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Member postings for Andy_G

Here is a list of all the postings Andy_G has made in our forums. Click on a thread name to jump to the thread.

Thread: Tapping M2 thread in plastic - which?
31/07/2022 13:15:11
Posted by Kiwi Bloke on 30/07/2022 23:52:20:

I considered Helicoil-type inserts, but assume that these might also cause galvanic corrosion, as would the use of metallic screws (perhaps Zn-plated would be OK), so we'd be back where we started pretty soon.

Stainless in aluminium is prone to glavanic corroson unless protected.

Using Duralac (barium chromate paste, I think) or Tef-Gel on assembly will prevent this.It is standard practice in the marine rigging world.

Either of these would be close to a lifetime's supply!



(I use Duralac, but it has to be said, it can be quite messy.

I think Tef-Gel originates in Australia, so may be more readily available to a 'Kiwi'  )

Edited By Andy_G on 31/07/2022 13:15:27

Thread: Strange problem with jack plugs
31/07/2022 12:57:03
Posted by Ian Parkin on 31/07/2022 12:39:37:

it has 3 nc switches one for each pole

Have you wired the switches to short tip to sleeve when the plug is removed?

This is usually intrinsic to the design of 3.5mm sockets, but needs to be wired in for the 1/4" socket in your picture - There needs to be a wire between the sleeve contact and the *switched* contact on the tip for it to function like the 3.5mm one.

Thread: Carl Zeiss Large Toolmakers microscope
18/07/2022 09:34:13
Posted by William S on 17/07/2022 20:24:17:

...what I really want to know is about the lens, am I right in thinking its a "condenser" lens, what is the exact purpose of said lens?, is it really necessary? (I'm guessing it is!) I only ask this as with a light source it the microscope seems works as it should even with the projection screen.


Yes, it looks like a condenser lens. Its function is as per Clive Steer's post above.

Everything will "work" without it, but there will be a lot of stray and scattered light that will reduce contrast, make edges indistinct and probably create other artifacts. (The fins on the inside of the tube are there to try and reduce stray light, so it was clearly a design consideration.)

This sort of application is very common, and I suspect that the lens will be a generic one, rather than something designed specifically for the microscope.

The key parameters are the outside diameter and effective focal length (EFL).

The condenser will probably be aspheric, which makes it difficult to establish the EFL without having the lens in hand. You will be able to get a good idea of EFL by measuring the axial distance from the lamp filament, at ~mid travel on the slider, to the plane of the back of the lens. This will be the back focal length (BFL). EFL will be somewhere between the front edge of the lens (BFL + 1.5mm from your sketch above) and the very front of the lens (BFL + 1.5mm +9mm from your sketch above) - more likely nearer the +1.5mm dimension (especially if the lens is spherical, rather than aspheric).

The good news is that the lamp is on a slider, so it is possible to accomodate some error / differences in EFL here.

With your lens diameter & EFL estimate, you can try and find a suitable replacement, for example from Edmund Optics:

Condenser lenses

If/when you find something that is likely to be suitable, you may be able to find a more convenient source for it.

I doubt that the material is anything special (the application is visible light only, and the power / heat levels are modest).

ETA: Anti reflection coating would be nice to have, but I doubt that it is critical here.

Good luck!

Edited By Andy_G on 18/07/2022 09:41:49

Thread: SCGT 06 inserts - do such things exist?
11/07/2022 10:00:17
Posted by Robin Graham on 10/07/2022 23:08:16:

I want to buy a couple of 45 degree holders for square indexable inserts - primarily for chamfering. [...]

Any advice (other than grind your own out of HSS!) would be welcome.


You can definitely get 10mm and 12mm holders for the SCMT / SCGT 09 inserts. (I have a 10mm one, but can't remember where I got it. The shank is ~10mm high x 12mm wide).

12mm holders are listed at RDG: 12mm

They also list a 10mm SCMT06 holder , but appear only to offer the SCMT06 inserts.

Edit to add:

My 10mm thick tool holder came from 'Chesterfield Machine Tools' on ebay. They now seem to have vanished...

Edited By Andy_G on 11/07/2022 10:22:25

Thread: Home made jet engine
09/07/2022 10:41:38
Posted by John P on 05/07/2022 15:35:50:

First of all you have made an impressive job on this Kamps engine design ,the EGT
is very low ,did you find out the top end speed and have you done any thrust measurements?


I seem to remember that before the availability of cast turbine wheels
inconel slices about 6 mm wide were around the £35 mark ,that was over
20 years ago ,

Thank you very much - That's an impressive collection you've got there.

I don't believe my EGT readings: I think the TC was waving about in the breze a bit too much to be credible. I will drill the NGV / tailcone to hold the TC nearer the turbine exit to get a more trustworthy reading.

