need help with boring

[artrans]

Hello fellow hobbyist I never used a boring bar in a mill or any where else for that matter.
And at the risk of being a very stupid question lets say you have a 1/2 hole and want to make it 3/4 or .750 how do you go about setting this up. I no things like this are not that easy to explain by writing. And also in anyones opinion is the best way of machine a part to exact size for example again lets take .750 and we want to make it .725 my problem is I take a cut measure and then need to take off more next thing I no its .720
again don't laugh but can you use the dial on the lathe. Zero that dial and trust it enough to take off .25 thosands going by the lathe dial. I mostly cut alum and brass. ??? ???

 

[ksouers]

Art,
Boring is just like outside turning, for the most part. There are some differences to worry about, like making sure the boring bar has clearance inside the bore. If it's touching anywhere it'll throw off the cut. Also cuts should be shallower than you would take on an outside cut.

Yes, the machine should be able to hold tolerance, just like outside. The big thing to remember is that boring bars are quite flexible, much more so than an outside cutter. You should make several spring passes when boring before taking any measurements, even with a shallow cut of just a couple thou. The bar will twist and push away from the work, so you need to make a few spring passes.

Kevin

 

[Brian Rupnow]

To bore a hole, you must first drill a hole larger in diameter than the boring tool. Set your boring tool up in the lathe with the cutting surface horizontal--if it looks like the heel of the tool is going to contact the inside of the bore before the cutter, you can revolve the tool slightly towards you and down at the front. (I may get shot for suggesting that.) With the lathe shut off, crank the boring bar in and out of the drilled hole while turning the chuck by hand to make absolutely certain that your boring bar clears everything. If it clears okay, cramk it out clear of the part and start the lathe. Advance the boring tool untill it is just barely inside the hole, and start to back up your cross slide untill you see that the tool is beginning to cut. Back the tool all the way out, then take a full depth cut. Run the boring bar in and out 2 or 3 tomes because it will, as Ksouers says, "spring" quite a bit. Back the tool out, roll the carriage back, and shut off the lathe. Measure the hole to see what you have. At this point you simply HAVE TO trust the feed dials on your machine. From then on, you just keep repeating the cuts and checking.

 

[joeby]

A few things to remember:
1) Keep your bar as short as possible to reduce flexing. (Read as little overhang as possible)
2) Use the largest diameter bar you can, making sure you have chip clearance. (Also helps with springing and chatter)
3) Feed by hand the last bit (.100" or so) in a blind hole to avoid gouging the bottom.
4) Set your boring bar just slightly above center. It will spring a little no matter what you do, the cutting edge will then drop closer to center instead of dropping below.
5) If chatter becomes a problem, you can try to adjust the tension on the toolholder, wrap a strip of an old bicycle innertube tightly around the bar shank, hang washers on the back end of the bar, etc. Of course, start with feeds and speeds, overhang, bar diameter, etc., and then get creative from there.

I know I've left a lot out; but just a few points to start with.

Kevin