I f you don't have the luxury of a taper turning attachment, you have to advance the compound by hand. This makes an even speed very nearly impossible. The depth of cut may be too deep for the diameter of the boring bar and the insert may not be sharp enough for finish cuts. The tip needs to be at or just above the centre line (+0.005&quot. I would use the biggest boring bar that would fit the hole. You can probably get a larger diameter bar to fit your lathe. Measure the height of your bar above the bottom of the toolpost , it may be possible to get a 16mm one if the 12mm one has 4mm of packing shims under it.

You don't have to pay much to get a reasonable 16mm boring bar. And CCGT09 inserts for aluminium will also fit it.

**LINK**

Good to eliminate problems one at a time!

I set the tool height using either a length of rod faced carefully to centre height as a gauge or the steel strip method as per photo. Sorry it's sideways!

The steel strip is pinched between the tool tip and a rod. If the tool is too high, the top of the strip moves away from the operator. If too low, it tilts towards the operator. The tool is centred correctly when the strip is vertical.

When facing, adjust tool height until there's no pip, or deliberately make one so the point can be compared with a centre in the tailstock:

If still not cutting reasonably with the centre-height correct, it might be the material: ordinary mild-steel isn't wonderful. En1a-Pb machines much better.

Other possibles, 850 rpm is too slow, the bar tends to chatter because it's on the slim side, and/or a sharper insert is needed.

I think I mentioned in another post that running before one can walk isn't the best way for a beginner to start! In this case, boring an internal taper is harder than boring parallel. Once boring parallel is mastered, it's easier to spot what might be going wrong with a taper. And one shouldn't be boring until plain turning is mastered.  When still at the unsure about tool-height stage, keep it simple. Only move to trickier jobs once reasonable results are achieved with the basics.

Dave

Edited By SillyOldDuffer on 10/03/2020 20:38:28

Tip radius looks big, if the last two numbers of the six on the box that begins TPMR or TNMG etc are 08 it's a roughing tool. You need 04 or 02 for finishing

My introduction to lathe tools with carbide inserts was about 15 years ago when I bought a toolholder with 10 tips at a local steam rally, I was instantly converted!

I then bought a set of five tools with inserts from a well know supplier. The tip were pure rubbish, if the purchase had been my first introduction to indexable tooling then without doubt I would have been put off for life.

Point of me mentioning this is that the tip in Steve's picture looks to be identical to the ones supplied in the set I mentioned, they have the same dull grey look and the same parallel 'gutter' around the perimeter.

Ian P

PS, I did not know much about tips when I made my steam rally purchase but I now know they were the fine grained polished type for machining aluminium and plastic. (which I now use for 90% of all my turning)

The one in the middle is a **GT tip and will give the best finish. 

But If you have a HSS tool use that instead as its much sharper, set the tool just above centre height, ramp up the speed, small depth of cut, feed in as smooth and slow as possible, use cutting fluid. You're welcome.

Edited By Hollowpoint on 11/03/2020 00:01:51

When turning the handwheel don't just use one hand on the handle, you should use both hands on the rim of the wheel which helps keep a constant feed but more importantly helps eliminate the risk of up/down movement as you crank the handle. Locking th eother slides that are not being used will also help.

Corner of the tip that first makes contact is what you want on ctr height.

I also saw that screw last night, if the tip is not held firmly down and back against the edges of the pocket it could be moving about get a proper sized replacement with the right CSk angle.

With that kind of bar it's usually mounted so the insert is angled downwards towards the tip to stop it rubbing. The flats on the body usually force correct orientation so I'm guessing this isn't your problem, but it may help someone else.

Neil

EN1A should also be OK to weld as there is no lead in it to give problems.

That's what I thought as well.

It would be interesting to know what grade of stainless is being used. Of the austenitic stainless steels 303 and 316 are easy to turn, whereas 304 is 'orrid stuff. In my experience 303/316 are fairly insensitive to cutting speed and depth of cut, although I'd regard the depths mentioned as rather shallow. The only golden rule is never let the cutting tool rub, as the steel will work harden, 316 especially so.

I don't understand why a flexible boring bar would cut a tapered hole. There might be some bell mouthing as the cut comes on. But once the cut is established, and the bar has deflected by whatever it wants, then the cutting process is stable and the diameter of the work won't change. The diameter might not be what you expect, but it won't be tapered. That's definitely my experience. It's counter-intuitive but the best way to kill chatter when boring is increase the depth of cut and/or the feedrate. The boring bar may deflect a little more, but if the cutting process is stable then the bar will not chatter.

Andrew

Definitely agree with setting up the topslide properly. Take the slide right off and clean the ways etc. Lubricate well including the lead screw and the mating faces of on the front and rear of the leadscrew bracket. Reassmble without the leadscrew and adjust the gib by feel as you slide the topslide back and forwards by hand. You neen silky smooth with no rock. Push it fully forward and refit the leadscrew.

This bit will sound daft, by have you tried starting the cut at the bottom of the bore? I've not tried it on an internal taper but I always seem to get a better finish on an external taper when using the topslide. I have no idea why.

regards Martin

Don't know the answer, but I'd agree that one generally gets a better finish on the way out, for internal bores at least. I haven't tried it on external tapers.

Andrew