Can I Get A Witness (Mark)

One of the constants of my small home machine shop, is the need to match up disparate equipment so that they can cooperate together, to "do good work*". Not an easy task, but yet another great excuse for more tool building. (did I mention how I like to build tools...?)

A great example of this, is how I connected the newest acquisition to the shop, the Divisionmaster CNC indexer, to my two Chinese lathes.

One component I had to make to fit the indexer, was this mandrel to fit the rear of the lathe bore:

The mandrel slips in the rear of the lathe, and then a cap screw pulls the expander back, and hey presto, its grips the inside of the bore. The indexer can then be strapped onto it with this bellows coupling, and the CNC rotary table is now able to directly drive the lathe:

As I mentioned last week, the tools from China are great for getting the shop up and running, but the QA can be a bit patchy. But would it surprise you to learn that the run-out of the rear of the lathe bore was 30 Thou? I can understand 4 or 5, but 30?! Was he boring it with his eyes closed? No space program for you my friend...
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Simply clamping the rotary table to the mandrel without addressing this runout was not an option. The eccentricity would pull the table off the axis of the lathe, and result in rotational error that would quite literally be printed on my clock wheels as a variation in tooth space. We don't want that, no sir.

The solution: Turn an eccentric mandrel, and then mount it so as to to cancel out the error.

Easier said than done. I made a nice little collection of near misses, before I came up with one that acceptably balanced the error, and then of course I had to find the correct rotational alignment to cancel the error.

Some "Practice" parts..

Which brings me to the title of the post. Like so many of these tools, you can often cancel out a truly dreadful error with your own fix, but it absolutely, positively has to be put in the same place every time. No problem - Just use a witness mark like this, which aligns the part with an existing feature on the rear lathe bore:

The result was a residual run-out at the back end, of about a thousandth of an inch. Which is where the bellows coupling comes in. It flexes enough to accommodate that small error, and the rotary table now sits as solid as a rock on the lathe center line, as it rotates through 360 degrees. So the superb rotational accuracy of the CNC driver and the rotary table is translated to my clock wheels, and now everybody is happy. Well I'm happy.

There's another example of a witness mark on the rotary table fitting too, to account for a very slight run-out in the Sherline tables' central hole. (Yes that surprised me too...)

The spigot fitting I made to fit onto the table, was milled in place, and this dot was marked at the zero degree position while it was still on the mill. Provided it goes back into the same place every time, I have a perfectly concentric spigot. 

By the way, here are some other bits and pieces of the lathe indexing set-up.

There is a stand-off rod, that attaches to the change gear banjo, as well as a plate that fixes onto the back of the rotary table, and a hand wheel to fix the whole thing together. I got a bit fancy with the shaping on that one... I figured if I'm going to be using this thing for the next few decades, it had to look better than a hacked off piece of scrap aluminum.

Be sure to check out this system in use in the Wheel Cutting video.

Thanks for stopping by,
Chris.

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* Yes, I'm a confirmed NASA tragic, the quote is from Gus Grissom.