Lathe chatter

[BelgiqueB]

Yet another novice question

Doing a boring operation, have to enlarge drilled holes from 33mm to either 50mm in C45 unhardened steel. My method was going up to 49mm in 3mm steps from the 33mm, measuring at 49mm and then doing the final 1mm to get a nice finish. This went ok for about 10 pieces and then all of a sudden when I took the last 1mm pass on one workpiece I got horrible chatter, the workpiece was ruined.

I then tried a different workpiece without changing anything in my setup and everything was ok again, any advice at what could have caused this? I was using a CCMT boring bar, problem is it only happens on the final pass and ruining the workpiece each time, happened 2 times out of about 30 workpieces.

A normal workpiece next to one with the horrible chatter

Insert specs

Setup

 

[RichardM]

Are they supposed to be the same internal diameter?

 

[Rrobson]

Difficult to say as there are a thousand reasons why a tool chatters, and the soul destroyer of every machinist!
When cutting something, lots of forces come into play. If it’s one out of 100, could be a material issue. If it’s happening more frequently then I’d look at the set-up and start by trying to increase rigidity. Be this by moving the tool further in the tool holder, locking the non-moving slides, using a 4 jaw etc etc.
Another thing I’d look to change is speeds and feeds. Are they suitable for the tool and the object being machined? Yes might be fine at diameter x, but diameter y has just the right resonance to give chatter.
Another reason could be depth of cut. Maybe look at holding the gear in a different way, possibly on the teeth with a bit of protection. The list goes on.

I’m surprised to see you’ve drilled to 33mm and turned the rest out, since it’s in the lathe I’d have gone to at least 45mm depending on how true it is, then taken a couple of final cuts.

 

[mike os]

I would suggest a larger boring bar to start, lot of stick out there

I would also suggest reaming if the finish is that critical

 

[BelgiqueB]

The thing is, the setup worked perfectly for 28 of the 30 holes, I find it very weird that on 2 of them it chattered.

Only drilled to 33mm because I don't have a larger drill , also buying a quality reamer that size would be too expensive for this job, and it's good practice for me boring out to + 0.00 - 0.02 mm tolerance.

 

[Pete.]

12 teeth on that gear so turn it around and hold it on the teeth tight against the chuck face. Check it's running true before cutting as even a small amount of runout will make it thrumm in use. Pull the boring bar back in the holder until it's only just long enough to cut through. Set it parallel to the work and check the tip with a magnifying glass because it doesn't look too healthy. Clean under the insert seat and make sure it's fully tight.

 

[rory1]

Scaling off your 50mm bore The insert in the boring bar looks like a ccmt09 rather than the box of ccmt06 that you have posted?

 

[Brad93]

Can you not get a bigger boring bar in there to take the finish pass?

Headstock bearings need nipping up?

 

[Agroshield]

The thing is, the setup worked perfectly for 28 of the 30 holes, I find it very weird that on 2 of them it chattered.

If it is two out of thirty, suspect the material. What is the source of the gears? Do they offer them in different material grades or heat treatment regimes of the same grade?

It might be worth dropping a note to the supplier of the gears, describing your issue. At worst, they will tell you to go away; at best, they might give you two replacements.

 

[BelgiqueB]

Scaling off your 50mm bore The insert in the boring bar looks like a ccmt09 rather than the box of ccmt06 that you have posted?

Good eye , i bought 2 boxes from a guy, both the 09 size, but apparantly one of the boxes is from 06 size and he must've filled it with the 09 size.

 

[BelgiqueB]

If it is two out of thirty, suspect the material. What is the source of the gears? Do they offer them in different material grades or heat treatment regimes of the same grade?

It might be worth dropping a note to the supplier of the gears, describing your issue. At worst, they will tell you to go away; at best, they might give you two replacements.

I've suspected this too, all the others went flawless, even the 2 troublesome ones went flawless when i took the 'deeper' 3mm (diameter) cuts, only when i took the finishing pass of 1mm (diameter) deep that i ran into the chatter issue. Especially since when i took another gear and bored it out using the exact same setup (even the same insert) I had no problems.

 

[BelgiqueB]

12 teeth on that gear so turn it around and hold it on the teeth tight against the chuck face. Check it's running true before cutting as even a small amount of runout will make it thrumm in use. Pull the boring bar back in the holder until it's only just long enough to cut through. Set it parallel to the work and check the tip with a magnifying glass because it doesn't look too healthy. Clean under the insert seat and make sure it's fully tight.

It was the only picture of the setup i had on my phone, the tip of the insert was broken off, the power went out during a cut and the tip broke off when the lathe stopped. When i had the chatter issue i had a good insert.

 

[Kram]

The only thing that changed was a new part in the 3 jaw chuck? Possible the part is moving in the jaws.
If one of the chatter parts was after the power cut broke a tip, that can be enough to throw part out of true and chatter.

 

[ukracer]

The only thing that changed was a new part in the 3 jaw chuck? Possible the part is moving in the jaws.
If one of the chatter parts was after the power cut broke a tip, that can be enough to throw part out of true and chatter.

I think he said he got another gear out and that cut fine AFTER the two troublesome ones.?

 

[BelgiqueB]

Correct, everything went fine when putting in another gear after the gear where I had the chatter issue, didn't change a thing, even the insert was the same.

The part I was doing when I had the power outage went without problems, after changing the insert offcourse .

 

[Johnnybravo]

Measure the boss on the gears that chattered, not that they are tapered and only tight on one spot, not along the whole length of the boss. Could also have been caused by a bit of swarf between the jaws and the boss. Check boss for marks.

 

[mtt.tr]

how thick is your sidewall between between the teeth? if it the material is deforming between the teeth as its to thin?


i say that as your second picture the bore looks pretty good

 

[Jimbobfrank0]

Yeah the bigger the tool the better also minimise overhang increase feed and depth of cut and slow down suface speed can usually help get rid of chatter. Though when its only 2 of 30 it may just be material usually at least double the nose radius of the tip for the final pass.