Delrin Pratten

[Geoffrey Ellis]

But here's an interesting observation. I have marks on my reamers that tell me: stop when you get to this mark. So I naturally stopped at that mark when reaming the Delrin. But later, I noticed the bore was a smidge smaller.

Yes, I had this problem repeatedly with Delrin. It is so flexible that it resists the reamer. On one memorable occasion I was reaming a longish body section (one-piece body on a Pratten) and it stuck fast on the reamer! I mean really stuck. I thought I'd never get my reamer back without somehow cutting the Delrin flute body off of it. My reamer is powered by a heavy-duty gear motor that is bolted to a workbench at the back of my shop. I was unable to pull the damn thing off the reamer using my own strength. In the end I had to take a fence-stretching winch (the cable type that have the ratchet lever on them) and use it to pull the flute off the reamer! This involved a LOT of fuss and bother, figuring out how to get hold of the flute body on the one hand, and where to secure the other end of the cable so that I could winch it tight enough to pull the thing off. It took a shocking amount of force to get that thing off the reamer. A never-to-be-forgotten lesson .

Part of the problem stemmed from using rod stock that was too narrow. If I have a 30mm diameter piece of Delrin that I'm using for a D flute, the narrower girth makes it more flexible. If I hold it with my hands when reaming (which I cannot do for any sustained period of time) I can feel the material ovaling and distorting as the reamer turns. Instead of making a cut, it merely pushes the material around. Using a thicker Delrin rod solves the problem, but increases the waste. The added material gives stiffness to resist the reamer (so it cuts instead of bends). But bigger Delrin means more expense and more plastic shavings.

[Sedi]

I never made a complete flute from delrin -- just some blocks for tin whistles, but that was already annoying enough. Filing, cutting -- it's worse to work with than either wood or aluminium and even steel (I made a few whistles from stainless steel).

[Conical bore]

If Delrin has this much difficulty in machining, I'm wondering why more flute makers don't pursue the carbon fiber option as with Carbony flutes. Maybe a different set of difficulties? Faster dulling of the reamers than with a softer material?

I'm not sure how these are made. I have a Bb Carbony flute, and the flute wall is much thicker than I expected, so it's also quite a bit heavier than I expected. Usually carbon fiber is associated with light weight, but I guess the layup here is to duplicate the embouchure and tone hole wall thickness of a wooden flute. The flute also has a couple of the tone holes slightly angled in the drilling to reduce the finger spread on this big flute. That would only work with a thick enough body wall.

There is a typical carbon fiber weave pattern on the outside, but looking at the embouchure and tone hole cuts, the exposed side material seems to be mainly smooth resin. Looks like maybe the weave is only on the outer layer for appearance, and the flute body is a finer-grained carbon/resin matrix?

At any rate, the cut of the embouchure and tone holes on this flute are very precise, no fuzzy edges. The flute is exceptionally stiff and non-bendy, as you'd expect. Hard to say how good it sounds because I'm still working on the different embouchure required to get those low notes, and I don't have another Bb flute to compare it with. When I can occasionally manage a strong clear Bb on the bottom, it sounds pretty good.

[Sedi]

If Delrin has this much difficulty in machining, I'm wondering why more flute makers don't pursue the carbon fiber option as with Carbony flutes. Maybe a different set of difficulties? Faster dulling of the reamers than with a softer material?

I'm not sure how these are made.

Carbon fibre is even worse . Impossible to drill holes with a normal drill, you'd need completely different tools -- like highspeed milling machines.
The Carbony flutes AFAIK are made largely with injection molding. You cannot drill the material and you cannot ream it either. So that's a completely different process.
Regarding the carbon fibres themselves -- they are embedded in some type of epoxy-resin. My guess is that they are hidden by a layer of that stuff at the embouchure and finger holes in order to not have the fibres sticking out.
I tried to make a flute once from a carbon fibre tube. It was a true pain in the neck but it had pretty great playing characteristics, even though I messed up one hole which I then painted over. It still worked great. I gave it away as a gift. I still got one of those tubes left but never bothered to make another as I'd need different tools.

[Geoffrey Ellis]

I never made a complete flute from delrin -- just some blocks for tin whistles, but that was already annoying enough. Filing, cutting -- it's worse to work with than either wood or aluminium and even steel (I made a few whistles from stainless steel).

This is interesting, since I've had a different experience. I found that things like drilling and turning Delrin was very easy. With the right cutters it gives a very smooth finish. But I've not done much filing or shaping of it by other means, so perhaps it would give me some trouble if I tried.

