FILENAMES:

Taphld.gif

DESCRIPTION:

This is a drawing of an extender for holding taps. Thanks to 
AL <alicecurtis@worldnet.att.net>. AL provided the following 
description:

>A quick and easy tapholder
>
>This project is handy when you have to tap a hole that
>a standard length tap won't reach.
>If standard diameter material is used, the tap holder
>can be held in a collet or a tapping head.
>The design can be scaled to fit almost any tap,
>it also is handy for tapping heads that do not have collets
>for the smaller taps.
>
>Example is for a 10-32 Greenfield tap, shank is .193, square is .152 .
>
>I usually start them several at a time by milling the square and drilling 
>the through hole.
>Then toss them in the bottom of your toolbox until needed.
>The material is not critical I favor stainless TG&P since it
>is common and end pieces can be used.(and I had a bunch of this stuff)
>
>The square drive is not critical since collets hold the shank not the square.
>
>The important part of this tool is the bore the tap is pressed into.
>This bore must be straight and true to the outer diameter. 
>It is also a press fit onto the shank of the tap .0005 to .001
>this means you must bore the hole or your tap will wobble.
>fortunatly you only have to bore it about 3/8 inch for small taps
>
>Many people do not have an accurate chuck or a collet holder for their 
>lathe, this is why Item 2 is shown.
>
>Spring Collet (Item 2)
>A spring collet like this can be made to fit in a three jaw chuck
>and bored true to the axis of the machine.
>to do this you need to first chuck the blank.
>rough drill it to about 1/16 or 1/32 under what the bore will be.
>take it of the chuck place in the vice cut three slots equal distant 
>in the work piece.
>as is shown in the drawing one of the slots must break through
>the others cannot
>chuck it back in the lathe, the jaws should clamp between the slots, 
>place a piece of shim stock in the slot that is cut through to the 
>drilled hole.
>tighten the jaws until the shim stock is clamped snugly.
>this will keep the spring collet in place while you bore it to
>the diameter you need ( in this case .375)
>when you are done boreing it to size loosen the jaws until you can
>just pull the shim out while the spring collet is still clamped snugly
>If the collet moves it will not be true and you will have to start over
>once this has been done slip the shafting into the spring collet
>and tighten the jaws tightly.we are now ready to bore (maybe we are already 
>bored)
> 
>Broach
>when the hole for the shank has been bored it is time to
>broach the square drive.
>the broach (item 3) is a tap that has the end ground square or
>concave instead of with the center pip.
>hand taps are ground flat on the end machine taps have a center.
>use machine taps.
>before and after are shown.
>mark the shank of the tap at the depth it will be pressed into
>the tap holder in this case 3/8"+ 3/16"= 9/16" = .562
>after grinding the end of the tap slip it into the bored hole
>mark with a file where one of the corners of the broach
>slips into the tapholder (this helps when replaceing taps).
>place the tap holder in a drill chuck and resting the end of the
>broach on an aluminum block press the broach into the tapholder
>the end of the broach will shave 4 lands in the .125 hole
>taking about .012" off each side this will prevent the tap
>from spinning when in use.
>Remove the tapholder from the chuck and useing a 1/8" pin
>down the through hole, drive the broach out.
>now you can press in a tap and you are ready to tap
>those pesky holes in the bottom of that casting.
>
>Math 
>to determine how deep to cut to make the square drive.
>Shaft Dia. .375 radius .187
> c=a*sinC
> a=radius
> C= angle 45
> c=.187*.707=.132
> ..187 - .132 = .055 
>depth of cut
>this would give a .264 square , not standard but close.
>take to .25 or .062 depth of cut