Strength of Cast iron

Hi folks,

I am looking for informed opinion on the strength of cast iron when subjected to an extracting force of about 16kg via either a M6 or a M8 bolt.

In MEW 251, Phil Dawes describes how he modified his SX2P milling machine by replacing the spring loaded side lifting arm by a counterweight system. As I have a Warco MiniMill (X2 clone) using the same system, I thought that it should be possible for me to do the same.

Phil says that he was able to attach the lifting chain to the milling head by means of the two rear bolts holding the motor to the head. As the photo below shows, I do not have those screws, and the two nearest screws do not have sufficient room to have a suitable bracket attached.

It seems to me that my alternatives are to either replace the cast aluminium spacer between the motor and the actual milling head with a longer spacer but this then poses problems with creating the spacer on the lathe, the lengthened spacer being 200mm long, or to fasten the lifting chain directly into the dovetail section of the milling head.

The flat surface of the dovetail is about 23mm wide where the rectangular cutout is whilst the vertical thickness is 11mm.

The second photo shows a close-up of the rear of the dovetail.

You will be able to see that there are caphead screws down the two holes, whilst in the middle of the cutout section can be seen a dowel which serves to ensure that the two halves of the head are correctly located.

As I see it, I can either use a single M6 thread or a single M8 thread into the top of the dovetail, hence the thread would be 11mm long. I suspect that M8 would be better as then the individual threads would be thicker, and therefore stronger. Alternatively, I could shape a piece of 6 to 8mm thick steel to fit inside the curved section with a clear hole through the cast iron. This would have the effect of compressing the cast iron and at the same time spreading the lifting load over a larger area.

Ideas & thoughts please. Incidently, I am proposing only using one screw through the cast iron. I feel that providing two screws, even if only M6 might weaken it too much.

MTIA

Peter G. Shaw

Unbrako have a good PDF on the subject **LINK**

I would expect even a small fixing like an 8BA screw to take 16kg in CI, if there are no shock loads.

An online calculator suggest that with 11mm thread depth in cast iron, the pull out strength for M6 will be about a ton, so you should be OK.

www.tribology-abc.com/calculators/e3_6f.htm

I used a yield strngth of 130 MPa isntead of shear strength.

Neil

I was always taught the maximum strength of a thread was 1 1/2 x diameter.

It depends on the shear strength of the bolt and of the tapped material. You need more engagement in a soft aluminum part than in hardened steel. It also depends on the class of the threaded hole, but to a lesser extent.

Also depends on how elastic the material is. Often the load is taken on only a few threads, but you don't need many for the thread to be stronger than the core of the screw.

Neil

If you mean the maximum strength that a thread can give in tension is achieved when there is 1.5 diameters of engagement, you may well be right. But I just wonder who else had to sit and think for a moment how the strength (tons per square inch, newtons per m squared) could possibly equal the diameter of anything.

In my experience, the guidance that three turns of thread do all the work is another way of looking at the same relationship. And somewhat easier to understand, perhaps.

Cheers, Tim

Peter - to try to offer an answer: There are two fairly big holes in the back face of the motor support block. It seems possible to add a plate bolted to these holes - simple bolts, nuts, washers - and then add your chain, hook, etc to the plate. This would mean that the strength of the attachment depended on -

a) the two bolts in shear - and they look like 10mm each, ie plenty;

b) the thickness and material of the plate - up to you and your scrap stock.

And in the event of failure of the extra support, no damage would be caused to the machine.

Regards, Tim

Edited By Tim Stevens on 11/05/2018 21:03:49

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.

Cast Iron is notoriously weak in tension as shown by the Tay Bridge and other disasters. Also worth knowing that cast iron has low fatigue strength in addition to low tensile strength. It's not safe to stress cast iron repetitively without bumping up the safety factor considerably.

Peter,
My X2 has been modified to use an air spring to support the head with the additional weight of a 3 phase motor. Two M8 cap screws into the cast iron rear housing/dovetail area have been sufficient for several years without any issues.

The photos below show the setup as it was before further mods recently, including moving the mill head from the lathe and attaching it to an SX1P X-Y table.

Andrew.

