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11-28-2009, 11:22 AM #1
Broken gear teeth - info on repairs?
I've got a backgear with a broken small tooth, and a QCGB idler gear with a broken tooth. I attempted to silver solder in filler on the idler gear today, with possible success - won't know until I get into machining it tommorow once it's cooled. These are both from my 10K frankenlathe.
Now, given the pain-in-the-assery of the silver soldering today, I'm thinking it would be much easier and more effective to just weld in some 308 stainless filler with my mig welder. With a nice long preheat, of course, and a long slow cool down.
Anyone have thoughts or suggestions to offer? I don't have an oxy-acetylene torch, I use a couple of propane/MAPP torches to silver solder.
11-28-2009, 12:01 PM #2
I would be tempted to try it. I have fixed lots of gears when i was a Millwright. The SS might be a little hard to file...BobBob Wright Metal Master Fab
Salem, Ohio Birthplace of the Silver and Deming Drill, all others are copies.
11-28-2009, 12:19 PM #3
Let us know how it holds up. I am in the same situation and could either do exactly the same as yourself with MAPP, or TIG in some filler. I was considering the latter but considered the disaster potential very high
11-28-2009, 12:59 PM #4
Brass works as well. Good wear charactistics, easy to machine, strong enough (especially if you are only fixing one tooth).
11-28-2009, 09:11 PM #5SmithSolar Guest
Best way is to make a new one out of 12L14
Next best braze and machine the tooth
11-29-2009, 04:33 AM #6
fix gear or buy new
i assume you used silver brazing rod not silver solder.
to avoid noise and damage to rest of gear train are you machining the gear teeth ? with a dividing head ?
i was missing a change gear on a atlas lathe. easiest thing was order a "boston gear" from Kaman (local distributor) who shipped and i received in 48 hrs. i just had to bore it out to bigger ID.
11-29-2009, 06:24 AM #7
I'd love to just buy a new gear, but the cost of restoring this lathe is starting to build up, and I'd like to save where I can. Does anyone know the DP of the gearbox gears?
Also, my plan for machining the filler was to make a sort of fly cutter with a slightly narrow shaped form tool to rough it out. I can use the teeth on either side to guide and then I can file or scrape it in.
11-29-2009, 06:27 AM #8
I have repaired a few C.I. gears and a couple of racks that had a tooth missing buy drilling the broken gear radially (or perpendicular to the rack) and pressing in a piece of brass round stock. File or mill to suit. Wide gears taking multiples. Simple, clean, and no heat required. Works for non-metalic gears too, There is just such a repair on the timing wheel of my tractors magneto.
I like the brass for it's ductility and toughness. Though I have not done any "load calculations".
11-29-2009, 06:59 AM #9SmithSolar Guest
I think it is 18DP
This not hard to make if you have the tooling
Take about 1 to 2 hours to mill the gear of this size each tooth take about 1 min to machine after it setup in the mill.
A small gear is just fast and better just to make a new one If this was a large gear brazing a new tooth would be faster.
11-29-2009, 08:08 PM #10
Did a bit of looking up, and the gears in my 9A gearbox appear to be 16 DP - used a formula from this page (http://www.gizmology.net/gears.htm) to figure up the DP. Based on the root diameter, these gears appear to be 20 or 22.5 degree pressure angle.
Interestingly, the pitch is the same as the backgears.
McMaster sells part 6325K18, a 32T 16DP 20* gear, 1/2" wide with a 1/2" bore for $27. Machine off half the width of the teeth, bore out for the press fit of the needle bearing and you're in business.
I left my backgear assembly at my in-laws, so I am unable to verify the pitch of those gears, but the gearbox gears mesh as well with them as they do with each other.
Still going to try repairing what I've got.
11-30-2009, 06:54 AM #11
I broke some teeth recently and managed to find them so my plan is to silver braze them back on. I also have an involute gear cutter that will work for the gear in question (because I am going to cut some other gears from scratch) so I figured I would use that to clean up any excess braze. There is also an area of 3 badly worn or partly chipped teeth on the same gear that I may try to build up and then re-cut.
