Silver steel or stainless?

[Chris R 1Participant @chrisr1]

..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

[Chris R 1Participant @chrisr1]

[JasonBModerator @jasonb]

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 3Participant @simonwilliams3]

 

+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 WyattModerator @neilwyatt]

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

[SillyOldDufferModerator @sillyoldduffer]

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.

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

[HOWARDTParticipant @howardt]

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.

[EmgeeParticipant @emgee]

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 1Participant @chrisr1]

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 

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

[JasonBModerator @jasonb]

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.

[SillyOldDufferModerator @sillyoldduffer]

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

[Anonymous]

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

[VicParticipant @vic]

I’ve threaded silver steel but 303 is much nicer IMO, it’s also very handy being rust resistant.

[Howard LewisParticipant @howardlewis46836]

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 1Participant @chrisr1]

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 1Participant @chrisr1]

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?

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

[VicParticipant @vic]

Can you take a picture of the part when it’s done Chris, we like pictures!

[duncan webster 1Participant @duncanwebster1]

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 TriceParticipant @christrice43267]

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.

[EmgeeParticipant @emgee]

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

[Enough!Participant @enough]

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.

[Chris R 1Participant @chrisr1]

I'm not that confident the tailstock on the lathe will be very near the axis…

 

ANyway you asked for pictures:

This is the bit of microscope, the one existing rod is in place, screwed in a the point arrowed. It IS steel of some sort that it goes into. As you can see the rod goes through a large hole. Quite why they break I don't know.

It's moving a filter cube in the fluorescence illumination head of a 'scope. There are two filters and a dichroic mirror in the cube. You use light of wavelength X, which reflects through the objective lens onto the subject, which you preload with fluorophores which emit at a different wavelength Y. None of X gets up the eyepiece / camera, so you only see your excited bit of subject, emitting in its own colour. Then you use a different excitation wavelength and cube and different fluorophore, for a different bit of your subject. The light source is usually a mercury arc lamp, though LEDs are replacing them because they're made with a narrow colour bandwidth.

Normally you tag a protein or drug or whatever so you can see where it's got to. You can tag cancer cells or TB cells, or things smaller than a wavelength of light. They emit so you know they're there.

Here's a picture c/o Zeiss of a bit of gut, with ten different fluorophores and sets of cubes/filters.

 

 

Edited By Chris R 1 on 25/01/2019 01:46:02

Edited By Chris R 1 on 25/01/2019 01:49:03

[Chris TriceParticipant @christrice43267]

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 !!

Emgee

Sorry, I skimmed. Reading back I see that. Great minds blah blah blah….

[Grindstone CowboyParticipant @grindstonecowboy]

How about a bush in that hole to provide a bit of support?

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Chris R 1 on 25/01/2019 01:39:15:

This is the bit of microscope, the one existing rod is in place, screwed in a the point arrowed. It IS steel of some sort that it goes into. As you can see the rod goes through a large hole. Quite why they break I don't know.

It's moving a filter cube in the fluorescence illumination head of a 'scope. [ … ]

.

That looks unusually poor engineering for Zeiss Nikon Chris … but I suppose they have 'moved with the times'

I suspect that the reason they fail is that they unscrew slightly, in normal use.

The bending stiffness should be maintained by face-to-face contact; but if the joint slackens the load will be taken by the screw.

Is there room to add a sleeve to the end of your new rod … to span the joint line

… it only needs to be a thin tube.

.

MichaelG.

[ tinkerer with microscopes ]

.

Edit: Just noticed the manufacturer's name on the cube !

Edited By Michael Gilligan on 25/01/2019 08:56:03

Edit: for the general interest

https://m.nikoninstruments.com/content/download/16085/353426/file/Fluorescence_Filter_Cubes_2CE-MRJH-3.pdf

Edited By Michael Gilligan on 25/01/2019 09:00:44

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