My new project ...Small diesel engine .

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Dec 29, 2016
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Viet Nam
Hi everyone !
My new project :

Hi All !
I'm stuck with the design : gears
Buy : the size I need is not available
DIY : I have never cut Helical gears
There is no size of the gears so I can't continue
Maybe I should find a way to cut the gears, then continue with the design
You have just shown 45deg gears which should be available in a range of sizes and tooth counts and they will mesh together.

What sort of sizes are you looking for?
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What sort of sizes are you looking for?
I am looking for gears with an outside diameter of about 15 mm for small gears
(I'm bad at designing and machining gears)
From the McMaster-Carr website:

The helical teeth on these gears stay in contact for longer than straight teeth, which allows them to transmit higher loads at higher speeds than spur gears.

For gears to mesh correctly, they must have the same pressure angle and pitch. All of these gears transmit motion in a straight line—make sure to get one left-hand and one right-hand gear. To change speed and torque in your assembly, pair two gears with different numbers of teeth.

Parallel helical components have a 21 1/2° helix angle so they only transmit motion in a straight line since the helix angle is so small. Made from alloy steel, these gears resist abrasion and are stronger than carbon steel gears. Their teeth are ground, so they're more precise than gears with rolled teeth. Pair them with a rack or a gear that has a different tooth direction.

Crossed helical gears, also known as screw gears, can be configured to transmit motion at a 90° angle. To transmit motion at a 90° angle, pair two gears with the same tooth direction.

The engine in TonyM's avatar has two helical gears, one on the crankshaft, and one mating with it at the bottom of the vertical shaft.
The two gears are the same diameter, but have a 2:1 tooth ratio, and roughly 30 and 60 degree helix angles.

JasonB has a really good design for the 2:1 helical gears of the same diameter.

As I understand it, you can cut helical gears using a standard gear cutter, with the right setup.
There are videos out there of how to do it.

Chuck Fellows some time back built a jig to cut helical gears. ill try to find his post with the plans but just incase i dont find them do a search for it as well. unfortunately Chuck passed away a few years ago.

You show two different diameters which I assume are because you want a 2:1 reduction so if we took an average of 15mm OD you could look at something like 20T and 40T MOD0.5 these would have OD of 11.5mm and 22mm respectively. Something like that could be had quite cheaply from the far east or for a bit more money someone like HPC in the UK
This German hobby supplier is quite reasonably priced and I've used their gears on a couple of engines, like HPC these are 45degree helix so any gear will run with another of the same MOD
You only need the reduction once so could have 20T on the crankshaft meshing with a 40T on the bottom of the vertical shaft. Then the top pair just need to run 1:1 so 20T/20T

As Pat hints if you need the two gears to be the same diameter but running at 2:1 ratio then it is less likely you would be ably to buy them off the shelf
Hi All !
An update .
After thinking... I'll never know if I don't do it...I decided to make the gears myself.
Some images are in the process of designing



Parts are designing
- Fuel pump system (including auxiliary pump)
- Governor
- Water pump
- Lubricating oil pump
You seem pretty adept with 3D CAD, can't you have them 3D printed in a suitable material?


I don't understand what you mean
I think he does mean exactly what he writes. If you can design the gears with CAD, then you can export them and print them. I think it might be a bit expensive if yo decide to use anything that is not normal plastic.

So there is clearly only one chance. "You have to build" a hobbing machine first. :)

Haha. Good success.

Greetings Timo
I'll add that 3d printed gears work quite well in my experience. I 3d printed a set of back gears for my lathe and they're still going strong years later.

If I were printing them I'd use bevel gears rather than crossed helical. They are more efficient.
I'll add that 3d printed gears work quite well in my experience. I 3d printed a set of back gears for my lathe and they're still going strong years later.

If I were printing them I'd use bevel gears rather than crossed helical. They are more efficient.
I must say your anecdotal evidence undermines the plan to provoke a hobbing machine build. :cool: ( that upsets me 😅 )
The lathe (not a particular small machine) would shred 3d prinded gears to bits and pieces. This is my educated guess, but I cannot proove it.
Printing gears on the FDM printer, is somewhat satisfying, but it is just not the same as a metal gear. I never really used printed gears for anything useful. ( admittedly same goes for most of the metal gears that I tried to make, haha ).


@Nerd1000: How would you print the bevel gears? Tooth pointing upwards? When the layers are perpendicular to the teeth spur gears become reasonable strong.

Greetings Timo
Timo, I definitely want to see the hobbing machine build!

Meanwhile, I have tried FDM printed gears in two applications, one with success and one with failure. The failure was replacing a gear in a heavy-duty paper shredder. I am not completely convinced that it is not feasible to print one that could succeed, if I were able to print with something like carbon-filled nylon; I'm also not completely sure there is not another issue at work that is causing the failure.

The place where I have a success is the gears that synchronize the rollers in my home-built slip roller. Two key issues with this success - first, the gears are large, module 4 spur gears. Second, there is not much torque on these gears; their main purpose is just to help get the material fed into the rollers. Once the material is feeding, the pressure on the material / between the rollers transfers the torque without really relying on the gears.
3D printed plastic gears seem to work OK as lathe change wheels provided you don't take silly cuts from severaral posts I have read, there are a couple of guys on UK e-bay that will print to order including the splined holes that some lathes need to their gears. Also handy if you need an odd tooth count for a one off job.

Would be costly to have them printed in metal or even send off and have 3D printed waxes that are then cast, Don't think Grey plastic gears would look right on a model

Pat you should be OK casting from those, the trick is to carefully wind the pattern out of the sand, the helix should allow it to unscrew.
Pat you should be OK casting from those, the trick is to carefully wind the pattern out of the sand, the helix should allow it to unscrew.
From what I have been reading, people heat the plastic up to get it soft enough to withdrawl from the sand, but avoid overheating the sand.

Apparently bound sand can take a lot of heating.
I think I was reading about sodium-silicate molds getting heated to allow removal of the 3D print.

I have flamed resin-binder molds with a gentle propane flame, and also burned off the moldcoat, without problems, so they are pretty heat resistant.

Hopefully I will not have to get the 3D print fully molten to remove it from the sand.

I am thinking about using a 2-part print, with a removeable round core that takes up perhaps 80% of the print, and then separately print the teeth with a little backing. Then I can remove the center part of the gear, and then heat and remove the teeth.

A bound sand mold I think will be as accurate as a ceramic shell, or almost as accurate depending on how well the teeth spaces ram.

This may be a case where excess binder is used, which makes for a very hard mold or core, but that may be desirable, unless the mold gets too hard and prevents expansion of the molten metal.