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Old 10-25-2010, 03:48 PM   #1
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Cutting new threads in SS shaft

Is it possible to use a hand die to cut new threads into a stainless shaft of about 1/2" in diameter? The threads on my bow thruster's shaft are starting to go away because the little zincs*are not that effective. They don't last long, are hard to change and*cause my monthly diver to lie to me about their status.

The plastic props slide onto*the bare part of the shaft*which has a*keyway. The threaded part is for the nut to hold the prop back on the keyway. I would think that a 1/2" of threads would be enough*for the nut to bite.

-- Edited by Doc on Monday 25th of October 2010 03:57:17 PM
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Old 10-25-2010, 04:42 PM   #2
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RE: Cutting new threads in SS shaft

Do you want to increase the length of the threads you have, i.e. thread further back toward the prop? Or do you want to cut threads of a smaller diameter than the root of what remains of the existing threads?

It's easy to thread a 1/2 inch shaft but how are you going to remove the old threads? Do you just want to "chase" the existing threads because they are damaged?
What size do you want to make the new ones?
How do you propose to hold the shaft while threading it?
If you have never run a die before are you sure you want to start with this project?
How much does a new shaft cost?

It might be more cost effective to just replace the shaft now rather than later when it fails and you lose the prop and overspeed the thruster as well.
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Old 10-25-2010, 04:48 PM   #3
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RE: Cutting new threads in SS shaft

Unles you are real good at it, cutting threads on the end of a shaft with a hand-held die can be an excercise in frustration. If you don't get the die started exactly centered and exactly perpendicular to the rod or shaft, you can end up with a threaded section that's not lined up or centered on the rest of the shaft or rod.

I found this out the hard way when trying to thread some new silicone bronze rod for our new anchor windlass intallation. Even the very experienced machinist who was helping us with this project had a tough time when he tried it. In the end he put the rod in his lathe and cut the threads with it.

So keep that in mind in addition to answering the questions Rick posed. I had plenty of rod stock to screw up with. It sound's like you need to get it right the first time.
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Old 10-25-2010, 05:01 PM   #4
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RE: Cutting new threads in SS shaft

You aLso need to make sure the pre thread diameter is right or the die won't cut properly if at all.
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Old 10-25-2010, 05:09 PM   #5
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RE: Cutting new threads in SS shaft

I want to make some new threads so the nut will tighten up on the shaft. Now the nut will just barely grab and the yard folks (out for bottom paint) say that there is not enough threads for the nut to grab. I would assume a smaller diameter shaft and nut when finished.

The alternative is to get a new shaft or an entirely new lower unit.
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Old 10-25-2010, 06:14 PM   #6
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RE: Cutting new threads in SS shaft

Quote:
Doc wrote:The alternative is to get a new shaft or an entirely new lower unit.
I have cut miles of threads with a hand held die and with a lathe but if it were mine, I would just replace the shaft and the seals. As long as the lower unit is apart, replace the bearings as well and you will have a new lower unit.

Compare the cost of the parts and how much you value your time and confidence you have in your ability to do the job right vs the cost of a new lower unit and let that decide which way to go.

Rethreading is not a great idea, if for no other reason than it is extremely difficult to machine the old threads off to a proper size and tolerance to take a new thread.

If you could somehow machine the new diameter, just holding the shaft while running a die will prove to be a real challenge without causing more damage in way of the propeller. It would take the fabrication of a special tool that keyed into the shaft to lock it solid because the die has to work in both directions

*
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Old 10-25-2010, 07:13 PM   #7
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RE: Cutting new threads in SS shaft

Quote:
Doc wrote:

I want to make some new threads so the nut will tighten up on the shaft. Now the nut will just barely grab and the yard folks (out for bottom paint) say that there is not enough threads for the nut to grab. I would assume a smaller diameter shaft and nut when finished.

The alternative is to get a new shaft or an entirely new lower unit.

You can "chase" (clean up) the old threads with the correct sized die and you can make the threaded portion longer, but you cannot use a threading die to make the shaft smaller in diameter.* You would have to turn it on a lathe to get it smaller and then thread it with a smaller die.

Since you asked here, it's pretty obvious that you don't have the equipment or skill to do this so your choice is to pay a machine shop or replace the part.

have you tried a new nut?*

*
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Old 10-25-2010, 08:57 PM   #8
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RE: Cutting new threads in SS shaft

Quote:
rwidman wrote:"You can "chase" (clean up) the old threads with the correct sized die and you can make the threaded portion longer, "
That's the correct way to do it.

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Old 10-26-2010, 04:21 AM   #9
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RE: Cutting new threads in SS shaft

Quote:
rwidman wrote:you can make the threaded portion longer,
I would be very surprised if there is enough length available to do that.
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Old 10-26-2010, 04:22 AM   #10
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RE: Cutting new threads in SS shaft

A bit of cutting oil will help as you lengthen the threded portion.

If you chose to replace the item , if bronze were a choice , it might be longer lasting.
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Old 10-26-2010, 07:26 AM   #11
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RE: Cutting new threads in SS shaft

What about something similar to a Helicoil like we used to use for a stripped spark plug?

Does anything exist that fits a shaft instead of a hole?
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Old 10-26-2010, 07:51 AM   #12
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RE: Cutting new threads in SS shaft

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rwidman wrote:you can make the threaded portion longer,
I would be very surprised if there is enough length available to do that.


We don't know.* You are probably correct, but the OP should be able to check.

I was really describing what someone could (or could not) do with a die, more than saying it would work in his application.

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Old 10-26-2010, 08:00 AM   #13
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Cutting new threads in SS shaft

Quote:
Doc wrote:Does anything exist that fits a shaft instead of a hole?
Yes, there are sleeves used to renew the worn surface of a shaft where a seal or bearing rides. They are too thin to thread and they require a close tolerance fit. Neither of those conditions exist in way of damaged threads.

The bottom line is none of the methods which might work can be performed with the lower unit mounted or intact. If you have to remove the shaft anyway, just replace the thing.


-- Edited by RickB on Tuesday 26th of October 2010 08:04:43 AM
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Old 10-26-2010, 01:55 PM   #14
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RE: Cutting new threads in SS shaft

I may have dodged the bullet. Just returned from the boat yard and the shaft had enough left for the nut to tighten.
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