Saturday, February 11, 2017

Machining Lessons Learned

Most of the art of machining isn't operating the lathe or the mill, but recognizing the proper order of operations, tooling to use, and limitations of the machines.  I have been trying to efficiently make a part that is 5/8"or 1/2" OD at one end (to fit in the drill press chuck) and 3/4" ID on the other to hold a tap wrench, with a 3/8" wide, 3/8" long slot to accommodate the handle on the tap wrench.  If you drill out the 3/4" ID end and cut the slot, the 3-jaw chuck on the lathe can't get a good grip.  (Draw three jaws at 120o and try to get two slots 180o apart to fit those jaws, even ignoring bending under chuck pressure,)  So start by turning down the other end first.  Once you reach 1/2" diameter, the workpiece starts to bend under load from the cutting tool, causing chatter and eventually the workpiece exits the chuck.

Solution, cut down to 5/8" diameter.  Reverse workpiece in chuck.  Use 3/4" end  mill in tailstock chuck to bore hole.  Use boring tool to make any final size adjustments.  Put workpiece in mill.  At about .1 inches.minute advance 1.5" long x 3/8" diameter mill  3/8" inch from end.  Since I need two holes, one 1/4"-20 threaded and another 1/4", I will change to center drill on the mill, and drill pilot holes for drill and tap on the drill press.

I tried to make this part from a 1" 1/8" wall tube and a 4" long, 3/4" OD rod turned down to 5/8" OD on one end, but getting a press fit into the 3/4" ID tube was harder than it sounded.  Drilling and tapping through the tube and rod and holding them together with 1/4"-20 bolts worked, but it was ugly.  I'm not even sure it saves much time over starting with a 1" rod and machining it as one part.

2 comments:

Will said...

Either there is no measurable inquiry's regarding this type of bit, or there is a materials limitation on the design. Taps tend to be brittle due to the hardness required. This may be a factor in a long form.

IIRC, most milling bit producers have an order system that takes custom requests. You might check with some of them to inquire if this is possible/feasible/cost effective. It's possible that they may know of someone who does this, if they aren't interested, or if it requires a process that they can't handle.

Will said...

Drat. That should have gone with the tap question post.