Set up a rectangular block in a 4 jaw chuck

[DaveH]

This is not the only way – not necessarily the best way, - just how I do it and a few others.

The block in question 3.2” x 2.8” x 2” thick.

I have already marked and centre punched where I want to bore/drill a hole or cut a boss.

A word about the centre punch mark. The punch I use is only used for this purpose I do not use it for general centre punching. The reason being it was ground carefully to an included angle of 60 deg. Also the mark is produced by taping a number of times and rotating the punch, all the time keeping it perpendicular to the face. If one is trying to centre the block + or – ½ a thou or so, then the centre punched mark needs to be done with care. Giving it one almighty whack isn’t really the best start!
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Here are the bits we are going to use.
A MT3 (non rotating) centre, and a “centring” bar. I call it a “centring” bar it’s that long thing at the bottom next to the rule, just to note it is a solid bar it is not sprung loaded.
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The “centring” bar is about 12” long, 12mm dia. It has a point at one end at an included angle of 60 deg. The other end is centre drilled with a centre drill (again 60 deg)
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This is the set up. The mag clamp in the foreground is to hold the camera.
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Before you get into this may I suggest you have a look at this video by Rob Wilson here http://www.metalworkingfun.com/showthread.php?tid=312

We are both doing it the same way, you can look upon the following as a slow motion version

The rectangular block is chucked up – try to get it as close to being central as you can, use the concentric circles on the chuck. Just pinch up in the jaws, the block needs to move.
The plunger of the dial gauge needs to be as close to the pointed end as possible, this is where the maximum movement is. I had to change the dial gauge to a 1” travel one to accommodate this, otherwise the block would hit the dial gauge.
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Now we can start to centre the centre punch mark. This is done in exactly the same way as we did for the round bar, see this
http://www.metalworkingfun.com/showthread.php?tid=196

Starting with jaw # 2 zero the dial gauge using the cross slide.
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See what dial reading we get for jaw # 4. The needle has moved 40 divisions.
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So by adjustment of jaws # 2 and 4 the block is moved (back) to give a reading of 20 divisions, half of the 40 divisions moved.
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Turn the chuck back to jaw # 2 and zero the dial gauge by moving the cross slide.
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Now turn the chuck to jaw #4 see what we get. Well you can see it is .001” out. Remember we have not fully tightened the jaws up fully yet. This .001” can be reduced by the final jaw tightening
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Do the same for jaw # 1 and 3.

During the final tightening do opposite jaws and watch the needle movement on the dial gauge. Here one needs to be careful, gentle, and watchful.

Now one does not always need it to be so accurate and if it is out by a thou or two – that may be fine.

Here the video when all done

You can if you look carefully see the needle does move a touch.






Now you can turn that 2” dia boss or bore a 2” hole.

Some important points to note.

The “centring” bar is the prime bit in all this. It must be straight, and it is easy to check to see how straight it is. When it is all set up zero the dial gauge turn the “centring” bar see how much the needle moves – it should hardly move at all.

This “centring” bar should be a snug fit in the non rotating centre and the centre punch mark, that is to say it will turn but there is no lateral movement. This snug fit needs to be checked often and after each adjustment of the work piece (very important). This is why spring loaded “centring” bars are used.

I use a solid “centring” bar, for a very good reason
I’m too damn lazy to make a sprung one.

Then there is the proverbial question should the “centring” bar be allowed to turn or should it not be allowed to turn.
Well if your “centring” bar isn’t dead straight, then don’t allow it to turn.

If the “centring” bar is dead straight, then you choose.


DaveH