The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => Lucas, Ignition, Charging, Electrical => Topic started by: RichardL on 07.09. 2020 17:10
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Starting the rebuild of the magneto for my A7 and looking for comments and advice on the dimple in the earthing brush comtact surface. All free-form thoughts, advice and humor are welcome. To me, the dimple on the rotating contact surface (about 5-6 thou deep) seems like it might coincide with where the spark fires due to the slip-ring contact.
Looking forward to ideas for fixing, or, for that matter, ignoring it, if anyone has done so with successfull results.
Richard L.
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well I don't know a lot about these things, except what I've just found out earlier with copious amounts of oil in mine, but before I put the comp maggy back on the bike I saw the similar or maybe bigger dimple again that has been there for umpteen miles------ actually a great big crater in the brass on the slip ring and it hasn't bothered the bikes performance or starting so I forget about it.
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People turn the brass down in a lathe, to make it level with the bottom of the dimple.
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What TT said.
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G'day Fellas.
But why is it there?
Cheers
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It's the place the return current lands up when the mag fires. So there'll be 2 dimples on a twin often.
Turning true is the answer. But don't go too hard at it, don't want to break through the brass and hit the steel dowel pins lurking under the surface. In very severe cases the dimple is deeper than the brass depth available to machine, so the best thing to do there is skim to the limit and then smooth out the 'on and off ramps' of the dimples with emery or whatever to make the best of it.
The dimples come back though - they're like acne.
A secondary earth brush on the rear of the cb backplate can maybe reduce the dimpling where one's fitted or could be fitted. On many mas there is ONLY a cb mounted brush and the armatures remain unscathed.
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Thanks guys. I was pretty sure the prescription would be turning, but thought I'd ask about the alternatives. Turning would be a breeze if the membership DIY machine shop I used when I did the timing-side-bush had not gone belly-up due to the pandemic. I always thought they were on thin ice. They went under pretty early-on.
The ramps on my dimple (calm down) are somewhat smooth already, but maybe just right for the brush to use the ramp for a motorcross jump. Groily (or anyone, for that matter), do you know the approximate total thickness of the brass cylinder?
Richard L
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... But don't go too hard at it, don't want to break through the brass and hit the steel dowel pins lurking under the surface.
Uuuuh, I guess wall thickness is not the issue. Do you know the approximate depth of the dowell pins?
Richard L.
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Could that spot be built up in brazing without frying the coil?
Richard L.
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Cannot give the depth but the photos show my armature drive end when replacing the capacitor.
It is a stainless part but the dowels are the same place on the brass ones.
I think the second photo shows it had been trued up.
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Juslian,
Thanks, so much. Very enlightening to know the sit inside the wall thickness.
Richard L.
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Back in the mists of time, when I thought I had the last remaining armature in the world, we stripped the drive end from the armature, removed the condenser and made a good repair with silver solder. Bronze welding was another option but we went for the least amount of trauma to the part. A light skim and a polish made it good as new. Removing material from the running surface will extend the carbon brush from its holder, whether this is of any consequence until the brush shortens enough to pop out is one of those things. The brush under a brass type points carrier, as suggested, sounds a much easier fix.
Bit different now, there seem to be more magnetos and parts available in various states of disrepair or running order than ever before.
Swarfy.
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That would be a special racing armature, Julian?
I've got an N1TT magneto with a stainless drive end.
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Yes it is from a K2FR.
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The ramps on my dimple (calm down)
*smile*
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Juslian,
Thanks, so much. Very enlightening to know the sit inside the wall thickness.
Richard L.
I may have jumped the gun in considering the equivalence of the pins on Julian's steel earthing cylinder with those on the brass version. In the latter, the pins Groily mentioned appear to be the small ones holding a brass cap on the brass contact cylinder (where my left index finger is pointing). Those pins appear to be about 0.050" from the surface, still OK, I believe, but something to watch.
Richard L.
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OK, finally able to report back on this. Since turning seemed the best answer, as validated by Mr. G. ("Mr. Bill"? Oh, no!), I set about trying to figure out the best way to do it with limited skill and machinery. First problem was protecting the drive-end threads while chucked-up in my drill press (aka, poor man's lathe). After a bunch of head scratching and searching my garage, stumbled on my drawer full of electrolytic capacitors. Aha! Aluminum body of the cap would serve as a sacrificial sleeve. (When finished, there was a very minor impression on the peaks of the thread due to the chuck jaws, but of no concern to me.) Next, how to stabilize the points end without the benefit of a live center. Solution, small 1/4"-drive socket attached to a breaker bar in a vise clamped to the drill-press table. I was a bit surprised how stable this turned out (and turned).
