Truing up the worm wheel

[skybadger]

Hi all

I am about to embark on a task to true up the dec worm wheel. It seizes against the worm on the other side if I adjust it for best fit on one side, so its clearly not concentric with the shaft.

Its a Beacon hill 8" wheel which is fixed to the mounting bush using three hex screws. My plan is to turn up a stub mandrel on the lathe to fit, fit the wheel and dial it into concentric using the dial gauge.

Any thoughts or issues before I commence ?

Is there an argument to doing this on the scope instead of the lathe ?

Cheers

Mike

[ChrisLX200]

Depends a bit on how the wheel was made - cut or hobbed. You're not actually going to able to get a DTI onto the relevant surface (i.e., the load bearing faces of the teeth). If you think you can improve things then by all means have a go but a better alternative might be to make spring-loaded worm carrier to hold it in engagement with the wheel.

ChrisH

[Peter Drew]

+1 for ChrisH's suggestion. 

[Stub Mandrel]

A 0.5 thou error in concentricity will give you a maximum 12-arc-second variation over a half rotation. This is a periodic displacement of 0.25 millionths of an inch...

It's likely that other factors make bigger contributions to he overall errors.

The sprung worm sounds a good idea, otherwise, two suggestions:

1 - lap wormwheel to the worm concentrating on the stiff section, but if the worm is steel and the wormwheel is brass the abrasive may embed in the wormwheel and wear the worm more than the wheel, which is not what you want...

2 - same technique as a griptru chuck - modify the fixings and register so the wormwheel has a tiny amount of play available to it. Fit the screws but don't nip up. Gradually advancing the worm into the wheel while it is making multiple rotations, let the wheel shift slightly to find its true position. Now nip up very gently and check.

I need to hob a new wormwheel for my home-made rotary able as I'm not happy with the existing one - I'm in awe of the accuracy of the wormwheels on even cheap scope mounts - if I can do half as well I'll be happy!

[ChrisLX200]

A 0.5 thou error in concentricity will give you a maximum 12-arc-second variation over a half rotation. This is a periodic displacement of 0.25 millionths of an inch...

It's likely that other factors make bigger contributions to he overall errors.

The sprung worm sounds a good idea, otherwise, two suggestions:

1 - lap wormwheel to the worm concentrating on the stiff section, but if the worm is steel and the wormwheel is brass the abrasive may embed in the wormwheel and wear the worm more than the wheel, which is not what you want...

2 - same technique as a griptru chuck - modify the fixings and register so the wormwheel has a tiny amount of play available to it. Fit the screws but don't nip up. Gradually advancing the worm into the wheel while it is making multiple rotations, let the wheel shift slightly to find its true position. Now nip up very gently and check.

I need to hob a new wormwheel for my home-made rotary able as I'm not happy with the existing one - I'm in awe of the accuracy of the wormwheels on even cheap scope mounts - if I can do half as well I'll be happy!

It sounds like you know what you're doing! Yes, you need to be working in '10ths accuracy really - depends on the diameter of the worm wheel, and you're absolutely right about the lapping of the harder material rather than the softer - most people often don't realise that the harder one gets worn during the lapping process :-) Nevertheless I had some success using Autosol Chrome Cleaner paste as the lapping agent when I made my own worms/wheels, just taking out the 'roughness' alone reduces the PE considerably. If the wormwheel is really so far out that it's measurably eccentric I would be tempted to bore it out and sleave it to correct the bore, but certainly your suggestion has merit and worth a try. The spring loaded worm carrier makes life much easier though.

ChrisH

[Peter Drew]

Fullerscopes used to spring load their worm carriers by inserting rubber washers between the mating faces. It seemed a bit "Heath Robinson" but it worked.  

[Stub Mandrel]

Fullerscopes used to spring load their worm carriers by inserting rubber washers between the mating faces. It seemed a bit "Heath Robinson" but it worked.  

That's actually a very good idea, as it introduces damping as well as consistent pressure.

[skybadger]

Thanks for the suggestions. I measured the run out on the available edge surface to be 50 microns. Or 5units on the dti where 1 unit is 0.01mm

So I have left it. The other thing I did is to adjust the coalignment between the worm and the wheel by0.5mm after measurement and that has made a difference.

The worm wheel is bolted to a hub so the intention was to true it on the bolts.I shall bear in mind the rubber mounts idea.

Cheers Mike