CNC BSPP threading

To BSPP or not to BSPT

British Standard Pipe Parallel and British Standard Pipe Tapered.

ISO inch unit based standard with 55 deg thread flank angle. How an inch based thread ended up as an ISO standard I do not know...

The sizes concerned for the Aurora are:

1/4 - 19 (dash 04)
3/8 - 19 (dash 06)
1/2 - 14 (dash 08)
1"   - 11 (dash 16)

possibly also:
1/8 - 28 






For externally threaded parts, the major diameter is nominal size plus 1/4", thus 1/4 - 19 is 1/2" diameter (and 19 tpi).

Full discussion of how to identify threads and for the "dash" naming convention are here and here.

Discussion of carbide inserts for cutting BSPP threads

BSPP and BSPT threads are 55 degrees, not 60.

A good visual reference guide to threading standards.

A good discussion on Practical Machinist of how to cut BSPT and BSP - which is the same as Whitworth. 

Inserts:

If you select the Whitworth or BSP profile Carmex inserts, they will cut the full profile. Only the "partial profile" inserts leave the crests as-is.

So, Carmex "11 IR 11 W" or "16 IR 11 W" [for BSP, BSF, BSP, BSP] or "11 IR 11 BSPT" or "16 IR 11 BSPT" [for BSPT] or the equivalent Vardex or other competing inserts should do what you want.

Supplier (at $31USD each!) - (possibly not the best grade of carbide/coating for non-ferrous metal).
https://www.grainger.com/product/CARMEX-Threading-Insert-4PRZ6

Notes - a full profile insert is specific to the pitch and will cut radiused roots and crests. A partial insert will cut a range of pitches but leave flat roots and crests.

Carmex inserts seem to have the same naming convention as the Iscar, so the 11 IR 11 W example translates to:

11- insert size 11 (which is 1/4" inscribed circle)
I - Internal
R - Right hand
11 - 11 tpi
W - whitworth

Also, there seems to be some debate as to whether the radius roots and crests are necessary ("only important to amateurs"). My guess is that a sharp root will decrease the strength of the part, but that is highly unlikely to be an issue at the low working pressure in this application.


Possible inserts:

Explanation of ISCAR threading insert codes.
Good explanation of threading concepts, geometry and terminology.

Partial profile inserts (i.e. one size for a range of thread pitches with non radiused crests and roots)

ISCAR - External 55° partial profile, laydown threading inserts.
16ER A 55 IC908 - part number 5902222 
TPI min 16 - TPI max 48 - which would exclude the 1/2 inch

16ER AG 55
External 55° partial profile, laydown threading inserts.
TPI min 8 - TPI max 48

any of these carbide grades would do:

IC 508 grade
IC 228 TiN PVD
IC 908

Full profile inserts:

11ER 19 W   for cutting 1/4 - 19 and 3/8 - 19
11ER 14 W   for cutting 1/2 - 14

holder for all of these(?)


SER 1616 H16

Presuming that one holder can accommodate different sizes of insert (the 11,16 and 22 refer to the length of one side of the triangular inserts which inscribe a circle of 1/4", 3/8" and 1/2" diameter respectively).

References for CNC threading 

excellent explanation of code for CNC threading inch parts - however, while the code is specific to the HAAS or whatever he is using, the concepts are the same for the canned cycles in Mach3.

https://m.youtube.com/watch?v=e03pTbEBuGg

And now in Metric...

https://m.youtube.com/watch?v=e03pTbEBuGg


NYCNC guy's guide to thread milling with HSMworks and a Tormach

https://www.youtube.com/watch?v=a43S2y7Ccy8


HSMworks importing form tools
https://forums.autodesk.com/t5/support/hsm-2017-form-tool-does-not-show-on-import/td-p/6311059


G76 canned cycle explained
http://www.helmancnc.com/mach3-turn-g76-threading-canned-cycle/

Lever part 2

Here are a couple more images of the process of machining the lever, which I managed to complete last night.

I attempted to limit the machining processes as much as possible to left hand cutting operations to keep tool changes to a minimum, but the inside radius of the handle needs another tool and is best cut in the other direction.

Here is the scratch pass of the threading operation. I paused after this to check that the thread pitch is indeed 1.5mm.

Then, some 47 passes later, the threading is pretty much done.

A fair amount of sanding and polishing later and here is the finished part on the right with the original and the steel from whence it came on the left. As a side note, the threading at the small end of the lever that keeps the handle in place turns out to be imperial 5/16 - 18. Probably because they could't get the rather exotic blind nuts from a metric supplier. This makes it the one single exception to the otherwise absolute metric rule of the machine.

Group lever part I on the new CNC lathe

Over the last few weeks, while I have been waiting for the foundry to setup and make the first casting, I have been converting a lathe to CNC in order to fabricate some parts in house. This seemed sensible for a number of reasons among which is prototyping, which is expensive to have done, but possibly also for some small-volume production. The plumbing parts for the boiler are good candidates as they are expensive and in a few cases impossible to find. Additionally, the ones that I can purchase from my local supplier (and from what I have seen online) have no material certification - which means that they may or may not contain lead. 

So yesterday, after close to a month of modifications and wrangling new software I started cutting my first part: the lever handle.

This is just a piece of hot-rolled steel that I had lying around. I will be using cold-finished 1018 steel or I may switch to 7075 aluminum if I can find it cheaply enough here in Canada. 

The first couple of passes taking off the nasty foundry scale before preparing one end of the stock.




Roughing passes on the straight and conical parts of the handle.

