Tap Holder for a RF30 Mill
By R.G. Sparber
When I want to drill and tap on my mill, the first step is easy. I mount my drill chuck, drop in the proper drill and down feed. Then things get awkward. I rarely have enough room between the bottom of the spindle and the top of the part to fit in my tap with tap handle.
Sometimes I fit a piece of round stock with a hole in the middle into my drill chuck and then use a small wrench to turn the tap. I then use one hand to down feed on the Z-axis while using the other hand to turn the wrench. If I forget to keep the downward force on the top of the tap, there is a good chance that the tap will experience a bending force.
Other times I move the table to the side and turn the tap wrench without top support. I am occasionally punished for this action by having the tap snap off. The tap snaps off because of side forces. If the tap wrench were supported from the top, this side force would cause the tap to bend and fail.
My solution to this problem is to make a series of tap wrenches that fit in the same vertical space as my drill bits. The tap fits in a blind hole in the bottom of the body. It is secured by a pair of set screws that are recessed into the body. The recessed set screws permit the entire body to slide up into a collet mounted in the spindle. The collet is drawn up into the spindle such that there is a smooth sliding fit between body and collet. The body is made of ¾" diameter CRS which provides some downward force during tapping.
Here you see the assembly removed from the mill. The body is free to slide in the collet. The collet is deep enough to permit the body to slide up until the bottom of the collet hits the handles.
The dimensions of each tap wrench depend on the tap so I do not plan to give any fixed dimensions. Instead I will explain what is important and leave the rest in your capable hands.
Here is a close up of the body. The axial hole is drilled to a depth equal to the distance from the top of the tap down to the top of the flutes. Most of my taps measured 1.4". Measure the diameter of the tap you wish to hold and select a drill that provides a sliding fit. The two cross drilled holes go all the way through and are tapped all the way through ¼-28. The upper hole is centered ¼" from the top of the body. It will receive the two handles. The lower hole is located such that it hits the center of the flats at the top of the tap. Most of my taps dictated that this hole be 1.2" from the top face.
Here you see my lathe set up to drill the blind hole 1.4" deep. I happen to have a home made DRO that can measure the depth of the hole so I used it. The hole depth really is not that critical but it is fun to use tools that are home made.
Moving the body to my mill, I drilled the two transverse holes.
Here is the product of 2 hours of work. On the far left you see a body with its two ¼-28 set screws nearby. On the right is a body with the set screws installed in the lower holes and its tap. Note the square shank at the top of the tap. The set screws press on opposite faces of this shank in order to hold the tap into the body and also to prevent rotation. I guess this is one way to form a square hole in the bottom of a blind hole.
In the center of the picture you see the assembled tap holder. The handle is made from ¼" diameter steel rod that is threaded for about ¼" with ¼-28 threads. I only made one set of handles because I intend to store the bodies without the handle attached. If this becomes a pain, I can easily make more handles. For small taps, it would be prudent to cut a pair of 1" long handles.
I welcome you comments and suggestions for improving this design. It is freely given to the metal working community in hopes of advancing the art.
October 11, 2006