Aah, I’m glad I’m not alone! I was thinking hard if I should write about that; my fear was a bit to look like a know-it-all. But after searching a lot about Steel constituents I was quite sure my gut feeling was right.

Well I think something like that is not a ‘keyboard slip’. Informations given for beginners should be correct. How about giving RPM figures with a resolution of 1 RPM? I already see the letters coming in:

“Help please, Mr Knight says I need 458 RPM to turn 1″ BMS, but my lathe only does 420 !!!”

Or to (nit-)pick a bit more:

Steel with carbon content from 0.05 % to 0.6 % is called ‘low carbon steel’

Steel with carbon content from 0.3 % to 0.6 % is called ‘medium carbon steel’ after Mr. Knight.

Question: what now is a steel with 0.5 % carbon??

(a possible answer could be: a ‘slip-of-the-keyboard’ steel — )

Greetings, Hansrudolf

Long ago and far away a certain number was dinged into my skull (I can still feel the bump there). The number is the speed for turning mild steel with High Speed/high carbon steel tools. In inches the RPM to use is 288/D where D is the diameter in inches of the billet you are machining. In metric it is 7315/Dmm, where Dmm is the diameter in millimetres.

These figures are guide lines. If you are machining a 1” bar and running at exactly 288 RPM you should increase the speed after every cut. If you are ‘facing’ something and think like that then you have big problems.

If you use the calculated RPM based on 288, and adjust your feed rate to get a suitable cut.

Remember not all lathes have infinitely variable speeds, but the magic number will give you a good feel for the speed to use. So the difference between 420 and 458 RPM is pure nit-picking. I would tend to use a speed around about 300 RPM for a 1″ billet.

No two bits of steel are exactly the same even if they came from the same billet.

David I keep well clear of the Alps the Bacco-pipes there are huge. It would cost me a year’s Bacco rations to lend one of them lads a fill !

Posted by Stub Mandrel on 01/10/2011 19:48:28:

Congrats to Mick for taking on a tough task. Being largely self taught, I find articles by tme-served engineers/machinists really useful – there’s always something to learn.

 

To paraphrase “You can please some model engineers some of the time, but you can’t please any of them all the time!”

 

Neil

 

Hi Neil,

 

Such articles are only useful if accurate. If not they should be challenged (see my comment below re. ‘peer reviews’). If one mistake occurs how do you know there aren’t more unless corrected?  Which do you prefer, accuracy or ‘no comment’.  It’s not about ‘pleasing’ anyone, but about accuracy.

 

With all respect to Mick’s article perhaps it should also be pointed out that white and grey cast irons are brittle full stop, while malleable cast iron is not. Also cast iron is very good in compression, hence all of those cast iron columns in Victorian buildings and steam engines etc (not to mention the Ironbridge) while it is very poor in tension, hence no cast iron ties or bolts!

Edited By Terryd on 02/10/2011 12:11:09

Thank you very much, Terry, for that above. It describes exactly the point(s) I wanted to make, partially unsuccessful as it seems. Well maybe I have a bad understanding of British humour…

Glad also that you commented about the ‘cast iron is brittle in compression’ claim.

I did not want to add insult to injury to bring this forward also.

Going back in shelter also (without bacco pipe, though ),

Hansrudolf