Liberty/Gemtec booster threads

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ar15eric
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Liberty/Gemtec booster threads

Post by ar15eric » Fri May 12, 2017 9:27 pm

Hello, this is my first post on this forum, but I have been active on F1S board.

Is anyone here knowledgeable about making custom threads? I'm trying to make a custom thread profile for Fusiom 360 of the Gemtec/Liberty booster. There is a drawing here that has some of the info -

http://libertycans.net/wp-content/uploa ... T-REV0.pdf

Using the info from here about thread design -

https://www.fastenal.com/en/78/screw-thread-design

I tried to get the thread dismemsions in the XML file of the following -

Code: Select all

    <Designation>
      <ThreadDesignation>Gemtec Booster</ThreadDesignation>
      <CTD>1.180-24</CTD>
      <TPI>24.0</TPI>
      <Thread>
        <Gender>external</Gender>
        <Class>3A</Class>
        <MajorDia>1.179</MajorDia>
        <PitchDia>1.151</PitchDia>
        <MinorDia>1.123</MinorDia>
      </Thread>
      <Thread>
        <Gender>internal</Gender>
        <Class>3B</Class>
        <MajorDia>1.184</MajorDia>
        <PitchDia>1.155</PitchDia>
        <MinorDia>1.132</MinorDia>
        <TapDrill>1.125</TapDrill>
      </Thread>
    </Designation>
  </ThreadSize>
Does this seem right or am I off?

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yondering
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Re: Liberty/Gemtec booster threads

Post by yondering » Fri May 12, 2017 9:31 pm

I have cut both male and female threads for the Liberty booster (making male FBA and female threads in a Form 1). I'll have to do some measuring this weekend, I forget what the dimensions were now.

Edit - misunderstood what you were asking, and shoulda looked at your first link before replying. Yes, those dimensions do look OK. I suggest though you might make the major diameter just slightly smaller, maybe ~1.176-1.178 to allow for error; there's no real advantage to being at the high end of the spec on major diameter for a male thread, and it can make threads feel tighter than they really are if you're slightly over.

ar15eric
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Re: Liberty/Gemtec booster threads

Post by ar15eric » Fri May 12, 2017 10:17 pm

yondering wrote:I have cut both male and female threads for the Liberty booster (making male FBA and female threads in a Form 1). I'll have to do some measuring this weekend, I forget what the dimensions were now.

Edit - misunderstood what you were asking, and shoulda looked at your first link before replying. Yes, those dimensions do look OK. I suggest though you might make the major diameter just slightly smaller, maybe ~1.176-1.178 to allow for error; there's no real advantage to being at the high end of the spec on major diameter for a male thread, and it can make threads feel tighter than they really are if you're slightly over.
Would you be able to verify it against your measurements? I don't know if liberty and Gemtec hold the same tolerances and I would like to do something that would accommodate both if possible.

ar15eric
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Re: Liberty/Gemtec booster threads

Post by ar15eric » Fri May 12, 2017 10:19 pm

I guess I should also explain how I came up with those dimemsions for the XML even though the drawing doesn't list all of them. I read from the fastenal page that pitch should be the halfway point between the major and minor. So I made the assumption with this math -

1.179 - 1.151 = 0.028
1.179 - (0.028*2) = 1.123

For the internal thread I noticed on fusion that thier 3A/B threads had a difference between internal/external of about 0.004 in the major and about 0.009-0.01 on the minor.

Does this sound right?

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yondering
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Re: Liberty/Gemtec booster threads

Post by yondering » Sat May 13, 2017 11:18 pm

Ah, I see. It's actually easier than that - look at a thread chart for other standard thread sizes to determine thread depth for that pitch, the difference between major and minor diameter is the thread depth. We're talking about 24 tpi, so look at the major and minor diameter for 5/8-24 for example - the double thread depth for 24 tpi is .0511". That is constant regardless of diameter.

Using that, your minor diameter is
1.180-.0511 = 1.1289"

You do need to use 1.180 for the major diameter, not the "major diameter tolerance" which is what you turn the OD to before threading.

I don't have an easy way to measure minor diameter on the male parts I threaded, but it's not something you need to know for threading anyway. It is useful for strength calculations though.

BinaryAndy
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Re: Liberty/Gemtec booster threads

Post by BinaryAndy » Thu May 18, 2017 3:49 am

There is a ridiculously useful online thread calculator here: http://theoreticalmachinist.com/Threads ... erial.aspx
It will give you all of the dimensions and tolerances for threads of arbitrary diameter and pitch. It will even give you the min/max and wire size to use for measuring external threads with the three-wire method.

It's interesting to me that Liberty's thread tolerances are just slightly tighter than the calculated 3A spec.

Your numbers for major, minor and pitch are right on the money, all near the middle of 3A/3B tolerance. If you figured all that out from that Fastenal page, I'm impressed. I'm not sure what Fusion actually does with the tap drill number, but I would think that should be the same as the minor diameter.
Andy Gamble
Binary Arms
07/02

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