E/G for gear making ?

[greybeard]

Moving slightly away from the use of E/G for cnc frames, I have a need for my current 'project' for a large number of worm drives.

The project is to produce hammered dulcimers for children at a cost low enough for me to donate them to the local school.
The wooden parts will be made on my cnc, which has just about a big enough work space to do it.
Traditional designs rely on the tuning pins being gripped so tightly by the hardwood sides that they resist the torque produced by the strings.
This of course means that the child needs to overcome this same torque in order to tune it, and sufficient control of the handle of a tuning wrench to get the tension just right.

I decided to avoid this by using the same tuning mechanism that is found on guitars.
Unfortunately this means I need about 30 worm drives per instrument.

Currently the cheapest ready made item I can find is about £0.70 per string from Hong Kong, and while this is a great price for what they are (I have a set of 6), there is no way I could reproduce them in steel and brass with my limited facilities.

It could be that I finish up using the Chinese supply, but I thought it would be fun, and possibly very useful to others, to see what potential there might be in looking for other solutions.

As readers may know, I have done some experiments with spin casting E/G, and remembering that the resulting surface was "glass hard", it put me to thinking that casting a redesigned recipe aimed at this particular problem might be interesting, and possibly rewarding.

At the moment, I shall concentrate on the worm gear( the part similar to a spur gear, but with skewed teeth), and taking one of the guitar heads to pieces gives me a suitable original to try and copy.

I have a sample of polymorph (InstaMorph, Shapelock, and Friendly Plastic in the US), a nylon-like plastic that is soft and moldable at 60 – 65°C, and as it can be used in 3d printers, I assume that I can melt it with care.
This I intend to use as a mould making material ( though I haven't yet tested its release from epoxy resin), and the mould will be housed in a carrier that is fixed to the vertical shaft of a small motor.
Running at 3000 rpm will give about 100G at the periphery, which is why I'm choosing to use a rigid moulding material, rather than rtv silicone rubber. A cap screwed onto the mould will prevent the resin been thrown round the workshop, I hope, and perhaps a couple of minutes at this speed will be enough.

I will then transfer the sealed mould onto a pair of rollers running at 200rpm, along the lines of a stone tumbling set-up, to keep the filled epoxy in place while it polymerises.

What I am expecting to occur is that the fillers will concentrate within the teeth of the gear, and the excess epoxy(plus any air) will migrate to the centre. After casting, this will be drilled to allow the use of a fixing screw onto the shaft which ultimately carries the string(stainless steel wire in this case).

The temperature cycle for achieving full strength will have to be determined by experiment, as with such a small mass, it will be quite different to anything I've done before.

As to the epoxy recipe, I think something along the lines of Moglice, might be worth using as a starting point.
The approximate percentage ingredients of Moglice are: epoxy resin 40-80, molybdenum disulphide 15-30, graphite: 10-15, aluminum: 5-10, copper: 5-10, alumosilicate: 8-15. (quoting from the published patent documents).

That's the plot so far, and if you think that's unlikely, wait till I spell out my ideas to make the worm itself.

For anyone interested in the maths, the tension on a typical string might be about 3-5kgs, on a 4.5mm shaft, and the pitch diameter of the worm gear will be about 9mm.

John

[RomanLini]

This may not be of help Greybeard, but I'm going to suggest a different approach. My electric guitar has a Floyd Rose bridge, where the tuning is done by small 4mm screws, one for each string. The string is clamped then the tiny screw can be finger turned to give the correct spring tension.

You should be able to mass produce a system like this very easily, just clanp the string at both ends then have a fine thread screw to adjust the string tension either by direct pulling or by a slight leverage as used in the Floyd rose;

[greybeard]

Ave, Romanlini.
Thanks for the close up pic, and I'll see if it can be simplified, and at what cost
I had considered something slightly similar, and simply put, was to separate the fine tensioning from the anchoring and rough tensioning.
One 5mm socket screw with locking nut, and with a hole through horizontally to act as anchor and give rough tension.
The string then passed under the head of a second 5mm cap screw which was positioned between two bridges. Screwing this down would give the fine adjustment, in exactly the same way that holding the string down with the finger would.
Having come up with this brilliant idea, I bought a copy of Musical Instrument Design by Bart Hopkin and found a similar idea. So nothing totally new, but it did give good control over tuning with an allen key. This approach got rid of the torque problems, and got rid of the need for hardwood side frames.

However, as a method to produce gears in workshops that don't have facilities or skills to produce them by the usual methods, might have general interest, I want to concentrate on the gears, rather than my own needs specifically.

Mind you, if there's a philanthropist engineer out there..........please pm me

Regards
John

[brunog]

Hi John,
"Intuitively" I believe you are on the right track in using a mix a "moglice" type blend for friction for the ease of binding the strings. The other important aspect that needs to be carefully looked at is tooth geometry. You might need to use stubyer teeth geometry (or maybe not)

The part are very small for a start, at least you're not making pocket watch gears!!

