Mini-lathe saddle fit improvement

December 2007

I started by carefully following procedures described on Marty Nissen project to fit the mini-lathe saddle with the bedways. Ten hours to achieve “perfect” fit! I did the same to the underside of the bedways where the saddle gibs ride.

Improving mini-lathe saddle fit with bed

On the back there is silicone carbide paper glued to the bed with double-sided tape. On the “vee”, as many paper as needed to grind with the same saddle-to-bed distance in front and back, right and left. Measures taken with a dial test indicator.

Improving mini-lathe saddle fit with bed

This is a critical job. Measures and the exact paper height is most important. A slow and precise ten hours job. Before trying this, read carefully all notes on Marty’s website. Don’t make any mistake, and in the end you’ll see how good it is.

Improving mini-lathe saddle fit with bed

Instead of running a end mill down the roof of the “vee”, I used hard silicon carbide paper with great result. As Marty wrote: “As you attempt to remove material from this area to lower the saddle a ridge is formed that prevents progress.”

Improving mini-lathe saddle fit with bed

Grinding, with silicon carbide paper, one side of the “vee”. It’s easier to grind one at a time, and most accurate. You can see paper heights on back of the bed to assure grinding to the exact angle.

Using studs instead of screws to tight the mini-lathe saddle gibs (or slide plates)

I used stainless steel studs instead of cap screws to tighten the slide plates. The original cap screws only use a small part of the thread in the mini-lathe saddle and as they are not very tight they will wobble and wear the saddle threads. Over time they will wobble more and it will be increasingly difficult to maintain slide plates adjustment. The stainless steel studs use the full lenght of the saddle thread. The studs are also held with Loctite to eliminate any residual wooble ensuring a much stronger connection between the saddle and slide plates, in addition to avoinding threads deformation in the long term.

New front slide plate is 6x18mm steel bar and rear is 10x18mm steel bar. The slide plates are tightened with self-locking nuts. These nuts don’t loosen easily.

To achieve a good parallel fit of the slide plates with bed, I used four set screws on two parallel lines. Two aligned with the studs and two along a line parallel to the outside of the slide plates.

The saddle gibs are adjusted using a micrometer, measuring from the top of the saddle to the front and back of each slide plate, while adjusting the set screws to get a very good alignment. With two lines of set screws the slide plates won’t wobble and the self-locking nuts can be quite tight.

Mini-lathe saddle gibs

I used this same adjustment technique to grind, with silicone carbide paper (glued to slide plates with double-sided tape) and oil, the underside of the bed where the slide plates ride, so they are now smooth and parallel with the top of the bed.

Using studs instead of screws to tight the mini-lathe saddle gibs (or slide plates)

With these additional set screws and the stainless steel studs the saddle gibs are now firmly attached to the saddle and they will not wobble or move. And they will not come loose easily.

I discovered by chance that grinding just a little the last tap of a 3 pieces tap set I can do “tight” threads, avoiding the use of locking nuts. The set screws on slide plate are M4 thread and they don’t need locking nuts.

For Sieg owners, perhaps a novelty is the oil groove on saddle contact surface with bed, but this is one more difference between the Sieg and RealBull. The saddle has two oil cups on top.

As the saddle was disassembled, one 6mm hole and a 4.2mm blind hole were drilled to prepare it for the next mod - the carriage lock. Saddle was also milled to prepare it for the cross-slide extension mod. This way I don't need to disassemble it later.

Thanks to Mike Cox

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