Strength of Cast iron

[Gordon WParticipant @gordonw]

I know the scales are not checked– my doc. says I am overweight.

[duncan webster 1Participant @duncanwebster1]

Peter, I've sent you a pm, no need to clutter this up with sums.

[Tim StevensParticipant @timstevens64731]

There is really very little advantage in using a screw with much greater strength than the material it is screwed into. A common or garden metric screw is already stronger than the cast iron. And if you set your torque wrench to tighten a 12.9 screw properly, all you will do is pull the thread out of the casting.

And incidentally, such high tensile screws are rather more subject to hydrogen embrittlement, if the right (ie wrong) circumstances arise.

Cheers, Tim

[Martin NewboldParticipant @martinnewbold]

Perhaps a slighly differant thought on this subject if you are as it seems worried about pulling out a thread.

Perhaps another question is how are you lifting it and what are you attaching chains too as if its a roof then I think you should be considering this is not good practice. Most heavy machinery is still walked by leavers into position. I am not sure why you want to lift it and just through ths into debate as if its heavy enought to break out the thread then why do this?

All the best

M

Edited By Martin Newbold on 13/05/2018 21:55:58

[Michael GilliganParticipant @michaelgilligan61133]

Posted by duncan webster on 13/05/2018 00:05:10:

you'd be better off attaching the chain in line with the M6 screw, can you screw an eye bolt in?

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Although it does not answer the original question of the cast iron strength …

Here's a useful page about eye bolts: **LINK**

https://www.s3i.co.uk/lifting-eye-bolt-grade-80.php

M6 eyebolt: direct pull [using suitable materials] is rated at 0.4 tonne.

MichaelG.

[ZebethyalParticipant @zebethyal]

I also went down the route of using a gas spring, and removed the original spring mechanism in the process.

Just drlled and tapped 2 holes to M8.

Above, you can see it on the original tilting column, I also fitted it on the solid column after fitting that.

cost of the spring was about £13.00 from Ebay.

[Neil WyattModerator @neilwyatt]

Seeing Michael's figure of 0.4 tonnes for an M6 eye made me doubt my pull-out figure for PLA of 250kg (0.25 tonnes).

I do assume that the 0.4T figure allows a margin for safety, and mine was a test to failure.

Just for comparison I looked up the UTS of PLA – 50MPA, for Cast Iron its 160 – 450

Using 50 in the calculator above gives a pull out strength for PLA of 400 kilos for M6 with 11mm of engagement.

My real world figure of 250 is probably reasonably accurate, given my test block was 3D printed and not 100% solid.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 14/05/2018 14:17:51:

Seeing Michael's figure of 0.4 tonnes for an M6 eye made me doubt my pull-out figure for PLA of 250kg (0.25 tonnes).

I do assume that the 0.4T figure allows a margin for safety, and mine was a test to failure.

.

No need to assume, Neil … The figures are adequately explained on the linked page.

MichaelG.

[JasonBModerator @jasonb]

Don't those figures relate to the working load of the eye bolt, can't see anywhere where it says what it was screwed into when tested?

I would be more interested in the quality of the iron used on the mill casting and the % thread engagement that the factory used.

Edited By JasonB on 14/05/2018 15:39:00

[Peter G. ShawParticipant @peterg-shaw75338]

Gentlemen (& Ladies, if any),

I am not an engineer per se. Hence the strengths of materials is something I know very little about. Indeed, hitherto it has been a case of using what looks satisfactory, except that for cast iron, it is my understanding that cast iron is relatively good in compression, but not so good in the other direction. Therefore I was somewhat concerned that I might pull out the screw. Fortunately, the opinion so far is that my M6 bolt into 11mm of cast iron will be more than satisfactory for a 17kg pull.

One of my original ideas, which still remains as a possibility, was to insert a plate inside the casting to spread the load and at the same time place the casting under a compressive load.

Since the original replies I have tapped the casting for M6 on the basis that if it does pull out, I can then drill M6 clear and then add a plate inside the casting.

