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BRIDGEPORT MILLING MACHINE REBUILD – PART V

 The Base Dovetail

The next phase of restoration is checking the dovetails on the base. The dovetail angles have to run parallel to each other to avoid binding as the knee travels up and down. The actual angles are not a concern, because the mating angle of the knee dovetails can be altered to suit. The first step is to examine the condition of the surfaces. In this case, both surfaces had good signs of the original flaking marks from top to bottom. The flaking marks where lighter in the middle portions of the dovetail angles, which indicates the most popular travel zones. The left hand dovetail of the base(looking at the front of the mill), is the gib face. This face showed less flaking then the right side, and was strong at the top and bottom. The right hand dovetail was quite consistent top to the bottom. The next step is to use the scraped straight edge to show the contact pattern. It was now clear that both dovetail angles had about 4″-5″ of good contact patterns at the top then faded towards the middle, and became stronger towards the bottom. The next check is to measure from one dovetail to the other. I used two dowel pins, 3/4″ diameter x 1 1/2″ long, one in each dovetail, and measured over the pins at various locations. The measurements should decrease in the middle area of the base dovetail according to the contact pattern. There was a size decrease in the middle of  .001″- .0015″. A point to remember at this stage of checking, this check dosn’t show which dovetail face has the wear, or how much wear each side may have.

THE BASE IS REPOSITIONED

DOWEL PIN FOR MEASURING

CHECKING THE SIZE OVER THE PINS

 

This base is in fairly good condition along the ways, which shows if the machine is kept clean, and well oiled with the correct lube, the mill will survive a long time. In a short period of time the right dovetail was finished. Measurements using the dowels were retaken across the dovetails, which showed a hollow in the middle of the gib side dovetail of less than .001″. Scraping is now being completed on the left gib face, and after each pass, measurements using the dowel pins are taken. When the contact pattern shows well across this dovetail face, and the over the pins measurements are consistent from top to bottom, you should be good to go.

BRIDGEPORT MILLING MACHINE REBUILD – PART VI

THE KNEE

The knee is the next stage of the rebuild. The flat and dovetail surfaces that support the cross slide and table are chromed. This is a factory applied coating. It is my understanding the surfaces are scraped and flaked, and then a thin layer of chrome is applied, which is intended to reduce wear on those heavily traveled surfaces. If those surfaces need to be re-qualified, the chrome has to go. Grinding the surfaces adds extra cost, as well as re-chroming. At this point I am undecided, a better assessment will be needed. Regardless the knee ways and dovetails used to travel along the base will need to be checked and corrected as required.

THE KNEE

WAYS THAT MATE TO THE BASE

 

The above picture is the flats that mate to the base. The dark grey section in the middle of both flats are the under cuts, which allows the knee to ride on the 4 pads at each corner. This reduces the amount of friction and the amount to be scraped. The base flats will be blued and the knee is installed in the proper orientation to gather a reading. Essentially the base flats are the new master for scraping in the knee. At this stage it is important to keep the cross slide ways on the knee 90* to the base ways. The precision frame square becomes a friendly tool to own which simplifies this stage of mating.

THE KNEE AND BASE ARE REUNITED

THE PRECISION FRAME SQUARE

 

It is important to check the base and the knee for level during the scraping of the 4 pads. The base could also be placed in the vertical position, and leveled using jack screws. A cylindrical square and spirit level on the flat ways would be used to ensure a perfectly leveled base. I have left the base on its back, to reduce the height required to assemble the knee for a reading. I would caution using the turret surface as a reference point to level the base because it is a machined surface rather than a scraped surface. I will qualify that surface after the base and knee have been completed.

CHECK THE BASE FEQUENTLY FOR LEVEL

 

SCRAPING THE KNEE

 

 

 

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