Need Help!- holding a 125 finish.....

[Stifler51]

how can i about getting a 125 finish ID of .500 +.004 -.001?? depth is 3inches, mat 4140.

[Glacern]

This could be difficult. You definitely need a solid carbide boring bar, and probably a good quality cermet insert. Take a look a Tungaloy's inserts.

Small corner radius, high SFM (1000+), low feed.

Does the surface finish have to be that high through the entire length of the bore?

[Delw]

cermet ccmt .008 rad, 3/8 solid carbide boring bar, then wrap a bunch of rubber bands around the boring bar close to the holder and around the part if possible.
the rubber bands will take the slightest chatter out.

a steel sandvik bar will work in a pinch with cermet inserts however your going to need to run 25-40% of recommended SF take a bunch of taper passes in your roughing probally around .010 ir .005 per side.
and rubber bands are the key along with insert above center height a tad.

make sure you use a split collet holder even if you have to make it. as the set screw ones just dont work that great

With cermets on bores that size run about 50-75% of reccomended SF if you run to fast you will get chatter on small boring bars.

you will have to program about .004 taper ie .002 per side

3" aint that much. hold in a collet if you can or full jaws, alos make sure your insert is just a tad above center line. carbide inserts sucks so dont waste your time as no 2 parts will be alike

a 32 finish is no problem and a 64 is cake.

oh yeah with a tolorance that wide open you can flip the part and blend in and get an easy 32 finish

Delw

[ctate2000]

Bore straight, ream for finish.

[ctate2000]

Buy an OSG MAX series drill and drill it. Holding both size and finish. 1/2 drill about $135 will last for a very long time.

[PixMan]

Boring? Reaming? Nuts.

You should have ZERO problems holding that size tolerance getting a finish twice that good by simply using a solid carbide drill at the correct speeds and feeds.

I strongly suggest a insert-tipped dril such as the Kennametal Ken-Tip or Walter Titex A931XHNI if you have coolant-thru option on your machine, or the solid carbide Walter Titex Aplha 2 if you don't. No pecking, just run it in. Perfect.