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8th Mar 2021, 03:41 PM #1I break stuff...
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Any brilliant solutions for repairing this integral key?
Reassembling the apron on the Graziano, and I've discovered the powerfeed worm key has a little wear:
IMG_1501_1400x1050.JPG
The problem is, the key is machined as part of the worm. They've bored 22mm (plus a bit) from one side most of the way through, and then probably used a shaper from the other end in conjunction with a dividing head or rotary table to create a bore with a key. This is the layout of the parts:
IMG_1502_1400x1050.jpg
As you can see, where the key is sits right under the bearing seat and threads for the locknuts. There is less than 4mm wall thickness here, and those bearing seats are, slightly surprisingly at first, tight - dead nuts on 30mm. This had me bamboozled for a bit, as it seems illogical given the taper rollers don't go 'home' against the worm, they're preloaded by the locknuts. The fit being a 'light press' fit seems counterintuitive, as if you overdo the preload you've got to give it a whack with a mallet to back the bearing off and try again, but I believe it's been done this way so that oil doesn't weep through between the bearing and the seat, and then leak out the thread of the nuts. Not, in my mind the best design choice, I'm almost tempted to polish down those bearing seats to a sliding fit, and put an o-ring either in the chamfer on the back of the bearing, or put an o-ring groove inside the spacer that the seal runs on.
Nonetheless, I'm trying to figure if I can do anything at all with this key. I do have a torch for the TIG which is capable of getting inside a 16mm tube, so I could potentially build it up with silicon bronze and then use the mill or lathe to 'shape' it. Problem I forsee is it seems pretty likely the heat will at best pull the journal out of round, and possibly warp the whole thing....
The apron clutch lever uses the same arrangement for a key, but it's just a rotating surface on the OD, so someone has milled a slot through from the outside, and pressed a key into the slot. Theoretically plausible here, but the whole thing is hardened. I do have some carbide endmills that are intended for this kind of work, so that may not be a issue. The key would also have to be relieved for the threads to pass over, and some form of sealant would likely be required under the bearing, unless I can get the fit just so and use an o-ring as mentioned before... Not convinced it's overly viable.
The only other options I see are:
1. Don't worry too much about it on the grounds it's taken 60 years in an engineering shop to get to this point, and the worm is providing a 4.16 reduction, in addition to a 6.871 reduction inside the apron up to the rack gear (making for almost a 30:1 reduction in carriage feed mode if my maths is correct). In theory there's not that much load on it, other than the slop may make it 'bounce' under certain cutting conditions (although with that reduction would likely be a small decrease in surface finish?)
2. There is a guy in Italy who supposedly has some parts for these machines, but I believe his prices are higher than Colchester pricing...
3. Speak to someone like Elanjacobs work, and have a new one made.
4. As per number 1, except make a very careful drawing of the thing, and machine a new one out of some form of prehard at some stage in the future anticipating a failure. I don't think I have the changes gears to cut worms on the AL335, so would need the Graz running to do it (which I do have all the gears for, and a bunch of DP/module threads listed on the table)...
If the key DOES fail, providing it doesn't tear up the feed rod, it's possible to make up a kind of bush that slides over the two locknuts on the tailstock side, retained in the locknut slots by set screws. This would protude out toward the tailstock by some amount (say 40mm) with the appropriate 22mm bore, and could carry a replaceable key. The slight catch with that under heavy cuts it may actually tighten up the two locknuts over time and overdo the preload, but it would be A solution, as it would fit in the space and not interfere with anything....
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8th Mar 2021, 04:24 PM #2Gear expert in training
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Instead of having a new one made, we could simply wire EDM a keyway in the bore to match the one in the shaft and glue a key in with our magic aerospace glue (even 680 Loctite would probably do and it's removable with heat that way). Waaaaaaay quicker and cheaper than making a new one.
Assuming it's worn down the same shape all the way along, I wonder if you could even make up a "shim" of the right cross-section and length and just tack it in place on the ends with the TIG (maybe a couple of tacks in the middle if you can reach). I know nothing about welding, so don't know if that would still be enough heat to warp it.
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8th Mar 2021, 04:48 PM #3I break stuff...
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I could probably do that first approach via 'shaping' in the mill or lathe, but with less than 4mm wall thickness to play with, I'm not sure how well it would hold?
I did think about the shim option too - I could certainly make a matching 'half key' that is longer than the existing key, and wraps around the ends of it to hold it in place. Possibly even glue it in, I guess. The only problem with that is it's only really addressing the slop, and not adding any significant strength back into it. I'm not actually too worried about the slop, what I'm more concerned about is shearing the key off under a heavy cut...
Both good ideas though, something more to think about.
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8th Mar 2021, 05:14 PM #4Gear expert in training
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8th Mar 2021, 05:32 PM #5Philomath in training
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Can you wire cut in a shallow keyway and then secure a new with some (countersunk?) screws from the outside? The keyway takes the shear load while the screws stop it twisting...
Michael
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8th Mar 2021, 08:21 PM #6Most Valued Member
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I wouldnt worry about strength so much, if it shears then I'd go with the complicated repair plan. You are going to need to be making some very serious chips to shear it.
Id say the shim is a pretty low risk improvement.
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8th Mar 2021, 08:55 PM #7I break stuff...
