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Harold Hall

Workshop Processes

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The Viewer may question why back cutting is evident in both directions as with the trailing edge being higher it should be clear of the workpiece.  However, with home workshop machines being rather light weight, vibration can exist causing the cutter to rise and fall.

 

On these lighter machines therefore, maybe there is a case for the error in the spindle to table angle to be rather more than the 0.025mm over 300mm, thereby increasing the gap between the trailing edge and the workpiece.  In any case, I would suggest that when making adjustments, rather than seeking perfection, that is zero error, at least accept some error so that a better finish can be achieve in one direction, rather than a slightly poorer finish in both.

 

Final thought

I realise that the content of this article does not set hard and fast rules for using your milling machine only some factors that should be born in mind when carrying out milling operations. For many, dare I say, its content will be new ground in terms of their knowledge regarding the machine's use. Do though go further than just reading the article but also get out the dial indicator and test where your machine stands in terms of its accuracy and make a record of these. Then, by observing the results you achieve whilst considering how your machine is set up, your understanding of the milling operation will be increased.

 

Having encouraged you to check your own machine it occurs to me that I have no experience as to how close our home workshop machines come to the standards laid down. Realising that these are predominantly budget machines perhaps it would be expecting rather a lot for them to conform fully. However, they should come close  Having removed the pile of shims from my machine, as mentioned earlier, I found my machine was within the quoted requirement in the left/right direction and only just outside back to front. Why then were those pieces of copper there I wonder?

Back cutting effect, sharp end mill, Tramming
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 6

Back cutting effect, blunt end mill, Tramming
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 7

The test pieces were 230M07 (En1A) steel and the cutters  running at about 500 rpm.

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Drawings

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Photograph 7 shows the same tests being carried out using a 20mm end mill that is very blunt. Evidence of back cutting is very apparent on the right hand cut but also confirms my earlier comments that a blunt cutter will not cut at every turn but only when the depth is sufficient for the cutter to get below the surface. This is seen by the back cut circles as they occur randomly. Whilst the left hand test is appreciably better it is still poor, a case for keeping your cutters sharp.

Test Pieces

Having explained the theory and checked my machine I decided to machine a test piece. Photograph 6 shows a test piece that has been machined both ways using a 14mm cutter and at a depth of 0.2mm.  On the right the direction is where the trailing edge is lowest and back cutting has taken place. When machined in the opposite direction with the trailing edge highest, seen on the left, back cutting is almost absent and the finish is improved. However, having been machined with a very sharp cutter both surfaces would be acceptable in many cases.

Do click on the pictures to see a clearer view of the results