Sunday, 2 August 2020

Linear bearing makes quick work of DIY tap follower

I had many M4 holes that needed tapping in a workpiece. Normally I use a cheap ratchet tap chuck from Banggood such as : 

but these do not guarantee the tap is orthogonal to the work piece. Normally I would take care when tapping with one of these and do it by eye, but with so many holes to tap I needed some help.

Without access to a tapping machine  (I wish I had the room for one, never mind the device itself) I looked into tap followers. These are tap holders on a sliding mount that is held in the mill/drill. An internal spring presses the tap gently into the work piece as the chuck is turned, ensuring the tap remains at 90 degrees to the work,

I didn't have the time to wait for one to arrive and so decided to make my own. I'm not too bothered about the spring part of the device. I will only be using it in the mill and to get the tapping started. A gentle hand pressure downwards should be enough to get it, and keep it, going.

To make my own I need:
A tap chuck.
Something to mount the tap chuck.
Something to allow it to slide up and down.

I realised that the simplest solution for me would be to use some 6mm ground rod and matching linear bearing I had left over from an abandoned project. The outer dimension of one of the bearings is 12mm and so a perfect size to fit in the 1/2" chuck of my mill.

I decided to sacrifice the ratchet tap I had. Dismantling it revealed this:

I simply sawed off the section just before the splines. So that's a chuck and a slide. Now to connect to the two together.

I grabbed an offcut of 25mm aluminium bar from my scrap pile and faced it off in the lathe before drilling a 6mm hole all the way through.

On the mill and using a V block, I drilled a couple of 3.2mm holes which were then tapped to M4 for grub screws to hold the ground rod and the shaft of the chuck in place.

I'm using the original soft jaws here. I always wash my hands after using these but I find lead sheet is the best for holding this sort of thing without marking it.

And that was it. A quick check in the mill showed me I had about 25mm of excess ground rod. A brief session with the angle grinder soon sorted that out.

After fitting the chuck and fixing with screws, I wanted to add a tommy bar and the one left over from the donor ratchet tap was ideal. In drilling a hole for it I wanted to also drill through the end of the chuck shaft to give it something to engage in to stop it from turning. A 7mm hole was needed for this and I used an accuracy block to allow me to clamp the assembly securely as I drilled it.

And a quick test in the mill showed all was well

All in all I have something that does what I want for a build time of about an hour. I did use my lathe but this could be made just as easily on a drill press. It's going to significantly decrease the time spent on my pending tapping operation, and will is a nice little tool to add to my collection.

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