Edited By mahgnia on 12/05/2018 06:48:30

Edited By mahgnia on 12/05/2018 06:53:56

Edited By mahgnia on 12/05/2018 07:04:26

If the material is proper cast iron pull out of threads shouldn't be a problem. Generally the thread form in shear will be stronger than the tensile strength of the core of the screw.

Some years ago I needed to fit some high tensile M4 (Grade 12.9) screws into an aluminium (6082) heatsink where clamping forces of several hundred Newtons were needed. I did some experiments drilling and tapping holes in an aluminium block using thread engagements of 70%, 60% and 50%. I started with the 50% hole, and the screw broke before the internal thread stripped.

I also broke a number of SHCS when replacing the feed gearbox cover on my horizontal mill. The sequence went, nearly tight, tight, just a little tweak for good measure, ping! The screws were steel and the gearbox cast iron.

Andrew

Remember in the 60’s, in my youth and well into motorcycles, I was refitting the cylinder head on my Matchless G80 and was tightening the cylinder head bolts, high tensile steel bolts, cast iron cylinder, and in those days I was usually over enthusiastic when tightening bolts, before I had a torque wrench, well one of the bolts let go with a mighty crack and the spanner flew out of my hand with the top half of the bolt. No damage to the cast iron threads of the cylinder which is a testament to how strong threads are in cast iron. I can’t remember having ever stripped or pulled out threads in cast iron.

Dave W

While still at school one of my nephews took up old beat up motor bikes as a hobby, so I bought him a set of spanners, and made sure I got the shortest ones I could, I don't think he broke any bolts, his half brother was into old cars(Morris Oxford etc), and he broke a few studs, but I got him sorted in how to get the broken bit out of the engine block. The first lad is a cabinet maker, the second is a diesel mechanic with his own business(and uses a torque wrench).

Ian S C

Big yes for torque wrenches or some method of limiting torque, I.e. shorter spanners. We learn from our experiences.

Dave W

I mounted an air spring onto my X2 with a simple post bolted to the rear top surface of the head using 5mm cap screws and it is rock solid , the air spring is way too strong for what i need and has enough power to drive the head back up to the top of the column if i let go of the handle ! Peter if you want a photo of it let me know .

Hi all,

Thanks for all the replies. Some answers.

Tim. The spacer appears to be a cast aluminium block, well not so much a block as 4 sides and a top about 6mm thick with a few ribs therein. It's lightweight, so I don't think it's either steel or cast iron. I have to admit that when it comes to cast aluminium or cast iron, I'm rather shy about putting any load on.

Richard. I have now drilled and tapped the casting (see photo - caphead screw inserted). I used the workshop cleaner to suck up any dust as it was produced so didn't see the dust. I drilled & tapped easily without using any tapping liquid or grease. I'm reasonably certain it is cast iron.

Andrew. I understand what you are suggesting but I would then need to modify the spacer to be able to use them.

XD351. That's just the sort of answer I was looking for. No photo required, thanks.

To all. I've now drilled and tapped for M6 as shown in the photo, eg the cap head bolt near the rack.

You will notice that I haven't got much space to play about with, and indeed having done it, I now find that the top part of the motor cover means that the lifting chain will have to be attached on the rack side of the screw. In addition, there is a mechanical stop at the top of the column which may get in the way, but that can be made shorter or even permanently removed if necessary.

Finally, I'm not an engineer by training. Everything I know has been learned either by reading and experimenting or just by experimenting. As in Tim's reply, I am somewhat leery when it comes to castings as my very limited experience suggests that they can be brittle and thus easily break, especially if subjected to a stretching or bending force hence my question.

Anyway, thanks again.

Peter G. Shaw

I made a pulley system with a couple of gas struts for my SX2P. Photos are in one of my albums. I used a wire rope and fixed this into a block which is fitted to the cable after it is passed through a plane hole in the top of the spindle bracket. It works very well and balances the head perfectly. The hole strut assembly drops into the column and held with some screws in the top face.

I also took the opportunity to pack the rack out on the column. This improved the movement of the head, whilst it didn't remove all the play from the pinion it was a lot better than as built.

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