If you are only repairing 1 gear, then you can probably afford to buy an involute gear cutter to do the job. The trouble is they have a range of teeth numbers that they will do so you need 8 cutters to cover the full range of gears, but I reckon just buying the odd one as you need it will be affordable.
Because other teeth are OK you don't need a dividing head or even a rotary table, you just need to devise a way of indexing and locking the gear based on the other teeth. Obviously this is easy to do in a dividing head or rotary table but it should be quite feasible to knock up an arbour with an arm for the indexing which can be locked in place with thumscrews or something.
11-30-2009, 07:05 AM #12
The pressure angle for SBL gears is 14.5°.
The easiest way to determine the Diametral
Pitch in this case is to count the number of
gear teeth, add 2; Then divide the total by
the outside diameter if the gear.
The backgear should be around a 16 DP and
the gearbox gears should be around 20 DP
11-30-2009, 02:07 PM #13
Webb, not to start an argument, but my gearbox gears match the pitch of my backgears - the left hand gears are definitely a finer pitch, however. Since you say the SB gears are 14.5 deg pressure angle, then the McMaster gears should be a perfect match. I think I'm going to go with one of those at least for the back gear - my only concern is the gear will only be 1/2" wide rather than 5/8". They are steel rather than iron, but the mating gear on the spindle will still be iron of course.
11-30-2009, 02:24 PM #14
If it's just one tooth I can't see why the repair can't just be filed to profile. I had intended taking an impression off other good teeth and simply filing to fit. Sure a bit fiddly, but I've seen plenty of other gear repairs that were done this way, and sure as heck cheaper than buying a specific cutter to do a total of 2 cuts! It is, after all, a back gear, not a Swiss watch we're repairing
11-30-2009, 02:39 PM #15
This is the most common method I have seen for repairing gears. I have also seen the broken tooth filed down and drilled for two steel pins. The pins are pressed into the gear and then ground to the tooth profile. Just some ideas.
Welcome to the club man repairing these old South Bends can get disparaging some times, just take it one piece at a time and you'll be proud when you fire it up for the first time. You kids and grand kids will be too.
11-30-2009, 02:41 PM #16SmithSolar Guest
10 year ago a quoted for cutting gears upto 16DP $10.00 for setup and $1.00 per tooth (upto 1/2" wide) and cost of a cutter in 12L14.
11-30-2009, 03:27 PM #17
I was wrong on the DP in my earlier post (too many
different lathes). But I did some research and the
DP for the gearbox gears is supposed to be 18 (not 16).
11-30-2009, 07:14 PM #18
I have an old post up on a gear repair that I made. Rather than repost it, you can look it up.
Strangely enough it is titled "gear repair" and dated 12-22-07. I hope you find it helpful.
11-30-2009, 11:48 PM #19
I fixed the small end of a Southbend 405 back gear by machining off all of the teeth, then buying a new steel gear for around $20 at McMaster-Carr. I bored the ID out to a .001 press fit, and installed it using an arbor press and red locktite. Worked out great!
12-01-2009, 05:43 AM #20
Dullvinci: Actually, bentwrench emailed me a link to your thread, I'm thinking that's the way to go for the idler gear repair.
Arclight: I'm assuming that gear you bought was also 1/2" wide like they currently sell? No problems with it? I'm thinking that's the way to go for fixing the back gear, as it'd be very tragic to crack or otherwise break the large gear on that cast assembly from heating/cooling it.
Webb: Was there any time period when SB did use 16DP gearbox gears? If I put in the gear parameters (32T, 18DP 14.5deg) into that calculator, it says such a gear would be 1.88" OD - mine is definitely 2.125" OD. Model on the gearbox is 644Z, and there is "80" stamped on the boss where the stud gear shaft comes out.