With the "lathe" set up, it was a straightforward process of using ever-finer files as the cutting tools. Actually, worked my way up through four coursenesses before settling on the main meat remover. Turned out there was no real threat of breaking through to the dowel pins. I'd show the photo, but it's basically the same as the one I previously posted.
The first photo shows the sacrificial thread protector before trimming to length. Second photo shows the "dead" center in the vise. Third photo shows the rig ready to spin. Fourth photo shows the main cutting file and the dimple after the test attack. Fifth photo shows the result after a final run of 600-grit paper. Sixth photo shows a small amount of chamfering at the points end due to the dead center. No problem fitting the brass points plate. (In retrospect, was that chamfer there before I started?)
I have a video of the armature spinning, which I might post to YouTube. If interested, watch this space.
Apologies if I'm irrationally satisfied with myself for getting this sucessfully done.
Richard L.
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don't worry about the points end Richard you got a result by using the tools available , I say gooood lad *beer*
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Thanks, Bergs.
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Just re-read most of this thread and realize that, basically, everyone recommended turning, not just G. Everyone, you were right.
Richard L.
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Really neat job Richard! Resourceful as ever.
Why not invest just a few Budweiser and Rye vouchers in getting hold of a small lathe though - you'd love it, especially now that the DIY & Teach Yourself shop you were using has closed down?
Life has many milestones, and for me Getting a Lathe was right up there. Of course, I'd like a bigger better one, and I'd like all sorts of extra refinements, but a basic one (as long as it has screw-cutting and backgear) covers 90% of anything we ever need. Over there, you guys can pick up elderly but decent South Bends and similar for sensible $$$, and the investment is repaid very quickly. Not to mention the huge saving in time when it comes to not needing to order up and wait for things one can make for oneself - often out of better materials as well.
Occupational Therapy for the middle-aged, that's what they are! Much better than basket-weaving . . .
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Totaly agree Groily. Richard you are in a country that simply oozes machine tools of every kind. Ive had a Boxford AUD ( South Bend 9A clone ) for 30 years & wish I'd got it 15 years earlier.
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Have been doing things to magnetos... My brass contacts were missing the little pip that the retaining arm locates on.These things as we know are like hen's teeth and the after market ones don't fit. In a moment of inspiration, I snipped the end off a plastic syringe and tried that. Not perfect but it seems to do the trick. Sorted, hopefully.
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G'day Richard.
Well done mate. Necessity is the mother of invention.
I have done similar many times, but yesterday I picked up my new lathe. *woo*
Cheers
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Hi All,
Well done Richard, reminds me of times past when I was latheless *eek*
For the last number of years I have had a second lathe, but a couple of months ago a gear failed in the apron gearbox of the Colchester, Of course when I dismantled the gearbox a myriad of other issues came to light
I now have all the bits to rebuild it just not finding the time. I have to plan the sequences of work a lot more carefully when down to one machine
I really miss not being able to do a quick job when there's a half done piece in the running lathes chuck
Am I just "lathe selfish" *conf2* *conf2*
John
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Never been a fan of Colchester. OK when new, but seemed to deteriorate fairly quickly. Now, if it was DS&G or Holbrook ......
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Hi RD,
Yes they be would be nice to have one but the weight of either would test my workshop foundations *eek*
John
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I bought a small chinese lathe 15 years ago after wanting one forever and can't believe how handy they are. First job was making a mandrel to press in gearbox bushes and since then I enjoy making special tools and parts.
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Having paid the appropriate import license fee for the use of the word, I'm quite chuffed that folks I respect appreciate my hobbyist effort to overcome my shop shortcomings. Thank you.
Not sure in what price range you guys are seeing the abundance of used lathes in the US, but, on eBay, the pickings are not that great. Yes, there is the occasional bargain, but the seller has moved it 2000 miles away just to thwart me. Otherwise, it'll be a giant, greasy old Leblond or Warner Swasey born during WWII and requiring me to buy a new house with twice the garage space in which it might sit. That cost might just overcome the bargain for the lathe.
Richard L.
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This one should do it (no, it's not mine) ....
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*lol**yeah*
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Now, if it was DS&G or Holbrook ......
Ooh - I used to be on a Holbrook when still a boy.
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G'day RD.
Reminds me of the same size Dean, Smith & Grace I did my apprenticeship on.
Richard, it's surprising what you can do on a bench drill. I surfaced an alloy head with a fly cutter mounted in the chuck. Who needs a mill!
