Freebore and Leade Surface Finish

[Taylor Stratton]

Chambering 6 identical 6.5 PRCs this week I noticed the surface finish in the throat and leade seemed worse than usual. I only recently started chambering at 190 and 300 Rpm. Both are better than the slower speeds I had been using it seemed to me but I really wish my lathe had something between the two. I tried a plethora of things in the final few passes between the 6 barrels, evaluating the throat after each time and I just can’t seem to get a very smooth cut on the lead angle. I tried to polish the throat on one of the passes before I was at final depth with some flitz back and forth with a tight patch and all it did was make the jagged throat easier to see the stair steps in it.

For reference these were cut with a Manson 6.5 prc reamer that only had 9 chambers cut on the first barrel and 15 chambers on the last. I drill within .100 of the shoulder to minimize how much material the throat has to cut and pre-bore within .025 of shoulder diameter. Once the shoulder starts to cut I drop RPM to 190 as if left at 300 the barrel work hardens as the feed rate is decreased. I wouldn’t think that the reamer is dull after that many chambers and its had no ill abuse unless 300 rpm will destroy them that fast. The one thing that seemed to help I did on the last barrel was get out a second reamer that is identical and brand new and cut the last .030 with it. This was a Brux and I ran no bushing on the reamer, the other 5 were bartlein and I did use a bushing. The Brux looks better but still not near as smooth a finish as I’ve seen on some of my previous work or others videos I’ve seen and it was a brand spanking new reamer. Am I just too zoomed in and worrying about too much (I really don’t think so) or is there a secret to getting a mirror smooth finish on the throat and leade?
I can’t figure out how to share a video so I just took a picture of the videos I have. First picture is of the original reamer with the throat polished with flitz, second is just a reamer finish at 190 rpm same reamer, the last is the Brux with the brand new reamer used for the last .030 at 190 RPM.

 

[Dusty Stevens]

Run a copper bar against the cutting surface. It has hit a hard spot IMO

 

[Taylor Stratton]

As in the reamer has chip weld? I always check the cutting edge for the last few passes with a copper penny and if there’s any I can’t get off I scrape down inside the flat edge of the cutting surface towards the center of the reamer with a short flat piece of a smooth Arkansas stone. So it’s not chip weld. Great idea though.

 

[GenePoole]

I don't think I'd call it a chip weld; more like very fine metal particles bonded to the edge. It might be a consequence of not enough cooling at that very small contact area with the chamber causing some kind of glue effect with the cutting oil. I used to get this a lot and did the penny trick to remove it, but since going to a flood coolant and thru-barrel flush system, I've not seen it at all.

 

[Taylor Stratton]

Well whatever yiu wanna call it there was nothing there before or after the final throat cut. I don’t have a through flush system but I use plenty of vipers venom cutting oil and once the shoulder starts in i only go .020 or less per pass. It could probably help a lot with surface finish all around with a flush system though to get rid of the chips and keep the temperature down/constant.

 

[DaveTooley]

Every reamer will leave a different finish there. Some smoother than others. As you would expect that finish/edge deteriorates with the age of the reamer. If you looked at any HS reamer under high magnification you would be alarmed. The cutting edge is very course from the grinding wheel. I use a knife edge stone bottomed out in the flutes to stone the face. That includes the body also. Occasionally I'll get random bits micro welded to the flute face on the body. Almost impossible to see without magnification but large enough to leave a faint ring in the chamber. If you want as good a finish as possible get a carbide reamer. They are ground with a different type wheel and have a much better finish. Be careful they will cut you.

 

[Taylor Stratton]

Man I look a look at the reamer and with my naked eye the leade angel on the 6.5 prc has a significantly more course grind on it on both reamers. Compared to other reamers from same make an others they are definitely sub par.

I chamber enough 6.5 prc i think I may try a carbide reamer. How do you guys judge a reamer throat dull to retire it? If it starts folding a burr over on the land that’s visible in the borescope?

 

[DaveTooley]

I used up enough reamers I can pretty much tell with my naked eye. In the right light when you look at the face of the flute the cutting edge will have have broken down to where you can see a shiny line/edge.
I'll add that I have never had accuracy issues with leade angles that were rough. With HS reamers the leade starts rough even with a new reamer and will get rougher towards the end of the reamers life cycle. Just a fact of life. Now if you roll metal into the groove, yes you would have accuracy issues.
Talk to Sara at JGS about a carbide reamer. They take a different size bushing compared to HS reamers. Very seldom do I use anything but a nominal bore size -.0005" bushing. So one bushing works for me on carbide.

 

[Alex Wheeler]

The majority of it is the quality of the reamer, feed rate, and oil flush. You cant fix a rough reamer. You can make a good one cut rough though. The lead has a lot to do with raw accuracy, this is important.