Yes.

The 'magic' helix angles give two gears that will mesh at 90 degrees with the same PCD, but one being twice the number of teeth than the other (2:1 gear ratio).

If both are 45 degree helix angle, for example, the larger gear will be twice the diameter of the smaller for a 2:1 ratio.

Gear ratio is all about the number of teeth, nothing else.

Gear diameter (for a given module / DP) is determined by no. of teeth *and* helix angle.

For skew helical gears to mesh, they must have the same *normal* module.

Lab worktops are often made of ‘Trespa’ - a trade name for some type of laminate. It’s horrendously expensive, but you may be able to buy an off cut. (It cuts & machines OK with TCT woodworking tools.)

Is that still the case with something as slow revving as the Seagull? (Asking, not arguing! )

FWIW, the Seagull's cams are machined as pairs and 'loctited' onto the camshaft.

Charles Lamont sets himself an impressively high bar!

I've also looked in depth at the Seagull and like you, discovered it is not at all straightforward due to the number of errors in the drawings and the marginal nature of the castings.(See also https://modelengineeringwebsite.com/Seagull_revisited.html ).

However,

As far as I know, there is no requirement to harden and grind the cams for this type of engine (although one may wish to for authenticity).

The end of this video shows how cams can be machined on a lathe:

In truth, as long as one is not looking at a high performance ("racing" )  engine, they don't need to be very accurate to get a running engine - maybe 5 degrees, or so? You could probably get away with 10.

There's a way of cutting certain cam profiles fairly accurately on a lathe using a series of arcs which is straightforward, but a little tedious.

Edited By Andy_G on 11/02/2021 17:25:29

Lots of people have successfully built Websters (it seems to be something of a rite of passage).

Have a look at the Kerzel hit and miss engine - similar open crank design as the Farm Boy, but also built from bar stock. (It does need some milling, which is why I didn't build it).

Plans are very good, and freely available.

http://www.floridaame.org/GalleryPages/g1h0106.htm

(edit for correctness! )

Edited By Andy_G on 08/02/2021 08:52:55

This is mine, with the vice centred on the available cross slide travel - you can see that it's a fair way from the centre of the tool post (the hole between the two allen screws):

It's fixed using the compound slide mountings on one side, and a couple of tapped holes on the other. The mounting holes are arranged so that the plate can be removed without disturbing the slide. I added a dowel to the back side which, in conjunction with the spigot on the compound mount, ensures that the plate goes back in the same place each time without needing to be trammed in.

I'm sure a proper mill would be better if you have the money/space for one, but this has its uses.

Edited By Andy_G on 04/02/2021 23:29:07

Edited By Andy_G on 04/02/2021 23:32:02

If yours is a typical mini lathe, you may find that the centre of the tool post won't traverse much further than the axis of the spindle, so unless that Warco attachment was mounted offset from the toolpost (in the manner of a cutting tool), you would be limited to using half the width of the vice for milling ops.

It's very cleverly done - even with the bottles side by side, it isn't obvious.

Where did you buy it from?

Not the same stuff that Mr Crispin ended up with, is it?

Edited By Andy_G on 02/02/2021 16:58:35

off topic: If you explore the Clickspring channel, he makes the metal, drill bits, files, etc. using contemporary technology. It really is remarkable.

Hi Jason - that’s the same one as I found (above). It does seem to work very well!

The 45 degree gears will mesh because they both have the same helix angle, and hence the relationship between the normal and transverse modules will be the same for each. It looks like you're on the right path now - just getting the handedness & pitch angles the right way around.

If you appreciate the Antikythera mechanism, and have a day to spare....

Clickspring Antikythera Construction

After encountering frustrations trying to get Fusion360 to draw a parametric involute, I've found this add-in by Ross Korsky which looks very promising:

Link

It also seems to include provision for adding clearance for 3D printed gears, etc.

Andy

______________________________________________________________

Relevant notes for interest:

General Usage Instructions

Once the add-in is running its button can be found under the CREATE menu.

Description of inputs

Gear Standards:

The true involute pitch and involute geometry of a helical gear is in the plane of rotation (Radial System). However, because of the nature of tooth generation with a rack-type hob, a single tool can generate helical gears at all helix angles as well as standard spur gears. However, this means the normal pitch is the common denominator, and usually is taken as a standard value (e.g. 14.5 deg or most commonly 20 deg). In other words if you plan to have your gear manufactured with a standard hob you will likely want to use the "Normal System" and a pressure Angle of 20 degrees.

