Squaring a Block on the Mill - 6: Clamping and Machining
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WARNING: Machine tools present a safety hazard. Improper operation can result in severe injury. These topics are for non-laboratory study only and are not to be used in conjunction with the operation of any tool or machine described herein. Never use a machine tool without the supervision of a qualified instructor.
Place the work piece in the vise. The video shows the work piece being placed with the good machined side towards the perpendicular rear vise jaw. Notice that if the vise is clamped closed that there is a gap of light between the part and the rear vise jaw. Since only the rear vise jaw is trusted to be square to the machine you must find a way to eliminate that gap of light by clamping the face of the part flush to the face of the jaw.

A round bolster bar is placed behind the part and the vise is closed again. Notice that the work piece "rolls up" onto the face to the rear vise jaw. Even though this work piece is obviously very far off square the method of closing the vise with the steel bolster bar will work anytime the angle between the front face of the work piece and the bottom of the work piece is equal to or less than 90 degrees.

Now machine a square surface.

Checking with a machinist square again you can see that your trust is justified.

It is a good idea to use the round bolster bar as shown regardless of the amount of apparent deviation from perpendicularity. This is because the front jaw of a worn vise will tend to raise upward when pressure is applied. This will also lift the work piece. Whenever possible you should use a bolster bar so that you can be sure you will not lift the work piece... regardless of the vise condition. R.S.
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label: milling machine, square, parallel, transfer precision, clamping, bolster, perpendicular, machinist's square