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Old 17th August 2006, 08:16 PM   #11
DaveM is offline DaveM  United States
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Is this what we are making?
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Old 17th August 2006, 08:17 PM   #12
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Default Re: Milling dead centre perpendicular

Quote:
Originally posted by bulgin
Hi Chipco3434, RetroAudio, DaveM, Willem Fick & bluebeard

A real brainstrust here I see!! My gratitude and thanks for all the useful suggestions and links. The problem with this little part has also stumped my fellow members at the Cape Town Society of Model and Experimental Engineers.

@ RetroAudio I think you homed-in on part of the problem - the hand-torque applied to my milling vise. After the turned 'blank' comes out of the lathe chuck, it is then inserted into a two-part 'holding jig'. The other part of the jig goes into a precision Jacobs chuck, taper fitted onto a threaded 16mm mandrel with no run out, which in turn is fitted into the collet chuck of the mill. The latter runs perfectly true.

At this time, the job, fitted into the bottom part of the jig is in a vertical position in the vise for the purpose of alignment with the top part of the jig.

I then manipulate the mill's bed until I have 'perfect' alignment with the vised job. In other words, the top part of the jig (in the chuck) has to have perfect alignment with both the jigged job and the bottom part of the jig as well. To make absolutely sure the job is centred, it is sprayed with engineers' blue and the top part of the jig rotated by hand until there are no marks left by this part of the chucked jig.

Now comes the part where all these shanoogles go wrong. Since the 5mm dia x 5mm deep circular cavity to be milled at the tip of a 6mm dia plastic roundbar must be angled @ 20degrees off the vertical, I am now compelled to remove the job (still held in its jig) and tilt it and then 'vice-up' again. I think (as RetroAudio pointed out), the 2nd revising before milling is where things go wrong.

The degree of 'offcentreness' of this cavity (where the cart's magnet goes) is so small that it cannot be seen by the naked eye, but causes all parts inserted to be off centre.

I have in the past (while the blank is in the lathe) pipped a 1mm hole in the tip, which I then lined-up in the milling machine with an identical centredrill in the mill chuck.

Even this is not foolproof as the centredrill and its holding components (The chuck) is steel and the job is engineering plastic of 6mm dia. I think the slightest touching of the job by the steel guide drill, deflects the plastic job.

If such a 1mm (maglite) type chuckable laser beam is available, the job can be inserted already angled and the beam positioned over a pre-marked deadcentre pip which I could colour with engineers's blue.

Btw, each of these little horrors take about 3 hours to turn and mill from start to finish. At present, the reject rate is about 40% and this you discover right at the very end.

As a further aside about the crucial job dia of 6mm, this was ultimately determined by the standardised 1/2" distance between cartridge mounting holes. If I make these parts for an alu bodied cart, I at least have 7mm to play with...

Thanks so much once more for all the kind info and suggestions. I will check them out one by one.

bulgin
Firstly, I am not sure I understand your problem completely but I will offer a few things to consider.

1) Are you using a dial indicator to prove positioning when the part is transferred from the lathe to the mill? That immediately solves the positioning issue.
2) If the hole is 5mm diameter and perpendicularity is critical, you must bore the hole. I have mini boring bars I use and frequently bore precision holes at 0.065 diameter, 5mm is no problem.
I'd have to see the part in question to really recommend a process.
PS: If you want you could also bore a precise hole in soft aluminum jaws in the milling machine vise to hold the workpiece.
This would solve 2 problems. Firstly, it would spread the clamping load better and also positively register the workpiece in the vise.
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Old 17th August 2006, 08:29 PM   #13
poobah is offline poobah  United States
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Bulgin,

Look again at the same page I sent you. There are also indicators that contain a small battery and light bulb. I have used these before... they will repeat to 0.0002" (0.005mm) easily. These complete a circuit through your mill to light up. They are very good edge finders.

Perhaps you could find an edge... on the FIXED part of your vise. Then move the table using a dial indicator, rather than the hand-dials, to arrive at your center position.

BTW... I have used electronic calipers to make a digital Y readout on a small mill... they require only a small battery. You can figure out how to cut, drill, mount, and modify the calipers for this... a "poor mans" readout if you will.

I do believe your vising may be part of the problem, but this should only account to about 0.001" (0.025mm) of your problem.



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Old 17th August 2006, 08:34 PM   #14
KBK is offline KBK  Canada
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What is always intersting to note, is that a millwright and scientist/engineer, by the name of Christopher Dunn, went to the great Pyramid of Cheops, and measured the Sarophagus with a millwright's square and edge, that he had brought with him. The square and edge he brought were better than a 10,000th of an inch accuracy.

According to what he found, the stone box, and the surrounding room, are milled to a level of perfection that is FAR beyond anything we can achieve today. Done at least 4000 years ago. For example, the inside tri-corner to outside tri-corner were perfect and in absolutely perfect alignment to one another. Not metal, where one can account for known issues and compensate for them.... but STONE.

Think about that for a bit.

So much for modern technology and mankind - touting it's achievements.

Methinks were are conducting ourselves in this world--the hard way. for no concievable reason other than our own shortsightedness,and foolishness to think that we, in this age, are the center of human development. Obviously we are not.
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Old 17th August 2006, 08:41 PM   #15
poobah is offline poobah  United States
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Myth... supported by the curators. Go measure yourself (they won't let you).

I have been to the ancient Aztec ruins... it is ENTIRELY possible to stick a razor blade between the stones, if not a coat hanger.

Nice workmanship... yes. Mythical and unbelievable... no.

The stories abound... how the Vikings were able to bend oak to such extremes. Someone finally analyzed the grain of such pieces. Evidently, the Vikings made astute use of bent trees.

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Old 17th August 2006, 08:45 PM   #16
KBK is offline KBK  Canada
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Not myth. different continent, different pyramid. different people, different erea, different design. Different reasons. Different technology.

Do not be foolish enough to think that your butt is the center of the universe.

As for the myans, their calendar is far more accurate than anything in use today. Dead accurate across thousands of years. They certainly weren't stupid by any means.

basically, open your mind, don't close it. Stagnancy is death, open mind is forward motion and life. Pretty simple, actually. always remember to doubt and put it through bull filters, but don't forget to put in the proper effort. One without bias.
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Old 17th August 2006, 08:46 PM   #17
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Back to the topic of the thread perhaps.
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Old 17th August 2006, 08:55 PM   #18
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Default Boring head

Bulgin, Have you ever seen one of these?
http://www.criterionmachineworks.com/
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Old 17th August 2006, 09:02 PM   #19
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does the drill initially contact the plastic workpiece at an angle other than 90 degrees?
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Old 17th August 2006, 09:19 PM   #20
poobah is offline poobah  United States
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Quote:
Petrie gives the dimensions of this coffer, in inches, as: outside, length 103.68, width 41.97, height 38.12; inside, length 84.73, width 26.69, depth 29.59. He stated that the mean variation of the piece was .04 inch.
I'm ok with 0.04 inch...
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