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Mini-lathe carriage lock

December 2007

Mini-lathe saddle lock

There’s not much to say about the carriage lock, except that many versions we can find on web are poorly designed and ineffective. The same with those sold for these lathes.

There are only two reasons to use the carriage lock. One is to stop the saddle to prevent movement along the bed. The other is to press the saddle against the bed to avoid wobble. Achieving efficiency in first is easy. To achieve the second the most critical factor is where the carriage lock presses the saddle against the bed.

If you look at the saddle as a rectangle, and if you want to avoid both the transverse and longitudinal wobble, then the best location for the carriage lock will never be one of the corners. As the saddle make contact with the lathe bed in two lines (front and back), the pressure must be applied between them. Moreover, when metal turns against the cutting tool, the tool is pushed down. This pressure pushes the left side of the saddle against the bed, raising the right side. So the ideal position for the carriage lock is on the right side of the saddle, at center.

A carriage lock should press the saddle against the bed, never the slide plates against the bed. It’s better to do so with a horizontal lever as a vertical lever easily conflict with the metal you’re turning, when you need to turn large stock. I really don’t think a carriage lock need a eccentric. I had glued, with double-sided tape, silicon carbide paper to the tailstock clamp plate to grind along the bed. A lot of paint that shouldn’t be there was gone. Now it locks and unlocks all over the bed turning the lever only 45 degrees. So the bed is enough good milled there, as it only needs 0,125mm to lock/unlock. A good designed carriage lock will avoid a lot of unnecessary forces on slide plates and they will keep good fit longer with no re-adjustments.

On the photograph, a very simple design that fits my RealBull non-H-shapped saddle. It has a pin that fits a blind hole on the saddle to prevent it from rotating and a washer to prevent pressure marks on the saddle. It presses on both sides (front and back) of the bed and the thread is M6. For Sieg H-shapped saddle, one like this one that LMS sells may be a good choice but it would be needed to drill and tap two holes on the saddle to mount it on the right side. You can also find and adapt a good carriage lock design on John Moran website.

Note: If you already have a “left side carriage lock” or a “corner carriage lock” you can test how efficient/inefficient it is with a dial test indicator (magnetic base mounted on the bed) measuring on the right side of the saddle (front and back) while facing steel, boring or milling using the milling attachment. Do some facing or boring on reverse too. To avoid bed vibration or any damage to the dial test indicator, do it on very low spindle speed or by rotating the spindle with a hand crank.

Thanks to Mike Cox

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