Shop Doc – Stop the Bore-Dumb

Today’s Machining World Archive: September 2006 Vol.2, Issue 09

Dear Shop Doc,

I have problems at times with indexable boring bars when boring holes on my lathe. It seems the diameter or surface finish isn’t as good as I expect it to be, and for this job, I need to have consistent diameters due to some very close tolerances. I have tried cutting at the speed and feed parameters recommended in the tool catalogs, but most of the time the tool chatters until I slow it down. I have checked the centerline for proper alignment and believe it is set correctly. Shop Doc, I need some help quick !!!

“Stop The Bore-Dumb”

Dear Bore-Dumb,

I believe we can steer you toward a solution rather quickly. The two topics that we will focus on are length to diameter ratio and tool nose radius. These topics contribute to the majority of problems in boring applications.

Indexable type boring bars are made of basically three materials—steel, heavy metal and carbide. Some manufacturers are getting quite sophisticated with these tools with dampening features and special designs, but let’s keep this simple. A 1” diameter bar hanging out of a holder 4” in length is considered to be a 4 to 1 length to diameter ratio.

Steel bars can be used in relatively shallow depth bores such as 4 to 1 ratios. Heavy metal bars can be used in bores up to a 6 to 1 ratio. Carbide bars are used for bores up to an 8 to 1 ratio. The stiffness of the bar is the key factor. The bar must be capable of hanging out that far and still be sturdy enough to not have the cutting forces affect the tool adversely while in the cut. There is a cost involved—steel is cheapest, heavy metal is slightly more expensive and carbide is the most expensive.

Tool nose radius selection is crucial regardless of what bar you use. You need to know how much material you will leave for the tool nose radius on the insert you wish to use. The correct amount of material per side will be equal to or greater than the radius of the tool. This enables the material to be engaged completely around the radius of the tool. Always try to leave more material in the hole per side than what the selected tool nose radius is. If you need to use a larger radius, leave more stock per side. This engagement stabilizes the cutting forces of the tool and provides a smooth, consistent cutting action resulting in consistent diameters and tolerances.

Also, you might want to try out the wiper insert technology available today. It allows faster feed rates and better surface fi nish due to the design of the tool nose radius and clearances following the cutting tip.

One last thing to remember—Because the tip of the tool plays a huge role in creating successful bores, the center line of the tool must be as close to the machine center line as possible. If it’s not close, some really weird pressures can occur, giving you negative results.

Good Luck.

Jim Rowe
Application Specialists / Medical Accounts
Mahar Tool Supply, Warsaw, IN

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One thought on “Shop Doc – Stop the Bore-Dumb

  1. Mike Mendola

    Chip control is another important factor. Unbroken or stringy chips can score the inside of the bore and/or deflect the bar so that it cuts oversize. Most insert manufacturers offer a variety of chipbreaker types to help with various materials. Also, use a boring bar with a thru-hole to apply coolant at the point of cut and to flush out the chips. The deeper the bore, the more important this is.
    Implied in your article, but not directly stated: Use the largest diameter bar available, consistant with having enough room for chip evacuation.
    Finally, check the pocket of the bar and the components holding the insert. Any movement in these may cause an over/undersize condition and surface finish problems.
    Mike Mendola
    GKI Inc


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