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How would you bore this?

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cdhknives

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#1
I am working on a cone baffle for a 22lr suppressor. I have the form 1 completed and in hand, BTW. I want to keep the parts count down by making the cones with integral skirts but am having a heck of a time with boring that deep and small. Should I just make the cones and skirts separate and be done with it? I got these done but the 1/4" bore still needs to be reamed or bored, and boring it while still on the bar is better for concentricity. I just can't see how to get that deep, that precisely, to open a .025" hole to about .028" without using a very thin tool...which will flex/chatter/etc. I need 8 of the aluminum ones shown on the far right of the pic. The tube is 1" OD and the bore on the cone is 1/4" for reference.

 

mikey

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#2
Can you simplify what the job is? You need to go from 0.25" to 0.28" ID, right? How deep?

I suspect you're looking at a carbide bar, either solid or inserted. Solid bars can go small but small bars are limited in bore depth by their small shanks.

If this was me, I would use a Circle Machine CCBI187-4-5R carbide boring bar. Widia, who now owns Circle, has the equivalent bar. This bar has a 5 degree positive lead to reduce radial cutting forces. It also has flat-topped inserts (no chipbreaker) that have a positive axial rake to reduce tangential forces. This 3/16" bar can go nearly 2" deep and hold tight tolerances. With an insert with a 0.002" nose radius, it should cut accurately and without chatter with a depth of cut equal to or greater than the nose radius. I use this bar to bore 1/4" ID holes in steel, brass and aluminum and I know it will work well to depths of about 2".
 

cdhknives

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#3
I need to open the .25" drilled hole in the nose of the cones shown to .28" from the backside before I part them off the barstock to maintain best concentricity. The cones are about 1" long.

IOW , bore up a .25" hole to .28" an inch deep in 7075-T6 aluminum.

Since this is a one time job I would prefer to grind a tool if at all realistic. I have a very good 2x72 belt grinder but the depth vs tool diameter has me concerned. The tool you reference is almost $200...very steep for a hobbyist like me for a 1 off job.
 

cdhknives

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#5
That is much more doable. Good find. I spent 15 minutes earlier (don't tell my boss!) searching Amazon and MSC for something like this to no avail!:)
 

Silverbullet

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#8
If you use 5C Collet to machine the parts you'll have no problem boring or reaming the holes. In fact you can turn them and bore from the small end. At least I would . Wasn't thinking do you have collets on your lathe . If not I guess a boring bars it if you don't like reaming at there cost it's a toss up .
 
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cdhknives

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#9
My little lathe will only pass 3/4" through the spindle and 1/2" with 3C collets and a threaded tube closer. A 5C collet chuck would be for very short stock only so I never bothered to buy one...but yeah this one time it would be ideal. Even an ER style chuck would work I think...again, something I have not bought to date.
 

ericc

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#12
Would it be possible to make some sort of piloted D-bit to do this specialized job? I don't have any experience making tools like this, but it sounds like it should work.
 

derf

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#13
A 3/16" endmill works quite nicely. I use a 3" solid carbide 3/16" 4 flute end mill to make the final cut on muzzle brakes.
 

pontiac428

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#14
I understand wanting to keep your parts count down, but is it really worth the effort? Having the cones and spacers as separate parts allows you to tinker with tuning (via spacing and changes in spacing along the length). The cone mouth on most modern cone-baffle designs is neither round nor concentric, and there is good information on that particular facet of suppressor design on the web. Anyway, extra effort is warranted when there is something to be gained, but unless you really have fun doing it, make the job easy on yourself!
 

cdhknives

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#15
Tradeoffs...LOL. Separate cones makes for double the interface surfaces that need to mate up. Yes I SHOULD be able to machine to that close of tolerance but the difficulty goes up no matter what. Making the spacers different length for tuning has been considered and is interesting. FWIW I am fully engaged over at silencertalk.com but for basic machining help I feel I do better here. Ultimately the cones will be clipped, but that is the final op...and might get done on the belt grinder since all I have for milling is the Atlas mill attachment and no jig for holding fragile cones.

My fear of baffle strikes and the federal laws concerning (forbidding without expensive licenses if you take a strict interpretation) repairs have me being VERY cautious with alignment and concentric bores.
 
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