I want to get the EGT and a reliable RPM reading before I run it again. I've got a hall effect sensor working, but am waiting for a suitable magnet to arrive from China before I can try it out on the engine. The intention is then to measure the thrust and run it up to 100,000 RPM. (After that, it'll probably sit gathering dust !)

I did try and buy an Inconel disc - it's difficult stuff to get hold off. I was quoted £75 for a 15mm length of 2.75" dia 718 (+£18 carriage + VAT). - that was the shortest I could get. I did have the offer of a couple of slices of a 6xx Inconel, but after the donor had killed two bandsaw blades without making much progress on cutting the bar, the offer was politely withdrawn.

Thanks for the interest,

03/07/2022 21:26:15
Posted by John P on 02/07/2022 17:26:35:

It is perhaps not worth making a turbine wheel from solid these days
unless you already have the material and the means to mill them ,back then
ready made wheels were only just appearing from the GTBA when they had a
batch made for members of the organisation. sell ready made wheels from about 66 to 82 mm dia from
about 165 Euros which suit the KJ 66 engines and also the Gerald Rutten

If there are any aspirations towards high performance, then a commercially cast inconel turbine is a necessity. Unfortunately, they are probably less available now that they were ~15-20 years ago, as the co-operative DIY efforts have morphed into commercial entities, and the supply of parts to hobby builders has dried up. (There is nothing available via the GTBA - I am a member.)

About the only non-Chinese source is Jetmax, as you say, but taxes and shipping pretty much double the advertised 230 euro cost of a 66mm turbine wheel (and you'd probably want the matching inconel NGV which would cost the same again.).

There are cast turbine wheels advertised on Alibaba and other Chinese sites which are about half the cost of the Jetmax ones.

In any case, I thought that was too much money to sink into a project for me (I didn't want a jet engine, particularly: I wanted to *try to build* a jet engine) - the Kamps design is one of the few still available that will tolerate a home made turbine (even a stainless one), albeit ar reduced performance, which is why I chose that design - I was very tempted to try one of the original FD3 variants (since this is what had sparked my interest, back in the 1990s) but the Kamps is a much better design.

If you just want a jet engine, the answer is to buy one! :D

There is a GTBA project engine coming to fruition that uses a radial inflow turbine (rather than an axial turbine as used in most of the model jet engines) that would allow turbocharger turbines to be used.

02/07/2022 22:51:36
Posted by Buffer on 02/07/2022 19:37:26:

That's really good and I love the details like the engine mount, that really makes it stand out from the others.

Did you make your own spot welder too. If so which plans did you use or was it the Gas Turbine Builders Association spot welder made from a microwave.


Thank you!

Yes, I did end up making a spot welder, after trying and failing to TIG everything - I didn’t follow any specific plans, just used a larger than usual microwave transformer (from a very old commercial microwave) and replaced the secondary with the heaviest cable I could fit in there. It’s controlled by a 555 timer IC and a solid state relay. It packs quite a punch and welds 0.5mm stainless in less than 0.1 seconds.


A little more info

Edited By Andy_G on 02/07/2022 22:59:59

02/07/2022 14:07:25
Posted by Brian John on 02/07/2022 13:44:29:

That is quite stunning ! How long did it take you to make the turbine wheel ?

Thank you!

The turbine wheel didn't take as long as I thought it would - it was split over several sessions, so it didn't seem too bad:

  • Cutting the blank and turning the profile;
  • Slitting the disc into blades;
  • Putting the initial twist into the blades;
  • Cleaning out between the blades and the initial shaping (with an angle grinder);
  • The first go at smoothing and shaping the blades;
  • Second go at shaping, and carving the concave faces with a dremel.

Each of those took an afternoon, pretty much (some afternoons were longer than others!)

More details of the process I followed here:

Turbine wheel blank

Forming the blades

02/07/2022 09:31:18
Posted by JA on 01/07/2022 10:29:27:

Have you made the jet pipe yet?

I've made the tail cone, but it only appears in the first video - I want to confirm that the exit temperature is under control before running the engine up with it in place. I think that people who fly these things sometimes use an internal exhaust duct that they call the jet pipe - I don't have any plans to put this in a plane, so I haven't made one of them.

Thanks to all for the kind comments.

01/07/2022 10:05:38

Thank you!

01/07/2022 09:28:03

Inpired by comments on the 'See through Jet Engine' thread, here's my engine:




It's based on the plans in Thomas Kamps' book and eveything except the compressor wheel (a turbocharger part) was made in my shed on my Chinese 7x14 lathe.



The turbine wheel was hacked out of an Hastelloy X disc with an angle grinder and a Dremel, but came out well, I think:



There's a fairly detailed write-up of the build on my website



First start-up

The subsequent run using liquid kerosene fuel



For the real massochists, an video of me mumbling as I assemble the parts:

I'm currently waiting for parts for a Hall-effect RPM sensor before I run it again.