I am aware that numerous people have used gas springs as used on car boot lids, indeed I have a set of plans for using one from a Peugeot 105 series. The article by Phil Dawes that I referred to in my original post states that the force generated by a gas strut is non-linear. I have no way of checking that, so I can only go by what Dawes has said, hence the idea of using a counterbalancing weight which should be a linear force, and which does actually make some sort of sense to me. The basic idea then, is to attach a cycle chain to the top of the milling head, route it over the top of the column via a pair of wheels, and then suspend a weight of, in my case, 18-19kg on the end of the chain. The reason for the extra weight over the actual weight of the head is to provide a permanent up force on the head. So, no connection to garage roofs or indeed anything else.

Finally, the idea of using a lifting eye is fair enough – except that I had already drilled and tapped for M6 when that point was made. And, somewhat unfortunately, discovered that when I reassembled the head after drilling and tapping, that the outer cover of the motor prevents a straight pull on the screw. My fault, I should have assembled first, had a good look around, and only then disassembled for drilling & tapping. What that means is that the tapped hole is of the order of 6mm too near to the motor so I am now considering how to make a suitably offset shackle. My first effort is going to be to use a piece of aluminium channel, however, I have a suspicion that it might not be satisfactory due to its physical size. No matter, I'll get out the knife and fork and make one out of steel if necessary.

As I said at the beginning, I'm not an engineer per se. Indeed everything that I do is aimed totally at increasing my knowledge, ie learning by experimentation, and if along the way I manage to do something that works and is useful, then hooray. Of course, I also learn by making mistakes, and hopefully, I learn from those mistakes.

So thanks for all the suggestions and thoughts.

Peter G. Shaw

[Michael GilliganParticipant @michaelgilligan61133]

Posted by JasonB on 14/05/2018 15:36:56:

Don't those figures relate to the working load of the eye bolt, can't see anywhere where it says what it was screwed into when tested?

.

Try again

[quote]

Super alloy steel construction.

Fatigue rated x 1.5 WLL for 20.000 cycles.

Proof tested – 2.5 times working load limit.

100% Magnaflux crack detected.

Safety factor 4:1

Red painted finish.

We recommend only using Grade 80 Eye Bolts with Grade 80 Components.

 [/quote]

.

MichaelG.

Edited By Michael Gilligan on 14/05/2018 15:53:56

[Michael GilliganParticipant @michaelgilligan61133]

Posted by JasonB on 14/05/2018 15:36:56:

I would be more interested in the quality of the iron used on the mill casting and the % thread engagement that the factory used.

.

So would I, Jason … Which is why I prefaced my link with the statement: "Although it does not answer the original question of the cast iron strength …"

MichaelG.

Edited By Michael Gilligan on 14/05/2018 15:59:05

[JasonBModerator @jasonb]

That is what I saw Michael, where does it say those loads apply when it is screwed into cast iron?

[Michael GilliganParticipant @michaelgilligan61133]

Posted by JasonB on 14/05/2018 15:58:56:

That is what I saw Michael, where does it say those loads apply when it is screwed into cast iron?

.

Jason,

It does not and it reasonably could not !!

I hope you have read, and understood, my follow-up post of 15:58:05

MichaelG.

. 

Edit: Evidently I was not sufficently clear, last night, when I wrote:

M6 eyebolt: direct pull [using suitable materials] is rated at 0.4 tonne.

Edited By Michael Gilligan on 14/05/2018 16:10:19

[JasonBModerator @jasonb]

Yes I saw the follow up. Looks like Neil missed your comment which is why I posted the question as it seemed he was thinking the 0.4T related to the pull out strength of an M6 fixing in CI

[Michael GilliganParticipant @michaelgilligan61133]

Posted by JasonB on 14/05/2018 16:09:56:

Yes I saw the follow up. Looks like Neil missed your comment which is why I posted the question as it seemed he was thinking the 0.4T related to the pull out strength of an M6 fixing in CI

.

