Lathe mod - tapered saddle gibs

This photo shows one reason for wanting to make tapered saddle gibs - the gibs supplied with the lathe are brittle and can crack rather easily when adjusted. In addition, adjusting the gibs properly is made more difficult by their locations under the saddle where they are hard to reach. Various solutions have been proposed for addressing this issue, including using shims to adjust the gib height (see referenced links above). I used the shim method and it worked for while, but is even more difficult to re-adjust than the original gibs.

Since I clearly had to address the problem, and didn't want to buy new gibs that were likely to break again, I decided to try my hand with making tapered gibs.

Gib Brackets

I started with 0.5" x 0.875" 1018 cold finish mild steel rectangle bar. The bar was cut to length (4" nominal) and faced in the mill on one side (the side that would be facing the bottom of the saddle). I tried to mill off as little as possible - just enough to make sure the side was flat (about 0.010"). The ends were also milled flat to clean up the rough hacksaw cuts and to adjust the length.

The front and back sides of the bracket were then milled to the appropriate width. The bottom side was not milled.

The milled pieces were center punched to mark mounting hole locations, and then drilled on the mill. Since the holes were 40mm apart on my lathe, I believe I could have just marked the center hole and measured out from there on the mill. Your lathe may differ of course, so don't depend on my measurements if you try this build.

In any case, the holes got drilled - everything was pretty easy up to this point.

Front Gib Bracket

The next step was to mill out clearance space for the gear that engages with the rack under the front of the lathe. I used a ball end mill instead of a regular end mill. This was not strictly necessary, but I think it gives it a more attractive appearance, and eliminates crud-catching corners.

Be sure to mill the correct end of the gib bracket!

Here you can see the front gib bracket being test fitted (the saddle is upside down). Only one of the three mounting screws is in place.

It's a pretty good fit, although I could have made the clearance space a little narrower (but no harm done). It may look in the photo like the mounting screw is interfering with the gear, but in fact the gear is well out in front of the screw.

The final step in making the gib bracket is to drill and tap a hole in the end for the gib adjustment assembly. There is not enough clearance on my small mill to do this with the bracket held in the vise, so I had to improvise the work holding method shown at left. I clamped the bracket end-up onto a 1-2-3 block which was fastened to the mill table.

The only things good about this method is that it worked, and that it used the tools I had on hand. A webbed angle plate would probably have been a better choice here, but I didn't have one.

I should note that the location of this hole is rather critical, in order to ensure that the adjustment washer fully engages with the gib.

Mounting Screws

Speaking of mounting screws, I substituted M6x1 hex-head bolts and lock washers for the original socket cap screws. This worked out pretty well, and I found these easier to tighten using a regular wrench than working under the saddle with a hex wrench.

Milling the Tapers

The next step was to start milling the tapers. Rick Kruger's original build used a taper jig, but I opted to go with a custom made 1° angle plate using this method. You can see the small aluminum plate I used to the left. I won't go into too much detail on this plate, because I opted to make a longer plate later in the build (as I will explain later).

I did use this smaller plate to successfully mill the taper in the gib brackets, and to mill one of the gibs, but I had problems with the second gib. It was easy enough to make a longer angle plate, and it worked much better, so I would definitely recommend the longer plate to anyone following this method.

Unfortunately I neglected to take pictures of the milling of the gib bracket taper. However, it was fairly straightforward: The bracket was positioned on top of the angle plate in the mill vise, so that it was held at the proper angle. A section of the bracket was then milled out to create a tapered cut in the finished piece.

Do remember to reverse the angle plate for the front and rear gibs, so the tapers will be running in the same direction when mounted in the lathe.

Milling the Tapered Gibs

The gibs were made from pieces cut from 0.375" extruded C360 (Half Hard) brass square bar, 6" in length each. The pieces are intentionally cut long so they can be cut to size after milling.

As was pointed out in Rick Kruger's original article, expect some warping when these pieces are milled. I experimented with heat treating the brass before milling to reduce this problem. It worked well enough that I would recommend doing it - it's very easy, and does no harm at the worst.

