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Silver steel or stainless?

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Chris R 124/01/2019 11:59:51
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..or either?

I need to make a 3mm male thread about 5mm long on a 6mm rod. It's an actuator on a microscope. It (originally) had a knob on the other end - not really necessary. The rod is about 100mm long and not well supported, so the original threaded part tends to snap off.

I have access to a small Chinese lathe in a makerspace, and dies.

What's the best material for ease of working, + strength. How about 303 stainless?

Thanks

JasonB24/01/2019 13:15:40
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Of the two 303 stainless will be a lot easier to thread. If you just want an easy to work material then brass will more than likely do.

Simon Williams 324/01/2019 13:28:41
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+1 for stainless for me, but do make sure it's 303 grade which is nominally free machining, you'll likely get a much nicer thread. Lubricate the die well with something like Rocol RTD.

You can get free machining silver steel, but I've never worked with it. Ordinary silver steel would be a perfectly good material, though cutting threads in it is sometimes prone to tearing. But for making any thing scientific related go for stainless every time.

You could make a stronger job by drilling and tapping the end of your 6 mm rod with a M3 hole, then screwing in a long grubscrew such as a hex socket grubscrew (steel). These are made out of a very high strength material - or you could use an A4 stainless screw which is 316 grade by another name, so tough stuff but much the same strength as 303. A grubscrew in a tapped hole doesn't give the same concentrated stress raiser at the transition of diameters as making a male thread so it will probably be stronger. But be careful that the bit into which this screws isn't prone to damage - sometimes you end up moving the problem around!

HTH, can we have a picture of the finished repair?

Simon

Edited to correct my error over grades of stainless - 316 = approx. A4 grade, 304 = approx. A2 

Edited By Simon Williams 3 on 24/01/2019 13:34:27

Neil Wyatt24/01/2019 13:42:40
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Silver steel isn't very corrosion resistant which makes it unsuitable for unpainted/plated microscope parts unless you can guarantee a nice dry environment.

Neil

SillyOldDuffer24/01/2019 14:10:07
4587 forum posts
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I'd be inclined to try Silver steel and to heat treat it. The long unsupported rod breaks because it acts as a lever and puts a lot of stress on the rod just where the 3mm thread weakens it considerably.

Hardening a silver steel rod won't make it stronger but it might make it more resistant to fatigue cracks by turning it into a spring that distributes the stress rather than concentrating it. For the same reason it might pay not to cut the usual relief on the inside end of the thread, instead cut the thread into a chamfered end. ie Prepare the rod as on the right rather than the left.

microthread.jpg

I'd make the chamfer longer than shown in the drawing.  The latest version of Freecad on my machine is unstable and buggy - I didn't push my luck after I'd got the model nearly right!

The other possibility is to recognise the thing is going to break again and make a few spares while you're at the lathe.  Usually copies take a lot less time than making the first one.

Dave

 

 

 

Edited By SillyOldDuffer on 24/01/2019 14:14:01

Edited By SillyOldDuffer on 24/01/2019 14:16:13

HOWARDT24/01/2019 14:29:38
438 forum posts
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If threading a short length to a shoulder is a problem then add a washer, thickness to suit to give the right length of thread. That way there is no need for an undercut and you can put a good radius to the face and a corresponding chamfer on the washer.

Emgee24/01/2019 14:44:43
1157 forum posts
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Posted by Simon Williams 3 on 24/01/2019 13:28:41:

+1 for stainless for me, but do make sure it's 303 grade which is nominally free machining, you'll likely get a much nicer thread. Lubricate the die well with something like Rocol RTD.

You can get free machining silver steel, but I've never worked with it. Ordinary silver steel would be a perfectly good material, though cutting threads in it is sometimes prone to tearing. But for making any thing scientific related go for stainless every time.

You could make a stronger job by drilling and tapping the end of your 6 mm rod with a M3 hole, then screwing in a long grubscrew such as a hex socket grubscrew (steel). These are made out of a very high strength material -

Simon

Plus 1 for Simon's suggestion of drilling and tapping M3 then use the steel grubscrew.

Emgee

Chris R 124/01/2019 15:07:14
29 forum posts
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Woo, 6 answers, all a bit different! Thanks.

I reckon I'll try 303, no relief, and a couple of washers to bridge over the unthreaded mm or two..

I don't think the stuff it's screwed in to is tough. Good idea to make 2

Maybe I ought to waist part of the unseen part of the rod down to 2.5mm so I don't get another bit of broken off thread to winkle out  sarcastic 2

Edited By Chris R 1 on 24/01/2019 15:08:50

JasonB24/01/2019 15:40:22
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Dave, would you not temper that Silver Steel down as if it was just hardened it would be very brittle and more likely to fracture than if left in the supplied annealed state.

