18" barrel old delrin bushings????
All right its time to get my 18″ barrel in. I have a older one with the delrin bushings and only 1 anchoring hole for each bushing. Should i drill out for the second and just leave it unthreaded?? I think this is my best bet, given what i understand.
Its my understanding you can’t tap Delrin with a conventional tap, true or false? A special tap is needed.
I’m cutting slits in the front bushing with a cutoff wheel 1/8″ deep or so. This is to vent the bushing and aid in noise reduction. Seems a better bet then removing the delrin and trying to get it back on if even possible. For now i’ll try the delrin and see how it performs, maybe i’ll get a set of bushings from Tony in the future. Thanks for the replies.
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Haven’t seen a big deal with accuracy using the delrin mounts or even combinations though some have commented with their thoughts. The coefficient of linear expansion is different for the two materials where aluminum expands at around twice the rate but I don’t know if that is a major issue. Fit may be a stronger influence re. Stability/clean surface contact in the mount system.
BTW some plastics expand like crazy,, depends upon the material in question.
As I noted earlier it seems to make sense to isolate what loads and the normal force by providing a clear path for the grub screws to anchor the barrel. Also find it irritating messing with alignment and invariably if one looks closely a tight path ends up binding the grub screw so the load force is skewed into the mount.
Just some thoughts, others may have different observations or comments.
PS Remember someone discussing mount/securing about a year ago along similar lines??
The POi shifts may not have had anything to do with the bushings, as we now know a more common reason for frame flex and POI issues. With that said i’d like to try the delrin and see what kind of accuracy i get. Being a plastic i’d tink it’d be alot more stable to thermal changes then metals. We’lll see.
I think i’m going to go WOK’s way and just drill them out and leave them unthreaded, but use a bit that is a bit undersized so i get a self tapping affect, not fully threaded but will allow or so thrmal expansion without biinding. If i screw it uo i’l get a set from AF.
Why do the bushings need to be threaded? Don’t the set screws simply push against the barrel?
Wouldn’t the threading the bushing make it so as the set screw pushing down on the barrel it in turn will push back against the threads in the bushing essentially pushing the bushing away from the barrel and taking away the roundness of the bushing?
I would not thread the bushing and allow the set screw a clear path through the bushing straight to the barrel.
Allright the link states that a bottoming out tap will not allow me to tap a unthreaded hole, as the tap lacks the taper to succcesfuly start the threads?? See i do my research, thanks so far friends, can i get some clarity on this?
Possible solution. Get the hole started with the tapered tap, grind and then finish taping till its bottomed out. Other solution is drill a hole and leave it unthreaded?? what do you guys think?? Sorry to be a pain in the arse, but machineing is new to me. I like to do things right, but is it necessary to tap the secondary holes? I’m not trying to cut corners, but if threads are not necessay why do it. Seems to me that i have 1 hole threaded on each bushing does it matter if the secondary holes are threaded. I’d drill the hole tight so the grub screw fits snug. What do i do??? 😕
You are not starting the threads as you will use the threads in the frame to guide the tap.
You can do this any number of ways if you like.
Hey Bd! You know, the alu. bushing on both of my bbl’s just have a close fitting hole for the set screw. (We call them set screws in this country. Grub screw is a UK thing). I can think of no reason to thread the bushing for the screws that go thru the frame. The screws that secure the bbl in the bushing however, is threaded. It’s only real purpose is to keep the bushings aligned while inserting them into the frame.
If you want to thread the bushings you can use a standard tap, and tap the threads (start), then grind the end of the tap down. But this is more work than you need to do. If you are using derelin any way. Just drill the holes slightly smaller than the set screws. Derilin is a soft pliable material, so the screws will basically self tap. Leaving tighter threads than if you had tapped them. Of course, alu. bushings are a different story. would require tapping.
