Boring Bar Issues..

[stevemannellParticipant @stevemannell]

Evening Chaps

 

I seem to be having a major issue with boring holes on my lathe. As I make a cut It’s making a horrible screeching noise, the cut looks to be interrupted with a terrible surface finish.

To give you some idea of the set up…

I’ve using a Southbend 9a lathe. The boring bar is mounted on the back of the tool post in a quick change too post holder (Not a round boring bar holder) I have drilled a 12mm hole,  the stick out is about 3 inches.  (Although tonight I tried sticking it back through the chuck so I had no stick out and still had the same issue) I’m trying to bore to 12.67mm. The material is EN1A Leaded. (Although I was having the same issue with Aluminium at Christmas) I’ve tried HSS boring bars, Carbide boring bars with inserts and braised carbide tools and still getting the same issue. I have checked the tool height and its on centre. Its not tool sharpness as I used some brand new inserts. The boring bar is parallel to the hole I’m making.

Any ideas or suggestions to try and resolve the issue would be greatly appreciated.

Cheers

 

Steve

[bernard towersParticipant @bernardtowers37738]

Blunt tool, above centre line, not enough of boring bar cut away below the cutting edge or spindle speed too high?. Possibly

[MikeKParticipant @mikek40713]

Is this a shop-made boring bar, or a purchased unit?  Is it rotated to provide clearance?  I found this out recently.  My purchased bar is rotated to provide cutting tip clearance, and my shop-made bar rubs from lack of clearance.

[DiogenesParticipant @diogenes]

Hmm – what size bar are you using – for a 12mm dia. hole, I’d be starting with an 8mm bar, so not sure how you could initially have had 3″ of stick out?

In any case, a common source of rubbing with some boring bars seems to be that the end of the screw protrudes through the bottom of he head and fouls the work, worth a check.

If you run the tool up to the hole and check from all angles with a flashlight and loupe if you need to..

 

[JasonBModerator @jasonb]

If you had 3″ of work sticking out the chuck, how much boring bar did you have sticking out the toolpost. Generally the more tool stickout the more likely you are to get a high pitch chatter which sounds like the tool is screaching.

If it is more a graunching noise then look to see if part of the holder is rubbing or the bottom edge of the cutter bit/insert.

Not taking enough of with a general purpose insert can also have it riding out of the cut try a **GT insert if you are using an insert tool with small corner radius if not a blunt or poorly ground HSS one will act the same.

[stevemannellParticipant @stevemannell]

Thanks a lot guys.

I think the issue was the tool height was above centre. I’ve dropped it down and it now seems to be fine.

 

[Howard LewisParticipant @howardlewis46836]

If it is possible, reduce the speed. Always a first try when chatter happens.

Boring bars are thin, and flexible, the more unsupprted length, the more flexible

(Assumes, as said that the tool is sharp and mounted with the cutting edge on centre height.  If it is not, you are on a hiding to nowhere!)

Sometimes, a light cut will chatter when a heavier one will keep the tool “dug in”.

Currently, I am boring a 3 inch long brass bush.  It means noting the position relative to Jaw 1 of the 3 jaw, ready to use the same position when boring from the other end, to maximise concentricity.

Fortunately it is a slack tolerance job!

Not ideal, but when needs must!

If there is room, you could try wrapping the shank of the boring bar in lead, to lower the natural frequency, and to damp vibration.

Howard

[Author]

Posts