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Carrier block counter bore


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#1 20Gun

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Posted 18 May 2020 - 04:50 PM

Ok , I was off to a good start. Then managed to scrap the 75.00 brass carrier block. I was using 27/64 counterbore for the bolt holes, I should of known better. Soooo I'm starting over on all 3 pieces. My question is how to get a clean, accurate, square cornered  flat bottom hole? Reamers all seam to have a chamfer. I wouldn't expect accuracy from a flat bottom drill bit, am I wrong?. Boring head would leave a tool clearance angle bevel at the bottom. I have heard of a flat bottom D-drill, not sure about it's geometry ect. Seem none are on the market. Am I missing something here? I have a Bridgeport, and  do not have a coolant system for chip evacuation. Any help would be greatly appreciated!



#2 maccrazy2

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Posted 18 May 2020 - 06:19 PM

Make it in 2 pieces pinned together. You can use a boring head and go all the way thru since the shell carrier portion is not in the way.
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#3 20Gun

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Posted 18 May 2020 - 06:59 PM

I saw some people who have done that in the forum. Trouble is, I already have the new carrier turned to length, and diameter, boss bored, and 2 press fit sockets for hex stock. If I cut it in half both pieces will be too short. I didn't go the two piece route because, I wasn't sure if that would create a headspace issue with the additional free floating part, that could slide fro, and aft.



#4 Larryx

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Posted 18 May 2020 - 07:26 PM

My production on that particular part was three. Finally got it right on the 3rd try BUT I followed the groups collective wisdom and went the 2 piece route the third try.  In a conversation with Paul Moore quite a while ago, I asked why they used brass for the internals and he responded that they were trying to replicate the look of bronze which was the material on the original guns. 

 

For what is is worth,

 

Pur postings on this forum,  I did cut the back off one part and used the front off a new blank and the interface between the two pieces was not really measurable or visible.  There is no free floating part in my build as the  nuts at either make a sandwich which functions as a single piece. 


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#5 Sparky_NY

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Posted 18 May 2020 - 08:34 PM

Yes,  the 2 piece is the way to go.    Getting the bolt bores straight without a drill walking and dead on location is of extreme importance.   The best way to accomplish that is by boring the holes.   Another side benefit is IF you goof something up,  the brass piece isn't as big.


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#6 Cutter

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Posted 18 May 2020 - 08:57 PM

For what it’s worth.
There is enough linear tolerance between the Bolt and Carrier
for some adjustments in many cases.
The flats on the Carrier are 1.250 long while the
flats on the Bolt are 1.300 long, also keep in mind
the Bolt only travels 1.175
 
Remember as Paul Moore once posted “ The drawings are for reference only “

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#7 Swarfmonger

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Posted 19 May 2020 - 03:35 PM

I just spun down the sides of a 7/16 endmill to .420 on a surface grinder with a spin fixture and it worked great. Make absolutely certain that you are over the roughed out bore since doing it this way will change your location if you are off. If you do not have the equipment to modify it yourself I’m sure any endmill sharpener could do it for you for a minimal cost.
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#8 Swarfmonger

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Posted 19 May 2020 - 06:18 PM

I am new here and I’m certainly not trying to start an argument, but I am baffled as to why so many people think building that part in two pieces is such a great idea. In my opinion the chances of creating a subpar part just went way up by doing it that way. I did all of the milling on that part using nothing more than a quality 6 inch mill vise and a couple of gauge pins and combined it with the above mentioned cutter and a little trigonometry. No indexing heads or rotary tables of any kind were used. The end result was a perfect part with very little effort. Again, not trying to start an argument, I just wanted to share a different way.
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#9 20Gun

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Posted 19 May 2020 - 06:57 PM

With the two piece carrier. I would have to buy another piece of brass, I can only get 1/2 the carrier out of the new piece that I have ready to go. Plus I have to buy an additional hex bushing part for the extra 2 press fit sockets. The counterbore I was using was a 3 flute 27/64 .4219 (within tolerance) but bored a .434 hole, opps your fired! Curious why would an endmill produce a better hole? How would a shop do this with strict adherence to the print without f-up? I was a machinist 40 years ago with some schooling, and only 2 yrs experience. At that time all the work went to Japan, and shops wanted 5-10 yrs experience, they could get it easily. So I went another direction, but I kept my tools. I knew I'd finally get back to it.



