Locating a part on a milling plate

[stang5197]

What do most job shops and hobbyist use when locating a part from existing holes.

For example I have plate that where I am machining 4 holes and a slot in the vise.

Next I mount the part on a milling plate to machine the out side profile. Normally I take some socket head cap screws and turn the heads down so that they would fit in the 4 holes that I just drilled.

The cap screws locate the part by the holes and to clamp the part generally I tap the center of the cap screw so I can clamp the part. Is this how most shops would do this? Turning the down the heads is really that hard but tapping the centers is very time consuming.

[Geof]

On the milling plate I put an accurate counterbore around each tapped hole to a depth of about 0.15"; this is for a dowel bushing to align the part. Then I turn dowel bushings that fit this counterbore and also fit the hole in the part and use these in two of the holes to align the part. This means that the bolts threaded into the tapped holes can be used to clamp the part.

If it is necessary to face the top surface I use two clamping bolts and program to miss these in the facing. Then have a stop in the program to allow bolts to be put in the holes that have been face around and remove the other bolts to finish the facing. When this is done i use the dowel bushings in all the holes.

[stang5197]

Geof,
The concept makes sense, but where do you buy dowel bushings? I can't seem find them at McMaster Carr or MSC.

[Geof]

Originally Posted by stang5197 Geof, The concept makes sense, but where do you buy dowel bushings? I can't seem find them at McMaster Carr or MSC.

I make my own to the size needed.