Cheers
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Hi All,
Richard
That's the way, a small size lathe with decent features costs way more than a large ex workshop one
The law of supply and demand *eek*
Last evening I saw a clean looking 13 x 42 DSG *dribble* *dribble* for sale here in Ireland but its 300 odd miles away, and the latest Covid lock down regulations limit travel
There are quite a few You Tube guys posting on lathe work and you can lean a lot from them
joe pyzynski , Abom79, bbloc, Keith Fenner , David Richards are worth watching
John
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This one should do it (no, it's not mine) ....
Perfect for turning up fork staunchions for the chopper!
Richard
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Know what you mean, John. My B17 Holbrook weighs in over 3 tons and is one of the best lathes around for screwcutting / taper turning but a bit of a bugger when needing to turn something 2 mm or less in diameter. Could really use a small lathe at times, but they cost more than I paid for the Holbrook and I resent paying unnecessarily high prices. The choice is limited, too - either worn-out Myfords (when you can find them) or new Chinese models. Not keen on the layout of modern lathes, though. Guess it's a case of what you're used to. In my case, Harrison, Triumph, Colchester, Myford, Ward (capstan) etc in my early years
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Close to the end of my mag recondition and EasyCapping it became obvious I would need some body shims. Boy, did my garage get clean looking for them. Not sure if I lost them or if they were never there. I could have bought them from a UK source, but kinda expensive and maybe 10 days to get here. On advice from Groily, decided to make my own. Now, this would almost not be worth mentioning, except for the material I stumbled on in the auto parts store, specifically, 0.003"-thick adhesive-backed aluminum strip. I think the pictures tell the story: set the body on the strip, it sticks, trim the shim. I used the back of a spoon to smooth it out after trimming. Turned out I needed two pieces. Armature turns free and end float close to zero. Net cost for 25% of the roll, about $1.25. Many thanks to Groily for good advice.
Second to last photo shows finish from using my Harbor Freight soda blasting tank.
OK, last picture is a home-made tool for a different mag rebuild task. Call it a guessing game.
Richard L.
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Richard. That use of Aluminium tape is something I never considered, but I'll certainly give it a go next time. I have some left over from insulating internal stud walls with polyurethane slabs. The tape is used to bridge and seal air gaps between the slabs. A good use for leftovers. Cost was about £8 for 50 metres from Screwfix or Toolstation.
Swarfy.
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Don't know what the Runcible Spoon is for Richard (maybe you use it as a lever to hold the armature spindle by the little depression while setting the timing or something?) - but the ali tape thing is very good thinking!
Self-adhesive stainless steel tape, available in a lot of fine thicknesses, is also excellent stuff - for centralising camrings in sloppy housings and getting firing intervals and points gaps back to sensible.
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To find something like this aluminum tape in an auto parts store you must avoid all offers of help from the staff. Wandering the aisles in deep meditation is required.
The object in the bottom photo was originally the extension handle to a hand-held vacuum, not the common spoon I used for smoothing (which could have been anything), but happy to learn of the Owl and the Pussycat reference. Used in the rebuild, not for timing. I'm sure this will become a "so what" to those with more than a couple of rebuilds under their belt.
Richard L.
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Since any intelligent person has lost interest by now, I may as well reveal that the tool in the bottom photo is shaped to fit inside the drive-end bearing outer race. Thus, the tool is used for inserting the race inside the insulating washer without the magnet interfering.
Richard L
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Lacking in intelligence, I'm well into this. I had decided to turn up a bit of broom handle or similar in the wood turning lathe to do the same. Being particularly idle, I dug out a spare body with a good bearing and oil seal and used that instead. It did the job.
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To find something like this aluminum tape in an auto parts store you must avoid all offers of help from the staff. Wandering the aisles in deep meditation is required.
Yup, the UK standard outlet for motoring bits is Halfords, a national chain. The staff trying to be helpful will ask what vehicle you want a spark plug for. Telling them it's for a 60 year old motorcycle and it will definitely not be listed in the manufacturers catalogue causes them to go out the back and have a little cry.
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My eldest who also rides my bikes, works at the local branch. They love it when he takes a bike to work and he is trying to ensure the staff there at at least know these old machines are still out there and being used.
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I rode my 1913 Douglas to the local agricultural engineers to get some tractor bits. The storeman came out, looked over the bike and said we do have proper plug caps you know. Don't think they would look right on the Douglas.
On the same bike, setting off in the local town one sunny day I got a round of applause from bystanders as it puttered into life. 2 gears, no clutch, every start is a push start.
It's really important that old bikes are seen out and about. Anyway, I just love riding them.
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Fascinating story and wonderful bike, but, possibly, less fascinating than when the lady behind you in the yellow dress and her dog got on the green bike that was also behind you.
Richard L.
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Wow, that Douglas is superb! *smile*