[choppero]

I am gunsmith/CNC machinist. I make a muzzle breaks for rifles and need to see some thoughts on drilling and threading. Currently I have a CNC Masters 1440 Lathe and a Tormach PCNC1100 mill. The lathe will drill a very nice straight hole through 4140 steel if I first start the hole (21/64 for .308 bullets) with the tail chuck and finish the 2.5 in deep hole with chuck in the tool holder. Then Change to 29/64 drill started in the tail chuck and finished 1.4 in deep with drill in holder. The table and cross slide can't start a good hole(flexing and moving). I then finish with 1/2 x 28 single point thread .55 deep. They come out just fine but take a long time to make one. My question really is can I get the same level of concentricity with the mill. I am familiar with thread milling. It seems as it should and take half the time.

[djr76]

why not mount a drill chuck to a tool post morse taper holder? Be much faster then the tail stock, assuming you have a quick change tool post.

[choppero]

why not mount a drill chuck to a tool post morse taper holder? Be much faster then the tail stock, assuming you have a quick change tool post.

I do that. Just didn't say it in the way you stated it. The CNCMasters lathe is nice for a gunsmith like me that needs the versatility of a gear head lathe with the automation of a cnc. With a drill hanging way out from the center of the cross slide the drill will walk. So I start the hole about a half inch deep with the tail chuck and finish with a programmed drill operation like you suggest. I have been told in other forums, that the mill drill cycle is the least accurate as far as runout is concerned. It takes about 30 minutes to drill two holes and thread on the lathe and about half the time in the mill. What I have to have is dead nuts concentricity to the bore. The thru hole is only .02 over bullet diameter. So thread and hoe must be true.

[MetalZilla]

If you have to deep hole drill you may want to check this out: Drill Peck Calculator for Deep Holes!
You can control the pecks to reduce at various depths as the drill gets deeper into the part.