30+inch+TROK+primary+hogging+out+and+fine+grinding

Beveling and Hogging out the blank March 2001 The machine is finally complete. The first task was to bevel the edge and to grind the edge flat and circular. The nipped edge was out of circle by 4mm. This must be made circular so that when the mirror is put back on the table its exact center is in the center of the axis of the table. If not then the mirror and tool will attempt to create a new radius of curvature. This will create a new series of zones on the mirror. It must be centered to around 0.5mm. Attempts to use a grinding wheel and drill fixed at 45 degrees to the edge worked but was taking a long time and was VERY noisy. Too noisy for the neighbors. So I took it to a local firm who had a large machine, but the cost was £180 to edge and bevel. I returned and altered the machine to have a table speed of 1rpm and made a simple rig to hold a 6inch carborundum wheel 90 degrees to the edge with a water feed from a lemonade bottle as used in the hogging of the original 30inch. This worked fine but took around 48 hours to complete in 4 sessions.

You can see the motor output shaft is direct to the table giving a high table speed. The carbo wheel is mountd on the wooden frame below the water bottle feed. The machine was returned to its previous state and made ready for hogging out. I originally decided o use an 18inch-glass tool for the hogging out process. I began to use this in a traditional manner with BM80 grit. The BM80 grit broke down very quickly and did not seem to work that quickly. After a few hours I stopped work with this tool and purchased 2 x 20Kg barbell weights from JJB sports to use as hogging out tools instead. This method would be to use the 1inch lip of the outside edge of the weight as the grinding surface. There has been much on the web about this recently and is similar to using metal washers as I have used before that also seems to work quickly. We had decided not to use the more aggressive method of the radius arm grinder for this stage due to the cost of the blank and also that it is very difficult, if almost impossible, to get hold of this stuff anymore. The barbell weights are stated to be 4 x quicker than using a tile tool, which I believe, is quicker than using the glass tool also. Therefore we would give this method some time to evaluate how long it would take. We also hoped that this method although longer would produce a better initial radius of curvature…Time will tell… Saturday 24th March 2001 Work began on the hogging out with 2 x 20kg weights. This proved a little too heavy for the overarm motor and we reduced the weight to 20kg plus around another 5kg of other weight to act as spacers between the 20kg weight and the overarm. The motor still gets warm and I will be looking for a 1hp motor over the next few weeks. The BM80 was used and again seemed to breakdown quickly. The action of the Barbell weight seemed to be quicker. The weight was still bedding in after an hour's work before stopping for the day.

The above image shows the 20Kg Barbell weight 'doing it's stuff' to the 30inch blank. The sagita at this point was 2mm. You can also see that the edge is not perfect but is mostly smooth and is only out of circular to less than 1mm.

Sunday 25th March 2001 45mins of hogging was done with BM80. Monday 26th March 2001 30 mins with BM80 and 30min with Carbo80. The BM80 has now been exhausted Tuesday 27th March 1 hour with Carbo 80. The center now appears to be getting worn down a little with the sagita being a little more than 3mm. 6.5inch from the edge the depth of curve was 50thou which corresponds to 2mm. This corresponds approximately to a 30inch F15. The Carbo 80 is quieter than the BM80 but lasts a longer on the wets (approx. 3-4 mins). Saturday 7th April 2001 Hogging out complete. The sagita was measured to be 7.1mm. We decided to halt any more hogging and let the fine grinding tool complete the job. This tool would take some time to 'ded in' and would remove the remaining 1mm or so left to get the curve to just under F6.

