Slot milling.

[websterz]

I do a lot of work in annealed 4140 round stock that requires 1/4" x 1/4" slots milled along the length of the material. I have done this in the past with endmills but the results have been marginal at best. Obviously cutting the slot full depth/width in one pass leaves me with a parallelogram cross-section. Cutting the slot undersize and finishing with a second pass works but is a pain in the butt. I am wondering if I would have better luck using a side cutter. My mill is a Grizzly G0704 (BF20 clone) and has enough power to make the cut. What about a staggered tooth Woodruff cutter? This is a small scale production situation so tool wear is an issue. I obviously don't want to get one part out of a cutter and have to regrind/replace. Carbide is preferred but small diameter cutter bodies that will run 1/4" inserts are rare as hens' teeth. Any thoughts?

[DaveH]

Cutting the 1/4" full depth in one pass with a 1/4" end mill may be a little to much for it.

What I tend to do is set the depth of cut to 1/2 the cutter diameter, so in your case 1/8" and do two passes. The 2nd pass however is in the same direction as the 1st pass. ie after the 1st pass the work is traversed back to the start and the depth of cut added and cut again.
It does mean two passes but sometimes the feed speed can be increased, also it can be beneficial to keep the slot clear of chips.

Just some thoughts that may be of help.

 
DaveH

[Lew_Merrick_PE]

Websterz  --  Let me restate things so I can either demonstrate that I know what you need to do or expose my misunderstanding...

You are fluting round bar with ø.250 inch slots that are .250 inch deep.  Correct?  The material is 4140 alloy steel in the annealed condition.  Correct?  Your mill has the power to make the cut, but it lacks the rigidity to do so accurately in a single pass with an end mill.  Correct?

41XX series steels are known as chrome-moly steels.  Molybdenum is a particularly gummy element.  In the annealed condition, it makes for rather "mushy" cutting.  I prefer to machine it (whenever possible) in a lightly heat treated condition (say Rc35 nominal) as this makes the matrix around the molybdenum rigid enough to minimize the "mushy" nature of the moly.  In addition, if the ends of the flutes do not need to be perfectly full round, I would suggest that using a ø3/16 or ø7/32 end mill to rough the slot and another, dead sharp, end mill to finish it a worthwhile thing.  Using the correct cutting lubricant may gain you quite a bit.  I have been using TapMagic Gold with very good success.

I do not believe that a periphery-cutter is going to gain you much unless using it increases the rigidity of your set-up.

Does this help?

[BillTodd]

Try slot drilling instead.  i.e. plunge the slot drill full depth while stepping the material along a few thou" each cut.

Bill