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Machining cast iron

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Robert Mullan19/05/2009 11:44:46
12 forum posts
I have a Stuart 10V as my first machining project, I also have the book (I can't remember the exact title off-hand but it is something like 'Building a Vertical Steam Engine'). The instructions for facing the cast iron base in the book say to take a deep cut for the first one to avoid blunting the tool.

How deep?
David Clark 119/05/2009 12:53:14
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3357 forum posts
112 photos
10 articles
Hi There
Stuart castings are excellent.
I doubt you need to worry about the tool going blunt.
When that book was written I suspect they were still using carbon steel tools.
Modern high speed tools should be fine.
regardsDavid
 
Robert Mullan19/05/2009 12:56:35
12 forum posts
David,

Thanks for the helpful reply.

Rob 
Ian Lee19/05/2009 22:09:42
53 forum posts
Hi Robert
The reason why a deep first cut is take on cast iron is that quite often the surface skin on an iron casting can be quite hard as sometimes the casting may have become "chilled" and contain hard spots, a deep cut usually gets under these chilled area and cuts under them to remove the area.
 
As David says most modern cutting tools will cope, particulary the carbide tipped variety and a deep cut will not be needed, also modern foundry practices also reduce the prelavence of hard spots
 
Regards
Ian
Alan Gordon21/05/2009 13:54:59
2 forum posts
Hi Robert
 
As a relative beginer and having built the 10V i know exactly what your worries are. The question to me how deep should be the 1st cut ?
The above answers are of course correct but I still took the minimum 1st cut and it was within seconds that I had broke through to the soft stuff.
I made a few cock up's to say the least and i am sure I kept the balance books of Stuarts looking good with all the replacement castings I had to buy, but were are all on a learning curve.
best of Luck.
 
Alan
John Wood126/05/2009 15:10:32
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116 forum posts
Hi Robert
 
I am also a relative beginner and whenever I come to turning something unusual I adopt the 'gently first and see' method. On a casting I would start with a light cut (lets say 10-thou or so) at a fairly slow speed and see what happens. If the lathe handles that OK try a bit deeper next cut and so on until you notice the work protesting, then you know you've gone too far.  You can do the "deep cut" recommended in your book when you have a better idea of the capabilities of your equipment.
 
With iron castings I always start with a sturdy high-carbon roughing knife tool at least until I am through the 'crust' , that way, if there are any hard bits which damage the cutting edge then it's an easy job to regrind, only when that's done do I use a carbide tipped tool because, if a tip hits a very hard spot it's likely to chip and thereby ruin the expensive insert.
 
All the best
John
mgj06/06/2009 23:28:18
1017 forum posts
14 photos
Some engineers suppliers will sell single tips. Myford (used to?) sell  sets of inserted tip tools of 10mm sq shanks. They don't use the standard CCMT tips rather the squashed triangle WNMG-06-04 negative rake tips. (Thats the Sandvik number) They come with a .8mm tip radius and are brilliant for cast iron, as well as general turning, so long as the tip radius is not a problem. Produce a near mirror finish, and very economical because you get 6 points per tip, being double sided.
 
They do suffer a bit in iron because of the abrasion, and there is a KR designated tip specially for iron, but they are not so easy to come by, being a bit more specialised, wheras the WNMG tips are amongst the commonest used in industry.
 
The only problem is that most negative rake tools are quite a bit beefier than would fit in most model engineers lathe  - which is why the Myford tools are so handy. You can take much bigger cuts compared with the CCMT 06 tips. 150 thou or so at 600 odd rpm in mild on a Super 7, whereas the standard CCMT tip often gets unhappy at 50 thou. They won't do the 250 thou that a steeply raked ground up knife tool will do in free cutting mild, but even so they will shift some metal without the vast quantities of ribbon like swarf that the ground tool will make.
 
The extra thickness of the WNMG tips also makes them much stronger and less likely to chip in the cast skin during an interrupted cut.
 
So if you can find a set of those Myford tools it would be well worth it.
 
How deep. I'm afraid I don't start at .010 because with a ground up tool you are in the skin all the time. Sometimes that skin is no problem at all, and sometimes its like concrete, and you don't know till you have knocked the edge off your tool in a few seconds! (Especially with some of the cheaper grades of high speed steel which seem to be doing the rounds) I always start with .030 - .050, but as suggested I keep the speed and feed down. One wants to be sure that the casting is securely held of course, (with 3 points of contact if its on the faceplate.
 
A 6" lathe will handle much bigger cuts , but those figures are what I use  on the Super7.
 
 

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