A cast steel channel to substitute an extruded one

[arunmrao]

I have been asked by a customer to explore the possibility of substituting a U section channel wall thickness 8 mm and 2000mm long by a cast product. This is used in a medical equipment over which is fitted the rack for sliding the bed.

As the quantity is small sourcing such a product is difficult also forming by bending a sheet has not produced good results. a flatness of 0.5 mm across the length is expected.

I plan to cast a channel with extra stock and later machine all the faces to the required dimensions. I shall be checking ultrasonically for any defects.

I would like to know the precaution to be taken in using such a product and will it ever fail catastrophically( a scene I do not want to imagine).

 

[gieter]

You will need a hugh amount of machining allowance in order to have it withing the 0.5 mm flatness spec. Even then I doubt if you will meet it. Also to make it castable in a sand mould you will need extra thickness to avoid cold shut.

What kind of alloy are you talking about and what are the exact dimensions?

 

[arunmrao]

The grade of steel is WCB and the stock I am maintaining is 3mm all over. The wall thickness cast will be 14 mm ,I am still concerned about flatness, will post machining pressing operation assist? Cold shuts I can handle by providing multiple gating and pouring simultaneously at the two ends.

 

[gieter]

It's my guess that with only 3 mm allowens you will need to press it before machining.

I wish you good luck and hope you quoted enough.

 

[unclesyd]

Going with a casting and that much machining your got your work cut out.

Is possible to find mechanical tubing close enough to your dimensions and machine it?

 

[arunmrao]

Unclesyd,geiter,
thanks for your encouraging response. I have a difficult task cut out, but considering the fact I may be considered for future casting requirements I am going through this grind. However, what are the pitfalls I might encounter,please advise .

 

[unclesyd]

I was thinking of the potential problems in machining and finishing the part in particular
the holding of the work piece and the stress set up while machining. As mentioned straightening along with stress relief maybe required for the part. I would look for having to do an intermediate stress relief after the “hogging cuts”prior to final straightening and machining.

I see you know a little about he foundry side so this maybe redundant. On quite a few of our WCB castings we normalized and tempered after casting prior to machining especially were there was requirement for tight tolerances and stability.

Good luck and let us know how it looks when you break the mold.

 

[Metalguy]

Let me just add that if you are able to sucessfully make/machine this part and have decent quality castings, I strongly doubt you'd ever have a sudden brittle fracture problem. Cast steels in general can be very tough, quite unlike most cast iron.

 

[arunmrao]

Metalguy ,"Cast steels are normally tough--", it is with this optimism I am progressing with this assignment. Thanks for all your comments and suggestions and I shall certainly come back and share th results in 2 weeks time.

 

[arunmrao]

Unclesyd and all other members

Thanks for your helpful comments. I have just opened the mold and cleaned the casting. It has come out with very less bow(1.5mm max). I can handle this as there is adequate machining allowance. Next task is the difficult one ,machining and maintaining the dimension.

 

[unclesyd]

arunmrao,
Congratulations on a task well done in a record time.
That was nice casting job and as you stated that amount of bow is very manageable. Now stay on top the machinist and keep the tools sharp.

Thanks for coming back with a progress report.

 

[gieter]

Sorry guy's, been away for a while.

Well Arunmrao, how did it end up?

 

[arunmrao]

gieter,
the casting did not have any camber or bend. Also the machining did not pose much problem. I finished off the job with grinding operation.

 

[gieter]

Congratulations on a job well done!
The foundry industry can use successes as these.

Can you give a hint on the gating and feeding system you used?

 

[arunmrao]

Thanks geiter.
I have used side feeders of 2.5in diax4in ht. 5 such feederwere placed along the length equally spaced. For gating I used finger gating,with runner bar all along the length of the casting. I poured manually(normal practice for me) simultaneously at both the ends. This ensured proper fill and the side risers avoided hot spots on the top face of the casting. Thus even while cutting the feeder there was no bend. An additional feature was that i had provided 5 internal tie bars of 10mm thick ,these were subsequently removed by machining.