Picatinny Rail Arc

[AWLEAPHART]

Hello,
I bought a Nightforce Picatinny rail.I knew it had no holes but I didn't see that it had no arc cut in the bottom. Anyone know the arc and depth of cut for a Defiance Rebel action Short (mod 7).
Thanks
A Leaphart

 

[Fast14riot]

If it's a Rem700 clone, should be 1.35" diameter

 

[DickE]

Rear is bridge is 2" radius on Remington 700 action.

 

[Fast14riot]

Defiance can have either a Rem rear bridge or be full round

 

[AWLEAPHART]

Ill look when I get home. I think its round front and back.

 

[AWLEAPHART]

Anyone know how to make a great cut for a concave arc?
A leaphart

 

[Dusty Stevens]

What do you mean by the arc? If youre trying to cut it so that it fits onto a round action just use a ball mill thats the same diameter as the action

 

[ricco1949]

If you have a mill with a DRO or access to one you can cut the correct arc to match the action diameter using the DRO arc program.

I use a 1.250 inch carbide tipped core box cutter and achieve perfectly fitting bases. It takes about 10-20 passes to achieve an arc with no discernable ridges.

I bed the bases to achieve a totally stress free installation.

 

[AWLEAPHART]

Action is 1.350 Diameter so is that 1-11/32 or 1-23/64 and dusty where do you find a ball end mill that size?I can find large ones but not 1.350.
A Leaphart

 

[AWLEAPHART]

If you have a mill with a DRO or access to one you can cut the correct arc to match the action diameter using the DRO arc program. .

I do-I do. But I was wondering if it would leave those small chatter marks. I guess if i put it on smooth it won't. I need to do a test run. I have .500" ball end mill.

 

[ricco1949]

I do-I do. But I was wondering if it would leave those small chatter marks. I guess if i put it on smooth it won't. I need to do a test run. I have .500" ball end mill.

Your .500 will work you just need to enter more passes in the program to eliminate the ridges.....then bed the base too.

 

[Dusty Stevens]

A 1.250 is what i would use. Then i would lay sandpaper on the action and rub the base on it. But with a 1.250 and bedding i doubt youll ever see the difference

 

[jkohler]

Why couldn’t you use a boring head to cut the radius

 

[gme]

if your mill has a tilting head you can use a regular endmill, the angle depends on the size of the mill and the radius to cut, I do it all the time

 

[AWLEAPHART]

I must be over thinking things. The mill is a BP. So yes the head tilts. I missed around with the DRO. It wants to cut the full depth of the radius. I guess if I want .100 cut I have to make the cutter higher and fake out the DRO. I'm going to look in machnist book and see if I see any thing on tilting the head with a flat bottom end mill. I have a lot of those.

 

[geo.ulrich]

use a fly cutter and set up on a scrap piece to match radius, you can kick the head around to match radius then just fly cut. a small 1/2 boring bar with a tool bit will work

 

[GenePoole]

Tilting the mill's head with any round cutting tool is going to result in an elliptical cut, not a circular cut. This may be an OK approximation in cases like these where the radius is relatively large compare the the arc length, but keep this in mind.

https://en.wikipedia.org/wiki/Cylinder#Cylindric_sections

 

[geo.ulrich]

which is why he needs to play with a set up piece ....the elipse only gets to the worse the closer you get 180* of arc. a rail is minimal arc...

 

[ricco1949]

I must be over thinking things. The mill is a BP. So yes the head tilts. I missed around with the DRO. It wants to cut the full depth of the radius. I guess if I want .100 cut I have to make the cutter higher and fake out the DRO. I'm going to look in machnist book and see if I see any thing on tilting the head with a flat bottom end mill. I have a lot of those.

Yes you have to set the DRO radius of the circle at the appropriate height above the rail to achieve the desired depth of cut. Each DRO is probably different in program settings.