Shop Doc – Drilling a Deep Hole on a screw machine (depth of 2.250)

Today’s Machining World Archives April 2006 Volume 02 Issue 04

Dear Shop Doc,

We are a relatively new screw machine job shop that has just accepted a job in which we need to drill a significantly deep hole (a depth of 2.250). We are trying to estimate how fast we can run the job, but it’s difficult because of our inexperience at drilling holes at this depth on a multi-spindle. Any thoughts?

Sincerely, In Deep

Dear In Deep,

A few years back, we got a new job that was different than the types of parts we normally make here. The part was a steel fitting (12L 14 grade) with a hole that was .160 in diameter and almost 2.250 deep – very similar to yours. It was also a very thin piece, with a .025 wall. With very little experience in extremely deep hole drilling, I had to take an educated guess on how fast we could run the job on our 1-¼” 6-spindle National Acme. Estimating such jobs can be challenging due to the fact that the heat of the steel in many grades can affect the machining by as much as 15%, in my opinion. Upon setting the job, we realized it would run no where near the run time I had previously predicted. The part was packing with chips badly and snapping drills. Also, the process was heat treating the parts, turning them brown.

With the drills set perfectly, the machine was running an hour before all the drills broke. It had been running 50% slower than I had originally quoted the job. After a few days of doing all I could to make it run better, I decided to cut a valley into my main tool slide cam, just large enough for the cam roller to drop into. Then, I used springs to pull the main tool slide back when the roller approached the valley in the cam. It worked, and about half way through drilling, the drills pulled out, and the holes were washed out with oil. Then, the drills plunged back in to finish the cut. This kept the drills from welding up with chips in the holes. I was able to speed up the machine to our original quoted run time as a result. The machine was running with a high pressure system, but I decided to stay away from the coolant fed drills because they were $170 each, a price which would make the job significantly less profitable, considering that there were 5 drills in the machine. The most expensive drills I used were $20 each, and they did a fine job. After a few weeks, I decided to order a special cam that had a triple rise, with each rise being a progressively lighter feed, and the drop off in the cam running after the second rise. I also had the cam made 10 degrees longer than a standard in order to make room for the cut away drop off section, which needed to be wide enough for the roller to drop into. We also added a broken tool detector for drills, which kept the cost of broken tooling and sorting low. The job has run well for us ever since.

Hope this helps a bit.

Wes Szpondowski
Tool Room Leader, Wyandotte Industries. Wyandotte, MI