Boring an Al Alloy casting

[bp]

I've just started making a Sugden Special.  The first task to make the crankcase casting look something like a crankcase.  The first problem!!  This stuff machines like toughed steel!!  The only way I was able to get an acceptable finish while facing the back of the casting was to use a Sumitomo cutter, when I came to drilling and boring, my selection of (newish and all sharp, Suttons) drills didn't/couldn't make the full depth, and the hole looks like it had been chewed out by a termite.  So I tried boring the hole.....HA, it just laughed at me.  The little barsteward.
Looking at the faced off surface, the only acceptable bit of machining so far, its covered in tiny marks, porosity of some kind I suppose.

I no longer have the material spec, but from memory its a common UK high strength casting alloy.

I've looked at a Sumitomo boring tool, apart from the cost, the minimum bore is bigger than the crankshaft bush, so that's out.  Glanze have some really small boring bars..... are they any good.  Should I bite the bullet and get a solid carbide boring bar??
Currently I'm even thinking about machining a crankcase from solid 6061 T6, which I have and it machines beautifully, but obviously that's a fairly major decision as it involves a huge amount of work.
Anyone have any ideas??
cheers
Bill Pudney

[Jasonb]

Are you using any cutting fluid like parafin or WD40, that helps a lot on alloy.

Also one of the casting alloys LMsomething neads heat treating after casting to make it easier to machine.

J

[bp]

Yes, I was using paraffin, if anything it seemed to make it worse!!  The castings were supposedly heat treated. 
One thing that has occurred to me is that the one cutting tool that gave an acceptable finish, the Sumitomo one with the polished insert for alloy (from Arc Euro), has a lot of clearance and top rake, certainly more than any of the boring tools I tried, mind you that doesn't account for the lack of action from the drills.....
cheers
Bill Pudney

[Jonny]

Interesting i do a lot with 6082 T6 purely that it will anodise well.
Just recently i have had to change to a Glanze interchangeable boring bar bought many years ago, presentable with supplied tips but needed a few passes as they flex like no tomorrow with the proper H13 Sandvik tips, figure. Time to lash out on a solid carbide jobby.

Check tool geometry, might need highering lowering or twisting/tilting.

Castings are known as LM think its the silicon content. Usually castings i have come across machine like butter or free cutting alum with or without coolant.

Are the drills any good, theres a lot of crap out there the last 10 years. I only buy Titex or known to be good drills, you will be surprised! http://i1140.photobucket.com/albums/n563/Jonhareng/Jons%20machining/RamFixing9drilling.jpg?t=1294535967

Whats the symptoms you are getting?
Rough finish, burred up, tool tip build up, flexing away.
Or not cutting then suddenly digs in or vice versa suggests movement and or geometry.

[bp]

Most of the al. alloy I have machined has been 6061 T651, 2024 T3 and better and 7075 T6.  I had wondered if the silicon content (if any) in the castings was causing the problem. 
Specifics, the drills I have been using were all Suttons, and either new or near new.  They all had two sharp edges, but now they all look very second hand and are up for a good sharpening.   With the first casting it was not possible to drill more than about 75% of the depth required, the drills simply stopped cutting.  Yes I was clearing the swarf from the flutes.  The second casting drilled better but it was still REALLY slow going.
When trying to bore with a pretty rigid (10mm sq HSS, with minimal section reduction), the tool basically was pushed off.  Centre height was spot on, one of the uses for my centring 'scope!!  At one point the reduced section of the tool, about 4mm x 8mm in cross section, could be seen bending!!
The specific symptoms are ....
1  Extreme tool wear, good quality drills blunted in two castings.
2  Well ground "on centre" HSS boring tools pushed off with VERY rough finish
3  Very slow cutting.  They machine unlike any Al. Alloy that I have machined.
4  The only tool that worked had a VERY sharp cutting edge, with lots of front clearance and top rake.

Today I had a go at the second casting, this machined better, but it was still really hard work drilling the crankshaft hole.  The second casting shows much less porosity at the backplate face.  The porosity on casting No 1 would be enough to scrap it if I was still working and asked for an opinion.

Anyway I've ordered a boring bar with the same type of tip as the Sumitomo facing tool that seems to work.  If and when that arrives we'll see how that works.  I'm seriously thinking about the implications of machining from bar.....sad or what??
cheers
Bill Pudney

[Jasonb]

Any chance you have got sand inclusions, I had it once on a bronze cyl casting, tool would cut for a while then as it hit a bit of sand/dross lost its edge and got pushed off the work.

J

[sparky961]

There are lots of things that come to mind, but I was also thinking along the lines of sand inclusions.  I'm not sure what the tools you're using are made of, as I think you've referenced brand names.  Ordinarily, HSS works great on aluminum of any sort I've machined, but in this case carbide might be the tool of choice.

I've done some aluminum casting and found that my castings machine more like cast iron than something tougher like 6061.  Getting through the skin and any sand inclusions can do a real number on an HSS tool though.  As far as porosity, I do find many bubbles near the surface but not too many getting deeper down.  I don't find they usually interfere with the machining.

To each their own, but I haven't found a lubricant that is worth using on aluminum.  I machine dry with plenty of success and great tool life for hobby needs.

Some pictures of your troubles would go a long way to help people understand what's happening and help out.

-Sparky

[Jonny]

Bill i assume its a smallish hole you are trying to bore out if using a 10mm dia bar.
There are lots of rubbish HSS about that wont hold an edge i only buy the high content cobalt ones around 8%.
What i suspect is the hss bar or movement in the machine ie far from rigid and did have the same in an ML7 until scrapped.

Of course the casting could be another form of alloy but everything i have ever come across has been easier than 6082 T6 and much like machining chinese castings.
The Suttons drills are poor, very poor not a great deal harder than MS keep the heat out. Dormer, Titex you wont go wrong as what used to be exceptional brands years ago are no more, same with saw blades.

[bp]

I've found out that the castings were/are heat treated LM25.  The opinion of the supplier was that my difficulties have been caused by the inclusions from the dregs of the pour.  He is generously providing two new castings......hopefully from an earlier part of the pour!!
Interesting what you say about Sutton drills, as I have found that they are generally the best that I can easily get up here in 'Straya.  I'll have to have a look around for some Dormer and/or Titex.
cheers
Bill Pudney