Making and using the fine grinding tool

The fine grinding tool was made during and completed after the hogging out process. It was decided to use an 80% diameter tool to ensure it was large enough to prevent astigmatism but not so large that it would be difficult to move. We decided to use a wood base for the tool as had been used before on other mirrors made. I had made a plaster tool some years ago for the original 30inch but found that it was too heavy and cumbersome. The wooden base was made from 600mm wide worktop cut out into 24inch (600mm) discs. Five discs were used, each 38mm thick. The discs were joined with fibre glass resin, not waterproof PVA as had been used in the past. This gave a quick, strong, water proof join. The edges were soaked well in the resin also. Both the top and the bottom faces used the ‘working surface’ of the worktop to give a water tight surface. The face of the tool requires to be formed to the same curve of the mirror. This is done by a number of layers of car body filler pressed into the mirror with a ‘none stick’ layer between them. This layer in the past has been grease proof paper that is used in cooking. This time we used a thin layer of clear plastic sheet obtained form garden centres. This does not stick to filler and comes in wide strips. The final form of the tool needs to take the exact shape of the hogged out mirror. Other tools that had been made formed fine with just the filler, as there was not a large area to shape. This 24inch diameter tool is much larger and would not form a accurate curve using just filler. The approximate curve was then completed with a layer of fibre glass resin. The resin was painted thick onto the tool and a ‘blob’ poured onto the centre of the tool which was covered in a new layer of clear plastic sheet. The sheet had been ‘stuck down’ to the mirror with grease to ensure good contact and enables the bubbles to be pressed out. The tool is the placed on the mirror and the excess resin is squeezed out. When the resin was going off, I did get a little nearvouse about the tool ‘sticking’ to the mirror because of the close contact due to the layer of grease. In the end it only took a small amount of pressure to release the tool when the resin had only just gone off. I would suggest not to leave the tool too long. The heat generated during curing may have softened the grease enough. If left to cool the tool may have been very difficult to remove? The tool face was then left to fully cure. The face was then again painted with a thick layer of resin and then the metal, M27, washers (100 in total) were placed onto the surface and pressed well into the resin to ensure that they all were at the same height. The washers were cleaned first in methyated spirits and roughed up on the back with a course paper to ensure good grip. Time will tell if the resin is better than epoxy that has been used in the past. A worry of this type of tool is that washers may start ‘dropping off’.!! The completed tool was painted with gloss white for aesthetics. So far the tool has worked fine (no pun intended!). One puzzel is that on closer inspection, the 80 Carbo from Beacon Hill seemed to be finer than some 120 Carbo that I had. It was closer in grade under an eyepiece to 240 than 120?? After speaking to Barrie Watts he could not explain it either. He had been assured that hi 80 Carbo was 80. We decided to 'bed in' the tool with the Beacon Hill 80 Carbo until most of the washers had contact. This gives itself away by a darkening of the surface of the washers. This was completed within a few hours, but we continued with the 80 (180/240) to ensure a good radius of curvature was formed and the middle was deepened a little more to bring the surface to F6. We have now worked onto 320 Carbo from the 80 Carbo (which we think is closer to 180/240). The surface is now completed to 320 and has been packed away (11/05/2001) until completion of my new garage to enable completion of the project. Work will hopefully resume in August 2001.

27th October 2001
At last, the new garage is now complete!!!!! The new garage is clean, bright and has lots of space. It is now October 2001. The new machine was cleaned down and moved into the new garage. Some modifications made to the table support, electrics and a new overarm was made Steve King, myself, Geoff and Dave Owen managed to get some more work into fine grinding the mirror. The mirror tested out then to be F6 (15 feet focal length) using the Sun as the source. We need to get the mirror to around F5.6/F5.7 before polishing, otherwise the scope is going to be too long for the new ladder! The mirror underwent a number of 'pencil tests'. This involved drying the mirror and then marking the face with an indelible marker. A few strokes with the tool using BM202 would erase the higher marks leaving patterns that could be interpreted as to how to adjust the tool to ensure even wear and produce a good sphere. The image below shows how 'crude' the pencil marks can be.

The first 'pass' showed that the middle and edge were 'high'. This was due to the middle of the tool not touching as much as the edge of the washer tool. The reason that the edge was high was because that the tool stroke had been kept 'short' and the 70% zone had been worked more therefore leaving it 'low' in comparison. The tool was then moved out so the outer edge of the tool was wearing the middle of the mirror and the edge of the mirror. The centre of the tool was then positioned over the 70% zone so as not to take out 'too much'. The difference between the centre/edge and teh 70% zone was very small anyway. A few more strokes with the tool initially would have taken out the rest of the pencil marks anyway, so we knew we were not far off a sphere.