Dealing with lathe taper

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cdhknives
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Dealing with lathe taper

Post by cdhknives » Wed Nov 08, 2017 3:19 pm

My lathe is old and worn. No 2 ways around it. I can only get the taper down so much, especially around the headstock. I try to work in the most parallel area I can but the length of a suppressor tube means I will have some taper to work with...on the order of .002-.004" end to end. I can work with it...more trial fitting but I can get it done. The baffle stack will likely just not be rearrange-able as each baffle will be a thou or so different in size to maintain gap spacing with the tube.

My question: Is it better to have the taper open to the bore or open to the muzzle? I can see advantages each way. One way the stack is being compressed into the taper tightening it, the other it is being loosened. Each option leads to more stress in different areas. I figure having the taper open towards the end I will open to disassemble is obviously better, but that can easily be either end.

Or am I just nit picking about trivialities? I confess to a bit of OCD and I am completely self taught in machining...mostly from reading and trial and error. Much error.

Yes I know how to adjust it using a test bar. Yes I have done it several times. It is as good as it is going to get unless I set up to work 18"-24" from the headstock where the bed isn't worn and that adds different challenges.
YHM Phantom 30 cal, 22lr form 1 build, 45 cal form 1 in progress
Atlas 10x36

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fishman
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Re: Dealing with lathe taper

Post by fishman » Wed Nov 08, 2017 4:33 pm

Why can't you make all the baffles identical if you're making then one at a time?
300 blackout form 1: http://www.silencertalk.com/forum/viewtopic.php?f=10&t=137293

5.56 form 1:
http://www.silencertalk.com/forum/viewtopic.php?f=10&t=141800&p=955647#p955647

cdhknives
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Re: Dealing with lathe taper

Post by cdhknives » Wed Nov 08, 2017 4:52 pm

I can but I am fitting them to a tube that will have a slight internal taper...I am assuming the need to run a boring bar through to true up the inner face of my tube. That process is going to leave a slight internal taper. No way around it on my equipment. The idea is to build the baffles to account for said taper.
YHM Phantom 30 cal, 22lr form 1 build, 45 cal form 1 in progress
Atlas 10x36

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fishman
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Re: Dealing with lathe taper

Post by fishman » Wed Nov 08, 2017 5:00 pm

use a reamer to true up the tube, or dont true it up at all. i didnt on my tubes.

or chuck the ID of the tube and true the outside. who cares if the outside is tapered?

.004" isnt going to make much difference. If you're really worried about it, make all the baffles different diameters so that they fit in the tube equally well.

bore the tube, measure the ID of each end. assume a linear taper. calculate necessary baffle ODs. that will be plenty close enough
300 blackout form 1: http://www.silencertalk.com/forum/viewtopic.php?f=10&t=137293

5.56 form 1:
http://www.silencertalk.com/forum/viewtopic.php?f=10&t=141800&p=955647#p955647

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yondering
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Re: Dealing with lathe taper

Post by yondering » Thu Nov 09, 2017 1:44 am

Sounds to me like you should spend some time figuring out the cause of the taper and fix that. "Lathe is old and worn" doesn't get you anywhere; figure out what is worn and what you can do about it.

For example, sometimes the tailstock will become worn, especially on the side closer to the chuck so that it sits lower than the spindle axis. That'll cause a taper, but can be fixed by shimming it up, either temporarily or permanently depending on your abilities. Or maybe the headstock is actually sitting crooked on the ways and needs to be trued up.

There's a lot you can do there before just accepting that your parts will all be tapered.

Historian
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Re: Dealing with lathe taper

Post by Historian » Thu Nov 09, 2017 2:07 pm

My Atlas 618 lathe had a taper over the 18" that was measurable with
my DTI.

On a hobby site a clever gentleman described taking a SS roller bar from
and old printer ... incredibly straight with 2 screw holes
in both ends ... and putting it between two
hardened centers.

In my case a 2MT in head stock and a 1MT in the tail
stock. Running the DTI on the cross feed showed up
the how much off the tail stock was off. Multiple taps
with a plastic mallet set it dead on.

Starting e.g.,

<< https://www.youtube.com/watch?v=JT0zZuLO-84 >>

<< https://www.youtube.com/watch?v=PKvlWZ3rScU >>

<< https://www.google.com/search?q=printer ... vujwG_B32M: >>


<< http://www.chaski.org/homemachinist/vie ... hp?t=94081 >>

crazyelece
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Re: Dealing with lathe taper

Post by crazyelece » Thu Nov 09, 2017 3:02 pm

IME, using a boring bar long enough to run the entire inside length of the tube, will add taper on a perfectly trued machine from tool deflection alone.

Not too much mind you, but measureable.

With the amount you've posted, I wouldn't worry about it.

Make your baffle stack true itself up to the faces and don't worry so much about the sidewall fit.

No matter how you can draw it in CAD, you can't cut sharp corners and everything is going to have tolerances.

cdhknives
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Re: Dealing with lathe taper

Post by cdhknives » Thu Nov 09, 2017 3:43 pm

Seems like a long boring bar is going to be consistent. It is going to deflect when cutting, but a skim cut to true it up should be very light and after a couple of passes should be fairly consistent. Sharp HSS tooling will help...and I can get that done.

The taper is from bed wear from the original owner using it for grinding service. The cost to regrind/mill/true the bed is prohibitive. The only financially reasonable solution is to part out the lathe and buy another...and that isn't gonna happen any time soon. It is good enough for my skill level. I've spent years with this lathe and restoring it to working condition. It just isn't going to get any better realistically. Bed wear is about like a bent frame on a car...live with it or junk it but it'll never be perfect again. I bet I have run a test bar 20 times to set the tailstock offset. It is as good as it will get...the bed at the headstock is a couple of thousandth's narrower than the middle of the bed. No way around it and it isn't a linear transition.

So, with that...I guess my answer is to not be too picky about a couple of thou or to find a large adjustable reamer.
YHM Phantom 30 cal, 22lr form 1 build, 45 cal form 1 in progress
Atlas 10x36

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