How much extra to leave for machining?

Discussion in 'Pattern making' started by Quilbilly, Jan 13, 2022.

  1. Quilbilly

    Quilbilly Copper

    I am restoring a 1920 Stanley Steamer. I need to have new brake drums cast out of ductile iron for it. Finished they are 14" OD X 12" ID with a half in thick web. I am going to make the pattern and I know about shrinkage but I don't know how much I need to leave for machining. I was thinking a quarter inch all the way around. And help would be appreciated.
  2. Melterskelter

    Melterskelter Gold Banner Member

    That is a good question. I have never cast a brake drum or part very similar to one. If you can mold them so they cool more or less symmetrically I would not expect "much" distortion. I doubt you will need risers. I think I would try to sprue it on the center axis and maybe use 3 or four gates evenly spaced (120 or 90 degree locations). That said, it does not take long to machine off metal, but coming up short is frustrating.

    I would strongly suggest stress relief prior to machining.

    I think if you allow 1/4" on each machined surface you ought to be pretty safe. Maybe someone here will be able to speak better to that. I assume you are having a commercial foundry cast them. Do they have any suggestions?

    I did find this blog concerning the drums and see pics of the rather massive drum/hubs at about the fourth page. That is a heavy drum. The originals had to be grey iron. Are modern drums ductile? Somehow I thought they were grey.

    Last edited: Jan 13, 2022
  3. Billy Elmore

    Billy Elmore Silver

    We generally leave a minimum of .040" for machining. Any less and you get less than desirable outcomes due to surface hardness and surface defects. Usually between .040"- .062" would be good as far as machinability goes. If you have really coarse sand and castings are a bit rough you may want to add more or if you are prone to getting surface defects you may want to add more to get past them. As for slag and sand defects there is no magic depth to which they will disappear..if you have them they are usually throughout the entire casting, however they may be consistent from casting to casting, which will allow you to establish a base line depth for clean metal. In our case we are high volume and a minimal thickness to be removed is more profitable but we still have to make good castings so we have played around a lot with thicknesses just to see where we get the most bang for the buck. This is based off of thin wall castings. Thicker heavier parts with shrinkage issues may present more of a problem for internal inclusions but thicker parts are typically easier to machine in cast iron.

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