "Manual Machine Tools" would be good threads. Some idea of your experience and the precise details of the lathe would be helpful, as would a rough location. I've got a Harrison M300 and live near Cambridge, but that's not much help if you're in the north of Scotland. Is your lathe imperial or metric, short or long bed and with or without a gap piece?


Thread: Harrison wiring
25/05/2019 07:43:23

Many industrial machine tools do not use, or need, neutral. In theory a three phase motor running in star has no net current flow to or from the star point. So there's no point in providing a neutral. If a 230V rail is needed internally it comes from a transformer connected across two phases.

I don't have a lubrication pump on my M300? The coolant pump runs separately from the main motor, controlled by a push button panel on the headstock end.

I'd double check the mechanical linkage from the saddle lever to the electrical box. I've had issues with mine; the levers might move, but any slack means they don't move enough to operate the contactors.


Thread: VFD on a Boxford lathe
24/05/2019 22:11:38

Yes there is, but whether it's useful depends upon how hard you run the lathe.

In simple terms the torque of an induction motor is proportional to the current. So running at 50Hz with rated current the motor will produce a certain amount of power. Now if we run at 25Hz, without changing the belts, the VFD will ensure that the rated current stays the same. But since the frequency has halved the motor speed will be halved and hence the power at the spindle will also be halved.

Now assume we change the belt position (for convenience assume a 2:1 ratio) and run the motor at 50Hz again. Since the motor is running at 50Hz and rated current it will produce the original amount of power. But since we have a 2:1 reduction in speed via the new belt position the spindle will run at a speed as if driven directly by the motor at 25Hz. But the motor is generating the original amount of power, so if the spindle speed has halved the torque at the spindle has doubled. So belt drives via different pulleys allow speed changes while keeping the power transmitted a constant (minus the small losses in the belt).

So varying the motor speed with the VFD and not changing belts results in varying power at the spindle. Changing belts and keeping the VFD at a single frequency results in constant power at the spindle. That may matter if you want to turn large parts at slow spindle speeds, like a flywheel.

In practice one probably would do both, for smallish parts one would just change the frequency on the VFD. But for larger parts it would be advantageous to change the belts.

I'm well aware that VFDs can allow over-current operation, and hence more torque, but that's not a good idea in the long term due to heating issues in the motor windings.


Thread: Bookpress 5tpi Square thread help please!
24/05/2019 21:00:12

The 2.5 tpi setting requires me to transpose a couple of gears in the drive train. I did a quick experiment with the correct gearbox settings but not transposing the gears, which equates to 10 tpi. At 40 rpm it seemed pretty sloooow. So 40 rpm and 2.5tpi would be fine using the conventional half nuts. With the Ainjest I'd probably be up at 85rpm or more. The problem isn't stopping but starting, as you need to get the drive fully engaged before the carriage starts to move significantly. I suspect one may well get away without needing a travelling steady.


24/05/2019 16:44:59

That's not very nice. sad

But I wouldn't use the CNC mill, I'd screwcut it on the lathe - it'd be quicker.


Thread: Threading plastics
24/05/2019 10:58:59

Ah, thought it might be an RF application.

Many of the common plastics have relatively high RF losses. You might be better off making a frame (like a squirrel cage) from small sections of a low loss material such as polystyrene. Each rod can be notched with a file to act as a guide for the wire. The empirical Wheeler formula gives good results for the inductance of simple coils; I doubt the value would change much if the cross-section was polygonal rather than round.


Thread: Source of Machined Nuts
23/05/2019 16:26:58



Thread: Bookpress 5tpi Square thread help please!
23/05/2019 14:48:50

I'm in the single start thread camp.

Bookbinding presses are intended to apply, and hold, high pressures. A multistart thread would mitigate against that.


23/05/2019 13:56:27

Here's one I made earlier:


It's 8 tpi and about 1" diameter, cut as an exercise before I made the proper brake screw shafts.

The (HSS) cutting tool was simply a square parting off shape with the leading edge relieved to account for the helix angle. I made my tool a couple of thou less than the nominal 1/16" width. The thread is square, so you need to plunge straight in as the flanks are perpendicular to the shaft. Leave the top slide parallel to the lathe axis.

A square thread can bind on the crest, on the root and on the flanks. These are independent; fixing one has no effect on the others. Assuming you already have the matching internal thread the crest and root interference can be sorted by calculation, ie, select the correct OD for the blank and cut deep enough to provide some clearance at the root. That just leaves the flanks. That's why I made my tool slightly narrower. Having cut the thread you can advance the tool a known distance (a few thou) with the top slide and shave that amount off one flank. Try the nut for fit and repeat as required.

I can't help with the lathe settings (my lathe will cut 5 tpi) but you might be able to select 10 tpi and fudge the drive train by a factor of 2?


Thread: Source of Machined Nuts
23/05/2019 11:37:14
Posted by Alain Foote on 23/05/2019 11:23:12:

.........but although picture suggests machined there was nothing to confirm this!

The picture is a CAD model; probably little resemblance to reality. At the price I'd be surprised if these nuts were anything other than formed.


Thread: Hello
22/05/2019 16:13:49
Posted by Haggerleases on 22/05/2019 15:51:43:

I'm guessing that's a car?

I thought that too. But then it occurred to me that no-one in their right mind would want to restore an Austin Princess (had one of those) or a Morris Marina.

Turns out it's a 3.5" gauge locomotive by LBSC. thumbs up

David: Welcome to the forum. I can't help with details of the model, but the design has been mentioned here before. It may be worth a forum search.


Thread: What do YOU call it?
22/05/2019 16:06:24
Posted by Haggerleases on 22/05/2019 15:49:39:

............sometimes just sitting weighing up what comes next and maybe browsing a technical book or something, no agenda.

I do that too, but in the study. I call it design. smile


Thread: An old Shaper found in Phuket Town
22/05/2019 15:08:27
Posted by Speedy Builder5 on 22/05/2019 14:51:38:

Andrew, I worked for the Cincinnati Milacron Electronic controls div which had a UK centre in Bedford, Caxton Rd. That then moved over to Bigglewade in about 1980 as the Machine tool division moved up to Birmingham.

That's interesting too. When I was in the Bedford model engineering club in the early 1970s one of the members gave me a lot of help, and allowed me loose in his workshop on Saturday mornings. I think he worked at Brookhurst Igranic, which made heavy duty electrical control systems for cranes, mines and the like. As I recall the company was taken over by Cincinnati Milacron, although I'm not sure when?


Thread: What do YOU call it?
22/05/2019 15:02:09

Errr, I don't. If I'm in the workshop it's because I've got something specific to do, find or make. Time is precious.


Thread: An old Shaper found in Phuket Town
22/05/2019 13:26:10
Posted by Speedy Builder5 on 22/05/2019 11:47:35:

I used to work for Cincinnati at Biggleswade..........................................

Interesting; my syndicate partner in my big glider (Nimbus) did an apprenticeship at Cincinnati in Biggleswade in the mid to late 1960s.


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