Hi Guys,

I said that I would do a write up on the Key way Broach tool that I made to cut key ways in pulley bores. So far I have made two of them one is designed to cut a four mm wide keyway and the other designed to cut a three mm keyway. The three mm one is the most recent one.

In both instances I have used easily available square HSS tool blanks, which I've obtained from China via Banggood very cheaply. Compared to the prices that the tool suppliers want for them here they were very cheap and surprisingly good quality, being accurately square and 200 mm long, so you get a lot of HSS tool steel for your money..

I've also used pieces of round bar from the scrap bin or salvaged from old printers and photocopiers for the tool holder ram. The round bar size has been chosen to suit the bore in the pulley that I wanted to cut a keyway into. Use whatever size suits your need, turning it down to suit if needed. Non of the dimensions that I given are set in stone, just what I used for mine, use what ever fits your need.

The piece of bar that you are going to use for the tool holder ram, needs a slot cutting into its length to suit the width of the keyway that you want to use and is machined to be as deep as it is wide, it also needs to be long enough to go through at least twice the thickness of the pulley plus enough material for it to be held in a chuck. I used my drill press as a vertical slotting head, so the bar has to be able to seat up against the back of the chuck. This is done so the chuck jaws don't have to cope with the pressure of the round bar used as a ram.

The length of the tool slot wants to be at least twice the thickness of the pulley because the tool bit is held in place by a suitable spring placed a short way down from the top. This allows the grub screw fitted behind the tool bit to be used to push the tool bit outwards in order to set and control the depth of cut. Normally a keyway would be cut in both the pulley and the shaft, the depth of which should be a few thou deeper than half the width of the key.

It should be noted that the piece of bar used as the ram has a hole drilled in the bottom end and a cross hole drilled into it at the bottom of the tool bit. This was done because I found that swarf from the cutting got forced up behind the toolbit and prevented it retracting into the slot, so those two holes provide a means for the swarf to escape and fall out.

So far I've cut aluminium, bronze, brass and steel with these broaches without any difficulty, plenty of lubrication is needed for aluminium and steel. I used diesel fuel for aluminium and Temaxol for steel.

The drawing shows the sizes for the broach that I used for the 3 mm wide keyway.

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These pictures show the drawing and two pulleys that I keyed using this tool.

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These pictures show the parts used. The spring is any old one that will fit tightly around the shaft. The actual cutter is a short length of 3 mm square HSS tool steel blank with the end shaped as a cutter.

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These pictures show the broach in use.
Attached Images




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