Gallery, Projects and General => The Design Shop => Topic started by: Chuck in E. TN on May 29, 2014, 01:15:10 PM

Title: Custom tap
Post by: Chuck in E. TN on May 29, 2014, 01:15:10 PM
 [font=]I am making a custom 1/4" x 40tpi tap for a friend. He had the change gears for his lathe disappear during a move.
 I have the threads cut, and a 5 deg taper on the end. I'm getting ready to mill flutes and the square end for the tap wrench. Can I cut the flutes with a regular 1/8 end mill? I don't have a ball end that small. How deep should they be? He'll be cutting threads in cast iron. I have to also make a matching screw, so I will be taping a piece of hex steel stock for a test nut.
 I have printed out an article on making custom taps with just a filed obelisk (think Washington Monument) shape for the tapered start and no flutes, but I think flutes would be better.
Title: Re: Custom tap
Post by: tom osselton on May 29, 2014, 02:28:36 PM
So why a 1/4 x40?
Title: Re: Custom tap
Post by: Chuck in E. TN on May 29, 2014, 03:07:09 PM
It's what the plans call for. The plans are for a model airplane engine, running diesel. The plans are from a guy in PA.  MLV something. Can't remember his name.
Anyway, got the tap made and heat treated, a nut made with the tap, and am now working on the screw to match the nut. I think it's for metering fuel.
Title: Re: Custom tap
Post by: mklotz on May 29, 2014, 04:18:02 PM
For $15.60, I'd just buy it...
Title: Re: Custom tap
Post by: Chuck in E. TN on May 29, 2014, 06:05:48 PM
Where's the fun in that, Marv?
I went ahead and made the tap, cutting the flutes with a 1/8" end mill. I hardened it, but did not temper it. I made a nut from some 1/2" steel hex I had in the junque box and cut it's thread with the just made tap.
I had to go slow with lots of cutting fluid, but it worked!
Title: Re: Custom tap
Post by: Lew_Merrick_PE on May 30, 2014, 10:35:02 AM

Stop and think about this for a minute.  The purpose of flutes on a tap is to clear the chips axially down the length of the tap.  Because of fracturing and curling, a chip will take up as much as 50% more volume than it required as solid material.  Depending on the type of tap being used (i.e. plug vs. taper vs. bottoming) up to 6 threads (taper) or 4 threads (plug) do the cutting.  If you allow for the cross-sectional area of the thread being cut X the number of threads doing the cutting X the "expansion factor", you should have enough area (reduced by the number of flutes in use) for your fluting.

The fact is that the thread is being cut in "stages" such that you are really only disposing of (about) 2X the cross-sectional area of a thread.  The excess allowed in the previous paragraph merely provides a safety margin on the cut.

The square corner of the flute is going to create a stress riser that may well cause the tap to break prematurely.  Except in the case of spiral-fluted taps, the radius only has a small effect on the ejection of chips -- it is more to do with preventing stress risers.  The main thing in the design of flutes is to get the rake angle right for the cutting.