Brownells Gunsmith Tech Corner - July 2009

Keep Your Turning Tool On The Centerline 
- by Guy G. Lemieux © May 2009 Armatt G. S., Inc.

We use a South Bend 9” engine lathe in our Gunsmithing repair and the making of obsolete parts that are no longer available on the market place. The top of the turning tool must be on center to remove .010” when we dial up .010”. A properly ground turning tool will produce a good finish if the tool is on center and quickly removes the chip being cut from the turned surface. Many times we will re-cut a surface to take off a chip as small as .001”. You cannot do this without a tool that is not on the centerline.

We also do boring in our Gunsmithing. We have saved at least three of our customers when the Ejector Rod Bushing, which is a thin walled heat treated piece of steel, on their Charter Arms Cylinders has become stuck in the Ejector by boring it out instead of having to replace the Ejector. We used a small carbide boring tool to cut the Bushing out. You cannot do a precision bore unless the cutting surface is on the centerline.

Some of the lathe men working for me used the tail stock center to set their tools. Upon occasion I have used my scale to set the centerline. Our lathe is set up with a four way tool post and we keep it set with a turning tool, cutoff tool and a 45% chamfer tool. When changing to a boring or other tool we needed a quick positive way to set the tool on center.
 

Center Line Gage

We use a copy of a Hardinge setting tool.  We tool a piece of steel bar and drilled it through.  We then tapped a hole in one end for a screw.  We used a ½ -20 thread because we had a ½-20 socket head screw to use.  Put a cross tapped hole to lock it in position.  We cut down the head from the thread side to use as our reference setting surface. 

Now it is simple to check a new tool, by placing the bottom of the gage on the cross slide surface and placing the tool beneath.