Today's goal is to try to finish the profiling and threading.

First foundry casting

The first casting from the foundry! The square block at the top is the riser which provides a reservoir of molten metal that is drawn into the casting as it solidifies and shrinks. There seems to be a few slight dents in the cast, likely the wax pattern was dinged before it was put into the investment. They should come out in the polishing, though are couple are pretty deep. Now it is off to for machining.

This leads me to a discussion of the material. This is a functional prototype made from a brass alloy similar to C69400, which is not an ideal material for a couple of reasons. The first is that the lead content likely exceeds allowable levels (0.3% in the US). The second is that C69400 has no additives to prevent (or rather retard) dezincification, the process whereby the zinc is stripped from the metal leaving a weak and brittle copper sponge which can ultimately lead to failure of a part at pressure. Dezincification resistance is especially important for espresso machines because of the operating temperature, the acidity of coffee and often, due to the use of water softeners, the mild salinity of the water itself. 

From the excellent copper.org website:

[i]Sand-cast faucets and other plumbing components have traditionally been made from leaded red, semi-red and yellow brasses. The most common plumbing brass, C84400 (also known as 81 Metal or 81-3-7-9) contains nominally 7% lead. The most popular red brass, C83600 (85 Metal, 85-5-5-5), contains nominally 5% lead. Permanent mold and pressure die castings of plumbing components are also commonly made of the leaded yellow brass alloy C85800, which contains nominally 1.5% lead.[/i]

Lead is added to brass to improve machinability. It acts as a lubricant and causes the chips to break into small pieces while it is being cut. Worse still, because of the way the lead crystals form as the liquid metal solidifies in the mold, the concentration of lead is highest at the inside surface - i.e. where it comes into contact with the water. The unfortunate conclusion is that it is highly likely that both my machines (and indeed all vintage espresso machines), help me meet my recommended daily dose of lead in the morning. This was just the way things were was until California passed its law in 2006. Since then, considerable effort has been made to find alternatives to leaded brass.

1st group body prototype

The prototype of the group body is done. Although there are a few minor surface blemishes, the overall finish looks to be on par or better than the two vintage groups that I have. Certainly, the lines are a little crisper. A closer inspection of the photos indicates that there may be an issue or two to address with the secondary machining processes. I've requested a few more photos and some QC measurements. The part may have to go back for a little more machining before it is shipped.

Coffee project goals

The goal of this project is to build an 'update' to the aurora, which is my favourite vintage single group machine.

There are several variants of the aurora, at least four that I know of, which were imported to in North America by various resellers. I am lucky enough to have two of them. The one in the picture in my first post was made in Milan and imported to Canada. The one below, with a different boiler and HX, was restored by Orphan Espresso. I believe that it was assembled and sold in the US by Termozona, a company from Connecticut, who added the "Europa" branding. I'm not sure how much of the case of the Europa is original, but the design and build quality on the European import are better.




Dividing the machine into its component assemblies, the plan at the moment is as follows:

Group - everything that I have read about this group is positive. The main casting contains over 7lbs of brass and consequently has plenty of thermal mass to maintain temperature stability. It makes a killer shot and I prefer its aesthetic qualities over other lever designs I have seen. Consequently, this, apart from a change in a few of the materials for reasons that I will go into later, will remain faithful to the original design. As this part of the project is the most difficult, this is where I am starting.

Boiler - after the group, the boiler is, imho, the most important part of the design as it determines how well the whole machine performs. Here again, I want to stay true to the original design while benefitting as much as possible from the performance improvements that are offered by contemporary control electronics. So, much as I like the project that EspressoForge is leading that pairs an Ascasa Thermoblock with a lever group, I am going with an old school boiler and heating element, with an HX.

Control - this is the area where there is the most room for improvement over the original design which, depending on whether it is an autofill variant or not, uses a Gicar and Sirai pressurestat. This last is a rather blunt instrument with a deadband, when it is new, of around 0.2bar (considerably wider when the diaphragm of the stat gets old). Although I may start with a pressurestat as it will work "off-the-shelf", ultimately the control will be handled with some flavour of micro-processor which will drive the element using an SSR. A starting point for this may well be EspressoForge's combination of the arduino + TC4.

Autofill - yes. Fairly straightforward with a solenoid valve.

Sight gauge - not currently planned. The sight gauge adds a lot of complexity to the plumbing and the case design which would be eliminated by providing feedback from the water-level sensor with an LED.

Case and chassis - with the goal of making a commercial quality machine that can live on the counter top in the kitchen, I am going to try to slim down the profile and footprint a little while remaining faithful to the styling. The drip tray and sump designs also have room for improvement.

Cold Aurora in February

One particularly cold February morning, I got up to discover that the auto-fill circuit on my Brugnetti Aurora had failed. As I opened up the Gicar controller a few days-without-proper-morning-coffee later to figure out what was wrong (an old and faded capacitor), I realized that I had now either fixed or maintained every system on the machine except the lever group. So I took that apart too and replaced the spring and the gaskets. As I was putting it back together I started wondering about what it would take, given that the original Brugnetti company no longer exists, to make more of these fantastic machines.

A few days ago, after nearly three months of research, drawing, discussion and negotiation, I received these photos:



... a wax positive of the group body which, over the next few days, is going to be encased in plaster and replaced with molten brass.

The photos are also a sign, at least to me, that this project is now well and truly underway and that it is time to share it with the generous people on this forum, whose advice I have greatly benefited from since my obsession with coffee began. I am a long, long way from a working machine, but you have to start somewhere.