It could be the gear train, though I wonder if you have a tooth off the rack ? It’s a 2A ? Noel.

I don’t know the model, but one thing that could cause a problem of this kind is swarf damage at some point on the taper the knee gibs slide on. By damage I mean a piece that has got trapped and smeared into the dovetail.

You have given a clue in that this occurs at the same point each time. Perhaps that is where to look for a more detailed and searching examination.

Regards  Brian

Could you describe the mechanism from handle to knee.

AIUI, the handle turns a bevel gear, which then turns another bevel gear connected to a shaft. At the other end of that shaft is a spur gear that engages in a rack that moves the knee.

If one of the keys holding any of the gears onto their shafts is dodgy, it could be acting as a one way clutch.

Mark the rack and pinion as they are now it and then reinstall with the pinion mark 180 degrees rotated. See if that shifts the problem to a different place in the knee’s travel.

Lie the mill on its back and remove the rack (or pinion). Make sure the knee slides smoothly up and down the full extent of its travel.

My vertical milling slide does that and I reckon it’s a leveraging issue induced by the weight distribution

I can feel where it’s sticking via the gib adjustment screws, undo it slightly and it drops and stops sticking… for a while

Apart from the “old and worn out” issues it may be worth leaning the unit over slightly to one side or the other by a few degrees, or forwards/backwards to change the vertical weight pressure bearing down on the slides

As above, it may well be a circular pitch gear.

Start with the handwheel for the knee. How much is the total movement for one revolution? In other words, how many graduations are on the dial? What is the ratio of the bevel gears connected to that handwheel? Count the teeth on both. From that, we can work out how far one revolution of the 20t gear will move the knee. That will tell us the specification of the gear.

As a sanity check, measure the outside diameter of the 20t gear and write it down.

A rack should be simple enough with a flycutter. Turn the vertical head sideways and use the X-axis for spacing the teeth. Gash the teeth first with a slitting saw very close to the tooth root width. The challenge will be the opposite one to that most often faced by Centec owners – in this case you will likely have too much daylight between cutter and workpiece, so might need to raise the workpiece on the table to reach.

Basically the one in your first photo is a 2A.

The 2B is a good bit taller and the knee is operated from the front

The 2 has no gearbox and smaller than the 2A.

Later 2A had bigger leadscrews and the two slots for mounting the motor were removed.

You can get all sorts of cogs and racks on ebay and amazon now, some of which are steel. Might be useful to look.

If you fit a dro to the vertical it doesn’t matter what pitch the rack and pinion has. This makes it much more likely that you will find a commercial replacement

On 25 December 2023 at 21:12 duncan webster 1 Said:

If you fit a dro to the vertical it doesn’t matter what pitch the rack and pinion has.

If I carry my car around on the back of a tow truck, it does not matter that the engine is broken.

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In the best traditions of this forum, making wild speculations on the basis of no evidence (trying to blend in with many of the regulars here), might I suggest 0.200″ circular pitch for the gear (hence it would be 15.708 DP and 1.4006″ OD). That would mean a worm wheel of 40 teeth.

Making a 0.200″ pitch rack would not be too bad. I would do everything I could to save the gear. Does the full width of the gear teeth mesh with the rack? If not, reversing it on the shaft or moving it so an unworm part meshes with the new rack might be good enough.