[Sedi]

I just didn't like how it behaved when sawing a piece off or trying to get a nice edge on the block. Wood is much easier to work with. Even an aluminium block was easier to make. Even just getting a clean cut in a straight line free-handedly was annoying. The blade just didn't wanna go through it in a straight line.
I guess it might be my inadequate tools since I just do all this at a hobby-level. But I really didn't like to work with it.

[Terry McGee]

Heh heh, I've struck the "stuck reamer" thing with Delrin too. I ended up tapping the reamer out using a brass rod offered down the far end, with the blank sitting on the jaws of a wood vice on the work bench. It's a reminder to have those reamers really sharp! But also a reminder that we are trying to do the impossible - taking a cut the full length of a long flute body! Imagine if you were hand-planing Blackwood cross-grain on the bench with a plane whose blade was 320mm (just over a foot) wide!

"Hey, that guy is really well built. I guess he works out at the Gym every day?"
"Nah. He's a wooden flute maker...."
"Woah. Step aside, don't make eye contact. He looks like he might have had one of those "stuck reamer" days...."

[dcopley]

But here's an interesting observation. I have marks on my reamers that tell me: stop when you get to this mark. So I naturally stopped at that mark when reaming the Delrin. But later, I noticed the bore was a smidge smaller. Thinking about it, I can imagine two reasons:
- the poly would rather slip past the cutter than cut, and
- being more flexible than wood, it was able to swell a bit to avoid the cut.

This is because of the higher thermal expansion of Delrin (several times higher than wood or aluminum). The Delrin heats up and expands when you are reaming and then shrinks back when the reamer is removed and it cools down.

[Terry McGee]

Interesting, Dave. So probably all three effects are in play - slippery, flexible and high thermal coefficient. We don't stand a chance!

[Panic attack subsides] I wonder if letting the blank cool (pop it in the freezer?), touching up the edge on the reamer and re-reaming it might help.

[Geoffrey Ellis]

Interesting, Dave. So probably all three effects are in play - slippery, flexible and high thermal coefficient. We don't stand a chance!

[Panic attack subsides] I wonder if letting the blank cool (pop it in the freezer?), touching up the edge on the reamer and re-reaming it might help.

This might be worth a try. One of the tricks to boring and reaming ebonite is to freeze it first. One has to be careful because the freezing makes it brittle, but it counteracts the heat of friction, which is not great for ebonite (it will soften and get gooey).

[dcopley]

Interesting, Dave. So probably all three effects are in play - slippery, flexible and high thermal coefficient. We don't stand a chance!

[Panic attack subsides] I wonder if letting the blank cool (pop it in the freezer?), touching up the edge on the reamer and re-reaming it might help.

We try to keep things cooler by hooking a shop vac to the back end of the lathe spindle to pull cool air through. This also helps clear chips from the reamer though you still have to pull it out and clean it off a few times during the reaming. If the bore does not have a major change in taper along its length you can manage the thermal shrinkage by just reaming a little further into the bore. Probably not as easy with a one piece Pratten style bore and that is one reason we like to keep with a two piece bore for Delrin - not to mention the difficulty of trying to ream 12 plus inches of Delrin all at once.

I'm not sure we should get too concerned about this though. Wood flutes tend to shrink slowly over the years, possibly getting oval in the process. Delrin flutes do their shrinking all at once at the beginning and then are stable. They all probably end up in about the same place.

[Loren]

Funny Terry, I had a similar thought about icing the Delrin for reaming. Or maybe just moving production out into space.

This is the point in the discussion where I start to wonder about the specific processes different makers use, and how much that plays into issues, or lack there of, when working with Delrin. Specifically the reaming issues.

Unfortunately I never got a chance to make any polymer flutes or recorders where I worked, apparently the Baroque crowd don’t cotton to no new fangled plastics. Well, not unless it looks like dead animal tusk….. but it does make me wonder how likely sticking of the reamers would be with our process, and what may work better, or be even more problematic. A few questions for those makers who care to answer - either publicly or privately, no need to give away any trade secrets regardless:

1. How oversized is the a section when you are reaming it? I can’t remember exact numbers 15 years on, but we reamed sections when they were way oversize, you’ll see why in a moment. I expect wall thickness will have some effect on how a piece reams.

2. What type of reamers are you using? We used a large tapered scoop reamer to hog out most of the bore, then finished with a multi-fluted final reamer and I wonder if such a reamers would be more, less, or just as likely to get stuck. Also wondering if a single fluted reamer would yield better or worse results with regards the issue of the bore ultimately ending up undersize after the first final ream. (Oxymoron much Loren?)