Here the brass bar is mounted in the mill vise. For the first milling, the bar is mounted flat on parallels, as the initial step is to mill two sides parallel to the proper width. The pieces should be milled slightly on one side, then flipped over and milled on the other side, and so on until the proper width is reached, so as to even out stresses and reduce warping. Since the two sides are pretty much evenly milled at this stage, there is really no warping.

For my lathe, I had to make the front and rear gibs slightly different widths, as the gear rack on the front took up a bit more space.

I note that ends of the gibs are not well supported in the photo at left (I only had short parallel plates). However, I was able to make this work by taking very light cuts, but I should have used a smaller diameter end mill.

Update [Dec. 2016]: The support method shown to the left is really not very good - the bar should be supported along it's full length.

As noted previously, I was able to successfully mill the gib brackets and one gib using a short 1° angle plate, but this was far from ideal. I therefore made the longer 1° angle plate (from 3/16" mild steel) shown at the left. This 7" long plate provided much better end support, although it did allow for some front-to-back flexing; this was alleviated by taking light cuts, and by switching to a smaller diameter (5/16") end mill. It was also necessary to advance the table quite slowly as the ends of the pieces were reached.

Removing Warp

Even with prior heat treating I still experienced some slight warping when milling the taper. Unlike the parallel sides of the gib, when milling the taper a relatively large amount of material has to be removed from one side as compared to the other. I checked for warping after every (taper) milling step, and removed any warping using the vise set-up shown to the left. I cut two short pieces from a piece of L-shaped extruded aluminum (shown at either end of the vise at left), and put a scrap piece of round brass bar in the middle. By tightening the vise I was able to apply controlled "anti-warping" to the piece to straighten it out.

For more information on this topic see Relieving Stress in Brass by Heat Treating.

Another trick I found useful as a final step in fitting the taper, was to use the mating gib bracket to hold the gib in the mill vise for a final very light milling to ensure a good fit.

In this method, the gib bracket is set up on parallels, and the gib is fitted into the gib slot to that the tapers cancel out. A short piece of 3/16" drill rod in front of the gib helps to hold it tight in the vise. The top of the gib is then milled "flat" with the lightest possible cut. In order to mill the entire surface of the gib, it is necessary to make a very slight cut into the gib bracket, but this very shallow cut is of no consequence.

Adjustment "Washer" (Round Nut)

I departed from the original Rick Kruger design by developing a different type of gib adjustment. The design is basically a thick threaded "washer" (technically, a round nut with set-pin holes) that rides on a screw and fits into a slot on the gib; the washer is locked in placed with a hex nut. Six holes are drilled into the periphery of the washer to enable using a 1.5mm hex wrench as a lever to make adjustments.

The washer is made from 0.5" diameter stainless steel rod. Since the spacing of the adjustment holes is not critical, a simple paper template was created; this is wrapped around the rod, taped down, and used to mark the location of the holes. The required template is included in the plans above.

The rod is first mounted in a lathe, faced, and a central hole is drilled (#28 drill) to be tapped later for an M4x0.7 thread.

The photo at left shows the peripheral holes being drilled, first with a center drill and then with a standard drill. I fastened a 1-2-3 block to the milling table to act as a stop block so I could center drill a hole, rotate to the next position, and so on, and then repeat the process with a standard #50 drill.

Once all of the peripheral holes are drilled, the rod is returned to the lathe and the end is cut off (0.125" thickness). The resulting "washer" is then tapped for an M4x0.7 thread.

The photo to the left shows the adjustment washer in place on the front gib (the apron is removed in this photo).

Here is another view of the front saddle gib, again with the apron removed.

Here is a view of the rear gib saddle bracket with gib and adjustment assembly in place.

A closeup of the adjustment mechanism on the rear saddle gib. In use, to adjust the gib the hex nut is loosened and a 1.5mm hex wrench is used as a lever to adjust the "washer" (actually more of a round nut with spoke-holes).

Note that with this adjustment design, it is not necessary to use locking screws as in Rick Kruger's design. Instead, the socket cap screw part of the adjustment assembly is just screwed firmly into the end of the gib (thread locker could be used also, but so far I have not found this necessary). After the gib is adjusted with the "washer" the hex nut is tightened to lock the adjustment in place.