Boring Noise & Finish

[PaulR]

Decided to use an hour of 'shop time to practice something I have done for 10+ years: set up for an offset hole using a DTI, then drill and bore it using a new indexable tool. After drilling to 10mm, I started boring using very light cuts of no more than about 5 thou and while everything seemed to go ok and the final hole is accurate enough, cutting was accompanied by a very high pitched squealing and the finish looks like a knurled surface (!) although it feels pretty smooth.

This was 1/4" thick mild steel, a new carbide insert in a 8mm bar with an overhang around 10mm and tried with and without cutting oil and at low and medium speeds. I'm guessing this is chatter but I can't see where it's coming from unless its wobble caused by the offset. The only bars I've used in the past were the chunky type with welded on carbide tips.

Any suggestions/tips please?

(I didn't try at high speed as the piece was near the front of the jaws, not backed up and offset which seems like a recipe for disaster  ).

[crueby]

Can you show some pictures of your setup as you ran, bar holder, part in chuck, etc?

[PaulR]

Can you show some pictures of your setup as you ran, bar holder, part in chuck, etc?

I took the part out for the photo so I've just placed it back. I've just measured the tool overhang and it's more like 17mm so not excessive. The bar is held on the flat upper surface by the tool post screws.

[Vixen]

A 8mm boring bar with a CCMT 06 tip can only bore a minimum hole diameter of 10.5mm (some brands say 11.0mm min). Below 10.5mm, the lower edge (corner) of the insert rubs on the work. This appears to be what's happening in your case.

You either want to start with a bigger hole or use a smaller diameter boring bar and insert.

Mike

[Jasonb]

Also try taking a deeper cut, it keeps the load on the tool rather than letting it move about at high frequency which is what the noise is. smaller 0.2mm tip radius on the tool also helps.

[Vixen]

Ark Eurotrade quote a minimum hole size of 11.0mm for an 8mm bar. Check their website for other boring bar minimum hole sizes.

Mike

[PaulR]

Ark Eurotrade quote a minimum hole size of 11.0mm for an 8mm bar

Doh! I've just been and measured the bar - it's 6mm not 8mm (I think I had 8mm stuck in my head as I couldn't bore an 8mm hole with this bar even after shortening the screw that holds the insert - it's in one of my build threads but damned if I can find that post now!). So, the tool definitely wasn't rubbing.

Jason, I wonder if the noise or the finish could have been caused by the screws on the tool post? The holder I made for the bar has two grub screws on top which I haven't shortened ie they're proud of the holder's surface so they tend to get in the way of the tool post screws depending on how much of the bar is protruding. In this case two of the toolpost screws were on the holder  and two were touching the bar itself (not tightened down on it) which might have been causing some vibration.

I'm going to make a tailstock die holder in the next day or two so I'll get chance to use the bar in anger shortly. I'll experiment some more in the meantime.

[PaulR]

... In this case two of the toolpost screws were on the holder  and two were touching the bar itself (not tightened down on it) which might have been causing some vibration.

Looking at my photo, I think I see what the problem is - I've set the bar's holder at the rear (right side of the tool post rather than the front so the two tool post screws in the photo are the ones toughing the bar, while all the support is at the back. I know why that's happened - I put the bar in the wrong way so I could check the height against a tail stock centre then flipped the tool around rather than the holder. In a nutshell, the tool was being held at the far right rather than the far left. 

[crueby]

Good find, hope that solves it! Sounds like that is the type of bar where the end near the insert necks down? If you can, get all four screws in the tool holder clamped onto the boring bar, if its long enough. Checking the height was one of the things I was about to suggest, sounds like you've already done that. I've found the height can make a big difference, too hight or too low messes up the cutter geometry.
 

[cheepo45]

I have found that setting the bar about .003 - .005 above center helps a lot.
I scribe a light line on the face of the part with a O.D. tool that cuts perfectly to center (no nub), and then set the boring bar height to the top side of the scribe mark.
It's pretty easy to see if you have a good magnifier.
 Also use the smallest nose radius tool that you can. I like the .008 radius cutters.
If all else fails, slow your RPM way down.
 Scott

[PaulR]

I have found that setting the bar about .003 - .005 above center helps a lot.
I scribe a light line on the face of the part with a O.D. tool that cuts perfectly to center (no nub), and then set the boring bar height to the top side of the scribe mark.
It's pretty easy to see if you have a good magnifier.
 Also use the smallest nose radius tool that you can. I like the .008 radius cutters.
If all else fails, slow your RPM way down.
 Scott

Thanks for this. What's the thinking behind setting it a fraction higher, a bit more clearance?

