Boring Noise & Finish

[Jasonb]

I have a SELPR style holder that will cut from 7.5mm upwards but it does use a very small insert that is less than the usual CC** 06 size inserts.

https://www.glanze.co.uk/product/miniature-ccmt-selpr-boring-tool-790550/

Many of the holders have the insert at an angle and if you look closely at them you can see the angle gets steeper as the minimum hole size decreases, a selection of mine. The very small one is on the right, 6mm shank middle and 8mm left



The table here gives the actual angles of their bars

https://www.arceurotrade.co.uk/Catalogue/Cutting-Tools/Lathe-Turning-Tools-Indexable/ARC-Indexable-Turning-Tools/ARC-S-SCLC-R-95-Boring-Tool-Holders

[PaulR]

However a CCMT insert will always rub in  a 10mm hole with whatever size boring bar you fit it in.

OK, maybe I was wrong; although there seemed to be enough clearance it's not at all easy to see. What's the best solution for a sub 10mm hole then, a round bar with a little 'pencil' of HSS through it held by a grub screw? I think I made one of those in the past.

[Jasonb]

The type you describe would practically go down to about an 8mm hole if the bar were 6mm.

Less than that you really have to start looking at solid HSS or carbide. I have both but these days with the carbide being so cheap from the likes of Ali it is not really worth thinking about HSS. Plus they will cut to the bottom of  a blind hole. This is one of many to choose from.

https://www.aliexpress.com/item/1005006350444458.html?spm=a2g0o.detail.pcDetailTopMoreOtherSeller.4.1c25TURkTURkQE&gps-id=pcDetailTopMoreOtherSeller&scm=1007.14452.396806.0&scm_id=1007.14452.396806.0&scm-url=1007.14452.396806.0&pvid=36239d1a-06e7-4e4c-b910-4f6ab66b3c9b&_t=gps-id:pcDetailTopMoreOtherSeller,scm-url:1007.14452.396806.0,pvid:36239d1a-06e7-4e4c-b910-4f6ab66b3c9b,tpp_buckets:668%232846%238110%231995&pdp_ext_f=%7B%22order%22%3A%2275%22%2C%22eval%22%3A%221%22%2C%22sceneId%22%3A%2230050%22%7D&pdp_npi=4%40dis%21GBP%216.22%214.41%21%21%218.02%215.69%21%402103856417465137576225419ef0de%2112000036862161358%21rec%21UK%21855056161%21XZ&utparam-url=scene%3ApcDetailTopMoreOtherSeller%7Cquery_from%3A&search_p4p_id=202505052342376699050339357514095487_3

[PaulR]

I have a SELPR style holder that will cut from 7.5mm upwards but it does use a very small insert that is less than the usual CC** 06 size inserts.

I see you have the bars held directly by the toolpost screws. I though there might be a tendency for the tool to rotate if it wasn't centred under the screws so I made a holder from a bit of scrap steel with grub screws on top. The holder is probably too short though as it'll only allow two of the TP screws to contact (tightening the other two would tend to bend the bar downwards). Maybe that's part of the problem as well...

[Jasonb]

The flats on the bars should be enough to stop any rotation and will also set the bar at the optimum angle.

[GRAUBELE]

To clamp round or square turning tools neatly and at the correct angle,
two different turning tool holders are used.
One has a flat support surface for the turning tool,
while the second has a prism as a support surface.
This ensures that the cutting insert is always at the correct angle (clearance angle and rake angle).

Dieter

[Chipswitheverything]

For boring quite small holes that need that process rather than reaming, the various small boring tools and holders for which GH Thomas gave designs , in the Model Engineer magazine and also in his workshop practice book, are convenient , and work very well. The tools can be made up cheaply from odds and ends of bar and from silver steel, and tailored to particular requirements.  If the silver steel small boring tools themselves are hardened suitably and then honed to a nice sharp edge, the finish from them, with some cutting fluid and moderate speed can be excellent.  Apologies for poor quality of the rather ancient photos of a few of my tools of this sort! Dave

[PaulR]

Thanks all for the replies, the details and photos are a great help. 

[ShopShoe]

Possibly you could use some blue dye on the tool and inspect after a trial cut to locate areas of undesired contact. I did that a few times when I was first learning to grind my own tools. A good magnifier is also helpful for visual inspection.