SillyOldDuffer24/01/2019 16:21:30
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Posted by JasonB on 24/01/2019 15:40:22:

Dave, would you not temper that Silver Steel down as if it was just hardened it would be very brittle and more likely to fracture than if left in the supplied annealed state.

Yes, it's a balance. Silver steel as supplied is quite soft. I've noticed stressed mild-steel tends to give at the bending point, which I think is due to stress concentrating where the metal is plastic. By hardening it, the rod becomes much more springy, which might extend its life by spreading the stress over more metal. I agree over-hardening (say in brine) would be just as bad as leaving the metal soft, because it would then be too brittle, again allowing the stress to concentrate in a narrow zone by not bending.

My experiments with silver-steel haven't been at all scientific. Normally I just cool it in tap water to make a hard wearing surface, but that's more laziness than skill! I only temper when making cutting tools. Not tempering leaves the steel springy and rather brittle, tempering reduces brittleness but also the springiness, and it's springiness that  might be helpful here. Much depends on how the microscope user breaks the rod. I imagine a lot of gentle bending adding up to a fatigue failure rather than a lot of sharp wrenching, but perhaps the rod gets bumped and I'm wrong (again!)

My feeling is that the properties of silver steel could be exploited in this application, but I admit I don't know where the sweet spot is - or even if heat treatment would make a worthwhile difference.

Dave

Edited By SillyOldDuffer on 24/01/2019 16:24:25

Andrew Johnston24/01/2019 16:34:24
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From my experience cooling silver steel in tap water, or brine, without vigorous agitation doesn't result in anything like full hardness. I doubt hardening and tempering will have much of an effect in this application. The fundamental problem is one of poor design. The best solution of those mentioned is to insert a preformed thread from a SHCS or grub screw.

Andrew

Vic24/01/2019 18:16:18
2205 forum posts
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I’ve threaded silver steel but 303 is much nicer IMO, it’s also very handy being rust resistant.

Howard Lewis24/01/2019 18:33:44
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Suggestion.

When you turn the rod down to 3mm dia, use a tool with fairly large radius. This will avoid the stress raiser of a sharp corner, which will lessen the chances of a fatigue failure.

If the rod has to butt up hard against a shoulder, use a plain washer so that the washer is the actual clamping face.

Any chance of being able to make up and outrigger bearing for the rod?

Failing that, can it be shortened to reduce the bending moment at the interface? .

Howard

Chris R 124/01/2019 19:00:54
29 forum posts
18 photos

I didn't expect so much discussion!

The 303 rod is ordered via ebay.

 

FWIW I've never had a lot of faith in my abilities in ad-hoc handheld heat treating carbon steels, especially with thin sections.

The guff about looking at the colour, tells you about the surface oxide, not much about the structure inside, unless it's under some specified conditions. Cut up you can see the effect you were after, but often not very deep into the thickness or not to the extent desired. I was a metallurgist once upon a time.

It's OK for a fellow in a factory who does a thousand chisels a week...!

Edited By Chris R 1 on 24/01/2019 19:01:33

Chris R 124/01/2019 19:10:28
29 forum posts
18 photos

Posted by Howard Lewis on 24/01/2019 18:33:44:.

...

Any chance of being able to make up and outrigger bearing for the rod?

Er, sorry? embarrassed

Like a sleeve? It goes through a hole. I'd better take a picture...

Vic24/01/2019 21:48:35
2205 forum posts
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Can you take a picture of the part when it’s done Chris, we like pictures! laugh

duncan webster24/01/2019 23:55:57
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303 has a yield stress of 450/650 N/mm^2, probably towards the top end with small diameter drawn bar. This is pretty good stuff, 650 N/mm^2 is 42 tons/sq.in. If you over-stress it it will bend, if you over-stress hardened silver steel it will snap.

Having said that it's a pretty poor design, cant you somehow support the free end of the shaft, or make the 3mm bigger

Chris Trice25/01/2019 00:01:36
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Just chucking in the idea of drilling and tapping the end and then screwing in a long high tensile M3 machine screw with the head subsequently cut off.

Emgee25/01/2019 00:06:09
1157 forum posts
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Posted by Chris Trice on 25/01/2019 00:01:36:

Just chucking in the idea of drilling and tapping the end and then screwing in a long high tensile M3 machine screw with the head subsequently cut off.

Chris, You are the 3rd to suggest that or similar so must be a good idea !!

Emgee

Bandersnatch25/01/2019 00:48:32
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Posted by Emgee on 25/01/2019 00:06:09:
Posted by Chris Trice on 25/01/2019 00:01:36:

Just chucking in the idea of drilling and tapping the end and then screwing in a long high tensile M3 machine screw with the head subsequently cut off.

Chris, You are the 3rd to suggest that or similar so must be a good idea !!



It's certainly the way I'd do it.

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