Lee Martin
Hunter
There has been a lot of discussion of late on cylinder throats. I thought I’d share how we spec ours using a new 45 Colt conversion.
Let me start by saying we never finish the throats with a reamer. While they work they’re limited by: 1) Reamers can “walk” which compromises concentricity, and 2) Reamers leave machine marks which abrades the bullet.
Here’s a close-up of a reamed cylinder throat. The grit particles skew the view somewhat but the machine marks are noticeable between 6 and 9 o’clock.
This cylinder was reamed undersized at 0.450 – 0.451”. Final dimensions will be achieved using a Sunnen Hone. For those not familiar with hones there’s no better way to achieve throat concentricity. Hones use an abrasive stone and a self-truing process. Unlike grinding, which allows wheel imperfections to be imparted on the object, honing follows a complex path. The stones simultaneously enlarge the hole radially while they oscillate axially. And since the stones erode towards the desired geometry there’s no need to true them. So what does that mean? Basically the honed component achieves better concentricity than the machine tool that created it (ie, the reamer). Better yet our Sunnen Hone is accurate to 0.0001” and can return varying textures to include near-mirror finish.
Our Sunnen Hone:
This gun will see 0.452” cast bullets so the desired throat is 0.4525”. That extra half-thousandth allows the bullet to pass without material upsizing or constriction. Using a micrometer a gauge is set to indicate at 0.4525”.
Here’s video of the hone in action:
http://www.youtube.com/watch?v=Orppwq02CbY
All throats were checked against the “indicated” gauge. The dial at the top tells you if you’re off or not. If the hole falls to the left of the zero, it’s too small. If it falls to the right it’s too large. All of our throats zeroed dead on which means 0.4525”. Now you may ask why we don’t use pin gauges. It’s simple. Pin gauge sets are to the thousandth. In other words, they don’t indicate where I’m landing in between 0.452” and 0.453”. This gauge does and like the hone is accurate to the ten-thousandth.
But this cylinder isn’t quite done. Our finishing stone, which damn near mirror finishes the throat, is a bit worn. We’re ordering more and will final pass the throats. Since the bores are pretty smooth as is the polishing pass will remove very little; maybe a 0.0001” or so. I’ll post a close-up of one when we’re done.
Let me start by saying we never finish the throats with a reamer. While they work they’re limited by: 1) Reamers can “walk” which compromises concentricity, and 2) Reamers leave machine marks which abrades the bullet.
Here’s a close-up of a reamed cylinder throat. The grit particles skew the view somewhat but the machine marks are noticeable between 6 and 9 o’clock.
This cylinder was reamed undersized at 0.450 – 0.451”. Final dimensions will be achieved using a Sunnen Hone. For those not familiar with hones there’s no better way to achieve throat concentricity. Hones use an abrasive stone and a self-truing process. Unlike grinding, which allows wheel imperfections to be imparted on the object, honing follows a complex path. The stones simultaneously enlarge the hole radially while they oscillate axially. And since the stones erode towards the desired geometry there’s no need to true them. So what does that mean? Basically the honed component achieves better concentricity than the machine tool that created it (ie, the reamer). Better yet our Sunnen Hone is accurate to 0.0001” and can return varying textures to include near-mirror finish.
Our Sunnen Hone:
This gun will see 0.452” cast bullets so the desired throat is 0.4525”. That extra half-thousandth allows the bullet to pass without material upsizing or constriction. Using a micrometer a gauge is set to indicate at 0.4525”.
Here’s video of the hone in action:
http://www.youtube.com/watch?v=Orppwq02CbY
All throats were checked against the “indicated” gauge. The dial at the top tells you if you’re off or not. If the hole falls to the left of the zero, it’s too small. If it falls to the right it’s too large. All of our throats zeroed dead on which means 0.4525”. Now you may ask why we don’t use pin gauges. It’s simple. Pin gauge sets are to the thousandth. In other words, they don’t indicate where I’m landing in between 0.452” and 0.453”. This gauge does and like the hone is accurate to the ten-thousandth.
But this cylinder isn’t quite done. Our finishing stone, which damn near mirror finishes the throat, is a bit worn. We’re ordering more and will final pass the throats. Since the bores are pretty smooth as is the polishing pass will remove very little; maybe a 0.0001” or so. I’ll post a close-up of one when we’re done.
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