How to polish drawing dies after milling?


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    Default How to polish drawing dies after milling?

    What sort of tools and grits should I get for polishing 4140PH after milling for drawing dies? What are reasonable grit steps? Irregular shape with some 0.200" radius corners but overall opening is 5".

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    Community Moderator Jim Dawson's Avatar
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    Default Re: How to polish drawing dies after milling?

    Die maker stones. https://www.mcmaster.com/diemakers'-stones/ I always kept a dozen or so in a kerosene bath on my bench.

    How to polish drawing dies after milling?-stones-jpg

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    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: How to polish drawing dies after milling?

    I have a radius at the top of some plates and some small corners. Think a rectangular box with radius corners and a radius at the top edge. So I will need some round stones - they seem to come in a lot fewer grits and variations than square ones. The semiflexible rectangular ones have eased edges which seems like a desirable thing, Any suggestions on grit steps - I doubt I would need them all. If I get a good finish with milling is 320 a reasonable starting point? Then 800 and 1200 or more smaller steps? Most of the round jump from 320 to 800. I also noticed that many are oil filled but you said you keep them in kerosene so probably not the oil filled.

    I was thinking emery as it's flexible for the contours or small buffing wheel and compound, perhaps with a dremel tool.



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    Default Re: How to polish drawing dies after milling?

    Quote Originally Posted by Jim27 View Post
    I have a radius at the top of some plates and some small corners. Think a rectangular box with radius corners and a radius at the top edge. So I will need some round stones - they seem to come in a lot fewer grits and variations than square ones. The semiflexible rectangular ones have eased edges which seems like a desirable thing, Any suggestions on grit steps - I doubt I would need them all. If I get a good finish with milling is 320 a reasonable starting point? Then 800 and 1200 or more smaller steps? Most of the round jump from 320 to 800. I also noticed that many are oil filled but you said you keep them in kerosene so probably not the oil filled.

    I was thinking emery as it's flexible for the contours or small buffing wheel and compound, perhaps with a dremel tool.
    You can still use a rectangle stone; you just grind the shape you want on the stone, depending on the milling you may want 220 to start from 320 you will want 400 and 600 before you can go lower, you may find 600 is enough for a drawing die, yes you should only add the cutting oil when using these stones, soaking stones like this is not required, WD40 can be used it is not ideal but not as messy to use as the honing oils

    There is an art in die or mold polishing, you can rough with rotary wheels, for a drawing die it's not so bad but you have to follow the flow of the form when doing the finial finishing so the draw lines follow the flow of the form, or it will show on the part

    You can use wood sticks (like paint stirrers cut to the size / shape you need) when using compounds diamond lapping /polishing compounds work best
    Gesswein are the best stones for doing molds and dies they also have the correct oil to use, you can find some of their stones on Ebay, some being at a good price

    Oils that work well also is ATF

    https://www.gesswein.com/abrasives/f...SAAEgIdW_D_BwE

    Mactec54


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    Default Re: How to polish drawing dies after milling?

    I have seen CNC polishing a few places. Seems like that might be a good way to do it although for my case the direction would be perpendicular to the metal flow direction. Maybe do that and then do the final polish by hand in the flow direction? Have seen foam bobs that would require polishing compound and diamond embedded bobs with some sort soft backing. Getting grit on the machine is probably a concern. Here's a link to someone that does it on softer material. Any thoughts?





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    Default Re: How to polish drawing dies after milling?

    Quote Originally Posted by Jim27 View Post
    I have seen CNC polishing a few places. Seems like that might be a good way to do it although for my case the direction would be perpendicular to the metal flow direction. Maybe do that and then do the final polish by hand in the flow direction? Have seen foam bobs that would require polishing compound and diamond embedded bobs with some sort soft backing. Getting grit on the machine is probably a concern. Here's a link to someone that does it on softer material. Any thoughts?
    If you have a High-Speed Spindle, you can burnish the surface with a ruby or carbide ball no cutting compounds needed this is the best way for in machine polishing, you take micron passes until polished this also makes a harder surface on your part

    Mactec54


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    Default Re: How to polish drawing dies after milling?

    I have seen processes where a hardened ball is pressed though an aluminum or similar soft metal part to get a smooth and accurate size for a bearing or bushing. Very simple process using only a small arbor press. I also see that there are tools with multiple hardened rollers to accurate size and smooth tube bores. But it seems like spinning a hardened ball and moving that over the surface with the needed positional accuracy would be very difficult to do in a typical machining center. I could be wrong but the very small tolerances needed with a rigid tool seem very difficult to achieve and especially for a home shop. Perhaps some compliance could be designed into the shaft of the tool to make it workable without very high end equipment. An interesting approach. Is this a practical approach for a small shop?



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    Default Re: How to polish drawing dies after milling?

    Hi J27 - I've experience with polishing dies for aluminium tube drawing and automotive press tools. The smoother the finish the better and as Martec says the final passes need to be made in the direction of draw. We finished tools with diamond paste, I forget the grit was decades ago. So down through the paper grits to 2000 plus then move to SiC grit then to diamond. When trialing tools they did not have final finishes but it is noticeable how the diamond finish produces better surface finish to the prior grits. I expect it comes down to the lubrication film thickness. If the surface roughness is higher then the film thickness you have surface contact. Once the roughness is less then the film thickness then you have hydrodynamic lub so no metal to metal contact. The finish is a mirror finish to achieve this.

    The machinist use a tool like a jigsaw that held mop or stick and he set the tube dies in the lathe and it rotated slowly. That's fine for a round tool. The car parts are odd shapes so had to be polished by hand. takes time but patience gets you there. The use of surface burnishing is more about setting the surface into compression then the finish itself. If you have a tool that needs to make millions of parts they will fatigue and the surface will flake off at some point ruining the tool. Look up hertzian stresses. So by working the surface and creating a compression zone the tool lasts a lot longer. Same principle as peening welds. I expect they are still polished after setting...Peter



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How to polish drawing dies after milling?

How to polish drawing dies after milling?