Grafting feedscrew (Q from a new guy)

[Pete O]

So nice to know there is a definitive answer that everyone agrees on
Vinny, steel grades are still a bit of a mystery to me but I believe it is 1030 (sure everyone's going to tell me I did the wrong thing here...)

Had a bit of a setback today, on close inspection of the internal acme thread I discovered that I now have a nice bronze paperweight. The thread form is weird on one side, I think my boring bar must have rotated in the toolpost part way through the threading job. The back side of the thread has two distinct angles to it, my friend with the mill spotted it straight away when I took it to his place to mill the external shape of the nut. Will probably have to wait til next week to make another, glad I'm not relying on this for a living- or paying myself by the hour for it. On the positive side, my friend has some tapered reamers and pins- good man to know!

[Sunset Machine]

Many ways to skin a cat. Btw, Loctite 680 is made for heat shrink fits. I've found that it greatly improves the strength. Vice and hammer test.

[Pete O]

A shrink fit with a tapered pin seems to be the way to go then.
In heating the female part to achieve this, how concerned should I be about the gear teeth? Am I likely to end up with annealed teeth, or is quenching with a wet rag as suggested by f350ca enough to harden them again?
I have some loctite 640 retaining compound, will have a look at their website and see if this is suitable.

[f350ca]

Check the corner of one of the teeth with a file, they probably aren't hardened. If you apply the heat to the bored out sleeve quickly, as in large hot flame the gear should stay fairly cool anyway. Had a look at the specs on Loctite 640, they say its for shrink fits, but must be referring to warming a bearing or housing in oil, its flash point is 200F.

[Highpower]

(04-27-2015, 09:03 AM)f350ca Wrote: Had a look at the specs on Loctite 640, they say its for shrink fits, but must be referring to warming a bearing or housing in oil, its flash point is 200F.

As does Loctite 680, which says it is for slip fits, with a max clearance of .015", where as 609 calls out a max clearance of .006", and 640 is .1mm (.004")

[Pete O]

Looking at the tech data sheet for 620 it looks like the goods for this application, can be used in shrink fit applications- heat the collar, apply to the pin and use high press rates (which I assume means assemble quickly).
The torque load on this shaft is only what I can apply with my wrist, is it not? I don't imagine the axial load is particularly high either?
I won't cut the shaft until after I have made the nut (hopefully second time lucky) which I hope to achieve early next week.
I have to confess to a stupid rookie mistake- I discovered the reason for my failed internal acme threading effort- I ground the tool with the clearance angles the wrong way, i.e. same profile as the tool I ground for the shaft. It was rubbing the back off the thread the whole time. Did I mention I haven't been doing this long?

[stevec]

Pete, I, personally, would not go for a shrink fit, too much chance of it seizing during assembly (pressing) a good "fit for fit" sizing and a taper pin is all you need.
If you experience problems with your "press/shrink fit" you may scrap both pieces.

As you mentioned the maximum torque applied during use will be what you can apply with your wrist.

Just my opinion, for what it's worth.

Steve

[Pete O]

Thanks Steve, I have been mulling this over and thinking about where I would end up if I have problems assembling the shaft with a shrink fit. A size-for-size slip fit with some 620 and a taper pin would be far less likely to end up an expensive mess. I don't have the capacity to cut gears to replace the drive portion of the shaft if I damage it.

[Sunset Machine]

You guys just described why I like to taper the shaft and the hole that it fits. It's easy to control by sneaking up on the fit with a file. When it assembles about 3/4 of the way, it's time for heat and glue. Try a practice piece and see for yourself sometime.

Btw, Loctite 680 is just a thicker 609 to me. It stays put better. If you heat things up too hot it will smoke on assembly but seems to work anyway. Spit sizzle hot and a little more seems to be just right. It melts at 250C (480F) and will re-fuse when cooled. I don't see anything to be concerned with.

[TomG]

I'm a little late to the party, but it still seems like a good time to throw a monkey wrench into the whole shebang.

Most lead screws have a joint with a slip fit that are then pinned with a soft pin that will shear if some idiot runs the carriage into the headstock. If that happens, the pin will shear off rather than teeth on a drive gear. All of the aforementioned techniques will make a secure joint, but if you want some insurance, I would suggest pinning it instead.

Tom