Slim LDC

[subscriber]

Bruce,

I know that with Solidworks there are options for part files that can create them as an external surface, around a hollow shell.  This becomes obvious when looking at a section of the part.  I don't know what CAD you are using, but you may have inadvertently selected such a part configuration.

I have never heard of CF being threaded directly, but I guess there is always a first time.  Avoid contact with the fibers that are released when cutting, drilling or tapping.  You do not want them in your eyes, lungs or skin.

[WobblyHand]

I'm using FreeCAD for CAD.  Seems to be a bug.  If I draw pseudo threads, rather than true helices, the cross sections seem to work.  FreeCAD seems to have issues with doing the math on a helix for some reason.  Surprised that the cross sections no longer work, since it used to work.  It created a cross section of my LDC earlier in this thread, which had a subtractive helix in the design.

Thanks for the warning on CF.  It can be nasty.  Even just trimming the length on the lathe generates dust.  I usually run at lower speed and use a vacuum cleaner to get the dust as it is generated. 

I'll try a small internal threading tool for the threads.  Maybe it will work, maybe it won't.  Won't know if I don't try.  The ER-40 collet I'm using is a tiny bit snug for the CF tube, it scratches the polished exterior surface of the tube.  Think I'll use a 1" collet instead of a 25mm to avoid marring the finish.  But for now, I'll use the 25mm just to test the threading.  At least with a TCGT insert, the tube cuts and finishes reasonably ok, much better than say using a hacksaw.  Hoping an 11IR AG60 threading insert will do the trick.  Then I need to make a matching thread for the cap.  That should be fun to make.

[WobblyHand]

Bruce,

I know that with Solidworks there are options for part files that can create them as an external surface, around a hollow shell.  This becomes obvious when looking at a section of the part.  I don't know what CAD you are using, but you may have inadvertently selected such a part configuration.

I have never heard of CF being threaded directly, but I guess there is always a first time.  Avoid contact with the fibers that are released when cutting, drilling or tapping.  You do not want them in your eyes, lungs or skin.

The inside of the tube seems to be some sort of epoxy? resin.  Think it will thread.  That is if I can get down to the shop, my back has been hurting me today.

FreeCAD seems to have a bug in their section view.  I can break it under some conditions.  It is very sensitive to helices and their start or end points.  Why, I couldn't tell you, although the equation of a helix is relatively simple.  https://en.wikipedia.org/wiki/Helix  Of course, the section view is broken for the very case that I want to draw, a thread gutter for an inside thread.  Guess I'll have to make a hand sketch and get on with it...

[subscriber]

I suspect that the usual way to thread CF is to bond a metal ring to it that carries the thread.  I think CF is a bit brittle and that threading cuts off the fiber in short sections, so only the epoxy bears the load.

I could be wrong that the above is less true with random carbon fiber lay.  And potentially very true when organized in an unfavorable direction.  A bit like wood is strong loaded in one direction, but not the other; depending on grain direction.

[WobblyHand]

I suspect that the usual way to thread CF is to bond a metal ring to it that carries the thread.  I think CF is a bit brittle and that threading cuts off the fiber in short sections, so only the epoxy bears the load.

I could be wrong that the above is less true with random carbon fiber lay.  And potentially very true when organized in an unfavorable direction.  A bit like wood is strong loaded in one direction, but not the other; depending on grain direction.

You are right, but this is only for my personal use, and I'm trying to reuse stuff I already have.  The end cap is merely eye candy and really is non functional.  The threads will weaken the tube, but for this use, (on a low power device,) I don't think it matters.  For a 100+ FP AG, I'd do it a bit differently.  For one thing, the printed LDC wouldn't be  22mm in diameter with a wall thickness of 1.75mm... 

[WobblyHand]

Oh, this kicked my butt for a while, but I think it's pretty good for a first pass.  I'm just glad I got the threads to mate correctly!  Not that easy when nothing is standard.  Still needs a little clean up, but it worked out ok.  Can't say I'd ever consider this for making a bunch of these, it was more if I could make this at all.

