Member postings for Roger Hart

Thanks very much. Looking a bit deeper it seems that the 'coarse' size is what you usually get - unless described as 'fine'. So I ordered some up and will see what I get.

Trying to find a few M5 coarse thread 0.8mm bolts about 65mm long. Trawled fleabay and a couple of usual suppliers but no luck so far. Any ideas?

Thanks for the replies all. I think I was heating up too much. Instructions said boiling water which is probably a bit too hot, as the stuff cools down it gets more tractable and less sticky. Other instructions say 60 degrees C which looks more sensible. I'll know next time....

Recently used some mouldable plastic - where you tip granules into hot water and get a mass of clear plastic. Useful stuff but I found hard to manage.

Are there any easier ways to use this than attempting to handle a hot, incredibly sticky mass. I did my job OK but found avoiding blowholes and water pockets a bit of a problem. Even getting the stuff off swmbo's saucepan and ladle was a problem.

I would guess that the frequencies involved are pretty low and the presence of any metal surface thicker than about 0.1mm will screen off anything beyond - at least so far as detection is concerned. So the idea of a centroid might not apply, just the distance and area. Ferrous or not should not matter.

I like the idea of squirting cats eyes though.

Thanks everyone. I checked the invoice - Acetal Delrin. The job is not critical, made mostly from the junk box and a bit of Delrin rod left over from a previous job. I had some holes tighten up on the previous job - so tricky stuff. Actually the polished rod is only 1 thou under size (checked with Moore & Wright) - so less surprising it was tight. Thanks for the tip about raising a burr on the reamer, think I will try that. Next time - more thought first!

I am making a bearing for a 1/2 inch polished shaft in Delrin (black nylon) and find it is a bit tight and despite use of a 1/2 inch reamer (a bit old though) remains tight. The shaft measures about 2 thou under 1/2 inch.

I have tried freezing it an re-reaming (no good) and heating up the shaft (a little better), but short of a new reamer - any ideas?

I have had trouble with Delrin before - it seems to 'relax' after machining, is this just me or a general thing?

A couple of snaps of my attempt at a very simple glass lathe - in early stage.....

Bore is 1 inch - small but OK for my first try.

I would be quite interested. Having tried glass working (joining tubes etc) I would say it is a lot more difficult (for me) than it looks. Borosilicate is allegedly easier but more expensive kit needed and soda glass easily ends up a hopeless tangle - and then it cracks at the drop of a hat.

Currently working on a very simple glass lathe in an attempt to sidestep the difficulties of hand work. Think dual stepper motors and a sliding carriage plus leadscrew.

Tried an experiment, span up a toy gyro with 3/16 shaft using string. On the tacho this got to 4500rpm but dropped fairly quick to 3000. I reckon you would be lucky to do much better. Using the criterion of a gyro that will sit horizontal and do 1 rev/sec with a rotor speed of 2000rpm the 75mm diameter rotor is marginal - OK if turning a bit faster but losing energy fairly quick.

The snag is that a toy gyro to sit horizontal on a little tower is pretty challenging, the weight (mass) cannot be too much but you need the mass to get the gyro couple - and the practical rpm achievable is in the 2000 to 3000 region. This design criterion is quite different from designing a gyro compass.

As a kid I remember a pretty good gyro that was about 70mm diameter with a thin ish rim - but made of lead painted in shiny brass lacquer - probably not allowed now....

Take a look at www.gyroscopes.org and the maths tab. I plugged the 'as spec' numbers for your gyro wheel and chose 5000rpm and then entered the 'as scaled' (with a bit of guestimation). To get the same gyro couple it seems the rpm need to go to about 20000 or more. A bit of a problem with windage and bearing loss. This fits with my experience, I made a smallish gyro and the performance was pretty disappointing. I ended up mounting it in double gimbals as a demo. So, 75mm OD looks about the minimum in brass.

Most useful tools are:

Mandrel handle, Toolpost drill, Fingerplate (as from George H Thomas book)

Thanks very much everyone. I guess a bit of mercury slopping about was a small danger compared to all the other dangers. Nice to see what practical folk our forbears were.

The bourdon gauge was invented about 1849 but steam locomotives were around long before. So how was the pressure in the boiler measured? How did you know when 'enough was enough'?

Anyone fancy one of these. Take a look at summersplaceauctions.com (no connection) item 75 I think.

Not that expensive and a nice restoration job for the winter. A bit faster than a traction engine and the boiler test might be a bit exciting.

Not exactly a model, more an experiment in aluminium evaporation that worked. My thanks to friends and this site for help. The main vacuum pump is an old 1950s unit coupled to a modern turbomolecular pump. Most of the kit came from boot fairs and fleabay. Evaporation is from a 1mm tungsten TIG rod loaded with cooking foil. The transformer is an old microwave oven unit rewound with flexible copper cable 4mm dia. Feedthroughs are old stainless rods from scrap printers. For the economically minded the turbo pump splinter guard is a sink drainer. Yes I did use an implosion guard and eyeshields.

Teaching kids lathework sounds a job for the very brave. So how many times had this workshop had the inspectors run the rule over it - and did any of the inspectors have a clue what they were looking at? I hope they leave the teacher alone and carry on teaching lathework. Lesson learned, pay up and look big - there are very few problems a lawyer cannot make worse.

Sounds like under exposure to me. I use ready-resist printed circuit board. Exposed with mylar print (laser/mirrored) face down for 2.5 minutes in a 36watt 4 tube uv nail-gel lamp box. Tubes are about 100mm from board. Board/print sandwiched in 2.5mm glass. Developed in sodium metasilicate (30g in 1 litre water). Developer needs to be slightly warm or it is very slow, too warm and it strips the resist. I leave bottle on radiator for an hour before use. I tried caustic soda but found it too critical in strength/temperature. This works for me on fairly fine tracks. Experiment is my advice, maybe your resist is a bit thicker than the commercially applied stuff.

Apparently people did try running small turbines off the water main in the early radio days. The water companies didn't like it and used to trace the culprits using a listening stick to hear the distinctive buzzing noise.

Is there any advantage to the backshot approach other than a different direction of rotation?

Suck it and see is my advice. A USB camera is very cheap in the local supermarket, I don't think the usual astro ones are anything special (some used to get modified for exposure reasons but not so much now). You don't need the expensive low light astro cameras unless your application is very special. I doubt the USB cameras sold for microscopes are anything very special either.

Before getting too technical with the optics just offer it up to the telescope eyepiece and see if the image is any good. A USB camera is just like your eye - a lens with a silicon retina, nothing more.