Normal System: Pressure angle and module are defined relative to the normal of the tooth (i.e. defined as if the tooth was rotated by the helix angle). When defining a gear in the normal system changes to the Helix Angle will cause the gears diameter to change as well as the working thickness (and therefore the strength) of the tooth.

Radial System: Pressure angle and module are defined relative to the plane of rotation. When defining a gear in the radial system changes to the Helix Angle does NOT affect the gear diameter but it does change the "normal pressure angle" which may require custom tooling to have the gear manufactured (obviously this is not an issue if 3D printing the part).

Sunderland: The Sunderland machine is commonly used to make a double helical gear, or herringbone, gear. The radial pressure angle and helix angle are fixed at 20° and 22.5°, respectively. The tooth profile of Sunderland gears is also slightly shorter (and therefore stronger) than equivalent gears defined in the radial system.

Handedness: Direction the tooth appears to lean when placed flat on a table.

Helical gears of opposite hand operate on parallel shafts.

Helical gears of the same hand operate at right angles.

Helix Angle: Angle of tooth twist. 0 degrees produces a standard spur gear. The higher the helix angle the more twist the gear has.

Pressure Angle: The pressure angle defines the angle of the line of action which is a common tangent between the two base circles of a pair of gears. The short of it is this: leave this value at 20 degrees until you have reason to do otherwise - but know that any pair of gears MUST have the same pressure angle.

Module: The module is the length of pitch diameter per tooth. Therefore m = d / z; where m is module, d is the pitch diameter of the gear, and z is the number of teeth.

Teeth: Number of teeth the gear has. The higher the helix angle, and to some extent pressure angle, are the fewer teeth the gear needs to have to avoid undercutting. It is possible to create a Helical gear with a single tooth given a high enough Helix Angle. CAUTION: due to performance reasons, do not make gears with several hundred teeth.

Backlash: [experimental] a positive value here causes each tooth to be slightly narrower than the ideal tooth. In the real world having a perfect tooth is not often desired, it is better to build in a little backlash to reduce friction, allowing room for lubricant between teeth, and to prevent jamming. Backlash is allowed to also be negative which has no real world application I'm aware of but may be useful for compensating for undersized teeth which were 3D printed, etc.]

The angles need to add up to exactly 90 degrees (for a right angle drive). If they are 'off' the gears won't mesh correctly with the axes at 90° (think of trying to mesh spur gears with one axis tilted with respect to the other).

The diameter of each gear depends on the tooth count *and* helix angle for a given *normal* pitch. One is free to choose arbitrary angles (subject to them summing to 90° ) but the diameter of the resulting gears will vary. For the unique set of angles given previously, the gear sizes are identical with a 2:1 ratio.

Reduction ratio is set by the tooth count (only).

Edited By Andy_G on 31/01/2021 10:23:35

As regards making helical gears, have a look here which describes a (relatively) straightforward way to machine them on a lathe.

https://www.homemodelenginemachinist.com/threads/helical-gear-cutting-lathe-attachment.9199/

As regards modelling them, I think Jason has already given the answer - the gear profile needs to be constructed normal to the helix angle, not the face of the gear where you have different helix angles meshing.

I think it is only possible for gears with different helix angles to mesh if the normal module is the same for each (which means that the projected shape on the face of the gear will be different for each one).

I think it should be possible to model this on an oblique plane at the appropriate angle for each gear in your model then project the profile onto the face of the gear before extruding with rotation (but haven't tried it!).

Edit to add:

Actually it might be easier to calculate the equivalent transverse module for each helix angle and model the gear that way - see equations 2.13 and 2.14  here

Edited By Andy_G on 30/01/2021 13:41:04

Good idea. I think we should have a combined Welsh and Scottish mark - something like

C U JIMMY

Edited By Andy_G on 24/01/2021 10:10:12

That also could be the reason for the carriage springing off the bed. (It's not just cranking, as the movement can be seen at the end of the video where the carriage is stationary - 13 seconds on).

Yes, there's a part missing - there should be a plate there.

Have a look at the top of page 11 of this document:

https://www.arceurotrade.co.uk/machineguides/C3-Mini-Lathe-Dismantling-and-Reassembly-Guide.pdf

(The whole document may be useful)

The carriage should just sit on the bed with or without the plate. From your video, the carriage appears to be springing off the bed which is definitely wrong. I would be concerned that the lathe has been lifted by the unsecured carriage and something (probably the leadscrew) has been bent.