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Member postings for Pete Rimmer

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

Thread: A home-made gear - will it work?
23/03/2021 21:28:34

A 19DP hob or gear cutter would cut the teeth to a very close profile (ideal would be 18.85DP). I recently cut a 18 tooth thread chasing gear for a 6tpi screw with (I think) a 19.5dp hob, which worked perfectly well.

Thread: The Secret Science of Sewage
21/03/2021 18:50:22
Posted by MC Black on 20/03/2021 01:22:57:

The Victorian pumping engines at Crossness are being restored slowly but it’s a fascinating visit. I went with a group from the Institute of Physics London &South East Retired Members Section. The visitor centre has a great exhibition and a Café. Well worth going if one has the chance.

The Victorian plan was to store the sewage collected by the main Southern sewer in large underground tanks and then pump it all out into the Thames at low tide!!


A large amount of sewage is still getting into the Thames. Every time it rains hard it overwhelms the sewers at Blackfriars (where the surface water gets diverted into the sewer) and gets relieved into the Thames right under the bridge. This is why they are sinking a 60 metre shaft to divert the outfall into the new supersewer.



Edited By Pete Rimmer on 21/03/2021 18:52:14

Thread: Myford Fine Feed Tumbler Gear
18/03/2021 21:21:45
Posted by Howard Lewis on 18/03/2021 15:00:12:

This is puzzling!

Using the formula from Ivan Law's book on gears, OD = (tooth count+2 ) / DP, so a 20DP gear with an OD of 0.750 would have 13T . (13+2 ) / 20 = 0.750

The gear obviously has 12T, but 0.7465 x 2 = 1.493.

So it looks as if the gear is oversize for a 20DP involute form 12T, unless there was a Zero error.

(12+2 ) / 20 = 0.700

Are there any professional gear specialists out there to offer an explanation?


As Jason said the small tooth count gears are modified with a 'long addendum' to reduce the under-cut and strengthen the gear. The amount of profile shift depends on the tooth size and tooth count.

Thread: Part of a series on a two stroke engine
13/03/2021 21:30:45

Good luck to Mike and Becci I only met them once briefly at Woodbridge but they seemed like lovely people.

Thread: Superglue and Oil?
07/03/2021 19:19:04

Araldite is also fuel and oil proof once cured. The nromal cure stuff not the rapid one.

Thread: Where to find a *good* optically flat mirror?
05/03/2021 20:16:21

I want to thank everyone who has replied to my thread. I have secured a couple of Hilger & Watts mounted mirrors in great condition which I will be collecting this weekend.

04/03/2021 18:03:02

Thank you for the replies.

It seems I have a lot to learn about mirrors and optics. I spent a good couple of hours yeaterday trying to figure out what was the problem with my autocollimator. When I rotated the tube from vertical to horizontal the lines were blurring and fading almost right out but appearing again when it hit the end stops. I figured there was something wrong with the instrument when in fact it was the mirror all along. It must be polarised or something. I swapped to a different mirror and the 'fault' went away

02/03/2021 20:37:02

I've bought this mirror from a member who was kind enough to offer it in response to this thread. It's in fantastic condition, the silvered face is 55mm and it's 20mm thick.

I intend to make a cast iron mount to hold the mirror, I have the materials and the facility but what is the best way to hold it inside the housing whilst allowing for adjustment in the vertical plane? The sled will be 2-piece with the base holding the mirror mount so I can allow for horizontal adjustments there. I have to think about fine-adjusting the mirror vertically.

I've done reading-up online and there seems to be a bewildering number of options and considerations. I'd like to keep it as simple as possible.


26/02/2021 19:58:21
Posted by Neil Wyatt on 25/02/2021 11:01:31:

You can get them relatively cheaply from various sources as newtonian telescope secondaries.

Orion Optics is one option.

For small ones, Astromedia are great, scroll right down:

They should have something that will do the job for between £2.58 and £5.58 although there's P&P on top.