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Not quite sure what you're meaning here, but I think you're talking about Scotch keying from the ends (parallel to the feed rod)? If that's the case, a very difficult ask. The amount of meat is worse than I thought - I measure the wall thickness at the threaded ends to be about 3mm, and there's at least half a mm of thread depth. So no room to put any sort of screws, and any keyway cut into the bore is only going to be 2mm deep at best.... Although with Elans idea of EDMing out it, a 2mm depth down the full length and glue is probably going to be stronger than the 25ish mm long key provided from the factory... I suspect the setup charge for a one off on the EDM might be fairly pricey though?
I have to say I'm leaning towards this approach, the one thing giving me pause is getting the bugger out later on. But it occurs to me it's probably possible without having to remove the apron, just with a spot of lying on the back and swearing loudly.
Another hare brained idea I just had if it did shear, along the lines of my previous would be to float a key longer than the bore, and use a pair of sleeves over the locknuts at either end with a screw to retain each end of the key...
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8th Mar 2021, 09:12 PM #8Most Valued Member
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J&H you could bore out the old flogged out key and make a new (half) key, slide the new key and gear onto the shaft and mark the gear to the sides of the key, vee/prep the ends of the key and the gear clamp the key in place, maybe a small jacking bolt, and then just weld the key on the 2 ends, then clean it up as necessary, face the welds and maybe a bit of file work.
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8th Mar 2021, 09:13 PM #9Gear expert in training
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The setup on that isn't a complicated one; hold it in a 3-jaw chuck (we have a stainless one for the wire machine), touch off on the insides to centre the bore and run the program. The feed rate will be fairly slow given the length of the part and the material, but the toolpath is short so I don't think it would take that long.
We're in Dingley, you could always bring it in and see if it's a) a viable idea and b) cost effective. The real engineers will know more than me about it.
Precision Machining & Engineering | R&I Instrument & Gear Co
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8th Mar 2021, 09:53 PM #10Most Valued Member
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How long is that key (in the axial direction)?
Silicon bronze in a MIG that might be a viable option. Gives a bit more reach than the TIG, and I think would put less heat into it overall.
I've still got mine set up here with a roll in it if you felt like a drive.
If you put the bearing on the shaft before any heating (perhaps use a sacrificial one you're not replacing it) it would help with stabilising it and act as a heat sink. Likewise I'd have the locknuts on there and tightened for the same reason.
Other than that, is there sufficient thickness under the bearing journal to be able to cut the threaded section off, bore it out slightly and internally thread - then make a replacement piece to screw in with new key and external threads (one to screw into the new threaded bore, and the other for the lock nuts)?
A close fitting thread and some permanent mount Loctite and it would never go anywhere.
Steve
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8th Mar 2021, 10:15 PM #11I break stuff...
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About 25mm long - probably a little longer, but less than 30mm. The torch I have for the TIG would reach it no worries, it's a CK MR140 micro torch. I was wondering the same thing as you, whether the bearing might keep the shaft round and the heat down...
The second option is probably a no go as suggested - sub 4mm wall thickness in the bearing seat, doesn't leave much meat for threads, and I'm not sure whether it's through hardened or just case hardened, so threading it might be a crap shoot. But I like your thinking. That option might be workable if I took the whole bearing seat off up to the shoulder on the worm gear, and threaded the inside of the worm gear (depending on hardness). Somewhat more involved than I was really hoping to get at this point, but certainly filing it away as a possible repair method later - I should have thought of that one, but didn't.
Simple enough, especially as there's no need to clean the ends up, if they come in contact with anything the worm is the least of my issues. But if it's just welded on the two ends of a 1/4" wide key, I don't see it being any stronger than what is currently there, which is about 3mm wide by 25mm long?
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8th Mar 2021, 10:58 PM #12Most Valued Member
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If I've done my sums right you're looking roughly 3 tonne to shear a 3mm x 25mm piece of steel, multiply that by whatever gearing ratio the worm gives and you get what Caskwarrior said earlier - you'd be making some serious chips to shear it!!
I'm picking there's likely a feed clutch somewhere anyway?
BTW - I looked them up and that's a cool micro TIG torch you've got there.
Steve
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8th Mar 2021, 11:06 PM #13Most Valued Member
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8th Mar 2021, 11:08 PM #14I break stuff...
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Yeah, I'm sort of getting to the point where it's all too hard and I'll just see how long it lasts, which will probably be a fair while as you say. At least I've got some good ideas from here how to fix it if needed.
Not to sure about there being a feed clutch as you're thinking of it. There IS a clutch arrangement for the feed stops as you've seen in the project thread, but I don't know whether that also provides 'overload' protection - seems logical that it would slip at a certain load. On the later SAG 14, they had an adjustment knob under the powerfeed selector, and I've seen some discussion that this seems to be an overload in that it kicks out of powerfeed when this is run on the looser side. No-one seems to know for sure if that's what it is or not, and my SAG 180 is a little different in that it has a set screw under there which preloads a detent for the powerfeed lever - it's possible that this could serve that function.
Re the torch - I bought that thing early on in my TIG days, probably over ten years ago now, thinking it'd be good for inside merge collectors (I was building extractors at the time for a particular model of vehicle). Would you believe I've never even used the friggin thing...
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8th Mar 2021, 11:14 PM #15I break stuff...
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The play doesn't overly worry me, 99% of the time it's going to be loaded in one direction, and I don't think considering the almost 30:1 reduction through the apron that any 'bounce' that occurs will make much difference to surface finish. If I just put it back in I might jam a shim in there just because I can, but my primary concern was whether what remains is strong enough. Seems like a few are agreeing with one of my initial thoughts that I'm probably worried about nothing...
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