You saw last time how easy it was to get the correct taper on the holder part without the use of the jig.
So now I am going to prepare the bronze gib strip, in preparation to cutting the taper on it.
First off, down one corner of each strip I cut a very small chamfer using a 90 degree countersink bit. This is a perfectly acceptable method of doing it, as long as you don't try to remove too much material.
The reason for the chamfer is to prevent binding in the very sharp corner of the holder.
Now that I am getting close to the end, I marked the parts up so that they would be made as matched pairs, and by doing this, they will always be assembled correctly if ever they are removed.
These next four shots are concerned with a major issue I have in the way the original concept of these tapered gibs has been done, and IMHO very bad engineering practices have been used.
Let me explain a little.
The 5" long gib strips run up a tapered slope, and the only thing stopping them twisting to the side is the very small head of the adjusting screw fitting into a running fit slot. The whole gib strip could just as easily move sideways and hit the casting or jam up the whole saddle.
So I have come up with an easy fix.
From the side of the gib strip that sits against the upright side of the holder, and the part of the strip that runs against the slope facing upwards, I measured in 0.150" (half the width of the gib strip).
I then machined along the strip a slot 2mm wide x 1.5mm deep.
After that, the holding block was mounted up and I measured 0.150" in from the upright face. Then two 2mm holes were drilled on the tapered slope. The first one was just a bit further in from the end than the depth of the adjusting screw hole and another one, further up the slope, 2.5" away from the first hole. A bit of 2mm rod was stuck in the holes with high strength loctite, then each one was ground down to just below 1.5mm sticking up.
I am sorry for all the mixture of measurements, but I used what was available to hand.
After deburring the slots and the tops of the pins, I found that both jibs run perfectly up and down the slope, and there would be no chance of them slipping sideways when mounted on the lathe.
So now I was back on track.
In fact, when I first started to plan this job out, I had forseen a problem with holding the gib strips to their holders while I machined them. So I made the strip 0.005" wider than the face they were to run on, 0.305" instead of 0.300".
This is how the solution worked. The two parts were put into the vice, and everything was tapped down onto the parallel. Because the gib strip was wider than the slope, it clamped everything rigidly together in the vice.
It pays to plan ahead.
The extra 5 thou can be left where it is, it will not cause any problems in the future.
You can see on this shot how the gib strip sits up at an angle. By cutting it level on the top, I will have a perfect match between tapers, giving me a totally parallel lift on the top face.
So with a nice sharp cutter, I wacked up the speed to 1800 and taking cuts of 50 thou, knocked the jib strips down to size. The finishing cut was about 0.002".
A pair of perfectly matched holders and gib strips.
Notice that I have cut away part of the gib strip marking, that will be restamped before assembly.
Unlike in the article and plan write up, I have no problems with twisting or bending of any part. I think that is because I had clamped everything together for final machining.
This shot shows the angles produced.
Who needs a machining jig?
Here is the rear bits mounted up ready to be marked for cutting to length.
The same for the front.
Remember when I said I would make the holes in the mounting blocks a bit more adjustable. Well it was required. So when final assembly and fitting takes place, I will drill and dowel pin the holders in their final positions.
Not everything can be calculated out, so a bit of built in adjustment comes in handy.
So what needs to be done now is, strips cut to length and an adjusting slot machined, adjusting screws to be designed and made and the apron gear cutout to be machined on the front holding block.
These posts make it look like a very long drawn out process, but when you consider the problems and mods I am doing, it has extended it somewhat, plus I am only doing it in short stints of a couple of hours at most. A normal fit like this would only take a good day in the workshop.
Bogs
