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Thread: Compounds for lapping

  1. #1
    Boolit Grand Master

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    Compounds for lapping

    Ive been seeing a lot of posts on lapping polishing a mould and what to use so thought Id make a quick post.
    Lapping polishing compounds are by nature abrasive and vary as to use and grit. Lapping is a precision finish and can produce sizes to exacting size finish roundness. In practice a soft material in our case lead is impregnated with the compound and becomes a cutting tool. Lead or brass ill cut hardened steel in this manner. In tis earliest uses copper blades with sand cut stone blocks.

    the various compounds are
    Lapping compounds
    Silicon carbide, usually a green color. This is for carbides, steels hardened and soft, Grit sizes run from 80-1000 grit and bought by the grit size. Breaks down to stay sharp and can provide some very fine smooth finishes. It comes dry water based or oil based.
    Aluminum Oxide, usually a yellow color. this works best in softer materials aluminum brasses coppers and soft steels. Grit sizes run 80-1000 grit and again bought by grit size. Breaks down slower and is a harder abrasive. it also produces very fine finishes. Comes dry oil based and water based also.
    Valve grinding compounds are usually a form of silicon carbide compound oil based and running around 80 grit for coarse and 120 grit for fine. fast metal removal and decent finishes can be obtained but amount of stock removal makes it use in moulds limited.
    Diamond lapping compounds. color designates concentration of the diamond. These are used for very hard materials carbide glass and fine polished finishes yellow is the more popular all around here. It takes very little diamond to do a area. with a nylon brush in a dremil it will make mirror finishes.
    Simichrome and Flitz are metal polishes that when used as lapping compounds will provide nice finishes with little metal removal
    Tooth Pastes are a very light polish that work again for fine finishes and little metal removal.
    Comet barkeepers friend and scouring powders I class with the valve grinding compounds as being on the coarse side for this work.
    One I have been using to hand polish parts with ( a rag or towel and a dab on it) is nufinnish automotive polish. I suspect this will be similar to the simichrome or flitz maybe even a little finer.
    Automotive rubbing compounds tend to be water based. They run in the 1000-1500 grit range and normally are used to remove light scratches from Paints and Clear coats. They work here for fine finishes and little stock removal.

    In use impregnate the lap by spreading a light coat of compound on a steel plate and lightly roll lapp between it and another steel plate this impregnates the compound into the surface of the lapp.

    Remember in holes .0005 on a side stock removed equals .001 on the dia.

    In addition to the above compounds there is a way to use the grinding ash that builds up around your bench grinders also

    In a container of light oil gently sprinkle the grinder residue on the top of the oil
    At 5 mins gently pour off the oil whats in the bottom is the biggest coarsest grit.
    At 15 mins again pour off the oil into a container this is slightly finer.
    At 30 mins pour off
    at 1 hour.
    at 2 hours. You can continue this for the day and end up with your own lapping compounds from something normally thrown away. The Older Machinists Handbooks have this procedure in them to follow. It does work. I have found you et more of the finer grits from the surface grinder than the bench or pedestal grinders.
    Last edited by country gent; 04-25-2018 at 05:03 PM. Reason: addition

  2. #2
    Boolit Master

    Hickory's Avatar
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    If I have to lap a mould I will use Arm & Hammer tooth paste or Flitz.
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  3. #3
    Boolit Grand Master

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    It depends on what I need to accomplish. If the mould is casting small I start with a compound usually silicon carbide in 320 grit or finer. and finish with simichrome or tooth paste. All my compounds are oil based. So as I lapp I occasionally add a drop of oil.

  4. #4
    Boolit Master
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    I have only ever lapped two molds (one lee and one beat up H&G but found the comet/water trick worked well on the lee and the flitz worked well on the H&G.

  5. #5
    Boolit Master
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    Thanks gent -- you gave us a lot of info there !

  6. #6
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    I use 600 grit Clover compound.

    still gives me shiny smooth bullets without going to a higher grit (gotta love those Lee DIY molds)
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  7. #7
    Boolit Grand Master

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    Ill get out Dads older Machinists handbook out later and see if I can find the actual process for making and grading lapping compound. I don't think its In mine ( 1980 version) Dads is late 50s so it might be there. If not Grandpas does have it His is early 40s I think. Grandpas Machinist handbook has nothing on vee belts or vee drive pulleys in it. In reality in most shops doing lapping 600 is on the fine side. As you lap a piece the grit brakes down from use becoming finer a you go.
    Last edited by country gent; 04-26-2018 at 09:00 PM. Reason: addition

  8. #8
    Boolit Master
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    Most lapping compounds will work best if thinned down with some light oil.
    The clover compound works best for me if the compound is mixed with some ATF oil so the lapping compound is very thin in the oil mixture. Mix in a small plastic squirt bottle, add a nut to aid in shaking to mix when ready to use the compound.

