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| Making a granite surface plate |
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| Colh:
Thanks Rob and Jason for your comments. Lew for you thoughts and the book lead and Phil, Ade for the book search and the link. I will first have a go at checking flatness of the pieces of granite. If they are acceptable then nothing further to do except make a box with lid to house one of them, keep one as a spare and maybe gift the other to someone in need. Will let you know how the measuring goes. I downloaded Franklin D Jones' book and have some reading to do. A quick scan shows a lot of information and is what I was looking for. Ade How is the QCTP going? Followed your thread and have one the same on the To Do list. Col |
| AdeV:
--- Quote from: Colh on November 17, 2010, 01:11:44 AM ---Ade How is the QCTP going? Followed your thread and have one the same on the To Do list. --- End quote --- Col, I've not done any more to it since mounting it on the lathe, other than making a couple more tool holders. It's doing really well & has made it MUCH easier to change tooling - even if I am having to "multitask" some of the toolholders at present... One day, I'll get around to making the handles for it.... at the moment, I just use the bolt & stud. It seems to work OK like that. In certain circumstances, it does have a tendancy to be pushed around by the cutting forces, e.g. if using a form tool. It's OK if I really wind down on the clamp bolt - but the clamp is only an M10 stud; I plan (at some point) to increase that to something much more substantial - maybe 1/2" or more... but that means putting the old toolpost back on.... so I'll probably just wait until the stud breaks before I do anything about it... |
| Colh:
Hi Guy Checked the granite plates using the 300mm ruler from a Rabone CHesterman combination square set (set barely used, ruler still in orginal paper sleeve) as a straight edge and feeler gauges. Not looking good. The plates appear to be concave in one axis probably due to the polishing machine being out of tram. The worst had gap of around 4 thou in the middle of the 300mm rule with the best being about a thou. After reading what Franklin D Jones had to say, I think I might have a go at making 3 500mm straight edges (to reach across the diagonal of the plates)by fixing all three together, milling one side then scraping A to B & C, B to C, etc as per Mr Jones' writing. I will then use one of the new straight edges to test the plates again to confirm original checks. I think I will have to lap the plates against one another, again as per Mr Jones. Given the amount the plates are bowed I think I will start with the 240grit silicon carbide and then something finer to finish. Given that most things used on the plate will be in the order of 10s of mm long then the course finish provided by the course silicon carbide is negated in the first instance. A finer finish will be required if I want to use bearing blue to test flatness. May be a while before I get the this as I have to tram my mill and then make one of Bogs' tramming tools (two new DTIs should arrive next week while I am away for work). I am also half way through making a Harold Hall designed grinding rest. Will continue the thread when progress is made. Col |
| GerryB:
G.Day Cohl, You probebly have attempted by now the Grinding together of your Granite Plates and found that it will not work. I read a reply ferther up on getting in contact with an Astronomical Group who grind their own Mirrors. But they do not want the surface to be flat but one surface that will endup as the Mirror is Parabola surface as this is what is required. Two blank peices of thick glass are ground together,one on top of the other. The top glass results in the required mirror and the bottom peice is discarded. Due to the physics of the operation when two pieces are ground together you will not end up with nice flat surfaces. I don't know of any way that you might obtain a decient surface other than getting it done commercialy. Gerry |
| John Hill:
--- Quote from: GerryB on February 26, 2011, 11:40:23 PM ---G.Day Cohl, You probebly have attempted by now the Grinding together of your Granite Plates and found that it will not work. I read a reply ferther up on getting in contact with an Astronomical Group who grind their own Mirrors. But they do not want the surface to be flat but one surface that will endup as the Mirror is Parabola surface as this is what is required. Two blank peices of thick glass are ground together,one on top of the other. The top glass results in the required mirror and the bottom peice is discarded. Due to the physics of the operation when two pieces are ground together you will not end up with nice flat surfaces. I don't know of any way that you might obtain a decient surface other than getting it done commercialy. Gerry --- End quote --- Gerry, I believe you are right regarding grinding two surfaces together in that you will end up with a portion of a sphere, however if you grind three together giving equal time to each pairing you will wind up with three flat surfaces. There are only two shapes of surface that will fit and slide in all directions, flat and sphere. |
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