Thanks for clarifying that, Jason : Perhaps you would have been better addressing a question to Neil.

… I don't really think I can contribute any more to this discussion.

MichaelG.

[duncan webster 1Participant @duncanwebster1]

I think we're all getting a bit carried away. The figures quoted for the eyebolt will assume it is screwed into something strong enough that the eye bolt gives up first, how else could they test it.

​however I'm concerned about this offset shackle idea. If not careful it could increase the load on the M6 thread by a lever effect. It's still probably strong enough, but let's have a sketch.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Michael Gilligan on 14/05/2018 16:14:33:

… I don't really think I can contribute any more to this discussion.

.

… On reflection, perhaps I can:

We have, I think, established beyond all reasonable doubt that a properly installed M6 eyebolt would comfortably support Peter's 17Kg load. BUT the local quality of the cast iron remains unknown.

Although unlikely, it is feasible that Peter has tapped into a casting flaw …

Given the modest load under discussion, it should be a very simple matter to conduct a 'proof test' at [say] 80Kg.

MichaelG.

[ZebethyalParticipant @zebethyal]

Posted by Peter G. Shaw on 14/05/2018 15:38:14:

I am aware that numerous people have used gas springs as used on car boot lids, indeed I have a set of plans for using one from a Peugeot 105 series. The article by Phil Dawes that I referred to in my original post states that the force generated by a gas strut is non-linear. I have no way of checking that, so I can only go by what Dawes has said, hence the idea of using a counterbalancing weight which should be a linear force, and which does actually make some sort of sense to me. The basic idea then, is to attach a cycle chain to the top of the milling head, route it over the top of the column via a pair of wheels, and then suspend a weight of, in my case, 18-19kg on the end of the chain. The reason for the extra weight over the actual weight of the head is to provide a permanent up force on the head. So, no connection to garage roofs

 

As to the potential non-linearity of a gas spring: I have not noticed this myself.

With the weight of the head sitting on the spring, it seems pretty linear in both the upwards and downwards directions on my mill, requiring about the same level of force to move throughout the range of motion, regardless of direction of travel. It can be wound up or down using one finger on the rack feed handle albeit with a small level of resistance due to the gas moving in or out of the cylinder on the spring.

This maybe down to my choice of spring – I am using a 15Kg gas spring with 210mm of possible movement.

The mill head will also stay put without needing to be clamped wherever I choose to stop, due to spring resistance and friction in the rack assembly (obviously I would not think of machining like this), so acting as a pretty good counterbalance I believe.

Also because the threads on my spring go in from the side of the mill, I have no worries about them ever pulling out because there is very little tension on the threads, all the force is at right angles to the threads and cast iron is very good at resisting shear forces.

Far be it from me to stop you from installing a counterweight and chain – whatever works for you, I am merely suggesting an alternative that was cheap (IMHO), takes up minimal space, took less than 30 minutes to install and would still allow the tilting head to be used on the original installation if required.

Edited By Zebethyal on 15/05/2018 10:16:25

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Zebethyal on 15/05/2018 10:11:51:

Posted by Peter G. Shaw on 14/05/2018 15:38:14:

I am aware that numerous people have used gas springs as used on car boot lids, indeed I have a set of plans for using one from a Peugeot 105 series. The article by Phil Dawes that I referred to in my original post states that the force generated by a gas strut is non-linear. …

As to the potential non-linearity of a gas spring: I have not noticed this myself. …

.

This may be of interest: **LINK**

http://www.stabilus.com/fileadmin/download/Vehicle_GB_12_12.pdf

MichaelG.

[ZebethyalParticipant @zebethyal]

Posted by Michael Gilligan on 15/05/2018 11:02:56:

Posted by Zebethyal on 15/05/2018 10:11:51:

Posted by Peter G. Shaw on 14/05/2018 15:38:14:

I am aware that numerous people have used gas springs as used on car boot lids, indeed I have a set of plans for using one from a Peugeot 105 series. The article by Phil Dawes that I referred to in my original post states that the force generated by a gas strut is non-linear. …

As to the potential non-linearity of a gas spring: I have not noticed this myself. …

.

