Aluminum screw in stainless bushing. What thread depth to use to select tap drill?
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    Default Aluminum screw in stainless bushing. What thread depth to use to select tap drill?

    Duplicating these two parts. The assembly is a air valve control on a paint gun handle.
    The long screw is aluminum because of weight savings. This Sagola brand has the air caps
    as all aluminum. In contrast a Devilbiss has a stainless air cap. Weight makes a difference
    if your arm is at 90 degrees all the time ...

    dsc_1127.jpg

    The stainless hex bushing will be drilled or bored and then run with a M9-1.0 tap D5.
    The tap drill charts call out the size for the tap drills as 75% or 50% engagement.
    For aluminum (75%) the drill would be 8mm. For stainless (50%) the drill would be 8.4mm.

    Wear? I think the original aluminum screw is anodized and I have kept some silicone free grease on the threads.
    How tight can a thread like this be until the hardness of the stainless becomes a wear factor on the anodize?

    If the aluminum screw was not plated. A bad design?
    Last edited by rons; 09-10-2021 at 06:53 PM.

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    I left something out.

    For aluminum (75% engagement) the tap drill is 8mm

    For stainless (50% engagement) the tap drill is 8.4mm.

    I used a 5/16 end mill (.3125). Eight millimeter is .3149. So it's close.
    If it's not then I could use the stainless piece as a die and get whatever fit I want.
    That's another reason I want to do some of my own plating and see what kind of good
    fits I can get in the future.

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    To admin, thought this question was worth asking. Obviously it's too weak so just remove all this. Will make room for more jokes elsewhere.

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    Quote Originally Posted by rons View Post
    Duplicating these two parts. The assembly is a air valve control on a paint gun handle.
    The long screw is aluminum because of weight savings. This Sagola brand has the air caps
    as all aluminum. In contrast a Devilbiss has a stainless air cap. Weight makes a difference
    if your arm is at 90 degrees all the time ...

    dsc_1127.jpg

    The stainless hex bushing will be drilled or bored and then run with a M9-1.0 tap D5.
    The tap drill charts call out the size for the tap drills as 75% or 50% engagement.
    For aluminum (75%) the drill would be 8mm. For stainless (50%) the drill would be 8.4mm.

    Wear? I think the original aluminum screw is anodized and I have kept some silicone free grease on the threads.
    How tight can a thread like this be until the hardness of the stainless becomes a wear factor on the anodize?

    If the aluminum screw was not plated. A bad design?
    For a rule of thumb the minimum screw engagement is 1 1/2 times the diameter of the screw. This allows for the chamfer at the end of the tapped hole. And the chamfer at the end of the screw for full engagement.
    As long as the mating parts are lightly lubed, wear is not a factor. Aluminum plated? Why?

    Roger

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    Quote Originally Posted by rons View Post
    Duplicating these two parts. The assembly is a air valve control on a paint gun handle.
    The long screw is aluminum because of weight savings. This Sagola brand has the air caps
    as all aluminum. In contrast a Devilbiss has a stainless air cap. Weight makes a difference
    if your arm is at 90 degrees all the time ...

    dsc_1127.jpg

    The stainless hex bushing will be drilled or bored and then run with a M9-1.0 tap D5.
    The tap drill charts call out the size for the tap drills as 75% or 50% engagement.
    For aluminum (75%) the drill would be 8mm. For stainless (50%) the drill would be 8.4mm.

    Wear? I think the original aluminum screw is anodized and I have kept some silicone free grease on the threads.
    How tight can a thread like this be until the hardness of the stainless becomes a wear factor on the anodize?

    If the aluminum screw was not plated. A bad design?
    For a rule of thumb the minimum screw engagement is 1 1/2 times the diameter of the screw. This allows for the chamfer at the end of the tapped hole. And the chamfer at the end of the screw for full engagement.
    As long as the mating parts are lightly lubed, I doubt wear is a factor. Aluminum plated? Why?

    Roger

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    Quote Originally Posted by rogertoolmaker View Post
    For a rule of thumb the minimum screw engagement is 1 1/2 times the diameter of the screw. This allows for the chamfer at the end of the tapped hole. And the chamfer at the end of the screw for full engagement.
    As long as the mating parts are lightly lubed, I doubt wear is a factor. Aluminum plated? Why?

    Roger
    Well the Italians broke that rule or close to it.

    The screw in question is probably not going to be plated. If it starts to loosen up over time then plating would have prevented that.

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    Changing the tap drill changes how tall the screw threads are. A larger tap drill makes the flats on top of the threads fatter. Changes affect how much torque it takes to run the tap.
    It does NOT affect the pitch diameter of the thread or the clearance between the flanks of the threads. the perceived slop in the threads will be unchanged. The ultimate strength to pull the bolt thru the nut will be affected but nada else.

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    Here is some explanation.

    There are two paint guns. One came with an adjustment for air (bushing & alum screw). The other paint gun came with a plugged hole.

    I can copy the bushing & screw. The limiting thing in all this is that I could only find one 9mm tap. I tied to gently turn it into
    the existing bushing but it is not going through without a little effort. So if I make a second bushing with this tap the hole will
    be larger. The second aluminum screw might have to be larger and probably won't fit into the first bushing.

    Trying to get away from the old era where guns didn't have interchangeable parts. I mean old firearms.

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    I think you may be confusing the percentage of a full thread with the class of fit. The class of the fit will be determined by the die and tap or by your cutting depth if you single point them.

    This has nothing to do with the percentage of a full thread as determined by the diameter of the tap drill. A 50% thread can still be a very close fit while a 100% thread can be quite loose.

    The 75% of full thread is what is usually stated for the published tap drill charts. However, I looked into that number and exactly what it actually meant and discovered that it is only an approximate number, at best. The often quoted formula, diameter minus the thread pitch, will get you to the numbers in most published charts. But with many actual threads, you then must then select an available drill bit size and that can often change the actual percentage by as much as 5% or even 10% or more. And many sources do recommend as low as 50%. I think in your case anywhere between the 75% and 50% would work well. The 8mm tap drill would be the easiest to find and you probably already have one. An 8.4mm one may be a bit harder to locate or perhaps not at your location.



    Quote Originally Posted by rons View Post
    Duplicating these two parts. The assembly is a air valve control on a paint gun handle.
    The long screw is aluminum because of weight savings. This Sagola brand has the air caps
    as all aluminum. In contrast a Devilbiss has a stainless air cap. Weight makes a difference
    if your arm is at 90 degrees all the time ...

    dsc_1127.jpg

    The stainless hex bushing will be drilled or bored and then run with a M9-1.0 tap D5.
    The tap drill charts call out the size for the tap drills as 75% or 50% engagement.
    For aluminum (75%) the drill would be 8mm. For stainless (50%) the drill would be 8.4mm.

    Wear? I think the original aluminum screw is anodized and I have kept some silicone free grease on the threads.
    How tight can a thread like this be until the hardness of the stainless becomes a wear factor on the anodize?

    If the aluminum screw was not plated. A bad design?

  10. #10
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    Quote Originally Posted by EPAIII View Post
    I think you may be confusing the percentage of a full thread with the class of fit. The class of the fit will be determined by the die and tap or by your cutting depth if you single point them.
    You reminded me that I have to periodically change the x position when using a partial profile bit. To sweep the flat on the bottom.
    When I don't do it right the fit is tight or won't fit at all. Why did I invest in the partial bits. Because I didn't want a big collection
    of full profile bits. Height is less important than flanks being nice and tight.


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