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  #481  
Old 15-10-2021, 07:56 AM
Rod
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Quote:
Originally Posted by mura_gadi View Post
Rod, tyvm for the wire brush idea, I had been using the Stanley but that makes a lot of sense.
Hi Steve

The one I use is from bunnings and in this set:

https://www.bunnings.com.au/wire-bru...B&gclsrc=aw.ds

Only $3.50

Rod
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  #482  
Old 15-10-2021, 08:04 AM
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Sunfish (Ray)
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Thanks Rod , Steve and Werner. I have been using the Stellafane quarter circular stroke with a sub diameter but I will give the short straight a try . I have one of those wire brushes and it is true that the lap looks a bit shiny.

Last edited by Sunfish; 15-10-2021 at 08:38 AM.
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  #483  
Old 15-10-2021, 08:21 AM
Rod
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Meant to write short straight strokes mirror on top earlier. I’ve corrected my earlier post. TOT will probably work too but these days I’m finding MOT works best for me, probably because I use slightly oversized laps - usually about 6 mm larger.

Rod
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  #484  
Old 15-10-2021, 02:09 PM
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On another point , I have been wondering if the rib around the back edge of the cast Pyrex piece makes difference. The larger pieces I have a flat , but this Pyrex piece is cast in a tapered mould with a bottom with a ridge like a cup or bowl would.

Quote:
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Meant to write short straight strokes mirror on top earlier. I’ve corrected my earlier post. TOT will probably work too but these days I’m finding MOT works best for me, probably because I use slightly oversized laps - usually about 6 mm larger.

Rod
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Last edited by Sunfish; 15-10-2021 at 02:27 PM.
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  #485  
Old 15-10-2021, 04:45 PM
Rod
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Blanks with a rim like that used to be common. It shouldn’t pose any issues. By the look of it, it’s a standard 1:6 thickness ratio so what’s on the back is not much of an issue.

Rod.
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  #486  
Old 15-10-2021, 05:44 PM
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Thanks Rod. Gave the extra cut and wired star sub diameter a go as I had the lap on hand . Did not seem to be any different so I will try the MOT oversize Bartel again which I also have from the initial polishing.
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  #487  
Old 17-10-2021, 05:08 AM
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Hello,

When using a Ronchi tester, is the diameter of the returned beam on the Ronchi screen related to the diameter of the illuminated field given in "Newt for the Web"?


Thanks
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  #488  
Old 17-10-2021, 08:32 AM
Rod
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Quote:
Originally Posted by mura_gadi View Post
Hello,

When using a Ronchi tester, is the diameter of the returned beam on the Ronchi screen related to the diameter of the illuminated field given in "Newt for the Web"?


Thanks
There’s no relationship that I can think of. The illuminated field given in newt is mainly determined by the diameter of the diagonal mirror and to a lesser extent if you have a narrow aperture stop in the system (eg. small diameter tube).

Rod.
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  #489  
Old 11-02-2022, 02:23 PM
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Using hot channel cutters to reform a collapsing lap

FYI

Recently I posted about trying a thin heated taper metal tip to reset the grooves in my lap which had formed poorly.

The results were very good, using the heated tapered tip based on John Upton use of a hot channel cutter. There are some waves formed, but very few and easily cleaned off with the Stanley knife. Certainly made chipping away inside a plastic bag both easier & faster and the channels are far better formed and cleaner. Material loss from the lap is maybe 1-2 percent compared to the Stanley.

I used a long tapered metal screwdriver I had hit with the grinder wheel and a blow torch(small kitchen variety) to heat for 5-7 secs. I tried using boiled water in a cup but the screwdriver was too thick and the heat was lost after about 4"'s of channelling.

Using a boiling water pot to reheat the tip would reduce the chances of volatiles being released from overheating the tip, slightly longer but safer I'd guess.

A good size round skewer with an insulated handle might be perfect.

Normal reset on the cold press required etc.

The heat loss using the water in a cup was due to not being able to heat the whole length of metal. I would guess if you can immerse the required length of metal in near boiling water and re-press the lines in one go it would work very well.

