Tapered Saddle Gibs for the
7x10/12 Class of Lathes
by Rick Kruger
krugerr@easystreet.com
Copyright  June, 2001

The Asian made class of 7x10 and 7x12 lathes can be an amazing value of capability and function, especially given their price.  They are, however, lacking a bit in some aspects, including finishing and some design aspects and have often been much maligned.  Most of this machine's "flaws" are more from finishing than design.  Spending a little time cleaning, deburring and tweeking can pay huge dividends.  One true design flaw is the saddle gib.

The stock saddle gib consists of a flat steel(?) plate affixed to the underside of the carriage by three socket head cap screws (SHCS).  Adjustment of the gib tension against the bedway is supposed to be by two set screw/lock nuts that would push the gib away from the bedway.  It doesn't work and most simply hand tighten the SHCSs to get enough tension, but it is not stable.  Additionally, there is precious little room under the apron between the bedway and the leadscrew and the rack pinion gear, making use of common flat gibs with set screws impractical.

Several approaches have been attempted to resolve the stock gib problem, with varied success.  I had seen tapered gibs on some higher end machines but didn't think much about it.  While reading an article in "Lathe Notes Volume One" about gibs and gibbing I began pondering if it could used to fix the 7x1X saddle gibs.  After a bit of pondering, and realizing the only real solution for the 7x1X lathes was a gib adjusted at the end rather than under the saddle, I decided to try it.

The Taper

The actual angle of the taper doesn't appear to matter, but clearly parallelism and consistency of whatever the angle is must be important.  To ensure a consistent angle, I made a tapered plate to serve as a jig, on which both the gib material and the backing bracket could be milled to the same angle.  I used a small hardened pin to establish the initial taper, but anything could be used.  The angle turned out to be just slightly over 1 degree.

Making the Tapered Gibs

First, I made the gib, from 1/4" brass plate stock.  I knew there would be warpage as surface stresses were removed.  The gib taper was milled first.  It was then turned over and around 180 degrees to nullify the taper and the opposite side was milled to remove the surface skin.  As expected, this did not result in a straight gib, so it was bent to as straight as I could get it using my drill press as an arbor press.

The gibs were made longer than I knew they would need to be so they could be fitted to the finish dimensions and then cut-off.  Front. Rear.

Gib Brackets

Keystock, 1/2"x3/4", was used to make the gib ("L") brackets.  They were milled on the taper jig, but this is where I had to start being careful, as they are a non-identical pair.  I wanted the wider part of the taper to be at the rear of the saddle on both sides, so each side's "L" bracket had to be cut differently.  This meant milling on opposite sides as the stock was held on the taper jig

This difference had to be maintained throughout the remaining steps.  In addition to to milling the taper, I also cleaned up the mounting surface to be sure it was parallel to the "reference" surface.  The keystock I used was remarkably parallel, but not as close as it might need to be.  This was done by clamping the bracket in a mill vise, supported on the same bottom side as on the taper jig and milling the top surface.

The mounting screw hole pattern was made to match the threaded holes on the under side of the saddle.  Drawings are provided here of my parts, but you need to be sure the dimensions fit your lathe if you try to do this.

Adjusting Screws

The key here is that the gibs are adjusted at the end of the carriage rather than under it.  Adjusting screws on the ends of tapered gibs are a signature feature.  The screw heads need to fit snugly into grooves in the gib so the gib does not move significantly, forward or back.  I used M4x0.7 pan head screws as the starting point and turned the heads to fit the slots.  Clearance of about 1 - 2 thousandths was allowed.

Fitting the Gibs

Once the basic parts were machined, it was necessary to fit the gibs to each bedway.  The tapered gibs had been made longer than needed because I was uncertain exactly how thick each needed to be.  The initial gibs extended beyond both the front and rear of the saddle.  After fitting a gib to each side, they were trimmed, the front flush with the carriage, the rear extented for the adjuting screw.

Locking Screws

Gib adjustment would change through use if the adjusting screws are not locked in some manner.  Locking screws were provided, perpendicular to the adjusting screws on both the front and rear gibs.

Tapered Gib Assembly

One of the design issues was providing clearance for the carriage rack pinion gear.  Fortunately, this was not difficult, although it was close.  The entire assembly looks like this.

Drawings

Drawings of the taper jig, tapered gib and gib bracket are provided for those who might want to try this approach to improving the 7x1X saddle gibs.

Conclusion

It remains to be seen how well the tapered saddle gibs work in use.  They seem to be quite secure and seem to have improved saddle rigidity by quite a bit.  Inconsistencies in the bedway dimensions are becoming apparent, which were not otherwise realized.  If nothing else, the saddle gibs can now be adjusted to fine levels, which in itself is a substantial improvement.  Being able to do so easily from the rear of the saddle is an added bonus.