Threaded and grooved the inside of the CF tubing.  Had to grind an existing tool I had made for the groove.  The tool needed more clearance due to the 22mm ID.  Then I reversed threaded it using a left hand threading tool.  (Right hand thread  tool going towards the tailstock, lathe in reverse.) 

The cap was made of 1" aluminum, and turned down to 25mm, the OD of the CF tube.  Then I turned the thread area to 23.2mm, hoping my calculations were right.  I made a 1mm thread groove - which wasn't quite enough.  And I single point threaded it towards the chuck.  The thread pitch was 0.7mm and the threaded area is 3mm long.  Then I parted off the piece and cleaned it up.  Moment of truth came when the pieces threaded together!

Next, to cut the CF to length and to fit and glue the LDC in place.

[subscriber]

Looks like a tricky job.

[WobblyHand]

Standard machining.  Only tricky figuring it out the first time.  Had to fix a minor blunder due to a brain fart while machining as well.  Should I make another one, it should go easier. 

[WobblyHand]

Spoke too soon, minor issue which I can't quite figure out.  I have a gap between the cap and the CF tube.  Not sure where it is coming from, just can't determine the interference point.  It's about 0.015".  I have sufficient thread depth in the tube, and a thread groove past the threads.  Can't see any boogers on the cap that are big enough to matter (under a microscope).  I even took off 0.4mm of the cap thread length and I still have the gap.  I figure I will fill that area with adhesive.

Even with that, booboo it came out ok.  If I were to make another one, the barrel adapter would be made out of 1" stock, since I'd rather the adapter be flush with the CF tube.  Considering this was organically designed, instead from the get go, it's still pretty decent.  Looks a bit nicer with the CF tube.

Not going to lie, the machining was interesting to figure out, because I've never machined a piece like the aluminum end cap before.  It was even more interesting to attempt to fix the booboos.  When there's only 3mm of stock the piece is hard to hold!  I used a 25mm ER40 collet that had a 25mm piece of stock in it.  It was the same piece of stock that I had machined the end cap from.  So the diameter was exactly the same.  I pushed in 2mm of the endcap into the collet plus the 25mm piece of stock and prayed that it would hold.  It did.  If it wouldn't hold on, I would have super glued the two pieces of stock together at their face, while positioned in the collet.  Think it would have been easier with 5C collets, but I don't have a 5C chuck, so I had to make do with my ER40/identical slug diameter trick.

[subscriber]

Debugging is fun.  Not

[WobblyHand]

Finally figured it out.  Seems the top of the LDC has a couple of blurbs, roughness on the muzzle end.  I just need to shave off a little.  I will stick the printed LDC in the lathe and take off a hair.  I'm off by just a little bit 10 to 15 thousandths.  It's the only piece I can machine easily at this point.  Fortunately the end of the LDC has some substance, so it won't miss the material.

[subscriber]

Not unusual for 3D prints to have some lumps, and swelling where it was attached to the printer platen.

[WobblyHand]

False flag.  Error is not print issues.  The issue was the thread relief was insufficient depth.  I even drilled some holes in the cap so I could reuse a spanner wrench to tighten the threads, and it still was a no go.  The cap did not want to seat.

I recently had the same problem when I made the moderator adapter.  (Screwed up on the relief depth.)  So, I figured I could try adding a little more relief depth.  Inspection of the HSS grooving tool I had used for the groove showed significant flattening of a cutting edge.  Hadn't noticed that before.  So the tool was rubbing not cutting.  I found a carbide grooving tool, and slowly eased in the relief depth.  Took a couple of tries, but got it deep enough without really removing much.  Think I took off about 0.3mm.  That did the trick and the cap now screws in fully by hand and seats on the tube.  The thread relief looks a lot better too as it is now truly cut uniformly.

Since the aluminum cap seated in further, the printed LDC stuck out the back past the CF tube.  So I gingerly turned the LDC down at the threaded area by about 0.25mm to make things flush again.

I bit the boolit, so to speak, and super glued the CF tube in place at both ends.  Hadn't wanted to do that, but couldn't think of another adhesive that (I had and) would creep into the gaps as well.  The super glue I have is like water.  I'll give the glue a little time to cure and I will try it out.