If you have a fixed steady, use it to support the end of the job.

200rpm might, counterintuitively, be a bit slow for insert tooling. Try speeding up to 400 or so. Plus inserts dont like light cuts so you might try deeper cuts.

At that diameter and depth you might even get in there with a normal turning tool at a suitable angle which could be more rigid.

Try it taking cuts off the face working from middle to outer edge then a final cleanup along teh 80mm dia. Although a bigger bar would help you are not going that deep so overhang will be minimal. Somethines you just hit a diameter that at a certain speed starts to ring, modulating the varii speed pot can help.

200rpm when only at 40mm dia is slow, I'd be about 800rpm at the full 80mm dia, 1mm DOC

Have you checked the tip, sometimes a tiny flake comes off that's hard to see?

I now only use steel inserts on hard spots. Try CCGT, much sharper.

For an 18mm deep bore, it would be best to have only 20mm of the bar projecting and set the saddle stop to prevent the toolpost from hitting the work. As the nornal indexable boring bar has the tip height much lower than turning tools, a 16mm bar would be a better choice. The gibs on the saddle if adjustable and the gibs on the cross slide and compound should be checked. If you can change the insert for another grade, it's worth a try.

I am currently modifying a 25mm boring bar to fit in a larger toolpost which will be attached directly to the cross slide of the Smart & Brown model A which will greatly stiffen up the boring of holes bigger than 40mm. Even at 25mm diameter, the Sandvik bar is marked at 100mm for maximum depth.

Edited By old mart on 06/09/2020 15:59:27

Absolutely correct JasonB.

If you go to a smaller 0.2 radius insert, it will get rid of a lot of the vibration and has a reduced tool pressure when cutting. For best results with boring bars, you actually want them about a thou or 2 max above centre height. When on or below centre height , the bending load on the bar, makes it take a bigger cut. When slightly above, the load comes down to the centre height and has almost no increase in cutter load. Technically it is reduced by a smidgeon depending on the diameter of the work piece.

A big blob of 'Blu Tack' placed onto a boring bar works well to dampen vibrations due to it's strange 'Non Newtonian' properties.

This is not a joke, I learned enough about Blu Tack from an MEW article a few years ago to design a unique barrel tuner for a rimfire target rifle.

* Danny M *

I get my inserts from Cutwel and live by thier recommended feeds/speeds/depths of cut using anything between the min and max recommended depending on the strength of the setup and part, but almost never outside of this.

 You are perhaps not by coincidence running almost a similar speed/feed to what I would use on my NC5330 coated CCMT tips (I use Korloy CCMT09T308-HMP) – keeping in mind I'm on a Colchester with 20mm tooling.

Feed's are recommended 0.1-0.3mm/rev, …..check

DOC 0.5-3.0mm ….. check…. but 0.75-1.00mm might solve your problem if your lathes up to it.

However…… 200rpm………I would be aiming between 130-300 meters/min surface speed, you're under 30! As you approach 40mm you want to be SLOWING down to around 1000rpm (=130m/min) – if the chips aren't purple and bouncing off the ceiling, you're not doing it right…..

Worth noting…. my colchester has an automatic cutoff on the depth, so I can let it loose at 1000rpm&0.2mm/rev fairly care free, my Myford 280 has a graduated handweel on the saddle, when things are moving fast I mark the depth on the mm scale with big bright bold marker so I cut the feed when I get to the depth, requires concentration, and a knack……

Edited By Russ B on 07/09/2020 13:00:04

Edited By Russ B on 07/09/2020 13:03:30