#10 Cutter

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Posted 19 May 2020 - 10:46 PM

With the two piece carrier. I would have to buy another piece of brass, I can only get 1/2 the carrier out of the new piece that I have ready to go. Plus I have to buy an additional hex bushing part for the extra 2 press fit sockets. The counterbore I was using was a 3 flute 27/64 .4219 (within tolerance) but bored a .434 hole, opps your fired! Curious why would an endmill produce a better hole? How would a shop do this with strict adherence to the print without f-up? I was a machinist 40 years ago with some schooling, and only 2 yrs experience. At that time all the work went to Japan, and shops wanted 5-10 yrs experience, they could get it easily. So I went another direction, but I kept my tools. I knew I'd finally get back to it.

 

 

Drilling removes material , Boring straightens the hole, Reaming is for size.
This process is a standard machine shop practice.
Normally a counter bore is made for shallow depths. The counter bore will
follow the hole , so if the deep hole is not straight the uneven pressure will cause
the oversize condition. 
A boring bar or Swarfmonger’s method is best.

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#11 20Gun

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Posted 19 May 2020 - 11:19 PM

Thanks for the information. I think I'll try Swarfmongers approach. If I have problems I'll go the 2 piece route. Whats would be best end mill for this boring operation 2, 3,4, or 6 flutes?


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#12 Cutter

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Posted 20 May 2020 - 12:34 AM

I am new here and I’m certainly not trying to start an argument, but I am baffled as to why so many people think building that part in two pieces is such a great idea. In my opinion the chances of creating a subpar part just went way up by doing it that way. I did all of the milling on that part using nothing more than a quality 6 inch mill vise and a couple of gauge pins and combined it with the above mentioned cutter and a little trigonometry. No indexing heads or rotary tables of any kind were used. The end result was a perfect part with very little effort. Again, not trying to start an argument, I just wanted to share a different way.

 
This is my opinion on the two piece Carrier.
The bolt holes being straight through the carrier would
be the most challenging for a novice. A two piece carrier
makes it much easier to achieve this. The flats on the front
section only prevents the bolt from rotating not how straight
the bolt travels.  

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#13 Swarfmonger

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Posted 20 May 2020 - 12:50 PM

Thanks for the information. I think I'll try Swarfmongers approach. If I have problems I'll go the 2 piece route. Whats would be best end mill for this boring operation 2, 3,4, or 6 flutes?


Anything you have available will probably be fine but if you were going to purchase something to modify, I would probably go with the 4 flute for the sake of rigidity. And as Cutter pointed out that tool is to finish the bore only roughing it out with a twist drill first.
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#14 Swarfmonger

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Posted 20 May 2020 - 03:46 PM



This is my opinion on the two piece Carrier.
The bolt holes being straight through the carrier would
be the most challenging for a novice. A two piece carrier
makes it much easier to achieve this. The flats on the front
section only prevents the bolt from rotating not how straight
the bolt travels.


I guess I’m still not seeing how that’s easier for a novice, It still seems more complicated and time-consuming to me. Here’s how I would personally recommend anyone build that part in a non-production home shop environment:

Step one: complete all turning less the extractor groove. Leave the spud that will get the milling .020 oversize in both diameter and depth.

Step two: set up piece vertically in a mill vise and drill the small diameter holes straight through and rough in the step bores using twist drills and reamers. Reaming the roughed in bores while necessary on the finished part will improve accuracy of the next step. Use trigonometry to determine the exact locations of all the holes from the center of the part.

Step three: place 2 snug fitting gauge or dowel pins In two of the reamed holes, clamp the part in a mill vise with the two pins rested on the top surface of the jaw and mill the first flat. Continue rotating the pins around the hole circle and continue milling until all of the flats have been milled.

Step four: clamp part vertically in mill vise. Be sure to clamp on the flats you just mailed this time and that will automatically place the part in the correct orientation for you to now finish the bores to the proper depth using either the above mentioned tool or if that is out of your reach, Use a 27/64 twist drill for most of it followed up by a 2764 twist drill that has had a flat bottom on it ground on a pedestal grinder.

Step five: machine the extractor groove.

The above method should allow pretty much anyone with machine tools And novice skill to build that part in one piece in far less time than it would take to build it in two. A DRO on your mill would definitely be helpful but it’s not 100% necessary. If you are not familiar with the type of trigonometry it takes to calculate the bolt circle, there are apps available that will do it for you in seconds.

I personally think that would produce a better part than a two-piece one and faster to boot, but if someone is still more comfortable doing it in two pieces then they should buy all means do it that way.

That’s my two cents anyway.

#15 Roller

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Posted 20 May 2020 - 11:41 PM

Thanks for the information. I think I'll try Swarfmongers approach. If I have problems I'll go the 2 piece route. Whats would be best end mill for this boring operation 2, 3,4, or 6 flutes?

A jig bore reamer would be my choice if you don't want to bore.  Weldon used to make these, I don't know if they are still available, as manufacturing methods have evolved these days.  They are 6 flute.

 

Frank


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