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  1. #1
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    heres a DIY drawbar force gauge

    just to be clear, this is meant as a conversation starter and to get some good ideas for improvements.

    its an alternative to an $800 clamprite

    this is a fun beginners hydraulic project, and a useful tool, and there are lots of different ways to skin this cat so i think most people would enjoy making it. its also cheap and is a good way to get into making proper bore seals with orings.

    DaOne came up with this design in another thread. I just machined it before he did hehe

    Im not absolutely sure it works correctly. i cant really calibrate it with my drawbar because my drawbar has unknown springs in it at this time. and just putting a heavy weight on it doesnt really test it as it might work in a spindle..any ideas?

    Materials:

    toolholder that is "hollow" (coolant pass through). no modifications needed.
    pullstud (gets modified)
    threaded rod (cut to length)
    "cylinder" (block of metal, custom made)
    "piston" (block of metal, custom made)
    "bridge" (block of metal, custom made)
    various common fasteners
    oring (standard sizes..buna n is fine)
    pressure gauge* (a high pressure gauge from a welding regulator might work and people usually have one or two lying around unused)

    *figure out how big you will be making the piston (probably driven by what size oring you will be using). then once you figure out the cylinder bore ID (driven by oring size, again), you will be able to calculate the cross sectional surface area involved. use the cross sectional area of the cylinder, not the oring or piston. then figure out what kinds of forces you will be likely to measure on your drawbar, and get a gauge that will measure that much pressure, corrected for the cross section of the cylinder. you could just use a 5000 psi gauge and be done with it, but that might be an expensive gauge and not provide much resolution where you will be measuring. or maybe you have a few gauges lying around and want to make use of them (i.e. high pressure gauges from a welding gas regulator..or what have you..) so just take all this into account when designing your particular setup.

    use one of the many online calculators or tables to design your oring bore seal. this is very straightforward to do and its all based on tables. brainless! this type of seal is basically a static bore seal since it moves so little. its high pressure though so you should probably not just throw it together and cross your fingers. calculate the correct compression and gland dimensions for your oring!

    you should put a little grease on your oring so it doesnt stick to the piston or cylinder. i used spark plug dielectric grease. that may or may not be a good idea, i just wanted to get it over with. but i suppose you should make sure its compatible with your oring material lol

    for instance: a 1 square inch cylinder would be 1.128" diameter. so for every pound of drawbar force you would measure 1 psi on the attached gauge. if your going to be measuring 1500lbs of drawbar force, you should probably get a gauge with some headroom ...how about an 1800psi gauge in this instance?

    Heres how it works:

    The "bridge", the pullstud, the threaded rod, and the cylinder, are all bolted together during use and become one solid piece, not designed to flex.

    In the picture, the bridge is the C shaped aluminum thing with two screws in it, the cylinder is the large block with the blind hole bored in it, and the pull stud and threaded rod are..well..pictured.

    The pullstud threads get cut off with a lathe. Then the pullstud gets welded to the threaded rod. Now its free to move in the toolholder.

    So when the drawbar pulls up on the pullstud, it ends up transmitting that force to the cylinder block, which squishes the piston and cylinder together (between the tool holder and cylinder block). That force is transmitted hydraulically to the gauge. Fill the cylinder block with water or something. Bleed out as much air as possible somehow.

    You need to machine a passage from the cylinder to the gauge, obviously.

    Ill be putting in springs of known constant into my drawbar soon so hopefully ill get to try it out and get numbers that make more sense.

    This design needs to be improved!! Its very easy to make, but id make the following changes:

    -make the "bridge" out of steel, and make it thicker. It ends up taking the full force of the drawbar, and its lame when it flexes.
    -figure out a clever way to make the system air-free or easily bleedable.
    -mount the bridge to the cylinder in a way where the screws will definitely not hit the tool holder! mine come very very close.
    -an overall guideline should be to make it so the movement of the piston during use is as little as possible. i.e. you dont want the pullstud to be coming out of the toolholder because that makes the measurement inaccurate. so bleed stuff of air and tighten the pullstud into the bridge to the point where there is no play between it and the toolholder
    -DaOne has suggested he would have done it using a lip seal. that may be a better way! although this oring way seems to work.
    -remember you need some volume below the piston when its compressed! you dont want it bottoming out! although i think technically that may be okay, but i left about 1/16" fluid underneath the piston when its in working position.

    Sorry i know ive done a terrible job explaining it but if you look at it youll figure it out.