Nice project idea, post a lot of photos!

Best regards

Bruno

[greybeard]

Hi Bruno.
You're absolutely right about the possibility of changing the tooth geometry to improve it.
Photos tomorrow, I hope, will show one of my earlier ideas for cheap worm shaft sources that has an asymmetric profile for the worm.
As the load is only from one direction, this will perfectly suit my application.

First tests have produced success and failure.
I can melt the polymorph in a controlled fashion to produce a bubble free melt, and it gives a perfect impression of my trial brass gear. Slight contraction on cooling will be tolerable for my job, though not perhaps for others, makes separation needing a bit of effort, but it comes off cleanly.

The failure was the fact that my first test using fast setting epoxy fused completely with the polymorph.
I didn't use a release agent, I freely admit, and that may be all that is needed.
When I move on to incorporate the various friction reducing fillers, that too may improve the situation, so I'm not too unhappy with the first day's trials.

John

[DMF_TomB]

i once made a custom led light holder for an optical surveying instrument that i used aluminum filled epoxy.
.
got it out of the mold happy til i dropped it and it shattered on the floor like an egg.
.
i ended up using autobody filler found easily in auto parts stores. filled with fiberglass it is much tougher and durable material. it can be packed in a mold at room temp and sets up fairly quickly. just remember to heavily use mold release like wax or it will stick to the mold.
.
could be the epoxy just had too much filler in it that made it brittle. mechanical properties at different temperatures and pressures can change. teflon behaves like rubber when thin but like hard plastic when thicker or under load it can behave like rubber again.
.
rulon a mix of fiberglass mesh and teflon ?? hot press molded is usually used for plastic bearings (or graphite filled plastic)

[greybeard]

Hi DMF TomB.

I'm not sure how I might use this info in my very small spin casting set up, but thanks anyway.
It may well benefit others.
Regards
John

[greybeard]

Just to make it a bit easier to follow my ramblings, I've attached some first photos of the Chinese guitar head taken apart, and a close up of the parts I need to mould.

This is what I need in functional terms, but whatever I come up with, there will have to be a compromise between reducing the torque on the gear teeth by making the gear larger, and the fact that I need to keep the distance between the strings reasonably small.
The hammered dulcimer has two or more strings close together for each note, and they are struck by the hammer at the same time. About 3mm separation would be appropriate.
Then there will be a separation between each pair of consecutive notes of about 25 -30mm.
So to get this sort of layout, with the pairs so close together, I shall have each of the two strings tuned from opposite sides, the result being that an overall size of each worm gear assembly must be no more than about 20mm.

A worm gear of 10mm diameter, and a worm of 6mm will keep me in the ball park.

[greybeard]

In parallel with the idea of copying the guitar head, I thought I should look for alternative starting points.
The usual method of starting with a piece of all-thread converted to a gear hob, and cutting my own gears to be used with the all-thread as the worm, was ruled out as the time involved in reproducing them in batches of 30+ would make it too much of a chore.

As it seemed that making the worm would be the most difficult, I looked for sources, and found - - the hose clip !
Ebay produced 10 key powered worm hose clips for £1.89, photos attached.

One shows the clip as bought, and the other shows the beginnings of a worm gear made from the metal band plus a couple of washers.
I think this just might be a serious contender, so I've included it for comparison, and it might be food for thought for other readers.

[Kevin_Johnson]

If you want to try something even lower tech then obtain 30 small socket head cap screws or maybe button head cap screws. Some steel angle iron can be used to mount them in. Drill through the angle iron and either thread the hole or tack weld a nut on the other side. The screws can be cross drilled near the head to accept the wire -- just like safety wiring aviation fasteners. A jam nut next to the threaded hole or tack welded nut could be used to lock/hold the desired tension. You don't have to use the hexagonal keyed screws -- they just look a bit more elegant. Hex headed screws would work fine as well. Good luck.

[greybeard]

Hi Kevin.
See #3
John
EDIT
As a bit of background info, this is where I'm coming from.

[Kevin_Johnson]

Hi Kevin.
See #3
John
EDIT
As a bit of background info, this is where I'm coming from.

That's pretty funny!

Must be a good simple design!

A suggestion: I think your basic problem then is the key used to adjust the torque. Make a T shaped key with longer arms and use TORX fittings (Allen at your peril). I think most children would have no difficulty developing enough torque to snap an M5 or even M6 fastener in half if they were using a two-handed tool with 6-inch/150mm arms (like a tap wrench). I have installed tens of thousands of high grade M5 and/or 10-32 studs in the products that I manufacture. The torque spec I give for for the locknuts is only 45 inch-lb. The torque spec is so low that the click torque wrench is a screwdriver rather than beam type tool.