Using a large tap to cut a larger internal thread

**Frank S** · Oct 21, 2023, 07:38 PM

A few days before the thread was posted in tool talk using a tap to cut an external tapered thread.
(Jet BD-920W Operating _ Metric Thread Cutting _)
I had mounted my 1 1/4 12 TPI tap in a boring bar holder to thread a 2 3/4" nut For some reason I either didn't have or couldn't find my HSS cutter I had for threading the ID The only other ID cutter I have uses an insert, is too small the max TPI it is good for is 20 threads per inch.
To thread the ID of a bore with a tap there are a few very important things to consider and steps to follow.
never try to tap a blind hole unless you start from the depth and thread outward using the back side of the bore and running the lathe in reverse a tricky procedure for a novice and many seasoned machinists. for one reason you will be cutting from the bottom side of a cutter this will require the cutting surface of the tap to be a few thou. below centerline. enough about that.
to thread the regular way starting from the edge and cutting to the depth be sure to allow enough run through to compensate for the taper of the starting threads even a plug or bottoming tap will have a thread or 2 which are reduced in full OD.
Set your lead screw up to the correct thread pitch do not set up a carriage stop unless your spindle stops as well none of my lathes do.
Even large robust taps will have some flex in the shank, so I set my tool holder to a few degrees positive. if you set it up to the negative you will cut a tapered thread, but a positive angle will ensure the lead threads do most of the cutting.
When you reach the depth or distance of the thread you want back off the cross slide by running it towards the center before disengaging the lead screw otherwise you will probably 1 break the tap and 2 it will ruin the threads. After backing off the cutter disengage the lead screw and back the carriage out reset your cross slide only a few thousands more cut depth at a time. Depending on the diameter of the shank of the tap and the TPI you will have to make several cuts If you use your compound to increase the cut depth you will return your cross slide to zero and turn the compound out a few thousands remember the compound should be set at 60°from parallel to the bore when using to increase the depth of cut. always leading into the cut never cutting from the back of the cutter the same is true be it single point cutter or using a tap.
When you feel you have reached full thread depth check your work with the mating OD thread is is very easy to go from a class III thread to a class 1 or zero very quickly

Using a large tap to cut a larger internal thread-img_20231013_170635ts.jpg

Using a large tap to cut a larger internal thread-img_20231013_173702cn.jpg

Using a large tap to cut a larger internal thread-img_20231013_173823cn.jpg

no rotary table or super spacer no problem I have rotating tilting vice that works just fine.

Using a large tap to cut a larger internal thread-img_20231014_104111cn.jpg

Using a large tap to cut a larger internal thread-img_20231014_123131cn.jpg

**DIYer** · Oct 23, 2023, 03:33 AM

Thanks Frank S! We've added your Internal Thread Cutting Method to our Tapping and Threading category,
as well as to your builder page: Frank S's Homemade Tools. Your receipt:

Internal Thread Cutting Method by Frank S

tags: threading