Runout - it's the chuck.

 
The dial gauge and stand arrived today. I had taken the chucks off both the machines in preparation. Measuring the runout at the spindle nose (both external and internal) returned a value on the Metric Dial Gauge that was so small it was unreadable. Certainly less than 0.01mm or 0.0005 inches.
My heart lifted. The new machine is just as accurate as the old one.

Putting the chuck back on the SL and chucking a piece of steel rod gave a readout of around 0.03mm at 2 1/2 inches from the chuck. well within the spec of the machine.
Trying the same thing on the Mk4 gave around 0.08mm, quite a bit worse.

I stripped down the chuck again and gave it another clean up. I ran a brand new 14M x 1mm tap through to clean up the thread which brought out a lot of crud. Poorly cut threads eh? I also went back over the entire chuck with a magnifier. Sure enough there was an index mark! So badly executed that I thought it was a machining mark! I re-assembled the chuck carefully locating the jaws back in the correct order.
Lo and behold, the runout is now identical on both machines! 0.03mm at 2 1/2 inches from the chuck face. COOL!

Next task is to take everything out of the upstairs workshop, pick up the antique bench drill on Sunday and then sort and put everything back so that real work can be done.

One of the things I want to do is try and fit my 'Dremel' rotary tools to the toolpost of the Mk4 and make a punch and die set. I am still waiting for my small machine vice and the milling column for the SL to turn up.

Tinkering around

 

Whilst waiting for the dial gauge to arrive I decided to try a few things on the Unimat 4. I tried re-chucking some concentric ground tool steel bar and 'feeling' the runout. You could see it wasn't running true. About 20 thou out.
Then a brainwave. I completely stripped down and meticulously cleaned the chuck and jaws. Re-assembled it with the jaws all going back into their respective slots. Still the same ...... now ....
On the old SL/DB chuck, all the jaws are number stamped and the chuck face is also number stamped and the jaws must be installed in a certain order. This is because on a scroll chuck a flat thread draws all the jaws closed. In order that they all meet in the middle the threads of the jaws are all slightly different patterns..... the previous owner couldn't have been that stupid surely? I had been very careful to reassemble the jaws in the slots they came out of.
I dis-assembled the chuck again. Sure enough the jaws are all number stamped. However, try a I might, I couldn't find any markings of any kind at all on the chuck body.
I re-assembled the chuck moving all the jaws along to the next slot, but in their correct order. Blimey! The runout has dropped down to almost nothing! There is still a slight off centre 'feel' but that was using a piece already cut on the SL. if the two machines are out by about 2 or 3 thou then the combined effect of 5 or 6 thou is about what I'm getting. I'm a lot happier about the machine now. Even if there is a 2 thou runout it will be fine for the stuff I want to do.
I have decided to check both the machines once the dial gauge arrives and see where we are. If required I will fiddle around with the jaws until I am sure they are in the right slots and then mark the back of the chuck somehow.