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Harold Hall

Workshop Processes

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You may wonder why I have not mentioned the contributor by name, this is because I cannot remember, I do though remember that it was someone well thought of at the time. I also remember it was being referred to in relation to the Dore Westbury milling machine which had the facility for tilting the head,the workshop owner therefore had control of which way the head tilted.

 

Unfortunately, for most vertical milling machine owners, you are faced with whatever situation you find you have. That is of course unless you are prepared to make adjustments to the machine in some way. In the case of a typical mill/drill,shims between the column's base casting and the machine's base.

 

When I purchased my mill/drill I had to dismantle it to enable me initially to transport it and ultimately to install it as it was too heavy if remaining intact. I found that there was a lot of copper pieces between the column's base casting and the base of the machine. Rather like pieces of flattened water pipe. I could only assume that these allowed some adjustment if the nuts on one side were tightened more than the other. I did not like this and they were left out when reassembled. In any case, I could not remember which pieces went where, having dismantled the machine quickly at the supplier's. It was because of this that I checked the spindle's relation to the table at the time and decided that it was acceptable without them. I do hope that this approach is not being used on later mill/drills but see no reason why a very thin shim cannot be used to provide the accuracy required.

 

Workshop owners who have one of the milling machines with a tilting head will have a ready way of setting the spindle to the required position in one axis. If though it is calibrated for setting the head to varying angles, Photograph 2, under no circumstance use solely the zero mark for setting this position, this will not be sufficiently accurate, see later comments.

What the standards say

What then is the mathematical extent of the situation? Well British Standards give us some values for the permitted error of the spindle in relation to the table's surface. Photograph 3 shows the set up for testing the requirements which are that there should be no more than 0.025mm variation in the dial indicator reading over a distance of 300mm, that is with the dial indicator set at 150mm from the machines spindle and the spindle rotated through 180°. The error can be either way, that is with the table higher on the left, or higher on the right. For my suggested preference though, higher on the right would be the situation to aim at . The tests should be carried out with all three axis locked. To express this another way the angle of the spindle to the table should be less than 90° on the right and more than 90° on the left.

 

To quantify the effect at the end mill the above maximum error would cause one edge of a 10mm diameter cutter to be 0.0008mm deep compared to the other. Whilst this is very little its main effect would be in the quality of the surface finish if fed in the least preferred direction, most important when the work entails light cuts to provide a good finish.

 

When the test is carried out in the back to front direction the same level of error is acceptable but only in one direction, that is the front of the table high, that is the table to spindle angle in front of the spindle to be less than 90°.

Tilting Head Milling Machine, Hobbymat BFE65
Checking milling machine spindle alignment, Tramming
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