Thank you for the link Neil. I must confess to being pretty ignorant in the matter of mirrors and optics. It's difficult to come to an informed decision when you read some places that you need x quality and then hear that really only Y quality of mirror will do. I would never have thought of using telescope seconday mirrors with them being so thin in section.

I'm very appreciative and grateful for all the input, suggestions, offers and advice gained in the thread. Makes me glad that I started it because I've gained more than the offer of some suitable mirrors which was all I could have hoped for.

Edited By Pete Rimmer on 26/02/2021 19:59:26

25/02/2021 14:05:51
Posted by Bob Worsley on 25/02/2021 11:21:07:

H&W mirror is about 6" by 5" with corners cropped, about 1/2" thick, mounted in an aluminium casting.

For other metrology bits interested in a Watts Circular Division Tester? For measuring angles with circular graduation scale and microscope to read it. Calibration certificate says it goes to the few odd arc seconds. In a large and heavy wood box!

Yes I'll be interested in both of those Bob. I will send you a message, thank you.

24/02/2021 23:00:23

Thanks Clive, I've sent you a message. That one does sound just about ideal.


24/02/2021 19:39:44

If you do find the mounted one Bob please let me know. What size and thickness is the shadowgraph mirror?

24/02/2021 08:35:54

Clive yes I am buying an autocollimator for checking surface plates and machine alignment. It will only be used over short distances so I can suffer a bit of error in the mirror. I've got a couple of plates that I need to renovate, one is quite bad the other quite good. I've got a lapping plate and I've made a repeat-o-meter and found this instrument, now I need a good mirror to use with it.

23/02/2021 23:55:47

Thanks for the replies and suggestions. It's given me a lot to go on.

Comar optics do a 63mm 1/10 mirror which would be ideal except it's £120 plus vat. I was hoping to avoid quite that much expense but if I don't find a suitable mirror for less I will have to stump up the cash for that.


23/02/2021 20:03:17

Well Mart, they come in all sizes but realistically I am expecting to find one between 1" and 3" diameter.

23/02/2021 17:13:13

I put an advert on homeworkshop for an optically flat mirror but as yet have had no responses to it. These things seem to be thin on the ground and very expensive, if you buy the mounted ones.

I'm looking for 'thinking out of the box' suggestions as to where I can find a decent fractional-wave first surface mirror to use as a reflector for an autocollimator I am picking up shortly. I can make the sled to carry it and I'm confident that I can make a mount but I need to first buy the mirror itself.

I know that there are a few members interested in telescopy and other optical interests so I'm hoping that someone can suggest a source for a reasonably priced mirror. It must be flat, and first-surface, and fractional waveength, so half-wave or better.

I'm not a tightwad but I don't want to pay newor crazy ebay prices. Hopefully there will be someone here can point me in the direction of a decent mirror or a device or instrument where they re used so I can look for a surplus one.


Thread: Meshing gears badly with either different PA or with different DP
22/02/2021 00:04:33

For lathe change gears that you can adjust the centres on I doubt it matters much at all if you're in a pinch. Having seen all manner of worn, knackered, chipped, chunked and even running-out change gears runing perfectly harmlessly on lathes I think that if you have matching tooth sizes and enough clearance so they don't rumble then you're good enough. For fixed centres or long duty that's a whole different matter.

Remember, many gear pairs are profile-shifted one up, the other down which changes the effective pressure angle, and they work just fine.

Thread: leadscrew pitch
09/02/2021 18:30:24

The pitch is 1mm David.

Thread: 3D Printed Pulleys
07/02/2021 19:23:17

I should think that 3dprinting would be an ideal application for a pulley on a small lathe like that.

Thread: Cutting a Worm Wheel
04/02/2021 18:24:42

You could cut that worm with a MOD2 hob for only 2 thou per pitch error, which (IMO) is quite acceptable. I have a 32mm diameter MOD2 how which would do the job though you'd have slightly reduced contact.

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