Aligning the vertical head – tramming
One of the key alignments on a vertical milling machine is setting the vertical head so that it is square with the milling table. This has to be so it is square in the x-z plane and in the y-z plane. All vertical heads, except on the smallest machines, are designed so that they can rotate in the x-z plane. This is a very useful feature. But, of course, if it is used then the head has to be realigned afterwards. Some heads can also be rotated in the y-z plane. Again, if this is done then the head will have to be realigned in this plane afterwards.
A similar problem is where the head is not designed to rotate in the y-z plane but is not perfectly square when tested.
The process of setting the head so that it is square is known as “tramming”.
Consequences of misalignment
If the head is misaligned in the x-z plane then any rotating point on a cutter will cut deeper on one side that the other. This will be true whether the workpiece is moving from left to right or from right to left. If an endmill is being used instead of leaving circular marks it will leave “C” shapes facing left to the left or facing to the right. If this can be seen it is a fairly certain sign that the head is not aligned. If the workpiece is milled with an endmill with a series of parallel cuts in the y direction each cut will have a slight slope and will have a ridge that can be felt with the finger.
If a wide cutter is used such as a flycutter in the x direction then the surface will have a very shallow curve to it.
If the head is misaligned in the y-z direction and an endmill is used to cut in the x direction in a series of cuts then a series is very shallow slopes will be formed..
If a flycutter is used to machine the whole surface from left to right in one pass then the surface will be flat but will be at a slight angle to the bottom surface of the workpiece.
The most accurate way to check the alignment of the head is by tramming. This can be done by fitting a DTI to the spindle as shown. The rod can be any that fits the DTI. It can be fitted to the spindle using a drill chuck or any collet chuck. The accuracy of the chuck, in this case, is irrelevant.
If the head is square then the reading on the DTI should be the same when the arm is to the left as it is when it is to the right.
If the table of the milling machine was just a flat surface it is easy to see that as the DTI is rotated it should read the same whatever the angle of rotation. But there are the T-slots. The ideal solution to this is to have a flat plate that can be placed over the t-slots so the DTI can be rotated without the interruptions of the slots. However, the smallest ding anywhere under this plate will cause any readings to be unreliable. Another way of doing this is to use a sheet of paper. This is placed on the table covering the area that could be touched by the DTI. When the DTI is at a position of interest the sheet is slid away from under it. Having read the DTI the sheet can be slid back again. This can be done over and over again without applying any force to the DTI.
If the readings at the left hand and right hand positions are the same then the head is aligned in the x/z plane.
Heads that rotate or not in the y-z plane
Similarly, if the head rotates in the y-z plane then the readings in the front and back positions should be the same. The head should be adjusted till they are.
What if the reading are not the same and the head cannot be adjusted?
Usually the head can be removed from the column.The simplest problems is if there is dirt in the join between the head and the column. alternatively the surface at this join could be spoilt. this could happen if a screwdriver was used to force these surfaces apart.. This might have caused a ding on one of the surfaces. If this does not solve the problem it might be the bolts that connect the two part were not tightened properly. It might be found that adjusting the tightness of these bolts might solve the problem.
Otherwise place small pieces of shim in the joint. Use some soft metal so it is more compressible. In extreme case fit soft, copper washers on all of the bolts that hold the two surfaces together.
Setting the vertical head at an angle
If the head has to be set at an angle then the procedure is the same except that instead of the dti touching the milling table it is touching a sine bar. The sine bar is set to the required angle.