Grafting feedscrew (Q from a new guy)

[Pete O]

G'day, my first post here other than my introduction, been lurking for a while and you guys are already my go-to metalworking info source.
I have a big old Kobe lathe (have been unable to find out anything about the manufacturer, obviously Asian, I think Japanese) that my good lady gave me for my 50th birthday a couple of years ago. It has done a fair bit of work, has required a fair bit of refurbishment; the current project is the replacement of the worn-out crossfeed screw and nut. It is a 1" 5tpi, original is a square thread but I'm replacing it with Acme. I have already cut the screw, spent nearly all of today cutting the internal Acme thread in the nut blank- by far the most difficult lathe work I have tackled to date and I was very relieved to achieve what seems to be a working screw and nut . I have access to a friend's Bridgeport mill to cut the external dimensions of the nut, which will be my first effort at milling.
I'm seeking some advice about grafting my shiny new Acme threaded shaft into the existing drive section of the feedscrew. The drive gear is cut into the shaft itself, so my plan is to keep the outer section of the original screw, cut off the threaded section, and graft the new threaded section onto it. What is the best way to go about this? A picture tells a thousand words (but only if you can figure out how to post it):
   
   
   

The section of the existing shaft between the gear and the thread is (obviously) where I plan to bore. My thought is to turn the corresponding section of the new shaft to a diameter to insert into it and either loctite, braze, thread or pin or a combination of techniques to make it stay there.
The diameter of this section is 0.900". What would be a suitable bore size? I'm thinking bore it to 0.600" which would leave .150" wall thickness- would that be thick enough to not split if there is a press fit? Should it be a press fit? Should it be a slip fit plus loctite? Should I thread it? Also wondering if I should pin it- or would drilling introduce a weak spot? Are there better options that i haven't thought of??? Your thoughts please!

[Mayhem]

Something like that I would want to pin at the very least but there are more here with much more knowledge and experience than me, so I'd wait for their words of wisdom.

Nice job on the thread.

[Sunset Machine]

I'd bore a slightly tapered hole (3 times the diameter or so) and make a tapered end on the shaft. Heat the hole up sizzle hot, slather some loctite "press-fit" (680?) on the male end and assemble.

When you turn the male end, test the fit by chalking it (3 or 4 equidistant lines). Carefully insert it into the hole and give it a little twist. Check the contact pattern. Use a file on the spinning shaft to adjust. The assembly should lock hard even when cold, contraction after heating will lock it even harder and the loctite fills in the gaps.

[f350ca]

I'd go for a shrink fit, a pin through it would give you a degree of confidence but probably not necessary. Cut the old shaft at the thread then drill, bore and finally ream the hole to 5/8 to the depth of the gear. Boring it will ensure the hole is in line with the shaft, drilling alone won't. Machine end of the new thread 2 to 2 1/2 thou oversize. Heat the end of the old shaft but try and keep the gear cool. Slip them together and chill the gear with a wet rag. Looks like you could get an inch or two of depth, I'd never expect them to come apart with the loads you'd be applying.

[PixMan]

I would shrink fit but also pin it to be certain it never moves.

My concern in what I see is the pitch of the threads. The two screws have a dramatically different looking helix angle. Are they both the same or is that square thread one a double-start thread?

[Pete O]

Thanks for the responses. Regarding the apparent difference in the threads, it is a bit of an optical illusion- the acme threaded shaft is sitting flat on the bench, whilst the square threaded one is tilted due to sitting on the flange at one end. That and the fact that the crest of the square thread is so much wider than the crest of the acme. They are both 1" 5tpi.
Sorting through the replies, I'm hearing bore it to 5/8 as deep as the gear teeth, make the male section about .0025 oversize, shrink fit plus loctite. If I were to pin it, what sort of pin is used? How is this done?

[f350ca]

For an interference fit like I suggested you'll have to heat the shaft quite a bit, (probably shoot for blue heat) loctite would be evaporated. Can run the numbers for temperature and expansion if your thinking of going that way. If you were to pin it a tapered dowl would be best. You'd need a tapered reamer to prep the hole for the dowl

[Vinny]

What grade of steel did you use for the acme shaft?

[Dr Stan]

(04-24-2015, 04:37 PM)Pete O Wrote: Thanks for the responses. Regarding the apparent difference in the threads, it is a bit of an optical illusion- the acme threaded shaft is sitting flat on the bench, whilst the square threaded one is tilted due to sitting on the flange at one end. That and the fact that the crest of the square thread is so much wider than the crest of the acme. They are both 1" 5tpi.
Sorting through the replies, I'm hearing bore it to 5/8 as deep as the gear teeth, make the male section about .0025 oversize, shrink fit plus loctite. If I were to pin it, what sort of pin is used? How is this done?

If you heat it as per suggestions the Loctite will evaporate and I also think the 2 1/2 thousandths on a .625 shaft is extreme overkill.  I was taught (years ago) to use .0005" to .001" per 1.0" of diameter.  So at the outside I'd be using .0005" of interference.  After wards I'd also pin it, probably with an appropriately sized taper pin.

An extremely good joint would include a straight knurl on the male part of the fitting at .001" or even a little more over the knurls.  A diamond knurl will also work, but Lord help you if you get it started crooked.  Shrink fit them together (heating the female part only) and it would be almost like they were welded.  Freezing the male portion would not hurt.

[PixMan]

Actually, if shrink fitting two pieces together I would believe a smooth to smooth surface interface gives infinitely more contact area than a knurled surface. That is the whole purpose of a shrink fit, to make two pieces effectively become one. If you have a knurl and want the maximum contact area you would have to have a much greater differential in size to engage the "peaks and valleys" of the knurl, wouldn't you?