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Old 26-09-2016, 08:36   #1
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Shortening Drive Shaft

I need to shorten mine by about three inches, there is too much shaft sticking out behind my Cutlass and this may be contributing to a drive train vibration problem that I have had since I bought the boat.
My coupling seems to simply be slid onto the shaft and attached by means of a thru bolt going through the shaft, no keyway.
I'm thinking I can first drill though the shaft at the proper location and then with the boat in idle use a hack saw to cut mostly through the shaft, and finish cutting with motor off, slide the coupling back on and put the bolt through it securing the shaft to the coupling.

Will this work?

My other option is of course find someone and pay them.
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Old 26-09-2016, 08:45   #2
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Re: Shortening Drive Shaft

A hack saw will work yes. Not so sure about doing it from the motor running safety wise.
The wise way would be to remove it, saw it off and drill the hole with a drill press so you get the hole perpendicular to the shaft.
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Old 26-09-2016, 09:46   #3
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Re: Shortening Drive Shaft

That's an interesting idea you have, but I don't think you have to have the shaft spinning in order to get a straight cut. I did mine by marking the cut with wide tape, this gives you a straight line, then I just cut it with an angle grinder, drilled a new dimple for the keeper bolt and done. This should be no problem to do while in the water if you have the room.
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Old 26-09-2016, 10:23   #4
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Re: Shortening Drive Shaft

I was thinking with it slowly spinning that I could get a perfect straight cut, sort of like it was chucked in a Lathe, just cut the majority of it that way though, finish by hand.
Drilling the hole has me worried the most, I was thinking about clamping a drill guide to the shaft and drilling a 1/8' pilot hole first, then upsizing and reaming if that level of accuracy is necessary, but just looking at the bolt that is in there now, it does not appear to be a close tolerance bolt. Drill the hole before the cut as that way both ends of the shaft are supported.

I need a bottom job done this year, I may just pay the yard to do it, or at least get a price.
Last bottom job I had the shaft pulled and inspected for wear and straightness and a New Cutlass installed, so I don't think I need it pulled
Shaft is 1.25"
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Old 26-09-2016, 11:14   #5
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Re: Shortening Drive Shaft

The distance between the rear of the cutlass and the start of the shaft taper is called the "overhang" distance. Typically the accepted amount of overhang should not be greater than one shaft diameter, as this can cause shaft fatigue, wobble and cutlass wear.

Ideally you should pull the shaft and have any work done by a qualified machine shop. Then, when they reinstall the shaft coupling, they can take a light trim cut on the coupling face to ensure the shaft and coupling are true. Out of true couplings can also cause shaft vibration.

To me, using a hack saw on a rotating shaft sounds like a first class ticket to the emergency room.....way too many things to go wrong.

DougR
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Old 26-09-2016, 12:16   #6
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Re: Shortening Drive Shaft

Hey A64,
You can definitely do the shaft cutting as you describe. Use the coarsest, highest quality hacksaw blade available. Use oil. You must keep the speed very low, stainless needs to be cut at a slow speed. As you probably know, this cutting technique is sometimes done on the lathe and it works well.

I'm a little surprised at the lack of a keyway. Regardless, f the setup you've got has worked for a long time it's obviously OK. I would strongly recommend a precision reamed hole, fit as closely as possible to the bolt. The bolt should be a heat treated grade, I'd recommend grade 8.

If you've machined stainless you know that it work hardens badly. So use low cutting speeds and lots of feed pressure when drilling. You should make a drill jig to keep the hole straight. This can be as simple as a piece of angle(used as a V block) with a block of steel welded to it, with your guide hole drilled thru the block and angle with a mill or drill press to ensure it is accurate. Make your guide hole the diameter of the drill you will start with - and start small, it is much much easier to start with a small hole and work up. Clamping the guide to the shaft before drilling the first hole will help a lot. With each larger drill diameter you start by drilling the guide with the larger drill, drilling the shaft. Run the reamer at very low RPMS with lots of oil. It should only be removing a few thousandths, ideally.

It is important to fit the hole very closely to the bolt diameter. The reason for this is that the engine output is a series of pulses with each piston power cycle, the output shaft accelerating and then decelerating. Any play in the fit of the bolt allows the shaft/flange to pound against the bolt. A very close fit minimizes this. I'd shoot for no more than .002 clearance on the bolt diameter.
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Old 26-09-2016, 12:25   #7
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Re: Shortening Drive Shaft

I've not machined stainless, I'm just an old oilfield welder and hack mechanic, but I have drilled a many a hole, and work hardening is exactly what worries me, I know drill slow, with a lot of pressure and have a handful of good bits available and keep it wet with cutting fluid.
I feel sure there is no key, but I just did a curiously look, I think there is a bolt all the way through, I'll look closer this weekend
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Old 26-09-2016, 12:30   #8
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Re: Shortening Drive Shaft

I was thinking about using this as a drill guide, clamp it to the shaft
https://www.amazon.com/Gator-Tools-S...ds=drill+guide
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Old 26-09-2016, 19:36   #9
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Re: Shortening Drive Shaft

I looked at the tool. It will be difficult to make it work well because there is little to keep it aligned with your shaft. I suppose if you clamp it tightly you could make it work. The nice thing about a V-block is that it automatically centers itself on the shaft and resists moving. But at the end of the day anything that will support the drill straight will do the job.

