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  1. #11
    John,

    Can you give us an update on the results of the first pour?

    In particular - how is the alignment / flatness of the bearing mounting plates?

    I've been doing a lot of reading, and am pretty sold on epoxy granite due to the excellent damping characteristics.

    If I can cast in a linear bearing mounts with acceptable accuracy at home, and avoiding the need for post-machining, I'll be out the door for 20-40kg of epoxy.

    Also, can you share how much the Megapo

  2. #12
    Hi Pippin

    I will be posting the results soon, my workload has piled up so have not had much time for the machine.

    I have managed to prepare and undercoat the casting, there were more blemishes than I would have liked, I filled them with body filler and the part is now undercoated. The part is very massive too heavy for me to lift. Tapping it with a hemmer yields a dull click only, even on the metal parts. the damping propertied of mineral cast epoxy is clearly evident

    A prelim check with a straight edge indicates that the cast in position rail mounts are accurately located and co-planar . By eye I was unable to see a gap. However I will feel happier when I have tested them properly. Now that the part is clean I will set it up on jacks on surface plate to take some proper measurements.

    Regards
    John

  3. #13
    Hi All

    I have reviewed the accuracy of the casting and am very pleased. Link below.

    https://www.model-engineer.co.uk/for...?th=139042&p=2

    The maximum error is 0.0018" as cast, that will be reduced by lapping to high accuracy there is not a lot of metal to remove so the process will not take a lot of time.

    Regards
    John

  4. #14
    Thanks for the update John. Nice results.

    So, do you reckon it's best:

    a) To get ground stock (can be bought ground, but suspect $$$), predrill (most) holes and cast against surface plate as you have done

    or

    b) Use mild steel bar stock, cast in standard mold, not requiring surface plate, and pay someone with a big cnc mill to quickly machine the steel and drill the holes?

  5. #15
    Hi Pippin in answer to your questions A and B.

    Question A. “To get ground stock (can be bought ground, but suspect $$$), predrill (most) holes and cast against surface plate as you have done”

    I considered precision ground stock and phoned around to locate a source, it was available at a cost, quite a lot actually; however the supplier would not guarantee flatness on longer lengths. They would provide a tolerance for constant thickness but not flatness, however for this project only one side has to be flat the other side is buried in the casting.

    Laser cutting precision ground stock which is normally a water or oil hardening carbon steel is likely to make any hole or laser cut edge very hard. Also my laser cutter charges extra for precise positioning small pieces of material, they prefer to work with large plates. This hardening also occurs to a small extent with mild steel, making it hard going for high speed steel tools however I did not have a problem with HSS on the steel supplied by the laser cutter.

    If I was able to locate precision ground stock with a guaranteed flatness it would have to be machined to size and the 16mm holes for the cam pins would have to be drilled. These holes do not have to be located to high accuracy so marking out and centre punching in the normal way would be fine. The cams will compensate for any small errors.

    The 16mm thick standard mild steel plate supplied by the laser cutter has a reasonable surface finish free of the normal flaky scale you get on black bar stock, there is a black oxide coating and some mill marks. After cutting it is not flat there was heat distortion, bowing of the 16mm axis in particular. I mechanically straightened the pieces and then sent them to a firm with a surface grinder. This was not a success as they did not completely flatten then pieces. I suspect their operator was not up to the job. The big enemy is the magnetic chuck. Grinding long pieces requires packing the work so the chuck does not pull any bowing down before grinding a side then, then turning it over and repeating the process for the other side. This process may have to be repeated several times until any stresses are relieved and the piece will lay flat on the surface table. Even better would be to have the part stress relieved professionally another cost.

    In the end I hand lapped the errors left by the surface grinder. As the post above shows there were some residual errors.
    I have been working on them with the lap and now the maximum error is .001” over 800mm testing the part on the surface plate with an indicator. I am confident I can get that down to a tenth or so, however just turning the part 90 degrees would probably change that, gravity is a universal force. I guess I could set the part up on its airy point’s that would possibly reduce the measurement error however temperature is also a variable. My workshop has a skylight. We happened to have a hot day yesterday a high of 33 today the high was 20. As expected my error measurements changed with the sun shining on the part; the top face was heated and the face bowed up. Digging the depths of high accuracy is an interesting challenge.

    Question B. “Use mild steel bar stock, cast in standard mold, not requiring surface plate, and pay someone with a big cnc mill to quickly machine the steel and drill the holes?”

    That will work no problem, apart from finding a contractor that has a CNC machine and equipment that can handle large heavy parts and cost.

    It was tempting me to take that option, were it not for the desire to to find out what is possible using a surface plate, small lathe and basic tools. The laser cut mold is optional it could be made of MDF for one time use. The machine does use laser cut parts, they could also be made by hand a large time cost. If I manage to make a workable machine and demonstrate a method that worked I hope others will maybe create their own designs and contribute improvements to the methods I have used for this machine.

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