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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: Electric Traction Engine
13/11/2018 22:21:51
Posted by Ron Laden on 13/11/2018 09:23:26:

I went with 3mm sheet in the end, scale wise that is 7/8 of an inch, although I,m not building a scale model I would like to try and keep things looking realistic. I dont know the side plate thickness on a full size engine (varies I suppose) but I guessed at 1 inch, I could be way off but hopefully not.

More like 3/8" in full size, the same as the boiler plate thickness, give or take. In full size the hornplates are extensions of the boiler, not separate plates as in most models.


Thread: Thread cutting again
09/11/2018 19:38:19
Posted by Roger Hulett on 09/11/2018 16:08:54:

(Andrew Johnston), I was intending to use 55degree thread cutting inserts (from RDG) as I assumed that is the Whitworth requirement. Obviously incorrect,can you please advise further. I chose the 24tpi from the chart attached to my old S.B. lathe as it seemed fine enough to stop the cap unscrewing with vibration.

What I should have said is that you won't be able to get a full form insert for 24tpi Whitworth, ie, one that correctly forms the root and crest radii for a given tpi. The partial form insert from RDG will form the correct flank angle, but it won't form the crest radius, and the root radius will be too small. That leads to problems with depth of cut; since the radius is too small you need to cut a bit deeper to get the flanks correct. Of course for a one off it doesn't matter, although it it means both internal and external threads are a bit hit 'n' miss. Since the threads will be incorrect at root and crest they can't be called perfect, and will not seal properly.


Thread: Learning CAD with Alibre Atom3D
09/11/2018 17:34:43

Beaten to it. sad


Thread: Thread cutting again
09/11/2018 15:38:02

The theoretical thread depth for the Whitworth thread form is 0.64032738p, where p is the thread pitch. So start with whatever is the ID of the tube and cut the internal thread to the calculated depth. Then start with the OD of the plug at the ID of the tube plus twice the thread depth. Cut the external thread, using the tube as a gauge during the final cuts.

That's the theory, the practise is rather messier. For a start 24tpi isn't a standard Whitworth pitch, so it's unlikely you'll be able to buy inserts. So that'll mean grinding ones own tools. Inevitably that'll mean that the crest and root radii won't be precise. So the threads won't be perfect. If the OP is expecting the threads to be gas/liquid tight without sealant, dream on! For that you'd need to be using BSPT threads.


Yes, I know 3/16" and 7/32" Whitworth are 24tpi, but they're not in the original standard sarcastic

Thread: Learning CAD with Alibre Atom3D
09/11/2018 10:33:26
Posted by JasonB on 09/11/2018 09:57:47:

Another thing while learning is to save regularly then if you really do muck up close the part without saving and then when you open it again it will be back at the last saved position.

+1 - absolutely essential!


Thread: Cutting a keyway without a broach
08/11/2018 20:47:13

You just need a toolbit that looks like a short parting tool. Like these:

slotting tools.jpg

These tools and bars are actually intended for use in a slotting head on the vertical mill. But they could just as easily be mounted in the toolpost on the lathe and moved back and forth with the saddle.


Thread: Learning CAD with Alibre Atom3D
08/11/2018 17:35:00
Posted by mick on 08/11/2018 17:26:39:
I successfully reach the stage of creating the 25mm circle, however the circle isn't a solid circle but interrupted, when I click the green tick to deactivate sketch the circle disappears

Might be a reference circle (which is dashed) and is intended to aid sketching, not be part of the sketch itself.


Thread: Noisey MEM switch
07/11/2018 21:24:56

Even the slightest piece of dirt will stop the poles of an open frame contactor from mating properly, with resultant buzzing. Take it apart again and wipe the faces, clean them again and then wipe with a no residue solvent like acetone. Don't apply lubricant, the faces should be bone dry.

In terms of the magnetic circuit even a few thou gap is very significant.


Thread: ME taps and dies
06/11/2018 16:27:44

I've got a couple of ME style taps from Tap&Die and they've been fine. I wouldn't pay any attention to the chart on the website. I'd put more trust in the figures in the book by Tubal Cain.

If Fred's taps don't work in a 8.8mm hole then I suspect the taps. Certainly the taper tap grind looks odd; almost as if the leading edge of what should be full depth threads has been ground off.


06/11/2018 13:15:27

The tapping holes sizes are too small. They represent thread depths of 85-90%. Given that the tap will slightly extrude material, and should be slightly over nominal size, it's not surprising the taps are binding. I'd drill at least 0.1mm bigger, for the 1/4" thread and 0.2mm bigger for the 3/8" thread.


Thread: Cylinder boring
05/11/2018 16:21:10
Posted by Martin Green 3 on 05/11/2018 15:44:07:

I will hone the cylinders but has anyone any suggestions as to the finish I should be able to achieve?

I've just measured the surface roughness of the bore of one of my traction engine rear hubs. It comes out at 5µm Ra. On the face of it that isn't great, but the bore feels pretty smooth using a Mk1 finger. Comparison of the bore with a Rubert scale using the fingernail suggests more like 2µm Ra. Although it's the rear hub I'd be happy using the bore as is for a steam engine cylinder.

