Gear help needed please

Hi

Can anyone in Leicesteshire help with a printing request please?

I am a premises officer in a primary school in Hinckley and need to repair a broken gear in a projector. I've drawn it up in FreeCad and exported it to an STL mesh. It is a compound gear with 67 and 9 teeth although the 9 tooth gear may be incorrect. I had to work it out from the remnants of the smashed gear. It may be 8.

If anyone can help we would be extremely grateful. We would of course pay for materials used.

I've attached a photo and can give dimensions if anyone can help.

Could you add dimensions to your description, not least to ascertain if this could be made the old fashioned way. That would let me ( and maybe others) do some calcs about how to go about making said gear.

Sticking this in with the 3D printing stuff suggests that you're maybe expecting this to be made that way, rather than cut out of metal. What materials does this poorly gear run against, and is there a reason why a gear cut in metal would be unsuitable?

Rgds Simon

If machined, the smaller gear may need an undercut between it and the large gear or made as 2x separate gears. A bit hard to cut the teeth into a blind corner.

Paul.

Depends on the size of the piece, but I'd imagined it made as two separate components joined together. The bigger problem is that 67 is a.prime number

Simon

Edited By Simon Williams 3 on 14/12/2016 22:09:43

Depending upon size it would be a simple part to 3D print. Presumably the orginal part was injection moulded?

It would be difficult to cut the 9 (8) tooth gear with a dividing head and gear cutter, as it may need undercutting to avoid involute interference.

Andrew

If it's scrunched once, will replacing it with a printed gear give a better life expectancy?

Info from the OP as to why it failed would help us devise a reliable fix please.

Simon

Edited By Simon Williams 3 on 14/12/2016 23:45:09

Edited By Simon Williams 3 on 14/12/2016 23:45:35

If you are going to print it use ABS, it's far stronger than PLA and doesn't degrade

Hi Guys the whole gearbox is plastic. It got damaged when hit by a ball that shouldn't have been thrown. They are a common failure component looking on Ebay other sites available. as most of the same projectors have a failed gearbox. The diameter of the large gear is about 34mm by 3mm high. The smaller gear is 6mm high.

Not wanting to throw water on the bonfire, but with those dimensions, you may struggle to achieve decent tooth definition on the larger gear if 3D printed.

With a diameter of 34mm, that means a circumference of about 106.8mm and tooth tip to tip distance of about 1.55mm.

With a typical extruder diameter of 0.4mm and the fact that you will struggle to achieve a crest or valley that is not severely rounded at that resolution, I have my doubts as to how useable the print will be.

The 8 or 9 tooth part stands more chance of success as the teeth are much more defined.

I have printed several gt2 gears/pulleys that have 2mm tip to tip spacing, anything under 20 teeth was unuseable as the tooth definition was little more than a surface ripple and even with 75% of the teeth being wrapped by a belt, it still slipped. Pulleys of 40 or more teeth were useable, but only because of the high number of teeth being engaged by the belt - your application will be in a gear train, where only a few teeth will be engaged.

With the original being injection molded, a replacement may be better being cast from epoxy/resin after making the two halves out of some other material and making a mold with them.

Begs the question - how much is the replacement part vs cost of materials and time to produce a suitable replacement? Yes, 3D printing the part will be cheap, probably pennies in material costs, but will the printed part actually be useable?

You stand a better chance of 3D printing if someone has a powder based printer as this will achieve the required definition on the teeth, but I would then worry about the strength of the individual teeth.

Edited By Zebethyal on 15/12/2016 09:01:39

Is the exact gear size critical of is it just for a fan etc?

You can get a 70t 0.5 mod delrin gear HERE for £1.89 and you get a few 9T ones in these sets for <£3 (unfortunately the largest gear is 56 teeth).

It should be possible to bond them together somehow

Connection with schools - normally there is a network of connections between teachers. Are there no schools left in Leicestershire doing whatever came after CDT? Should be a sixth form wind-down xmas project - "oi, you , mend this or do extra PE."

It is going to be the slide changer I expect. Rebuild with Meccano gears is another possibility. Good to see you are still using the old systems and not gone all HDMI projector.

It's the gear mechanism which lowers the Mirror head on a short throw projector. Unfortunately it's not possible to bypass the assembly as the projector won't stay on without the gearbox working.

i am now wondering if one made with aluminium might not be better. I can do that myself. Any thoughts folks?

Edited By Bodgit Fixit and Run on 15/12/2016 14:38:51

Aluminium gears are tough and easy to make, that route would be my choice

Nothing wrong with alum gears for the application by the sound of it, but given it's a compound gear it may be hard to fasten the alum gears together reliably, and it would be hard to make them in one piece. There isn't much room for c'sunk screws or other fasteners, and I am not thrilled about epoxy on alum to alum joints. Depending on press fits in joining two alum gears would be a little risky too.

It may be easier to make each gear in brass and soft solder them together. If one has a locating spigot fitting in a snug hole on the other, accuracy of centres will be good. A tiny bit of solder and a good flux between the parts and it will be easy to join them. Brass is as easy or easier cutting than aluminum. (ask the clock blokes)

The result will be much better tooth form and strength than a 3D printed plastic gear, and soldered brass gears are easier to join together than two alum gears for good strength and reliability.

Metal gears in the first place likely would have survived the thrown ball!

JD

Oilon is specified for gears. Easy to cut and self lubricating.

Hi

I do have 2 x 3d printers at home. I have tried printing gears before but not had a great success in the smaller sizes. As already said in the thread the tooth definition does seem to disappear. If you put the STL file somewhere i would have a go but couldn't promise the result you may want.

I think you may be better going to the the 3Dhubs site. **LINK** It is a network of people with 3d printers who run jobs for people like yourself. You will be able to find a hub local to your location and maybe consult with them and obviously collect the result in person.

regards

Steve

+1 from me for brass two piece construction - make the small gear on a stub, make the bigger as a plate wheel and soft solder it onto the stub of the smaller. If using ABS I'd do the same, but solvent weld the two bits together.

Brass is my favourite though. If the OP is right and ebay is rotten with poorly projectors all of the same fault, why here's a little money spinner for someone!

Rgds Simon

Hi If you opt for Alu, mark the larger gear's 0 start or tooth 1 mount this on an appropriate bolt / axle place the second smaller gear and glue together so both 0 points are correct then remove axle / bolt, then drill and tap right through both gears either side of the center hole glue an appropriate size machine screw right through both gears, cut off the excess neatly each side of gear cluster and Roberts your uncle.

Rgards

Rufus

Make from aluminium and add a spigot to the small gear. Glue together with permanent type Loctite. (You could also sdd scotch keys (screws) for extra support if required.)

Paul

Some folks will know I'm reviewing a Dremel 3040 3D printer/using it for a 'first try at 3D printing' article. It only does PLA out of the box, but I've had a go at Bodge's gear, using his STL file.

Without spoiling the review, here's the result. It's less than 35mm across, so I'm impressed by this, if not by my photography. It's not imaculate, but I imagine it will work:

<edit> The larger gear meshes perfectly with stock 0.5 mod gears

Edited By Neil Wyatt on 04/01/2017 17:11:54