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Projects ... A3 Bolsters and Side Bearings 4/24/06

This page covers the machining of the Bolsters and Side Bearings ... Section 3.4 pages 18 & 19.


Although there is not much text on the bolsters they turned out to the most difficult part for me to make thus far. The tolerances are very tight and all 4 slots for the column also must line up (front and back side of the lower and upper bolster). Take a look on page 18 figure 3-20 the upper bolster ... the width is 0.316 0.001". The column width is 0.3115 0.0005" or say about .004" smaller. Adding to the problem there is a 3 taper that also must be considered. So all 4 slots on the right hand side of the bolster must lie within about 0.002" of each other, otherwise the column will not fit or will cause binding. The left side the same thing, fortunately left and right slots are not depend on each other. The issue then becomes ... how to cut these 4 slots then the other 4, then the taper so that they all lineup with each other and allow the column to fit.


Here is how I started ...
 

I placed each cut off section in my 4 jaw chuck and faced each end to obtain the required length, I tried to maintain a maximum 0.001" variation between all 4 bolsters. I decided I needed to use a few shims to help keep tooling marks to a minimum.
 
After machining to length I used super glue to bond the upper and lower bolsters together during the layout and machining. This way I knew each slot had to be in line. Now all there was to do is to cut between the scribed lines and I would have my slots to spec. That should be simple!
   

 
I milled all the flat work first then carefully milled to the layout lines on each slot. As you can see the left slot is done, to get the proper depth and to meet the tight spec of 0.621.001" I measured my stock thickness at 0.7473" ... subtracted 0.621" then divided by 2; this gave me how much I needed to mill off on one side( 0.0631"). But that is not really what I wanted. I then subtracted that from my stock thickness to obtain the thickness from the bottom of the slot to the other side or 0.6842".
 
  Here I am measuring the overall thickness of both slots which should be 0.621.001". As can be seen I am just inside the spec by 0.0001". It is important that the slot depth for each slot be the same and the final width be correct.
 
     

After milling the flat work I moved over to my very cheap angle vise. Before setting up with my newly machined bolster, I set up a test material to obtain the 3 taper, simple as it doesn't need to be exactly on. Done. So now a simple test ... how well do the columns fit the bolster? Great ... no binding at all. Seems to be a little loose though. Let's check the width of the slots ... okay now I'm in trouble ... the width were out as far as .010" over the spec. Way too much .... so I scrapped them ... started over. They were just too sloppy, I know I can do better! So what did I do wrong? As careful as I thought I was ... I just could not mill the taper without milling the sides too much. I even used red ink to so that I could see the edge ... did not help enough! So I started over with that in mind.
I did everything the same as before ... up to cutting the flat work. Instead of milling to the layout lines I left some material so that I could see the line when milling the tapers, also I cut the upper and lower bolster tapers at the same time. I placed my scrapped out material in between the two bolster. Now when I finished the upper I just reset the height to start a new taper for the lower bolster. Looking carefully you can see red ink inside the grove ... same idea when turning the taper for the bearing, leave just a trace so that I know the depth is correct. If I were to do it all over again I would just mill them in the angle vise and make the cuts at the same time ... no real need to move to the other vise as these are small cuts.
Another Test ... this time the fit is much better. All I needed to do was to file down a little material here and there to get a smooth fit with no binding. Much better ... slot widths all with spec.

The Side Bearings ... were simple ... no problems!
  Well ... here is the finished product looking closely you will notice that I have started a numbering system. That way I can put it back together and it will fit.




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