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  • friction fitting with dry ice

    Discussion in 'The main mechanical design forum' started by chofer, Jan 22, 2013.

    1. Twotenths

      Twotenths New Member

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      If you have an oven that you can set at 400 degrees F., The oven must be at temperature before you start and the rule of thumb is that you must put your piece in for 1 hour for each inch of thickness. If your plate is 1-1/4 " then you need the same amount in time as well. 1-1/4 hours. I would give it an extra 1/4 hour to be sure that it is heated evenly. The torch method is inconsistent and your piece will have different temps throughout. That can lead to warping or distortion of the plate. You can buy cans of liquid nitrogen that spray just like paint spray cans do. A .001 to .0013 is plenty for an interference fit at that diameter. You need to have everything perfectly set up to assemble because you will only have a very limited time to insert it. We are talking in seconds here. As soon as the materials come together it takes only a few seconds before they match temperatures. The bigger the temperature difference the longer you have to fit it. Too much interference will take those seconds away and you will be stuck in the same situation where you must force it through. Remember that if you take the plate out of the oven and lay it on a solid surface the contact will suck the heat out of the plate very quickly. Put down some type of heat resistant insulation material that will slow the transfer down before you take it out of the oven. We used to put pins in the freezer for a few hours before using the nitrogen spray. It is really a 2 person job. One takes care of the hot plate while the other keeps the nitrogen spay going on the shaft. Once the plate is laid down then the pin must go in right away. Have a mallet handy so you can push it in as fast as you can.
       
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    3. chofer

      chofer Member

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      I machined my plate so it has an .0025 interference, I heated the plate to 450 in the oven and froze the pin in dry ice, both for 1 hour and 45 minutes. It came together beautifully.

      I thank everyone for their help!!!
       
    4. Twotenths

      Twotenths New Member

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      I am glad that I could help. I have done it before so I know the best way to do it for the best result. The fact that you wanted to keep the interference at .0025 it was wise of you to raise the oven temp and set the pin in dry ice. My rule of thumb for length of soak time is quite accurate. I use the same rule for preheating hardened steels before welding. If you don't preheat you run a great risk of cracking the block. We have a very large oven and it runs continuous at 550 F. That temperature seems to be the best for welding purposes. If you look at the materials data information on hardening temps 550 F seems to fit well for most steels along the tempering curve. The literature tells you to preheat D2 to 800-900 F. In my experience when you heat the it to that temperature the base steel will deform even if you don't weld it. Once it cools it will not fit the machined pocket anymore. The steel distorted and you need to regrind it to fit. It has grown in size by a few thousandths of an inch and if there is an opening in it for cutting you will find the clearance has increased by the same amount as the block grew. I did a lot of R&D on the correct preheat temps. They are different for each type of metal but 550 F seems to work well across almost every steel except O1. If you want to keep a high RC number on O1 you cannot go above 400 - 450F or you will start to drop the hardness down from 62RC. 400F won't change it but 450F will drop it to 60RC and 500F will take it to 58RC. It is the only exception to the rule. For a proper welding technique you must preheat with my rule of thumb soak time. You weld it hot, When you are done you let it cool to below 200F. Easy to tell that with a drop of water. If it bounces across it is still above boiling. When a drop just tears and evaporates you are below 212F. You must then reheat it to 550F again full soak time to take the stress out of the welded material and the base metal. It will bring the hardness of both materials equal to each other. You take it out and let it cool slowly. No air blast to cool it or cold plate to lay it on either. Just use a few fireplace bricks to set it on and let it cool naturally. That will give you the best result. It was something that I forgot to mention in using an insulation material to put you hot block on. We always keep a small pile of fireplace bricks around just for that purpose.
       
    5. mvalenti

      mvalenti Well-Known Member

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      OK repost with pix! we love pix! Glad it worked for you!
       
    6. chofer

      chofer Member

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    7. ctrsz

      ctrsz New Member

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      heat on the base will make the hole smaller due to the expension of the steel.. that is why you failed. you can try cooling both the pin and the base
       
    8. anddrevvw

      anddrevvw New Member

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      Heat depends on the surface area of the plate.If the surface area is big then the change of temperature will take short time until to reach equilibrium.
       

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