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Thread: draw bench diy design

  1. #1
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    draw bench diy design

    Hey all,
    I am an engineering student studying towards a BEng degree. As part of my design course I am required to design a draw bench that is able to draw tapered brass tubes for french horns. The maximum diametre of the tube is 20mm. For the drawing I wanna construct a slider with two lead screws and a stepper motor at the end of the slider. For the tube I want to insert a tapered mandrel inside and I want to move the mandrel with the tube through steel die washers. The mandrel should be attached to a carriage moving with the lead screws.
    I have problems with selecting materials for the die washer, calculating thr draw force and therefore selecting the appropriate lead screws and motor. If I take the yield stress of brass, assuming that the drawing process requires a plastic deformation, I get draw forces around 10kN. This seems unreasonable since on a couple of youtube videos people were able to draw the tube by hand. I am really worried now since I cant start working on my project. Any help would be appreciated.

    Thank you!
    Sam

  2. #2
    Technical Fellow jboggs's Avatar
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    Do the processes shown on the youtube videos include impact, as with a hammer? That could be your difference.

  3. #3
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    I am linking the video for you, and I think not, they apply a quasi-constant force.

    http://www.engineersedge.com/video/D...-Machine_v1511

    The guy doesn't seem too exhausted by the process which suggests that the draw force cannot be in the kN regions.

    Cheers,
    Sam
    Last edited by Kelly Bramble; 11-24-2014 at 11:21 AM.

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    I do not have the background knowledge to comment on your calculated loading; however, by looking at their pulling assembly and making visual dimension estimates of the cable winch it appears to me that that assembly has an approximate 90:1 force multiplication from the handle to the cable (the combined drum to gear ratio of about 3:1 x a spool gear to handle gear ratio of about 5:1 x a handle gear to handle ratio of 6:1). This combined with the 2:1 ratio for their cable arrangement gives an about 180:1 force ratio from the winch handle to the pulling connection. Using this ratio your 10 kN pulling load would result in a 0.056 kN (12.5 lbf) turning load for the operator which I believe is not out of the reasonable range for someone doing this operation.

    This is a really rough estimate but it may help you resolve your quandary.

  5. #5
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    Thank you for your comment, it makes a lot of sense. I wanted to figure out how exactly they made that handle, and I was expecting a force multiplication of that kind, but with your detailed description I can now visualize it.
    Just to elaborate my problem. I am not trying to cheat on my coursework, we are getting graded on the CAD models which we design and we have been told to build the machine with that configuration. I mean with the lead screws and the stepper motor. My fundamental question here - after shedding some light on the pulley method - is: Why would it be beneficial for us to design a draw bench of that kind with precision screws? I suppose the pulley method is much cheaper and it requires less material as well. Is there any advantages that you can think of? Given the tube diameter and the functionality, meaning it is used for french horns, so it won't be frequently used. (I'm guessing that's what they meant by this information)

    Thank you again!
    Sam

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    I suspect that the main reason is that your instructor wants you learn about electric drives and their electronic controls as a part of this project; and, given our current exponential growth in electronics and robotic devices that is an valuable element in your education.

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    Thank you! So just to link this back to the original question. Is it then reasonable that with the leadscrew setup I need a 5 HP motor to do the same job as the cable assembly in the video? That means that the cost of the machine would be around 5 or 10 times higher and there would be no additional benefit to the powerscrew design. Benefits which are comparable to the extra costs. This makes no sense to me, there should be some similar design techinques for the stepper motor to achive a higher drawing force. This is where I got confused.

  8. #8
    Technical Fellow jboggs's Avatar
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    I have not seen the video but I do understand power train calculations. You say you need a 5HP motor. You need 5HP at the screw? Jalberts talked about various drive ratios. You can do the same thing with a screw drive. That's what speed reducers are all about. I used to have a professor that always told us you could lift the engineering building with a 1/4 HP motor if you have enough reduction.

  9. #9
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    Hey there,

    Although you haven't paid too much attention to my problem, you helped a lot! I could figure out the problem with your help!
    Thanks a lot, hopefully 20 years from now I'll be able to help others the same way.

    Sam

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