cutting a radius on a barrel breech end
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  1. #1
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    Default cutting a radius on a barrel breech end

    If I were to cut a 1.250 radius on a barrel just past the breech end would you cut a bit to do it or just use a righthand cutting bit. The radius looks like this one in the picture that I want to reproduce.
    40431405ia.jpg

    My lathe is manual not dro

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    Guy Lautard published a book with drawings and formulas for generating radii on mills and lathes. You measure for the center of the radius on your tool to the center of the radius on in your case the barrel. You can use a magnet with a 1 inch travel indicator without needing a DRO or Trava-dial. The attached photo shows how I came up with the numbers for generating a radius not in his book. The polished radius was generated in that style and sanded to a mirror finish.
    radius.jpg

    Another example with a milling machine, The Weaver base was made from round bar stock.

    savagebaseradiusrs.jpg

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    I call it "stepping" you calculate what needs to happen (to cross slide and carriage) to make a radius tool approximate a "stepped" radii that you finish later to suit your eyeball - the main reason for the several or many steps is to reduce or eliminate the possibility of chatter

    Here are two differing radii I put into the web/rim of a gear I was making Absolutely no DRO here - though it would have been helpful, I don't own such things

    dcp_1386.jpg

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    Yeah, chatter could be an issue. So the ideal tool nose radius would be to find the size that causes chatter then back down one step. :-) The larger your tool nose radius the less filing and polishing to finish, and the less steps necessary. In his book Guy sometimes uses a cut off blade, just more steps to approximate the finished radius and more polishing required. The book is worth while to get even if you only use it every few years. Most of the recent uses were with CAD drawings as in the first example. I snap to the intersecting lines and write down the X and Z coordinates.savbas3rs.jpg

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    Use the pivot on the compound to sweep it in. It can be a challenge to get the tip of the tool close enough to get a radius like this though. You dont have to set it perfect though, you've got the compound to fine tune it.

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    Using the compound to "sweep it in", after calculating close to what I need , I would back out the compound all the way, lock the cross slide in a particular place , start making a 'sweep" then moving the compound forward to keep making sweep cuts?
    Would it be wise to use a round nose lathe HSS bit? if so cut at what angles to make nice cuts in I presume is CM steel?

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    Any turning tool with a reasonable corner radius will work fine. The rake isn't optimal going 'backwards', but it's just gonna rub a bit of dust off anyway. You have to finish it with emery anyway.

    You can use the compound for cut depth, but I think it's easier to use it to fine tune the radius and use the cross slide for depth. Your last passes are for blending in, so the cross slide anyway. It's such a short section of arc the cut depth isn't going to be off theoretical perfect enough to matter. If it was full 180 the compound might be a better option.

    I've 'faked' a couple Palma profiles this way. The last one, I cut the radius first and then blended the tapers into it. I think the first, I cut the long taper, blended the radius in, and then blended the short taper into that. It sucks either way. You've only got one to deal with.

    As for scope bases, a 1" endmill with the head tipped 45° is about perfect for a Remington. But stepping it in is a lot less physical work. My Lagun has a 40 taper head ...


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