|CAST IRON TOOLS.
The first telescope rrirror I made I ground glass on glass. Since then I have only used cast iron tools. I have only worked six and eight inch glass and made 6"- f/8 and 8"-f/6 mirrors and 6" and 8" flats. I will explain my methods of making these.
I.cast two discs of cast iron, one for a convex tool and one for a concave tool. A template for machining the curves to was next made. I fixed a glass cutter to a strip of timber and measured back and drove a nail in at the distance of the radius of curvature. A radius cut was made on a small rectangular piece of thin glass and broken along the line. The two pieces were then lapped together with fine carborundum.
The curve of the cast iron discs were freehand turned to the templates. The convex tool is machined from the inside out and the concave from the outside in. Power feed is used on the cross feed and hand feed for the longitudinal. Bearing blue on the edge of the templates marks the high spots. Using a tool rest as in wood turning a high speed steel scraper was used to get the curve closer.
The two tools were then run together on the mirror grinding machine using different grades of carborundurn until there was an even marking on both. This ensures that the curves are spherical. Now I read in one of the ATM books, I can't remember where that when the tool is used on top then it should be 5/6 th. The mirror diameter. I made three convex tools and one concave of 5" diameter. The first convex tool is used for roughing out and the second and third tools for finer and finer grinding. The curve of the first tool changes with the roughing out, but the mirror should come off the last tool with the desired radius of curvature. The first tool can be put back in the lathe and the curve corrected, checking it against the concave tool with bearing blue.
Now the mirror is given a polish sufficient to give a reflection in the knife edge test. The centre of curvature can be determined and measured. If not close enough to the what is wanted the tools can be put back in the lathe and scraped to increase or decrease the radius, then run together on the machine, the mirror ground and polished and tested again. The process is repeated until the radius is to the desired degree of accuracy.
When satisfied that the radius of curvature was correct, I then channelled the convex tools. Before I had a milling machine to end mill the channels I would cut and bevel the grooves with a hacksaw and three cornered file. Not an enviable job. If the radius of a tool alters it cannot be corrected by scraping if it is channelled. I use a small angle grinder to do this and use the concave tool as the test plate.
Fig. 1 illustrates the end miling of the channells. A sleeve around the end mill controls the depth. As the table of the milling machine is traversed the quill is held down so that the sleeve is inncontact with the curface of the lap. This gives an even depth of groove.
I did not have a spherometer. I also gave up using sub-diameter tools and laps as I found I was getting zones on the mirrors. Also I found I had to make a full diameter lap for parabolising. I only use full diameter tools and laps now.
Making Cast Iron Tools for a Corrector Lens.
I shall describe now in some detail the making of cast iron tooling for a Maksutov corrector lens. I have now made several spherometers. ( refer.. Spherometers).
Patterns and castings were made; a convex and concave blank for the two lens surfaces. These were cast close to the desired radius to minimise the amount of machining. I did not have a piece of thin glass to make a template and doubted that it could be broken along so short a radius. I scribed the radius on a piece of 3mm. aluminium Sheat and cut close along the outside of the line. The convex piece had the scribe line on it and the curve was sanded to this line on a disc sander. The concave curve was filed and fitted to the convex.
The cast iron blanks were machined as shown in Fig.1. and Fig.4 shows how they were machined, and Fig.3. how they were checked with the templates. A fast power feed is used on the cross feed and hand feed for the longitudinal. With the convex tool the cut is from the inside out and from the outside in on the concave. This is not as hard as it looks and the curve can be machined close to the template. Marking blue is rubbed on the edge of the template and rubbed on the machined surface to get a marking. High spots are machined down. I concentrated on the convex tool. I did not scrape the surface as I described in making a chamfering tool. Instead I machined a sharp edge on an outer ball beaing ring and pressed it against the surface of the cast iron tool as shown in Fig.5. A lot of force is required but I was using a big lathe with a heavy saddle. A smooth surface was obtained.
The surface of this tool was blued and used to get markings on the concave tool. When a good marking was obtained I went to smooth it with the ring tool as shown in Fig.5. Disaster strusk. The ring dug in, the saddle of the lathe was forced back and the ring went flying. I had another go taking more care. The same thing, so I gave that idea away. I Put the tools on my mirror grinding machine and using the hand stroke lever applied pressure. I put my body weight on it but it was not enough. There were no dig-ins though. The trouble is that very little cutting takes place at the centre. If the convex tool was machined with a slightly longer radius and the concave with a shorter, the metal would be cut off the outside to bring it to the correct radius.
I gave that away and ran the tools together with carborundum powder on the grinding machine until I got a smooth marking over both surfaces. I could have left it at this, but I wanted to have a go at generating the curve on the milling machine. I made up a tool bar fitted with a tungsten carbide tool to fit in the Bridgeport flycutter. A rotary milling table driven by a variable speed motor with worm reduction and belt drive was set up on the milling machine table. A three jaw chuck was fixed to the milling table and using a dial gauge was centred exactly under the milling maschine spindle. The convex tool was set up first. A rule was placed on the machine table and the cutter brought down on to it. The spindle was turned by hand and the cutting tool adjusted until it was sweeping a 3-1/4" diameter circle. Calculations were made and the milling machine head tilted over to the required angle. The table was adjuster until the point of the cutter coincided with the centre of the cast iron tool. The turntable motor was switched on and set to a low speed. I cannot saw what speed it was. In all my years of using a lathe and milling machine I do not note what speed I use. I just know what is right . Looks or feel if you like. The miller was switched on. Now in this operation the cutter is not fed down as the spindle being at an angle the centres of tool and cutter will change. The table is fed up.
Because of the interrupted cut, a damper bearing on the outside diameter of the milling table would have been an advantage. The operation was beautiful to watch. When the concave tool was finished the same setting was used for the concave tool. This did not cut as smoothly as the convex tool. When I had finished I placed the two tools together and found that they were slightly high in the centre. This would be because it is very difficult to get the centres of tool and cutter to coincide exactly. Being high in the centre is beter than high on the outsides. As I intend to run them together with carborundum powder on the machine there will be less material to remove. However This will be done after they are channelled. Carborundum becomes imbedded in the iron and will take the edghe off the milling cutter used to cut the channels. And milling cutters are very expensive.
Now we come to the channelling. This is a bit of a problem and I have put a lot of thought into it. 1 want to make a nice job of it. After much thought I came up with the arrangement illustrated in Fig. 8. I have an angle attachment for my miller that I made to allow me to use small diameter gear cutters. But as it would not reach into the centre of the concave lap tool I had to make an extended spindle. On the end is fitted a 60° vee cutter. Outside of this is a steel disc that controls the depth of cut. This is mounted on a ball bearing so that it will roll along and follow the surface of the tool. The head of the milling machine is tilted to cut each groove at the correct angle. The cast iron tool is held in a chuck on the rotary milling table. This allows two grooves to be cut at right angles to each other with the same setting. In operation the table is fed across and the cutter held down by hand to follow the curve.
Warning: Do not "climb cut" unless you have a very rigid machine with no backlash in the feed screws. The cutter can pull itself forward and dig in and either damage the job or break the cutter. Use what is called the "conventional" method. Both are illustrated. Fig.9..
At the time of writing this I haven't made up this attachment.