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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: cutting spur gears on a mill
19/08/2021 21:32:47
Posted by brian jones 11 on 19/08/2021 16:37:38:
I belong to the school of thinking first and asking around before making chips.........

Unfortunately you seem to have mislaid the first step. If the tap is skewed by the helix angle to get straight teeth on the gear how does it drive the gear blank as there will be no tangential force. The tap drives a worm wheel because it creates teeth that are not straight and hence there is a force causing the blank to rotate.


19/08/2021 21:02:53
Posted by John Haine on 19/08/2021 15:40:43:

For larger tooth counts the tooth form will surely be involute as the tap is in effect a rack form cutter?

Fair point. The teeth on the worm wheel I free hobbed with a tap look triangular. But the wheel is quite small (~1/2" diameter) so any involute curvature will be also be small. The tap was UNF so the pressure angle will be 30°. Since the tap form is triangular the gear teeth will tend to a triangle rather than the more familiar rack form. No doubt gears made by the same method will run together but there will be significant forces pushing them apart.

The small worm wheel shown is for a speed adjuster on a Pickering governor, so I doubt it'll ever get turned:

governor with worms.jpg


19/08/2021 14:44:11
Posted by brian jones 11 on 19/08/2021 12:05:43:

Seems too good to be true

The method mentioned works fine for cutting worm wheels, although spiral flute taps are helpful:

hobbing worm wheel me.jpg

Getting the correct number of teeth is a bit hit and miss. One can free hob a worm wheel, but it needs to be pre-gashed:

worm wheel hobbing.jpg

For spur gears it's a useless method. First, the work would need to be swivelled to the helix angle of the tap in order to get straight teeth on the gear. Second, the work would need to be rotated at the appropriate rate, same as when using a hob. Third, the tooth form would be non-standard, and the gear would only work with other gears made by the same tap.

It doesn't take that long to cut spur gears by conventional methods, provide one gets on with it rather than pontificating on forums, two gears at a time in this case:

5DP Pinion Gears

If I want a quick gear, and don't have the appropriate cutter, I simply design it in 3D CAD and let the 4-axis CNC mill get on with it. As was the pinion:

pinion 21 teeth 20pa.jpg

For mating with this internal gear:

internal gear and pinion.jpg


Thread: Reducing drawings
19/08/2021 09:57:53

Posted by JasonB on 18/08/2021 19:42:16:

But so would the capacity of the engine have gone down by the cube law. Or is the momentum to the 4th power?

I expect your traction engine flywheel is quiet close to 1/4 the diameter of the full size.............

I think momentum goes down by the fourth power. Consider angular momentum which is:

radius x mass x angular velocity

If we assume a scaled flywheel, and the same rpm, the mass goes down as a cube law and the angular velocity (radians per second) stays the same. But the radius goes down linearly. So overall the angular momentum scales roughly as the fourth power.

I'm pretty sure my flywheels are close to 1/3 scale, the rim looks pretty thin although they seem to be roughly scale compared to pictures of the full size engines.

Ramon: I agree that the medium behaviour doesn't scale. That could explain why an engine that is significantly smaller than full size, say a tenth or less, might have a problem with scale ports/pipes.


18/08/2021 19:25:50
Posted by Ramon Wilson on 18/08/2021 18:22:48:
.........with no compromises save air/steam passages which are enlarged if the part allows for better air flow........

That's interesting. I'm in the process of re-designing the steam ports , slide valves and ultimately the valve gear for my one third scale traction engines. The port sizes are mostly copied from full size, scaled as per the model engine. I didn't think the ports needed enlarging. My reasoning went along the lines of the port areas have gone down by a factor of nine but the volumes are down by a factor of twenty seven. So the flows are reduced well below the reduction in area. Calculations for the model engines seem to indicate flow rates and Reynolds numbers well below what I would expect for a full size engine.

Of course I could have got my reasoning wrong - what does the team think?


18/08/2021 19:15:01
Posted by JasonB on 18/08/2021 18:39:44:
.....never had a problem with the flywheels having to be scaled differently, what is the reason that it won't work?

Presumably because the volume of the rim, and hence the mass, has gone down by a cube law. However, for a model, probably running off load, I doubt it will be significant.


18/08/2021 15:02:20
Posted by Ramon Wilson on 18/08/2021 13:55:55:

....reduce dimensions by say 50% you quarter the volume of the part....

Not so, if linear dimensions are halved then areas are reduced to a quarter, and volumes are reduced to an eighth, of the original. Basically an inverse square law for areas and an inverse cube law for volumes.


Thread: Is there such a thing as an 'external reamer'?
18/08/2021 10:42:07
Posted by Rob McSweeney on 18/08/2021 09:55:16:

...making of 'running down cutters', which are very close to the woodworking plug cutters..............

That's what I did when I needed to machine an enclosed spigot:

valve body and tool.jpg

I'm not convinced a roller box would be suitable. They're designed to machine long parts to diameter, not short spigots. Herbert did sell one indended to machine to a shoulder, but I expect they're in rocking horse territory. If I was making the part on the repetition lathe I'd use a knife tool and just set stops for diameter and depth. For a lot of parts I'd machine in two stages, a plunge cut to rough out and then a finishing cut on diameter and shoulder.


Thread: Which is better Thompson or er collets
15/08/2021 10:33:57

Posted by Clive Foster on 15/08/2021 10:01:03:

I have broken the end off a 1/4" end mill by blindly following the leave a gap instructions........

Me too, after that I followed the manufacturer instructions.


14/08/2021 22:46:55

Posted by Martin Kyte on 14/08/2021 22:29:12:

Thats why leaving a small gap is the way to use the chuck.

