Holding a 90mm optical tube to a rectangular bracket.

I have an 80mm refractor telescope which came screwed with two screws to a bracket that has a dovetail foot. I replaced the foot with telescope rings to make it all firmer. But now I need a lighter system and seem to have lost the original bracket.

I can do the dovetail and the rest of the bracket but I need to produce a nice clean 45mm radius groove along the top of the bracket (around 30mm wide) to sit the optical tube in. I have a milling table for my mini lathe and was hoping to mill a long internal radius slot along the top of an aluminium block.

A fly cutter with a long arbor could do the job but the lathe is a bit flimsy IMO. I was thinking I could use a long sort of boring bar, supported at the tail stock end to hold it steady and use a flying cutter (carefully).

Alternatively, I could try to form a flat plate round a suitable pipe to get the curve and sit it on top of my bracket.

This is the sort of thing I could imagine would be needed to fit a cylindrical boiler for a steam engine so people must have already solved this problem. Problem is to choose the best search terms for information straight from the internet.

Any ideas?

Edited By andrew lyner on 04/11/2019 17:34:53

Between ctrs bar with work on your table of if not enough room then have work vertical rather than flat

That's how I would do it.

Neil

That's what I had sketched out and just the result I want. Thanks.

My table is vertical and not as firm but I could go very slowly. There is screw adjustment that way. Did the feed in the picture use changing shims? Ah yes - feeler gauges - smart. Fitting to my cross slide could be hard;

Is that between ctrs bar something to buy? Could be expensive so it would have to be DIY. I was wondering about the problems of holding the tool and making a square hole but I guess a round hole and a round/square insert would be doable. Was thinking to cut a slot in the bar and sink the tool in it. Strong enough if the bar is solid enough I suppose.The picture doesn;t show the tool clamping unfortunately.

IF I could get hold of a 90mm grinding wheel, that could do?

Feed was by adjusting the cutter in the bar, shims and packing are to get the part to the correct height.

If you have the work on a vertical face then you can just set the cutter to dia and feed in with the cross slide

Bar is just any old bar, ctr drilled at one end for tailstock support. Drilled midway for a 1/4" HSS toolbit and at 90 deg to that for an M5 grub screw. You can drive it with the chuck rather than actually between ctrs. Round toolbit will also be fine and can be ground from an old ctr drill or milling cutter.

Not sure how closely it matches your requirement, but this is an acetal tube mount for (IIRC) a 76mm Bushnell spotting scope set up as a finder for a 7 or 8" reflector. I did it for a friend who's interested in measuring angular separation in multi-star systems.

You can see how spindly the lathe flycutting setup was, but it didn't have too much alli to remove to bring the finder mount seat parallel to the main OTA. I'd already cut off the cast-in tube as it was way too small, but hadn't thought about the draft angle there would have to be on the inside. Considering how heavy the Bushnell was and how feeble the mount looked I'm a bit surprised I got away with it at all, but last I heard he was very pleased with it.

I think I was doing a 45mm radius too. If you're starting with cheap materials without much work already in them it can be worth a few risks.

This is how I did it. Same idea as @JasonB's The work piece had a hole in it and sits on the tool post. There are no T slots on the mini lathe cross slide so this seemed to be the only way to clamp it.

I re-purposed an 8mm drill for the cutting tool

It was all firm enough and I was using about 300 RPM with small nibbles.

The result is not bad at all. Right curvature, fair finish and in the right place!!

I guess I could make a fitting for the cross slide with T slots but that would be hard, I think. Is there anything available on the market?

Thanks for all the input chaps.

Whoops. That picture didn't post. I just tried copy and pasted it in. I could see it fine on my copy as I edited it. I'm looking for an option to insert an image but the window I get is gobbledegook. I need a file select window and there doesn't appear to be one. Any ideas?

I trip out when asked for a URL. That's something from the dim and distant past afaic.

Edited By andrew lyner on 14/11/2019 23:28:18

OH boy - you have to use an album. Still it does work.

You can get small milling tables. The one here you could cut off the lugs and use a some counterbored cap head screws to fit it on top of the cross slide.

Alternative would be a vertical slide mounted along the axis of the lathe.

Neil

Possibly worth pointing out that standard dovetails, as fitted to the larger scope, normally have two raised, rounded rails which makes it possible to fit them to a large range of diameters.

Neil

I might understand a picture better - but the owner made it clear he wanted to use the diecast stem, for the standoff. None of the rings he had were big enough for the Bushnell. Yes, I could've machined something more solid but I was doing it as a favour, and I'd just finished doing a crosshair illumination drilling of the inner box of a c.1910 Watson astromicrometer for him, without destroying either of the spidersilk crosshairs...

I have a length of the dovetail already but the SW Equinox ED80 has a very short length of tube between focusser and retracted dew shield so it has to be stood off from the dovetail. the shape of the dovetail extrusion is quite cleverly arranged to take a range of radii but I cannot take advantage of it. Also, that short length of contact would appear to require a nice big contact area (the two M6 screws in the tuber are only separated by 13mm or so which makes it all a bit ' sensitive' if the whole setup is to be nice and tight yet minimise stress on the tube.

It beds in really well to the radius I've made. It should 'do' I think. I must see how the star trails look.

Yes. I saw that table design. That would probably be more useful than just buying an angle plate (I spotted that in my trawl through Chronos etc..

The vertical milling table I bought is the standard one afaics. It's pretty sturdy but the cross slide movement will not allow it to present along the line of centres.

I am learning that there are many perfectly acceptable 'bodges' in this work as long as you use your head and don't try to eat too much at a time. They tend to take a lot longer to set up than what you can buy. That seems to be the main drawback. I guess it could be different if the lathe were a proper workshop size, doing workshop type jobs,

The recommended cutting speed for aluminium implies that a fly cutting tool will be spinning at a truly scary rate. OTOH, too low gives its own problems.