ShopShoe

[PaulR]

Possibly you could use some blue dye on the tool and inspect after a trial cut to locate areas of undesired contact. I did that a few times when I was first learning to grind my own tools. A good magnifier is also helpful for visual inspection.

ShopShoe

Good tip, thanks!

[Overbuilt and Overkill]

Your first picture shows a classic case of what's usually referred to as tool chatter Paul. Unfortunately the complete answer or cause isn't always simple, or in some cases have an easy solution since the cause might be due to varied or multiple reasons. Boring is particularly susceptible to it's creation for the obvious reasons of having to use a less than rigid and extended cutting tip position. Think of it as the boring bar being much like a tuning fork. While it's the most likely cause, that's still only one of possibly dozens of other reasons including parts within the machine tool itself, or even the parts being machined and there cross section or shape. So the cutting tool, machine or part itself may be part or even a combination of the causes. Then you start getting into the extremely complex frequency of vibration issues. https://www.secotools.com/article/120669?language=en And even that is very simplified.

Fortunately we only need a basic understanding of how it happens and what the causes could or might be. But even something as hard to diagnose as slightly out of adjustment spindle bearings could in some cases contribute to the issue. So in reality, there's no one single answer that applies in all cases. But let's suppose that the lathe has no other problems or shows that tool chatter when the more usual turning or facing tasks are being done. The first 3 items you can vary when boring are the rpm, feed rate and depth of cut will 'usually' but certainly not always move the tools shank to a different and non sympathetic frequency of vibration range. With boring, the rough rule of thumb is to first try reducing the rpm, and / or increase the feed rate. Increasing that depth of cut can sometimes also work. Fwiw, rarely does increasing the rpm, reducing the feed or depth of cut work. It's not impossible it won't, just less likely it will in most instances. Why that works? The boring bar is then being loaded differently and is then moved outside the tools natural frequency of vibration. (NFV)

When that doesn't work no matter what you try. The next step is to try to deaden that NFV. Something as simple as molding a piece of plumbers putty, plasticine or modeling clay to that extended tool shank, or even tightly wrapping it with a length of solder can sometimes work because it's then helping to absorb or deaden that vibrational frequency. Again there's too many variables for a single method to work in all cases. Even thin walled parts can also cause that chatter to show up. But sometimes something as simple as an elastic band or rubber cord wrapped around the parts O.D. might be enough. Excess or any clearances within the lathe slides can also be a partial contributor. For that reason locking any slide not actually being used may or may not help. Learning this cost me a lot of time to even figure out the correct search terms, and I still barely understand the basics. As you've found, you can almost always hear it happening once it starts, so that's your first indicator that some changes need to be made.

There's also no guarantees that it will only start with thin walled parts. I've personally seen it start to happen in the middle of a 12" long hollow shaft that had a wall thickness of over .625". Since I couldn't reduce the rpm anymore, increasing the feed rate in that case stopped it. But the middle 3" of that shaft had a different frequency of vibration than the two ends did simply due to the better head and tail stock support for the material type and part cross section I was turning. 

[PaulR]

Your first picture shows a classic case of what's usually referred to as tool chatter Paul....

Thank you for taking the time to provide that detailed answer and a great selection of tips for reducing vibration and getting rid of chatter. I'll experiment over the next few days and see if I can reproduce and eliminate the effect.

[PaulR]

Spent a little time boring a piece of random scrap steel (I think it's some nickel-steel variant judging by the shiny surface) with different RPM, feed rates, depth of cut and oil or no oil. I managed to avoid any vibration except when using high RPM when the finish was correspondingly poor. After lots of cuts I ended up with a finish I'm reasonably happy with; there are still some tool marks visible but the surface feels very smooth. Hopefully I should be able to get an even better finish using mild steel or brass.

[crueby]

Great!
Do you know what thing or things you changed that helped the most?

[PaulR]

Great!
Do you know what thing or things you changed that helped the most?

RPM not too high, tool well clamped down and carefully set on centre, touch of oil, cut of at least 2-3 thou but most of all a very slow feed. I was hand feeding it which probably made the tool marks a bit more prominent.