Friday, 3 June 2011

Oval Prongs in flip plus rotary milling.

The question I field more than any other when it comes to milling, is usually tied to when the person asking, is trying their first attempts at flip plus rotary pass milling, where alignment becomes super crucial for the first time. This post is to help you get a grasp on indicating where you error may lie. You need to grab a good gauge and measure your prongs, cut a job with a prong that has a exact measurement, say 1.5mm then measure which way round the prong is oval , and more importantly whether it is a THINNER oval, or a THICKER oval. (ie, prong is thicker or thinner than the 1.5mm it was meant to be)

Oval prongs usually indicate an issue with A or Z axis errors or alignments.  I will attempt to explain this in the pictorial below. They can however look like each other depending on the positive or negative value of the error. SO measure twice, sketch your oval on paper with its measurements and then read on.

For this exercise I drew a very simple exaggerated shank with four claws.

1: Prongs oval THICKER than the correct size ACROSS the shank.

This usually indicates an issue with Z0 in your flip cuts.  It was too high, or too low, if Z was too high (ie not cutting right down to 0) then usually your prongs will be oval across the shank.

In this example I used a Z error of 0.22mm, the cutter did not go down far enough. which in a flip will result in a  total error of 0.44mm, these numbers are highly exaggerated and the scenario is not actually 100% accurate, its for explanation only.

 this error from the top looks like this,

correcting for this is easy, reprobe your tool, check for chipped cutters, and cut a few test pieces and guage Z thickness. You may need to reprogram Z offset in your controller software.

If the prongs are oval in the opposite direction but THINNER  across the shank , in other words the oval presents in opposite direction, but the claws are too thin ACROSS the shank, then usually this will indicate your Z is cutting too far downwards in your flip.

The same corrective steps as above will be required.

2: Prongs oval THINNER than the correct size ACROSS the shank.

This error will usually indicate an error in Y in your flips,  or A  in your rotary pass. It presents as follows

and is usually the result of A being out like this

The first image shows a hypothetically thicker prong along the shank but since the prong has already been cut in the flip, by the time it comes upright for the pass in A  part is already "missing" and the opposite part will be cut away, leaving only the centre section which I shaded.

If you own a mill with a A homing setup, follow your mill instructions on getting that sorted out (I dont have much good news for you users tho, typically automated A setups in mills using wizards are the users who present this error the most) for the manual guys, index properly when you setup your hub/arbor. I have a variety of units that will allow automatic indexing but I STILL cut an indexing pyramid into my shanks to visually check alignment... anything with moving parts will like to give you a run for your money... trust your eyes.

Oval in the opposite direction but THINNER than the correct size indicates 

In mills with cheaper or badly produced rotary units this error can also be a backlash issue in the rotary. I find physical backlash error to account for less than 1% of these issues, usually its poor alignment.

3: Oval along both axis.

This is a tricky one, very tricky indeed.. it can represent a combination of the two scenarios above or could introduce an entirely new problem. Y axis misaligned in your flips.

It looks like some variant of this,

and is the most typical isseu reported because it has the highest likelihood of occurring, and unfortunately this usually means you were lazy and did not spend enough time tramming and dialling in your milling machine. 

Tramming and dialling in properly will solve ALL the problems above usually.

If there is enough interest I will expand on the area of tramming and dialling in your milling machine.


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