Edited By Andy_G on 01/07/2022 09:31:25

Thread: Wyvern Spark plug
23/06/2022 09:57:36

The NGK CM-6 plug is often used for engines of that sort of size (it is M10 x 1 thread).

I used one for my little (20mm bore) hit and miss engine:

They are readily available and reasonably inexpensive.

In the absence of any information to the contrary, I'd aim for the end of the plug body to be ~flush with the inside of the head, provided that nothing comes too close to the electrodes in that configuration.

Edited By Andy_G on 23/06/2022 10:05:01

Thread: Blacking aluminium
22/06/2022 12:47:27

"Can it be done a' la cheapskate?"

If you already have a suitalble power supply, sulphuric acid free anodising can be quite cheap and easy.

(My setup: )

If the part mentioned (150 dia x 18) was substantially a solid cylinder, I would use ~0.6 amps for ~2 hrs for a dyed finish.

If you look around the internet, it'll become obvious that people have tried pretty much everything under the sun as a possible dye. Some work, some dont.

Gettin a good black can be tricky. I use commercial anodising dye in a heated bath (55°C). The dye is available in small quantities on eBay for £10 to £15. It produces a good, deep black that has proved colour-fast.

If the dye is leaching out (as per the example above) it would imply that the sealing stage hasn't been successful (the process is Clean -> De-oxidise -> Anodise -> Dye -> Seal ) - sealing by immersing the part in boiling tap water for 20 minutes has worked for me.

Make sure that the process works on a test piece of the same alloy before committing to the part!

(I could do it for you if you covered the postage and could provide a witness sample of the same alloy to check the process first.)

Thread: Timing Belt
14/06/2022 11:20:33

“How are toothed belts sized?”

Generally pitch, width &number of teeth, but it’s a minefield: tooth profile also matters, and there are many. Pitch could be imperial or metric (0.2” is very close to 5mm, for example. )

Can you discern any numbers on the remains of the old belt?

Edited By Andy_G on 14/06/2022 11:27:37

Thread: Curiosity about an Aldi belt sander
11/06/2022 09:50:54

I’ve had the ‘Scheppach’ belt and disc sander for a couple of years and have used it pretty much exclusively for metal (ferrous and non-ferrous).

The dust extraction for the disc sander is assisted by a fan on the motor shaft, but the belt sander extraction is just a duct that doesn’t go near the motor.

I’ve mainly used the belt sander in the vertical position with the horizontal table. I didn’t use the disc sander and ended up adding a crowned roller to drive a narrow sanding belt instead.


I use the blue ‘ceramic’ belts (usually 80 grit) and find that they work well for cleaning up sawn edges and knocking the corners off steel & aluminium parts, as well as sharpening HSS tools & TIG electrodes (I did remove some of the plastic frippery for better access to the belt). I almost always have a vacuum cleaner attached to the dust port which does a good job of capturing the dust and probably helps stop it migrating into the housing.


I think they were £130 when I bought mine. 

[Just to add that I managed to break the drive belt the other week, but I would regard that as a consumable- a replacement was readily available for a few £.  ]

Edited By Andy_G on 11/06/2022 09:56:01

Thread: Cosworth V8 1:12 scale
29/05/2022 09:25:26


Very impressive work

[Edit: Duplicate post, but it bears saying twice, anyway.]

Edited By Andy_G on 29/05/2022 09:26:25

29/05/2022 09:25:25


Very impressive work.

Thread: Thermal condution paste - none adhesive
21/05/2022 09:30:11
Posted by Martin King 2 on 21/05/2022 08:51:40:

Any thoughts on the Zinc Oxide paste?

Cheers, Martin

The original was almost certainly a silicone based 'heat sink' compound, something like this (which does contain zinc oxide).

(Personally, I'd replace it with similar.)

Thread: Milling Table Flatness - What is acceptable
12/05/2022 08:20:13

It would be worth repeating the measurements with the leadscrew removed to rule out the possibility that it is lifting the table as the nut approaches the bearing.

Thread: M&W Microstat Micrometer
11/04/2022 18:21:37

We used to use them a lot at work (some LED, but mostly LCD) - They were branded Cadar though.

I don't remember there being anything special about the on/off switch. It's a while ago, but I'm pretty sure that they charged whether off or on, and could work off the charger if the battery was flat. They often used to be plugged in to the charger 24/7 which tended to kill the batteries after a few years (you only found out if you tried to take one away to use elsewhere - which it sounds like what has happened to yours).

I replaced the batteries in a number of them - they are just a stack of NiCad coin cells inside the end cover; easy enough to change. (4.8V from memory, but don't trust that).

I *think* that the LCD ones may have had a serial interface and clicking on the thumb button sent the reading to whatever was plugged into the other end.

They were very rugged and dependable for such accurate instruments - and they held their accuracy extremely well, even in a shiftwork production environment.

Edited By Andy_G on 11/04/2022 18:22:42

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