[Vixen]

I helps stop the bottom edge of the CCMT insert (not the boring bar) rubbing. Which is probably your problem all along.

Mike

[Admiral_dk]

I also agree with Jason about a too shallow cut.
A few weeks back I milled a few things for work, in my own shop.
Here I took a cut about 0.16mm in depth and got a mirror like finish on almost the entire surface. There were a few areas where the cutter (endmill) didn't touch - so I took another 0.063mm => the result looked ABSOLUTELY horrible - but strangely enough felt very smooth, when rubbing a nail against it .... Standard S235JR steel in case anybody wonders ....

Per

[crueby]

I've  noticed one of my boring bars, a larger diameter one, has the cutout for the insert angled a bit from the flats on the shank. Is this to give the insert clearance like that, without raising it a lot?

[Vixen]

Chris,
A CCMT insert has a side clearance angle of 7 degrees, which limits the minimum size of the hole it can bore, before the bottom edge of the insert rubs in the hole. Inclining the insert helps prevent the bottom edge rubbing. However a CCMT insert will always rub in  a 10mm hole with whatever size boring bar you fit it in.

Just saying

Mike

[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.

[Roger B]

Some interesting experiments   

Another option for small holes is to use an end mill as a boring bar. I set the cutting edge level and on centre height.

[Vixen]

What is the diameter of the finished hole? Is it still the 10mm diameter that caused the original problem, or larger?

Mike

[PaulR]

Some interesting experiments   

Another option for small holes is to use an end mill as a boring bar. I set the cutting edge level and on centre height.

That would seem to be a very rigid alternative but it begs some questions!...

1 Does it give a good finish?
2 It looks like you've got it in a square holder, is that right?
3 What sort of depth of hole could you get with a 6mm end mill?
4 Would a starting hole of say 7mm suffice?

[PaulR]

What is the diameter of the finished hole? Is it still the 10mm diameter that caused the original problem, or larger?

Mike

I started with a 10mm hole (max size in tailstock chuck). I mentioned earlier in the thread that I made a mistake in the OP - it's a 6mm bar not 8mm so a starter 10mm hole should be ok. I didn't measure the final hole, I just kept cutting until I achieved a decent finish as it was just an experiment on some scrap... which has been returned to the scrap bin as I'm sure I'll find a use for it 

[Roger B]

That would seem to be a very rigid alternative but it begs some questions!...

1 Does it give a good finish?
2 It looks like you've got it in a square holder, is that right?
3 What sort of depth of hole could you get with a 6mm end mill?
4 Would a starting hole of say 7mm suffice?

I have used this technique successfully and got a good finish in holes from 3mm diameter upwards. I got the idea from Jo or Jason, I don’t remember who.

I have a square holder for 6mm diameter shanks that came with a set of HSS boring bars and a homemade rectangular holder for 3mm shanks.

I am not sure how deep I have gone, maybe 15mm, but this was limited by the size of the job not the technique. The cutter needs to be offset a few degrees so that only the tip will cut.

7mm would work with a 6mm end mill. I always remove as much as possible by drilling.

HSS end mills will be less likely to chip or break than carbide however I use Proxxon carbide end mills for 2 and 3mm cutters as they have 3mm shanks and are easier to use in small spaces.

[PaulR]

That would seem to be a very rigid alternative but it begs some questions!...

1 Does it give a good finish?
2 It looks like you've got it in a square holder, is that right?
3 What sort of depth of hole could you get with a 6mm end mill?
4 Would a starting hole of say 7mm suffice?

I have used this technique successfully and got a good finish in holes from 3mm diameter upwards. I got the idea from Jo or Jason, I don’t remember who.

I have a square holder for 6mm diameter shanks that came with a set of HSS boring bars and a homemade rectangular holder for 3mm shanks.

I am not sure how deep I have gone, maybe 15mm, but this was limited by the size of the job not the technique. The cutter needs to be offset a few degrees so that only the tip will cut.

7mm would work with a 6mm end mill. I always remove as much as possible by drilling.

HSS end mills will be less likely to chip or break than carbide however I use Proxxon carbide end mills for 2 and 3mm cutters as they have 3mm shanks and are easier to use in small spaces.

Thank you Roger, that's very helpful, I'll pick up an end mill and give it a try.