3. Are you hand reaming or using machines to assist? If going the machine route, speeds and feeds? Our process basically involved chucking a custom made reamer in the lathe headstock and hand feeding the piece onto the reamer. A heavy duty chain whip was secured around the piece to be reamed to prevent it from catching and spinning, the leather wrapped handle of the chain whip being braced against the side of the ways and free to slide along with the piece being reamed. The use of the chain whip is what necessitated our having the sections quite oversize for the reaming process as the chain did tend to dig into the wood a bit.

I’m wondering how all of that would have worked with Delrin knowing it can be more “grabby” in some machine processes, at least in my limited experience with it. Also just wondering in general how speed and feed rates separately, and combined, affect the process of reaming Delrin, as opposed to wood. I can speculate on speed of heat build-up vs efficacy of cutting before that occurs, but I’ll leave the science to the engineers (Dave C.) and the practical outcome experience to the makers who have made a bunch of Delrin flutes, because I’m not a fan of speculation, not even my own.

[Loren]

Aaaaand, of course I’m writing while Dave is already posting.

Thanks for the additional info Dave

[Geoffrey Ellis]

1. How oversized is the a section when you are reaming it? I can’t remember exact numbers 15 years on, but we reamed sections when they were way oversize, you’ll see why in a moment. I expect wall thickness will have some effect on how a piece reams.

My own experience with reaming any material is that the bigger the better. Larger pieces mean less flex in the material being reamed, which translates to better accuracy. Plus holding onto the stock sometimes does require measures that will mar the surface pretty seriously. I tend to use hose clamps on round stock, and then hold it in a v-block set-up. But I think my reaming set-up is quite different from the norm.

2. What type of reamers are you using? We used a large tapered scoop reamer to hog out most of the bore, then finished with a multi-fluted final reamer and I wonder if such a reamers would be more, less, or just as likely to get stuck. Also wondering if a single fluted reamer would yield better or worse results with regards the issue of the bore ultimately ending up undersize after the first final ream. (Oxymoron much Loren?)

By trial and error (mostly error) I've arrived at this same solution. Rough out the bore, let it cool, then do a quick, smooth finish cut with a fluted reamer.

3. Are you hand reaming or using machines to assist? If going the machine route, speeds and feeds? Our process basically involved chucking a custom made reamer in the lathe headstock and hand feeding the piece onto the reamer. A heavy duty chain whip was secured around the piece to be reamed to prevent it from catching and spinning, the leather wrapped handle of the chain whip being braced against the side of the ways and free to slide along with the piece being reamed. The use of the chain whip is what necessitated our having the sections quite oversize for the reaming process as the chain did tend to dig into the wood a bit.

I've been reaming at 100 RPM because my gear motor driver is set to that. I actually think a lower speed would be better (half of that, maybe).

I’m wondering how all of that would have worked with Delrin knowing it can be more “grabby” in some machine processes, at least in my limited experience with it. Also just wondering in general how speed and feed rates separately, and combined, affect the process of reaming Delrin, as opposed to wood. I can speculate on speed of heat build-up vs efficacy of cutting before that occurs, but I’ll leave the science to the engineers (Dave C.) and the practical outcome experience to the makers who have made a bunch of Delrin flutes, because I’m not a fan of speculation, not even my own.

For Delrin, it circles back around to your first question. Thicker is better. Sharp tools, lubricant and either cooling the material, or simply taking a break. I do this with ebonite as well. I ream it to within a hair of the finished size, then I let it (and the reamer) rest and cool off. Then the final polishing cut. Seems to help with the expansion and contraction issue, since the second cut doesn't last long enough for any significant heat build-up.

[paddler]

I've had similar problems reaming delrin. I found that reaming at as slow a speed as possible, in my case 50 rpm, helped keep the heat down.
It also helped if I kept my reamer freshly sharpened, and if I greatly increased the frequency at which I removed the reamer to clear shavings,
cooling the reamer each time. The moment anything starts to bind the heat from friction starts to make the delrin stretch. If the reamer is cool,
then the delrin seems to stay cool. I have heard other machinists talk about needing to freeze delrin before machining when precision was important.
I think that would help in my case too, but I haven't tried it.

My flute billets are pre-turned cylinders, small enough to slide inside the headstock of my lathe, which I do during the gun-drilling phase.
For reaming I used to try holding them by hand, but it placed too much strain on my hands/wrists, so I now use a strap wrench in very
much the same way Loren described using a chain whip. My strap wrench is basically like a chain whip, but has a rubber strap instead of
a chain. It is self-tightening to a point, but will slip if the forces get too high (which I think of as a safety measure). The handle rests
against the side of the ways and can move freely lengthwise as the billet advances on the reamer.