Not according to Clarkson - see Section 4a:

Autolock Instructions


Thread: Random Thoughts on Steam Injectors
13/08/2021 16:18:40
Posted by Daniel Ackles on 13/08/2021 14:00:16:

How do you calculate the appropriate sized injector to boiler ratio?

No idea where your equation came from, but I'm not convinced it's appropriate. The size of the injector is determined by the amount of steam used by the engine. See the first post in this thread for calculations. If the boiler can't provide that steam, or can't accept the calculated injector flow without losing pressure, then the boiler is too small. It is normal to size the injector slightly above the nominal flow rate needed, so it can be used intermittently.


Thread: Hole diameters for single point threading
13/08/2021 10:19:29

Posted by SillyOldDuffer on 13/08/2021 10:00:11:

.......EN8 often tears roughly when cut.......

Agreed when turning, high surface speed often cures the problem. Strangely though the same isn't true when threading. Here's an M16 thread in EN8 I made earlier:

m16 thread.jpg

It's not a super smooth finish but works fine as a drawbar on the horizontal mill. The thread was cut on an imperial lathe, so I couldn't use my high speed threading attachment. Instead I was slow 'n' steady with the half nuts always engaged, 85rpm I think.

Incidentally H=0.866P is the theoretical depth of the 60° triangle, it's not the same as the actual thread depth, that's 5/8H, unless one uses Zeus.


13/08/2021 09:34:45

To expand on the reply by Tony; this is a 16ER-AG60 insert:


It is an external insert intended to cut a range of ISO metric pitches. The nose radius will be set for the smallest pitch in the range covered. For larger pitches the radius will be too small and the depth of cut needed will be larger than theoretical to compensate. However the outer diameter will remain the same; for internal threads the core diameter will remain the same.

In contrast this is an 16ER 2.0ISO insert, full profile intended to cut only 2mm pitch external threads:


Note the larger nose radius. I will be using this insert later today as I have some M16 threads to screwcut.


12/08/2021 22:11:57

When single point threading I ignore tapping tables and percentage thread engagement. For internal threads I turn the bore to the theoretical core diameter, ie, assuming 100% thread depth. When screwcutting I aim for the theoretical thread depth, checking fit with a mating part. Once near the theoretical depth I take fine cuts, may be a couple of thou off the diameter. When checking fits it is important to do a spring pass and clean the thread with a fine brush. Almost invisible dust or swarf can be the difference between a nice fit and loose. Most of my single point threading is done with full profile inserts rather than HSS, so I can assume that the thread profile is correct.


Thread: chinese lathe tailstock is 3/16" too high
12/08/2021 11:13:20

Posted by Neil Wyatt on 12/08/2021 10:40:29:

What is important is making sure it is level so it doesn't rise/droop when extending it.

The accuracy limits for my lathe gives tolerances for the parallelism of the tailstock barrel (with barrel extended and locked) compared to carriage movement that are biased upwards and forwards. Although the tolerances are small, 0.015mm and 0.02mm per 100mm in horizontal and vertical respectively. So if the tailstock barrel isn't perfectly parallel to the carriage movement it should be pointing slightly up and/or to the front.


Thread: Making Tapered Castellations in Aluminium
11/08/2021 15:29:59
Posted by Dr_GMJN on 11/08/2021 15:06:33:

Can anyone recommend a rotary table then for an SX2P Mill?

Not specifically as I'm not familiar with the mill, but offer the following points for consideration:

  • Get a bigger rotary table than you think you need, a lot of room can be taken up with clamps
  • A vertical/horizontal mounting table is useful - I use vertical as much, or more, than horizontal
  • My rotary table has a 1" parallel hole in the centre; to my mind that's more useful than a Morse taper, as it's simple to make location spigots and fixtures

To illustrate the points, here's a horizontal setup:

final drive gear cutting.jpg

Note the central rod; simply stock bar with a 1" spigot turned on the end. The clamps under the gear only just fit on the table. And a vertical setup:



Thread: Beam Engine - where to start?
11/08/2021 15:19:46
Posted by Roderick Jenkins on 11/08/2021 14:58:02:

Andrew, I have actually started with the governor drive gears- mostly as a test of my fading intellect(?) to understand the manufacture of parallel depth bevel gears

That makes you one up on me; I've never made parallel tooth bevel gears. sad


Thread: Making Tapered Castellations in Aluminium
11/08/2021 14:45:54

Given that the sides of the wedges are radial lines all that is needed is to offset the cutter by half its diameter, away from the wedge, while indexing round each tooth with the rotary table and the wedge shapes will appear. Opposing sides of the wedges need equal, but opposite, offsets. Each cut only goes to the centre, not across the diameter.

I expect Jason will knock out a quick Alibre drawing to show the method. smile


11/08/2021 13:31:16

The existing parts don't look like they've got a taper on the teeth. Machining straight-sided teeth is a simple operation involving a rotary table to machine the sides of each tooth in turn. If a taper really is needed, simply tilt the head of the mill, or the rotary table. Aluminium seems an odd choice of material. Steel would be better wearing and less prone to fatigue.


Thread: Beam Engine - where to start?
11/08/2021 13:23:53

I'd start with the drawings. Understand how the parts fit together, how each part will be machined and how it will be held for each operation. Identify which dimensions are important and which not. Note tools that will be needed but not avaiable, or change the design. Check for drawing errors, especially on important fits between parts.

I expect my parts to fit together even if they are machined months apart, so I make parts in the order than interests me. Early on I made the spur gears for my traction engines even though the engines didn't come together for several years afterwards. If machining in a conventional order I'd start with the base.


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