There have been many complaints of poi shifts with the derilin bushings anyway. Why not pick up the phone and ask AF for the correct alu. Bushings. Since they are what is being used by them now days, they may very well send them to you no charge. Even if they do charge, it isn’t much I’m sure. And they are a lot less head aches! And do a much better job!!!
Mike
http://en.wikipedia.org/wiki/Taps_and_dies
Allright the link states that a bottoming out tap will not allow me to tap a unthreaded hole, as the tap lacks the taper to succcesfuly start the threads?? See i do my research, thanks so far friends, can i get some clarity on this?
Possible solution. Get the hole started with the tapered tap, grind and then finish taping till its bottomed out. Other solution is drill a hole and leave it unthreaded?? what do you guys think?? Sorry to be a pain in the arse, but machineing is new to me. I like to do things right, but is it necessary to tap the secondary holes? I’m not trying to cut corners, but if threads are not necessay why do it. Seems to me that i have 1 hole threaded on each bushing does it matter if the secondary holes are threaded. I’d drill the hole tight so the grub screw fits snug. What do i do??? 😕
Forget the tap. You want to anchor the barrel and that’s it.. That’s done the grub screw end resting on the barrel. If you want the side hole as well as the bottom just make sure that you leave enough space when drilling the in the delrin so the grub screw will not get bound up with the delrin mount hole wall.
Delrin machines like butter if using a sharp tool as it was designed to.
BD. I have a hard time finding bottoming taps. I often just buy two regular taps, and grind the tip flat until it reaches the normal thickness of the threads. I can do this on my hi-speed belt grinder without generating much heat in the tap. the belts I use are either ceramic carbide of synthetic diamon. You can do the same on a bench grinder. Just go slow, and dip in cool water often. However you may not need to worry so much about all this. The 18″x.22 Bbl. I recieved from Pyramyd a few weeks ago came with the alunimun bushing in place. The 18″ Bbl. is quite a bit larger in dia. than the 12″ .
Mike
I agree Voltar, where would one get the bottoming out tap from??? All my taps/dies have been bought as needed brand Irwin. Don’t belive i’ve seen the bottoming out type before.
The set screws i was speaking of are the ones already there on the bottom. I planned to set them in place so i know exactly where to drill for the additional 2 side holes. Planned to drill smaller reference hole so i didn’t strip the threads in the frame. Obviously the drill bit is smaller then the threaded hole for the tap, just didn’t know if i ran a risk of doing damage to the threads in the frame. I would prefer to do it as you have stated. Less screwing around, thanks.
DO NOT remove the bushings to do this job
Drill the bushings while in place with the correct size tapdrill paying attention to the feel as you bottom on the barrel
Use a bottoming tap only and tap the holes taking care to feel the tap bottom out
DO NOT run the setscrews into the bushing without tapping the Delrin
You are at risk of stripping out the tube threads.
I say if you are going to do a job do it right!
Walter….
Don’t worry about bottoming out with the tap??
Wouldn’t have to take the bushings off then. I think thats what i’ll do. Already have the tap and bit, will secure the bottom grub screws and drill a small hole in the center of the side bushing screw hole. Then remove the barrel, drill, reinstall and then tap while the bushing is in the frame and tap through the secureing hole so the threads line up. Sound about right??
Delrin/HDPE will self tap, but it will expand slightly…
Just make the hole the same size as the screw shaft and thread that sucker in… 😉
getting it of the barrel and back on might be of some trouble. As its one in which the grub screws have been ground down, so there is no way of loosening it. Anyone tried to replace these bushings after they have been removed?? I know about the U shaped block that can be made to remove the bushing. How about getting it back on??
Take the bushing off the barrel and a normal tap will work fine. Delrin is very nice to work with and very durable.
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Aahhhh so many thoughts on the matter, tight, loose, threaded unthreaded. Worse case i have to purchase a new set of bushings 🙂 . I’ll get going on this asap. Want to se what kind of numbers i get with Sean’s valve and the 18″ barrel.