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Quick Change Tool Post

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q20v
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17-May-2017

As soon as I learned about Quick Change Tool Posts (QCTP) for the Lathe I knew I had to have one. They are beneficial from a turret tool post (what I have now) in that the individual tools are pre-mounted in tool holders, pre-set to the correct tool height, so tool changes (ex: from facing to turning) takes about 5 seconds, as opposed to about 5 minutes. The standard turret style tool post requires shims under the tool to set the height, and shimming for every tool change is what takes the most amount of time.

A few months ago I bought a Chinese QCTP from Amazon for $268, which came with 5 tool holders. I also bought 3 extra tool holders. The idea is to get all your tooling mounted in tool holders so that once their height is set, it's just a matter of swapping tool holders to change machining operation.

The Base of the QCTP I bought is supplied oversized so that it can be machined to fit any lathe. This chapter covers the machining of the Base. This project covers some techniques I've never used before so I wanted to proceed cautiously.

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Here's the block of steel, front and back. Brieanna and I attempted to machine the part on Sean's milling machine but we encountered technical difficulties and the end mill veered off on an angle. At that point I decided to slightly change the design so that I could complete the machining on the lathe. The faint lines are the layout lines I scribed to show the amount I needed to remove off the width (first picture - about 10mm each side) and the diameter of the raised boss (second picture, with the milling cuts).

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I have never used a 4-Jaw chuck before and it's the only way to hold a 4-sided shape in the lathe. With the Base chucked I ran the dial indicator across the length to make sure it was sitting square. After a few light passes I decided it was going to take wayyy too long to remove 10mm from each side, plus the chips were really launching off the part, which I assume is from the big interrupted cut. So I threw it in the vise and used the angle grinder to remove the bulk of the material, then finished off on the lathe.

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With the width reduced, I faced the Base to make the raised boss. The first step was to check that the base was sitting square in the chuck, and again I ran the dial indicator across the face in 2 directions to make sure it was. The other tricky part was ensuring the boss was accurately centered in the chuck. To do this I got it as close as I could using rough measurements on the chuck, and then took a bit of material off. With the boss roughed out, I took measurements on each side and compared them, if one was higher than the other, I moved the block accordingly and took another cut of the boss. I had to do this 3 or 4 times until it was just right. This is hard to explain...

Once the setup was complete, I continued to take facing passes until the desired thickness was achieved. Once I was happy with that, I finished the outside diameter of the boss

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Here is the completed base. Only 3 machining operations:
- face the sides to reduce the width.
- face the top to reduce the thickness.
- turn the boss to the correct diameter.

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I wouldnt trust those drawings, they dont have a stamp on them. - ripcurl May 18th 2017

I managed to hit all the tolerances I specified on my drawing, which for me is thrilling. I didn't want to remove the part from the chuck and find out it didn't fit in the compound, only to have to re-mount it in the chuck for more machining. All three surfaces I machined needed to be appropriately sized in order for this to fit in the compound, so it was a big relief when it fit the first time.

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