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Joined: Wed Aug 05, 2015 3:34 am
Posts: 35

Post Posted: Sat Feb 10, 2018 11:21 pm 
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Won't attempt to speak for Dustin but will offer my take since I'm planning to use a similar fixture. I'll start with the assumption that you want to maintain the radius of the front strap as closely as possible. To do this, you have to know the front strap radius. The blueprint spec is 13/32" or 0.40625. Some vendors use a 7/16" radius or 0.4375 (usually described as feeling "blockier" in the hand). Other vendors such as JEM will allow you to specify the radius you want such as 0.500. As the pros will tell you, the front strap radius on any given frame will probably not be true and will need to be cleaned up by removing material...this will affect the final radius.

To me there are two approaches to getting the offset for the radius you have (or the desired radius if you plan to change what you have), both of which require a DRO-equipped mill and both should be confirmed with a dial indicator. The goal for each method is the same...the frame must be positioned so that the distance from the outer edge of the front strap to the center of rotation of the fixture equals the actual or desired front strap radius. For example, if you have a frame with a 13/32" radius, in theory the outer edge of the front strap should be offset 0.40625 from the fixture's center of rotation.

The first method uses the fixture's center of rotation as the primary reference. Dustin and I both emulated Jerry Keefer's fixture design which uses a vertical shaft that goes through the magwell of the frame...the center of this shaft is also the center of the fixture's rotation. Jerry put set screws in this shaft that can be adjusted to serve as mechanical stops for the inner radius of the front strap. To get the desired offset, you can measure the thickness of the front strap where the set screws will make contact, subtract that thickness from the actual/desired radius, and adjust the set screws in reference to protrude from the shaft the exact amount required to serve as mechanical stops for the inner radius of the front strap. Example, if your frame has a 0.40625 radius and the thickness of your front strap is 0.100 at the front, you would want to adjust the set screws so that they protrude 0.30625 from the centerline of the magwell shaft.

The other method uses a mechanical stop in the mill quill and the centerline of the fixture. Start by zeroing your mill on the centerline of the fixture and zero out the DRO. Next, chuck up a long ground shaft, gage pin, drill rod, etc, move the X axis of the mill by half the diameter of the shaft and re-zero the X axis on the DRO...the goal is to have the fixture's center of rotation centered on the outer edge of the shaft when the X axis on the DRO reads 0.0000. Now move the table so that the X axis reads out the same distance as the front strap radius. Using the same blueprint example, DRO X axis should read 0.40625 (or perhaps -0.40625). Now put the frame in the fixture and slide it until the outer edge of the front strap is in contact with the shaft in the quill. At this point, you can adjust the magwell shaft set screws and the set screws that hold the frame in the groove in the aluminum platter.

There may be other methods...these are just the two that I'm planning to experiment with. As previously stated, the offset should be confirmed with a dial test indicator prior to cutting to confirm that the offset matches the desired radius. Strongly recommend reading up on what the pros have to say about truing the front strap radius...odds are you'll have to remove some material to get a trued front strap which will also affect the math and ultimately the hole spacing on the dividing head. Apologies if my terminology was poor or if this induced any confusion...hope it helps!

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Joined: Sun Apr 12, 2015 1:00 pm
Posts: 76

Post Posted: Mon Feb 12, 2018 1:44 pm 
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That makes sense... I was trying to figure out how to move the base "back" the correct amount. Your way seems easier.

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