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Harold Hall

Workshop Processes

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Setting a milling mc. spindle vertical to its table. Often known as Tramming, and the effects of errors on the  finish achieved.      Harold Hall

It was when commencing to use the lathe mounted milling head I had made  that I finally decided that it was about time to investigated an aspect of the milling operation that had been on my mind for a number of years. This was the effect of the milling machine's vertical spindle not being exactly perpendicular to the traverse of the machine's table.

 

Permitted Errors

I think that many workshop owners tend to overlook that even quality machine tools are made with permitted errors. For example, the precision three jaw chuck will not be expected to hold the workpiece exactly concentric but with less error than thel chucks that most of us use. It would be easy to assume that the lathe's cross slide will be exactly at right angles to the bed but this is not so and an error within limits has to be allowed, in this case, rather than having a plus and minus error an error is permitted just one way, albeit very small. The permitted error is such that a surface faced on the lathe can be very marginally concave, but not convex.

If such errors were not permitted then our machine tools would be astronomically expensive. Where then is this leading us to?

 

Well, in the case of the milling machine spindle, an error from the perpendicular is just one such a permitted error. It was when becoming involved in metalworking I gained the impression from reading an item from a reputable author that the permitted error was, rather like the lathe's cross slide, just one way and decided that I would investigate the reasoning for this and its effect on the machining operation. After 16 years I have at last got round to it.

 

Now having referred to the appropriate British Standards regarding the accuracy of milling machines I find that an error is permitted both ways, that is left or right. It maybe that the information conveyed above was the authors own considered opinion, a situation that I will elaborate on later, and to which I basically agree. With regard to the error viewed from the side the error is only permitted one way this also will be discussed in more detail later.

Know your milling machine

I will briefly introduce these pages by stating that they are not to say, this is how it should or should not be done, but to introduce some facets of the milling operation that are probably rarely considered. For some, this may even be new ground.

 

I would also add that they are technically more demanding than many related to using the milling machine and may need reading more than once to fully comprehend their meaning. However, I do consider that knowing the whys and wherefores regarding the aspect of milling being discussed will appreciably increase the pleasures of using a milling machine. I would therefore, encourage the viewer to reread the pages should they not make full sense first time round. Also, read a third time if required.

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Drawings