You're right on all counts on drilling the hole. If you keep those items in it will drill fine. Definitely start with a small drill because no one is strong enough on a hand drill to apply enough pressure to keep from work hardening when starting with a large drill bit.

One last point - instead of a high speed steel drill bit, if you make your starting hole with a cobalt drill bit it will give you more margin. They are noticeably more wear resistant than high speed steel.

Good luck on your project.
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Old 27-09-2016, 10:42   #10
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Re: Shortening Drive Shaft

I donít believe you will ever get the new hole in the shaft close enough to perfectly align with the existing holes in the flange collar. This is hard enough to do in the shop with good equipment. At best you will get a very sloppy fit. If you are willing to ream everything one size larger then you may have a chance to get satisfactory fit. You may get lucky and be able to use say a metric bolt rather than a standard and thereby not going much oversize.


You would have a better chance of success to rotate the flange and drill a new hole 90 degrees from the existing bolt hole. Drill one side only in the drill press, drill only to pilot size, put the flange back on the new shaft end and use the flange as a drill guide. It wonít be straight but you will end up with a close fit. Just donít rotate it 180 degrees because it wonít come close reversed. You may think you were very straight but the holes wonít lie.


Hone out the bore in the flange and make sure the flange still has a good tight fit on a clean deburred shaft.


Donít use standard drill bits to drill stainless steel. I have written about this several times. Stainless is not hard to drill with the correct bit cutting angles, slow speed and high down pressure. Donít ever let it get hot or you are screwed. Look on the net and find out how to grind the proper bit. Bits are cheap, grind some to cut properly in stainless and keep them around. Liberal use of cutting oil helps, mostly to keep heat down but if your bit is cutting properly you donít generate that much heat.



I almost never use a pilot bit if drilling stainless in the drill press or the milling machine. By hand it is harder to get the proper pressure so I would use a pilot. Use under 1/8 and they not only break easily but are hard to regrind so I would pilot with an 1/8th . Then go up to the roughing size bit. Donít drill to the finished size because it will be a very sloppy fit. Drill a 64th under and ream to final size.

Block the shaft up so it is absolutely solid and will not rotate when you drill. gives you a lot better chance of success.
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Old 27-09-2016, 10:50   #11
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Re: Shortening Drive Shaft

Quote:
Originally Posted by a64pilot View Post
I was thinking about using this as a drill guide, clamp it to the shaft
https://www.amazon.com/Gator-Tools-S...ds=drill+guide
My boats had enough length of shaft overhang to get a bullet zinc on them. One I had to use a collar zinc though. That's probably 1.5 -2" or so. Consider that.
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Old 27-09-2016, 12:04   #12
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Re: Shortening Drive Shaft

I'll leave the shaft connected to the transmission to drill, that will hold it. I intend to drill before cutting.
Yes I will use 1/8" Cobalt for the pilot hole, I figure have four with me.
I'll clamp the jig with a C clamp, that will hold it I'm sure.
Assuming the collar is a close tolerance fit with the shaft, as long as the bolt will have a tight fit, it will accept slight misalignment, a slight interference fit on the bolt would be ideal, slight interference, the kind you tap the bolt in, not hammer.
Zinc wise I have the Islers Divers zinc adapter, that puts a B prop zinc on, that is all the zinc I will need, I currently have a collar zinc on, cause I had the room , why not. But I intend to not have room when I'm finished, I'm shooting for an inch of exposed shaft, need to look at where the taper starts though.
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Old 27-09-2016, 12:05   #13
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Re: Shortening Drive Shaft

Your method will work. There are a number of jigs that use a standard drill that allows centering on a round shaft. I would be very surprised to find no keyway. The torque in shifting from fwd to rev should snap the bolt. Possibly a taper pin in place of the bolt would be better. After drilling a hole, using a reamer, size it to a standard taper pin and drive it home. Much tighter fit than a bolt. A bolt will always have a little slop.
Years ago I did a remote repair on a prop shaft with a rounded out keyway and thru bolt. Anything more than a careful shift would cause the bolt to snap. No machine tools were available. 250hp engine. I reamed the bolt hole (that was a damaged tapered hole) to take a taper pin 3/8" on the big end. To replace the keyway I reamed the keyway with a taper pin reamer until it was sized to a taper pin I happened to have with me. Taper pins have a standard taper so the next size is just a continuation of the previous size. If you ream a hole too big, you buy the next pin one size bigger.
Anyway, the repair lasted thirty years that I know of. Maybe still running.
Also, you need cooling. Often I have used filtered bilge water with a small pump to supply continuous cooling (so I don't have oil to clean up). Otherwise I find or make a pan to fit under the shaft to keep the oil out of the bilge and the same small pump.
150 years ago, almost all machine fitting was done by hand.
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Old 27-09-2016, 12:18   #14
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Re: Shortening Drive Shaft

If you've got a bottom job planned anyway, wouldn't it be easier to just pull the shaft out then and do it with a proper drill press?
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Old 27-09-2016, 12:27   #15
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Re: Shortening Drive Shaft

I figured on an aluminum turkey basting pan to catch oil and filings etc.
I won't rig a pump but it would be nice, I'll likely use an oil can filled with tap magic cutting fluid.
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