The hub is 4" wide and was bored in the lathe using a 16mm boring bar sticking about a bit more than 4". I used an ordinary insert, not an alumimium specific one. I can't remember the feedrate but it may well have been greater than the normal 4 thou per rev that I use as standard. Particularly with shallow finishing cuts a boring bar can chatter with too fine a feed rate.


Thread: Workholding on the faceplate
01/11/2018 08:14:17

Regarding the original picture I'd be inclined to move the balance weights in slightly, as it's not clear if they'll clear the bed. I'd also second the blocks at the front of the work to stop it twisting, to the extent that if it isn't done the work will twist.

Ho hum, I use step blocks quite a lot. Although not clear there are six clamps and step blocks holding this smokebox:


No problems so far. smile


Thread: Mystery Spindly thing
31/10/2018 20:53:54

At first glance it looks very similar, but not indentical, to the spindle cartridge for internal grinding on my Myford cylindrical grinder. The black "handle" on the left in the first picture is actually a slightly domed pulley to be driven by a flat belt, like this:


Does it have an rpm engraved on the body, from memory mine says 28800rpm.

Tomorrow evening I'll dig out the spindle and have a better look.


Thread: Electric Traction Engine
31/10/2018 19:53:31
Posted by Ron Laden on 31/10/2018 13:03:12:

Thanks Jason, at the scale I am using 1.625 the rear wheels will be 9.75 inches, if my sums are correct that equates to 189 rpm at the wheel for 5mph......

I think that's wrong. The circumference of a wheel 9.75" is (times pi) 30.631".

There are 63360 inches in a mile (*) so 5 miles is 316800 inches, in a hour for 5mph, which is also equal to 5280 inches per minute (divide by 60). Divide 5280 by 30.631 to get the rpm, equals 172.37rpm.


(*) I knew that without looking it up as it is the scale of the old OS 1" to 1 mile maps. thumbs up

Edited By Andrew Johnston on 31/10/2018 19:54:15

Thread: Dial Gauge Advice
31/10/2018 14:18:24
Posted by not done it yet on 31/10/2018 13:21:51:

I would make a guess, and say that many of those who use the pivot type, almost universally, already use digital read outs with their mills.

Yes and no; I mostly use the pivot type and I do have a DRO on the mill, albeit only one of my three mills. I'm not sure what conclusion you can draw from that?

It's somewhat difficult to use the plunger type when indicating on the inside of a hole. teeth 2


31/10/2018 11:46:15

My go to DTI is a Mitutoyo where each division is nominally 0.01mm. Indicating on machined surfaces when setting up in the 4-jaw chuck, or on the machine vice jaws, I expect to get better than one division. It doesn't take long, and if you're going to do something you might as well do it right. smile


31/10/2018 10:59:08
Posted by ega on 31/10/2018 10:48:10:

I think Verdict claimed that their pear-shaped probe mitigated the error.

I have enough things going pear-shaped in the workshop as it is without deliberately introducing more pear shapes. sad


31/10/2018 10:24:17

They're different animals, with different uses.

The dials with a telescopic needle are dial gauges. They usually have a significant travel, 12mm or more. They are used to measure parts or travels. They can be used in a stand on a surface plate as a comparator or on a machine tool carriage to measure distance. The latter was common prior to the introduction of DROs.

The dials with a pivoted needle are dial test indicators (DTI). They are used to set parts or fixtures true. While they are marked with dimensions they actual dimension changes with the position of the pivot. So they are not used for absolute measurement but to indicate relative movement.

I've got several examples of both, but use the DTIs orders of magnitude more frequently that the dial gauges. Two common uses for the DTI are setting work running true in the 4-jaw chuck and setting the machine vice jaws parallel to the table on the milling machine. My DTI lives in the workshop on a magnetic base ready for use, by contrast the dial gauges are in the tool store, aka the dining room.

If you're only going to buy one, get a DTI. And buy a quality one; there's nothing worse than a sticky needle on a DTI.


Thread: Taper turning
30/10/2018 16:04:25

I've never needed to offset the tailstock to cut a long taper, as I have other means of producing them. But if I were to do so I'd use a radius centre drill, in conjunction with a standard centre, which are readily available at reasonable cost.


Edited By Andrew Johnston on 30/10/2018 16:05:07

Thread: Learning CAD with Alibre Atom3D
30/10/2018 13:24:25

I'd agree with keeping sketches simple. Too many features in one sketch may well be a problem later if you want to change things. You may get the dreaded "Target not available" message, which I'm sure David can elaborate on.

Many of my sketches contain only one item. For instance if I'm creating a PCB for form and fit reasons the outline of the PCB will be one sketch and mounting holes will be another, so they're completely independent. Ok, not quite true as I use Alibre Expert (the program that is, not the user) so I have the "hole" function available, but the point is the same. Things like fillets and chamfers on the PCB would be added later, not included in the initial sketch.

I don't sweat dimensions and constraints to start with; I just draw a sketch roughly where I want it and the extrude. I then sketch, extrude or cut and so on until I've got the basic part. Then I will go back to the first sketch and start adding constraints and dimensions and so on through the design. Finally I'll add things like chamfers, fillets and external threads.

I wondered why Neil didn't use the pattern function. I assumed it wasn't available in Atom. I use the function a lot, both linear and circular patterns. If nothing else if I put all the items in one sketch I'd forget which one it was that I'd dimensioned. I also use the mirror function a lot. No point in creating both sides of a part that has plane symmetry.


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