  9. #9
    Boolit Master D Crockett's Avatar
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    thank you Country Gent that is info that most if not all of us that have cast for years can use I have a few lee molds for my black powder that could stand a little cleaning up D Crockett

  10. #10
    Boolit Master Maven's Avatar
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    Thumbs up

    [country gent's] posts #'s 1 & 12 should be made a sticky!
    Last edited by Maven; 04-27-2018 at 03:27 PM. Reason: new information

  11. #11
    Boolit Master
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    Quote Originally Posted by Maven View Post
    [country gent's] post #1 should be a sticky!
    X2, this thread is a keeper!
    ~ Chris


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  12. #12
    Boolit Grand Master

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    Read thru the procedure for grading making lapping compounds grits last night in Dads 1954 edition of the machinist handbook. Its a basic easy procedure. This will allow you to make your own lapping compound.

    Run your "grit" thru a mortar and pedestal and break it up.
    The handbook calls for lard oil or sperm whale oil for the next steps but one of the light oils Olive ( this is called for grading diamond dust) canola or JoJoba should work well also.
    Pour oil into small bowl and grit on top of it. Let sit undisturbed for 1 hour and gently pour off into a second bowl, you dont want to disturb the settled material. This is graded as 0 or can be "washed" and ran thru the mortar and pedestal again.

    At 2 hours again pour off the oil into the first cup (cleaned out) this is graded as 1 and again can be used as a coarse grit or washed and ran thru the mortar again.
    at 3 hours pour of and this is grade 2 this is where the useable grits start
    Every hour pour off and label settlings as you go the longer it takes for the grit to settle out the finer it is.
    The last and finest useable grit grade is after the 8 hours every hour is what settles out till oil is clear

    This an be done with the dust from your bench pedestal grinder, The dust powder from sanding with emery paper, Some house hold items.

    When using these home graded compounds use the same oils as you did grading to lapp with them.

    So now those piles of grit on your grinder are no longer trash and have a second use. For those who recycle.

    I may try in the future to "Grade" comet scouring powder in this manner just to see what I get as to grades of compound. Another to try would be red rouge powder.

    Keep in mind the oils viscosity used will affect the grades of compound a thick oil Will hold the finer grades longer and a thinner oil will drop it sooner. Kerosene or mineral spirits may speed up the process some. But this allows the average guy who only needs a small amount of compound a chance to make his own in small amounts.

  13. #13
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    I have switched to using diamond polishing compound almost exclusively. I just got 3 tubes of 50 micron diamond paste which is approx 270 grit (very coarse) for $3.99. Diamond paste generally gets less expensive as it gets finer. I have tubes with differing grits that were also very inexpensive.
    Also available are diamond impregnated rubber burrs. I get the ones that go on dremel type tools. They can be shaped on the dremel like turning wood on a lathe. Forming them to the whatever shape needs polishing. They polish very quickly so care must be taken to not to take too much material off. They also come in little wheels that are color coded for grit size. I do not use them on aluminum because they are too aggressive. I use them to polish swaging dies. They will polish a hardened steel swaging die in seconds, depending on how fast the dremel is spinning.

  14. #14
    Boolit Grand Master

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    We had individual and Set of cratex diamond rubber polishing sticks and wheels in the tool crib at work. We used them to rough polish and break sharp edges on carbide parts. Finish polish was with a Foredom grinder cupped nylon brush and diamond paste. The latter would give a very smooth finish with no polish lines showing. On certain parts this was important to not scratch parts or to keep them from sticking in the part. The industrial diamond is much cheaper than the natural diamond. Even the natural diamond is reasonable since it comes from diamonds with flaws or imperfections making them in useable for jewelry. I find the natural cuts a little faster and cleaner than the man made.
    Hardened steel die details and hardened gage parts were polished with hand stones and wheels. Normally not the India or stones were used to. These stones were made from grits used in grinding wheels and softer bonds very open pores. They broke down and stayed sharp. a few passes on a sharp edge would leave a visible line showing. Most stones are to hard and load instead of breaking down. Once loaded they don't cut. If you want to see a big improvement over the "stones sold in most hardware stores, go to a supply store for machining and buy a few of the polishing sticks from them. They are a lot better.

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