Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length.

[OMK]

Gents, first of all, apologies for this being a non-boaty question.
If you haven't already recognised it, what you're looking at is part of the tuning system from a bog-standard AM radio. The black circular piece is just an ABS cog with a smaller cog molded to the top. I need this top cog somehow removed, made flush with the lower one.
Would any of you know a good way to do this? Would anyone with the appropriate tools be willing to have a stab at doing this for me?

[Shipmate60]

PMK.
Is it a single moulding and what are the tollerences for the removal of the top part?

Bob

[wideawake]

Basically it depends whether you have access to a lathe I think.  If you do then easypeasy - if not it'll be tricky.  If the tolerances aren't too tight then I'd try using a hand drill as a makeshift lathe.  I can see a screw head in the centre of the gear so pass a screw through and tighten a nut on it to hold the gear firmly, mount in the drill chuck, run the drill fairly slowly to avoid overhetaing the plastic and shave the unwanted part off carefully with a sharp chisel or similar.

HTH

Guy

[OMK]

The sketch barely gives enough detail.
Regarding tolerances, the center 'core' of the top cog is a tad lower than the cog itself. I can't get in there with neither the u-meter nor the caliper, other than measure the thickness of the lower cog (0.08"), plus 0.09" top cog thickness. I would guestimate the 'core' to be approx. 0.06".
Not that that matters - it all has to come off.

0.09" is the tolerence target.

[OMK]

"I can see a screw head in the centre of the gear so pass a screw through and tighten a nut on it to hold the gear firmly, mount in the drill chuck..."

Thanks, buddy. It's a nice answer but I don't think it'll work because the hole on the underside is anything but round. If you squint another grok of the photo you might just make out where two sides of the brass spindle on that tuning capacitor are filed flat. The underside of the ABS cog is molded to suit that brass spindle. The two flat sides means it's night impossible to get an extension shaft on there. It might work if it were somehow possible to lathe/mill/gouge out the offending bit of molding first.
The drill press chuck goes to 13mm, but no matter how I adjust it, it just won't grip that 2mm shoulder on the underside.

[Shipmate60]

PMK,
You can do it by hand, but you will have to be very careful.
You can get a piece if glass and ensure it is glued or fixed to a flat bench.
Lay a new wet piece of fine wet and dry on this surface and gently rub the top gear off.
Checking frequently and adjusting the weight to get a square, flat surface.

Bob

[OMK]

"Lay a new wet piece of fine wet and dry on this surface and gently rub the top gear off."

Nice thought, but precision is the order of the day. The 'shoulder' in the sketch also protrudes through to the top side. It has to stay there. Don't forget the bit I described as 'core'. It abutts to the upper shoulder (read molded). Using glass paper might end up with the shoulder being out of true.

[wideawake]

"I can see a screw head in the centre of the gear so pass a screw through and tighten a nut on it to hold the gear firmly, mount in the drill chuck..."

Thanks, buddy. It's a nice answer but I don't think it'll work because the hole on the underside is anything but round. If you squint another grok of the photo you might just make out where two sides of the brass spindle on that tuning capacitor are filed flat. The underside of the ABS cog is molded to suit that brass spindle. The two flat sides means it's night impossible to get an extension shaft on there. It might work if it were somehow possible to lathe/mill/gouge out the offending bit of molding first.
The drill press chuck goes to 13mm, but no matter how I adjust it, it just won't grip that 2mm shoulder on the underside.

I hadn't noticed the flats!   Still possible though I'd have thought.   Get a bit of brass rod of the same diameter as the drive shaft on the variable cap.  Then mill (if you have a mill) or hand file the flats so that it fits into the gear.   Centre drill, drill and tap any convenient thread in the end with the flats to take a clamp screw and washer then proceed as I first suggested.   If that isn't accurate enough I'd suggest looking for someone close to you (model engineer?) with a small lathe.

HTH

Guy

[Rex Hunt]

Without wishing to appear too stupid........why not just use a flat file?

Rex

[OMK]

Because, regardless of how steady your hands are, there is every danger of the file removing parts of that shoulder. Glass paper/wet & dry/files, etc, would almost certainly do the job, but even more certain to mess up the shoulder piece.
Guy's suggestion would probably be the best way to go, hence this plea for any modeller's out there with the appropriate tools to have a bash at doing it on my behalf.

[wideawake]

Because, regardless of how steady your hands are, there is every danger of the file removing parts of that shoulder. Glass paper/wet & dry/files, etc, would almost certainly do the job, but even more certain to mess up the shoulder piece.
Guy's suggestion would probably be the best way to go, hence this plea for any modeller's out there with the appropriate tools to have a bash at doing it on my behalf.

Hi PMK

I'd offer to help with it, given that it was my suggestion, but my model engineering w/shop isn't accessible ATM.  I live a somewhat peripatetic lifestyle between my nb in the Midlands, my partner's place in East Sussex and my cottage near Cardiff.   I've spent very little time at the cottage recently and that's where the w/shop is set up ATM with my model boat w/shop at the mooring.  The plan is to move the lathe, mini-mill etc to East Sussex early next year.

Cheers

Guy

[SteamboatPhil]

Hi PMK,
I would be happy to do it for you, PM me and i'll let you have my address
Phil

[OMK]

Guy, as you see, SteamboatPhil has stepped in. Many thanks for your offer to help, but it's looking like I could be on my way to being out of the woods.
Thanks again, and good luck with your house move next year.
Thanks also to Mssrs Bob and Rex.

Phil, I just Rx'ed your post. You're a star.
PM heading your way. Standby.............

[sheerline]

Hi PMK, it sounds like Steamboat Phil has got this in hand but as I just read this post, I thought I'd just throw in an idea if it helps anyone.
If a plastic or metal billet is faced off in a lathe to form a mandrel and a small centre turned on it to fit the centre of your cog, three or four tiny  holes could be drilled in the large cog face and used to attach it to the mandrel. You would now have a nicely attached and centred gear which could then be machined to clean off the upper cog and leave your centre untouched.
Might be a useful idea and I guess Phil has already sussed it anyway.

[OMK]

Hey, Chris. Thanks for chipping in.
As it stands, the job is now in the hands of Phil. He's probably thinking along the same lines as you already, so whichever/whatever/however he decides to go about it I shan't interfere. I'm just grateful that he's offered to do it, to let him carry on and work his magic (lathes are a black art to me).