Toolpost Grinder Part 4: Fitting the Caps

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The time has finally come to fit the bearings and bearing caps into the toolpost grinder spindle.  In this installment, we cut the internal threads in the spindle body to accept the bearing caps, lock in the bearings and face the caps off flush.

Inside threading operations can be a little unnerving–especially since you can’t see clearly what you’re doing.  In this video, I show how to use a dial indicator to locate the bottom of the bore and cut a relief groove for the threading operation.

We want the threads to be as close-fitting as possible.  The bore has a register for the bearing and the cap has a register for the bearing, so everything should line up, but having a close thread fit will serve as yet another force that will tend to center and align the bearings properly.  Unfortunately, we have no good way to measure the thread depth of the inside threads.

Three-quarters of an inch divided by 28 is 79 thousandths
Infeed Estimate

There’s a simple rule of thumb for the flank length for imperial threads.  The flank length is approximately 0.75 inches divided by the number of threads per inch.  Since these are 28-pitch threads, the flank length should be about 0.027″.  Because we’re feeding in with the compound at 29.5 degrees, this is the total feed depth.

Of course, the actual feed depth depends on a lot of factors, including how accurately we can touch off on the surface and the actual radius on the end of the threading tool.  For all of these reasons, we won’t trust the infeed number.  We’ll just use it as an estimate and start checking against the actual part as we start to get close.  We know the bearing cap threads are on size, because we measured them with thread wires.

If you haven’t seen the previous videos, a playlist with all of the parts of this project is here: https://www.youtube.com/playlist?list=PLDlWKv7KIIr-uFMjbe7AlsHswL6qihPPU

If you’d like to follow along with the build, here are some links to some of the parts and tools used in this video:

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