How to make a plastic gear!

[John Hill]

I discovered (some time ago) a simple way to make plastic gears.


DSCN0196 by aardvark_akubra, on Flickr

A plastic blank is mounted on the lathe spindle and a steel gear of the required gear 'family' is mounted on a spindle on the tool post. The size of the steel gear is not important.

The method is really simple, the lathe is started and the plastic blank brought into contact with the steel gear which begins to turn. Applying a little pressure causes a rumbling noise but after a minute or two that subsides. Apparently the friction of the steel gear running over the blank has heated the plastic which becomes softer. More pressure is applied and again held until the rumbling subsides. This sequence is repeated until teeth are formed of the required depth.

That's it, except for trimming the edges of the plastic gear.

A few notes, the number of teeth is determined by the diameter of the plastic blank which equates quite closely to the PCD. Any size steel gear will create a plastic gear with the correct tooth form regardless of the number of teeth on the steel gear or the diameter of the blank. It is important to withdraw the steel gear from contact before stopping the spindle, otherwise the plastic gear will be deformed where it cools against the gear. Not all plastics are suitable but it is not hard to experiment.


cogs by aardvark_akubra, on Flickr


John

 

[wildun]

John,
That looks interesting and obviously seems to work quite well, but how can you determine the number of teeth you require on the blank without something to "guide" the teeth to the right position, ie will it work out correctly when you do one revolution on a smooth gear blank, do the teeth just "slot" into place automatically?
Do you have to have the diameter or PCD quite precise before running the steel gear against it, - ?

 

[Till]

Use a small blowtorch or heat gun to soften the tooth area of thermoplasitics and to create a better finish on the plastic teeth' surface.

 

[mcostello]

Brilliant!

 

[va4ngo]

This is very interesting especially if it will work on some engineering plastics such as Delrin etc. This loiks too simple to be true and i will give it a try

 

[kvom]

Delrin is pretty hard, but it would be great if it works. Please let us know how you get on.

 

[John Hill]

The number of teeth is set by the diameter of the blank which should be the PCD of the required gear, plus a little bit for squish allowance.

If you want to use heat to soften the plastic you must make metal plates to fit each side of the blank leaving only the bit you want softened exposed. I don't recommend adding heat unless the type of plastic requires it.

 

[canadianhorsepower]

The number of teeth is set by the diameter of the blank which should be the PCD of the required gear, plus a little bit for squish allowance.

If you want to use heat to soften the plastic you must make metal plates to fit each side of the blank leaving only the bit you want softened exposed. I don't recommend adding heat unless the type of plastic requires it.

Ok now if you put plates on each side, where is the remove or cut plastic is going to go? back into your plastic and youll never have a gear.
try this on a metal part (putting side) and tell me the outcome:hDe:

 

[John Hill]

Ok now if you put plates on each side, where is the remove or cut plastic is going to go? back into your plastic and youll never have a gear.
try this on a metal part (putting side) and tell me the outcome:hDe:

The plastic is not cut or removed, it is displaced to form the teeth. The bit below the PCD is forced upwards to form the tips of the teeth.

This is a heat forming process, with soft plastic just the friction is enough to form the teeth with some plastics friction is not enough so you need to heat the edge of the blank to make it pliable, if you apply too much heat the whole blank gets 'wonky' and we don't want that. The metal cheeks shield the blank from the heat and leave just the bit you want to soften exposed.

If you do not have the side cheeks a bit of the plastic gets squished out the sides so to allow for that the blank needs to be just one British Standard Tad greater in diameter than the PCD. Even with the cheeks a bit still gets squishes as the cheeks are smaller in diameter than the blank.

 

[wildun]

JOHN,
Maybe I missed it somewhere, but what spindle speed would you recommend (say with the metal gear in the picture) as being the optimum, or should we just experiment?

 

[John Hill]

Fairly slow I think. That blue one I did at about 200 rpm.

 

[Hopper]

Hmm, maybe this is the way to make the missing change gears on my ancient Drummond lathe.
Would the plastic be stout enough to take screwcutting loads as change wheels?

 

[kvom]

Hmm, maybe this is the way to make the missing change gears on my ancient Drummond lathe.
Would the plastic be stout enough to take screwcutting loads as change wheels?

Any deformation would be lateral, so if you make metal sides to the plastic gears I suspect they would work. One way to find out.

 

[cfellows]

Interesting process, John. Did you measure the finished gear to see if it had increased to approximately the correct diameter?

It would be interesting to see if this works to create an internal gear...

Chuck

 

[John Hill]

No Chuck I didn't measure the overall diameter but it has the right number of teeth and the plastic teeth mesh fully with the steel gear.

John

 

[John Hill]

Hmm, maybe this is the way to make the missing change gears on my ancient Drummond lathe.
Would the plastic be stout enough to take screwcutting loads as change wheels?

At the end of the day they are still plastic gears and not all plastics lend themselves to this method. The best plastic I have found has been ABS (what Lego is made from), 'engineering' plastics such as the white stuff that looks like nylon is not so good as it has too much 'spring' and the teeth do not form full depth although this might be caused by not enough heat.

I tried a piece of old black 'stuff' that came from an electrical switchboard, it appears to be layers of fabric and something like black pitch, surprisingly it made a quite good gear form.

What Drummond do you have? My 1908 flatbed survived with almost every original part including the quaint wooden insert traveling steady. The treadle is gone though.

 

[cfellows]

Hmmm, wonder if the black fabric stuff was phenolic? That's made with epoxy and either paper or canvas. I also wonder if this method would work on small pitches, such as 32 or 48 pitch...

Chuck

 

[John Hill]

Phenolic, is that the stuff that smells like horse pee when it gets hot? This was more like a hard pitch. I don't think it would be very good for fine pitch gears, but ABS would be.

BTW, I am not claiming that this is a 'mature process' but certainly the ABS gear surprised me how well it came out and of course it is ideal for the task I put it to.

 

[Hopper]

....
What Drummond do you have? My 1908 flatbed survived with almost every original part including the quaint wooden insert traveling steady. The treadle is gone though.

Mine's one of the new ones -- an M-Type circa 1939. My old man bought it in the UK in the 1950s from a secondhand dealer who had rows of them from a WW2 aircraft parts factory. But after he shipped it home to Australia it turned out the change gears were an incomplete set with a few extra oddballs thrown in! Other than that the old girl does a terrific job, specially since I found out on the Drummond Yahoo group the correct way to tighten up the headstock bearings.
There is something perversely neat about using ancient technology when CNC is all the rage!

 

[RichD]

There is something perversely neat about using ancient technology when CNC is all the rage!

Ain't that a fact! CNC is great...but it's important to preserve the proven techniques so we don't have to reinvent the wheel every generation.