This may be of interest: **LINK**

http://www.stabilus.com/fileadmin/download/Vehicle_GB_12_12.pdf

MichaelG.

Thanks Michael,

That confirms what I was seeing – the document states that "Unlike mechanical springs, gas springs have an extremely flat, almost linear characteristic curve and therefore allow a uniform comfortable adjustment or pivoting movement", the graph also shows exactly the same force required for both compression and extension.

[Neil WyattModerator @neilwyatt]

Just a thought, when I replaced my brake calipers I noted that they were attached with M8 screws into cast iron, I have cast iron chucks/faceplates fitted using M6 studs.

I think we are ALL guilty of overthinking this, the simple answer 'M6 will be fine' was all that was needed.

Neil

[John ReeseParticipant @johnreese12848]

Posted by richardandtracy on 11/05/2018 21:06:33:

If it is really cast iron, as opposed to cast steel, then the strength will be very much less than expected.

At the turn of the 1800's to 1900's, the max tensile strength for cast iron was considered to be 2000 psi, and anything stronger was cast steel. In modern units, this is 13.8 N/mm^2. So, the shear strength is 65% of that at 9 N/mm^2. This goes a long way to explain why Victorian machinery was so huge for its output.

Regards

Richard.

I would be very surprised if 19th and early 20th century cast iron poured by a reputable foundry would have a tensile strength below 20,000 psi. Even the lower grades if iron used today would have a tensile strength around 40,000 psi.

Cast steel is a much different metal having a carbon content probably less than 0.4% where cast iron would have 3.5 to 4.5% carbon. Cast iron contains flakes or nodules of graphite. Steel does not.

[SillyOldDufferModerator @sillyoldduffer]

Posted by John Reese on 15/05/2018 16:53:02:

Posted by richardandtracy on 11/05/2018 21:06:33:

If it is really cast iron, as opposed to cast steel, then the strength will be very much less than expected.

At the turn of the 1800's to 1900's, the max tensile strength for cast iron was considered to be 2000 psi, and anything stronger was cast steel. In modern units, this is 13.8 N/mm^2. So, the shear strength is 65% of that at 9 N/mm^2. This goes a long way to explain why Victorian machinery was so huge for its output.

Regards

Richard.

 

 

I would be very surprised if 19th and early 20th century cast iron poured by a reputable foundry would have a tensile strength below 20,000 psi. Even the lower grades if iron used today would have a tensile strength around 40,000 psi.

Cast steel is a much different metal having a carbon content probably less than 0.4% where cast iron would have 3.5 to 4.5% carbon. Cast iron contains flakes or nodules of graphite. Steel does not.

Apples and oranges I suspect!

20000psi is the breaking point of some well-made cast-irons. But you wouldn't run a machine made of cast iron at anything like that level. In practice there is always a hefty safety factor built in. How big it is depends on the material and conditions of service, but it's unlikely to be less than 4:1, and 8:1 and stronger is likely. As even top quality cast iron is poor in both tension and alternating stress Richard's 2000psi limit is very reasonable.

 

Edited By SillyOldDuffer on 15/05/2018 18:27:37

[Tim StevensParticipant @timstevens64731]

Regarding gas springs and their non-linearity –

By the very nature of a gas spring it cannot be linear. The pressure doubles when the volume is halved, and this means that the load doubles when the length is halved. Halve it again (ie half as much further movement) and the load doubles again. The same extra load for half the extra movement = non linear.

The length in all this is of course the length of the gas chamber, not the total length (which is longer because of the piston rod, and the end fittings etc.) But the principle is sound – gas springs do not obey Hooke's law, or as you might say, they are non-linear.

Sorry

Cheers, Tim

[Author]

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