Last edited by mura_gadi; 11-02-2022 at 02:53 PM.
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  #490  
Old 12-02-2022, 09:57 AM
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selecting materials for dob bearings

Hello,

Just a note on picking laminates and sliding and static frictions etc. I've been trying to think of a test rig that would be easy to try various laminates with small weight on a telfon base.

Using angular friction will give you an easy way of calculating the difference between sliding and static friction. Just means you need a ramp that you can measure the lift on or a "side by side" test of what starts to slip 1st.

**I was going down the path of pulleys and weights over a table edge etc... Using the angular friction might make getting a few bits of laminate tested at a store/workshop a lot easier. A clipboard and a inclinometer would be good, or a simple a good sized board and a ruler to measure the height with.

Easy way to get the best laminate available for you. If you're in a shop with test boards, your teflon based weight and an inclinometer, life could be sweet. At least you might get to test a few more laminates before being asked to leave...

Ps. A good bench mark to test against would be to record the incline need against an old LP record, which are a good cheap start to beat for the azimuth bearings on smaller dobs.

*********************************** *****

Part 2: I wanted to incorporate vibration/knocking into the "sled" to help reduce the static friction grip. Turns out "Jaycar" has a little pill case with a vibrating alarm. It takes 3AAA batteries and has a compartment for pills/weights and is pocket sized.

There normally around $12, Jaycar is clearing them out at $5 so I'm off to get one to try later. Should be a very close to useable after a few teflon feet, depending on the power of the vibrator. - Missed the sale, thought to make the sled with a small PC fan with one side of the blades removed.

Last edited by mura_gadi; 16-02-2022 at 06:01 PM.
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  #491  
Old 14-02-2022, 08:49 PM
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2x12" mirrors up in Townsville NQ

My current project is 2x12" mirrors that will be flexed for a binocular.
Hopefully they will work.
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  #492  
Old 26-04-2022, 04:48 AM
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Star test rig (a good use for old newts)

My optic tester got a nice step forward for my newt.

Bought a solid tube GSO newt the same diameter as my blank, old and trashed for $50.

Plans are basically cut out the middle section. 2mtr poles attached to the base and have the secondary cage slide as required. Gives my star tester a mirror cell, bob's nobs and a full secondary cage and hopefully rapid acclimation for $50 and some dowel/brackets.


Ps. GSO and SW are 200mm and 203mm respectively in the 8". SWer's optical assembly would be better as you can always shim down.

Last edited by mura_gadi; 27-04-2022 at 04:29 AM.
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  #493  
Old 07-10-2022, 07:17 AM
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Back to the polishing . Still have TDE on my little mirror. I have gone back to the glass tool and Al. Oxide with short circular strokes, alternating with the star lap in the centre and the lines are creeping toward the edge. I would go back to grinding but worry that I will end up back in the same position, but I suppose if I use the same straight circular stroke with the glass tool and step up a grade from aluminium oxide I will get there quicker.
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  #494  
Old 08-10-2022, 12:02 PM
Rod
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Hi Ray

Pleased to hear you have found a mix of strokes that are dealing with the edge. However, I’m worried about you polishing with aluminium oxide. How do you plan to clean your lap when you are ready to return to cerium oxide? Any residual grit will likely cause scratches and sleeks.

Rod.
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  #495  
Old 11-10-2022, 07:33 AM
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Thanks Rod. I thought of that as soon I started. The little star lap is disposable I suppose and I was intending to replace my pitch as it is the tar based one and Marc has given me some synthetic pitch. I should not use the oxide on my full size lap as you say.

The aluminium oxide is 1 micron 14000 grade . I have some 800 grade to see if I could speed it up with the tool and then repolish .

I would like to make a small kick wheel which would speed up the polishing, like the Belgian optician with his tiny double mirrors.

Quote:
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Hi Ray

Pleased to hear you have found a mix of strokes that are dealing with the edge. However, I’m worried about you polishing with aluminium oxide. How do you plan to clean your lap when you are ready to return to cerium oxide? Any residual grit will likely cause scratches and sleeks.

Rod.
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  #496  
Old 11-10-2022, 08:46 AM
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+1 on the kick wheel, I have a pergola as my wood supply, plenty of good wood shows up in gumtree amazingly cheap.

I'm planning on sandwiching bricks between the two boards(discs) over one concrete slab for the base weight. Should be easy to disassemble and relocate if needed.

Lots of people sharing good ideas on youtube, also mentions which bearing types you need...

https://www.youtube.com/watch?v=iJEr1M9QGGc

Ps. If the weather ever cracks 17degrees again I can restart my blank...

Last edited by mura_gadi; 11-10-2022 at 12:43 PM.
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  #497  
Old 11-10-2022, 06:32 PM
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I was thinking more like crank arms and treadles with a lighter flywheel but there is a single bar design The Leach Wheel, which is the gold standard kick wheel for pottery.

https://makezine.com/article/craft/t...treadle-wheel/

Quote:
Originally Posted by mura_gadi View Post
+1 on the kick wheel, I have a pergola as my wood supply, plenty of good wood shows up in gumtree amazingly cheap.

I'm planning on sandwiching bricks between the two boards(discs) over one concrete slab for the base weight. Should be easy to disassemble and relocate if needed.

Lots of people sharing good ideas on youtube, also mentions which bearing types you need...

https://www.youtube.com/watch?v=iJEr1M9QGGc

Ps. If the weather ever cracks 17degrees again I can restart my blank...
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  #498  
Old 11-10-2022, 07:53 PM
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Looks like the mustard, vnc.

I'm building mine with a view of it living outside, a removeable top and easy to hose down, and probably a lot closer to my level of handy man work... I am certainly KISS when it comes to making stuff, tbh.
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  #499  
Old 29-12-2022, 07:05 PM
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Applying finer grits

Hello,

Thought I post this as I found it very useful and easy to apply.

Previously I have been using the squirt bottle and fine grits 1600+ using a milky mixture and spreading the mix out by finger tips. Then slightly lifting the tool/mirror and twirling to even out the grits out further. It was fairly obvious that even with the twirl the grits were still concentrated in places. As the wet progressed it become more obvious and the wet normally stopped due to the areas of concentrated mix.

Using a thicker mixture closer to a single pouring cream and place a coin sized puddle# in the middle of the mirror, lower the tool down to spread the mixture out to the edge. I have a glass tool so it very easy to control the spread out visually and found that the mix was very evenly distributed. The wet lasted a lot longer due to not having concentrated mix areas. Using this technic I was able to use 14000 grit as the final pre-polish grit and that sped up the flash polish by a huge factor. *

Another bonus is the initial hydroplaning dissipates a lot faster and the grits start to work a lot sooner. A technic well worth trying imo.

Wouldn't work with tiled tool, but a +1 for a glass tool.


Steve
*This is not my idea but I can't find the text I was reading to quote the original author
#I was using about a $1 coin size puddle for an 8" mirror.


Ps. Well I have been polishing for under three hours now, mostly MoT and the mirror passes the laser test to within about 15mm of the edge. Will change over to ToT for the next session. Well under 3 hours to pass the laser test start to finish so far, (Big smile), so much less time than it took me from 2000 grit first time round.

Might get the first born under the tester before I go back to work in the New Year... Will keep going on the polishing after I get a good laser test edge to edge as the inverted 25mm EP still shows the occasional spot.

Hopefully TDE will be bearable and not take an age to work out.

Last edited by mura_gadi; 31-12-2022 at 02:22 PM.
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  #500  
Old 05-02-2023, 01:08 PM
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Sounds like it works. I am still using the squirt bottle but swirling around seems work to spread the grit.

I also went back to 14000 pre polish to fix my turned down edge using the circular edge technique and that was working all be it slowly. So I built the rotating part of a mirror o matic 14/ 20 and back to 800 grit, fixed problem in an hour. Then went slightly too deep then back to slightly to shallow in 20 min. Very effective short circular motion on the rotating platter at 35rpm for getting TDE down and the polish is still good enough to test.

Next , install the mirror o matic arms and on to the 12 inch once I can work out how to figure the 4 inch F4.
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