  2. #2
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    I like what your doing, but think you have made a slight error in you math on the size of the piston. In order to have a 1:1 relationship of force and psi, you need a piston of 1sq in. not 1" dia. Area is pi * r^2 and doing the math, that makes the piston diameter equal to 1.128" unless I have also messed up!
    Art
    AKA Country Bubba (Older Than Dirt)

  3. #3
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    Quote Originally Posted by Bubba View Post
    I like what your doing, but think you have made a slight error in you math on the size of the piston. In order to have a 1:1 relationship of force and psi, you need a piston of 1sq in. not 1" dia. Area is pi * r^2 and doing the math, that makes the piston diameter equal to 1.128" unless I have also messed up!
    you're right i was just blubbering out all that as fast as i could. the piston on mine actually ended up at 1.25" diameter which comes out to about 1.23 square inches as you would calculate..ill edit the original post to correct it thanks

  4. #4
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    Nice idea
    I have always wondered what the drawbar tension was like in my Chiron, seems ok but it would be good to be able to put some numbers to it, think I will make one of these up tomorrow
    Hood

  5. #5
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    Quote Originally Posted by Hood View Post
    Nice idea
    I have always wondered what the drawbar tension was like in my Chiron, seems ok but it would be good to be able to put some numbers to it, think I will make one of these up tomorrow
    Hood
    thanks! its really DaOnes idea. I wish hed chime in i havent heard from him in weeks.

    Please take pics of whatever you make and/or measure and post so we can ogle at them!

  6. #6
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    I only managed to get a couple of hours spare today so never got it finished.
    I am thinking my drawbar force should be somewhere around 1200 lb-f so I made the piston 1/3 in² as the gauge I found is 6000 PSI full scale. It is a huge gauge and dont even know if it works or not, if not I have some 4000 PSI gauges which should just about do, as long as the drawbar force isnt more than I expect
    Anyway I did it slightly different, I decided to make it out of a round rather than a square/rectangle. I have also put in a hole so I can hopefuly fill with oil then tighten down the screw. Time will tell whether it works or not but pics below of what I have done so far.

    Hood

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    Quote Originally Posted by Hood View Post
    I only managed to get a couple of hours spare today so never got it finished.
    I am thinking my drawbar force should be somewhere around 1200 lb-f so I made the piston 1/3 in² as the gauge I found is 6000 PSI full scale. It is a huge gauge and dont even know if it works or not, if not I have some 4000 PSI gauges which should just about do, as long as the drawbar force isnt more than I expect
    Anyway I did it slightly different, I decided to make it out of a round rather than a square/rectangle. I have also put in a hole so I can hopefuly fill with oil then tighten down the screw. Time will tell whether it works or not but pics below of what I have done so far.

    Hood
    looks awesome!! please keep us updated! how come you made the piston smaller? so 1200 lbs will make the gauge read 400psi with the 1/3 sq. inch piston?

    if i end up making mine again im definitely going to increase the width and height of the "bridge" as much as possible. it looks like you are using 1/2 or maybe 3/4" steel of some kind? thats definitely an improvement, but you may want to go up even thicker. once its tapped in the middle for the threaded rod that gets connected to the floating pull stud, it loses alot of strength but has to deal with the full force of the drawbar right at its weak spot, the threaded hole in the middle. the flexing is probably not that relevant really if its small, but any movement at all is sort of undesirable for this kind of device i think.

  8. #8
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    Quote Originally Posted by acannell View Post
    looks awesome!! please keep us updated! how come you made the piston smaller? so 1200 lbs will make the gauge read 400psi with the 1/3 sq. inch piston?

    if i end up making mine again im definitely going to increase the width and height of the "bridge" as much as possible. it looks like you are using 1/2 or maybe 3/4" steel of some kind? thats definitely an improvement, but you may want to go up even thicker. once its tapped in the middle for the threaded rod that gets connected to the floating pull stud, it loses alot of strength but has to deal with the full force of the drawbar right at its weak spot, the threaded hole in the middle. the flexing is probably not that relevant really if its small, but any movement at all is sort of undesirable for this kind of device i think.
    wait nevermind, i get it now! 1/3 sq. inch will actually triple the measurement, so 1200lbs will show 3600psi on the gauge, duh!

  9. #9
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    Yes it should act as an intensifier much in the same way as the air over oil intensifiers work on some drawbars.
    It is just 10mm square stainless but I am going to thread and then tig weld both sides so hopefully it will be strong enough, if not I will just make a larger one. I had originally intended to drill a hole through the cylinder and use a bit of 16mm dia bar but the bit Alu I had just wasnt quite long enough to do that so ended up opting for the square section bolted.
    I am going to machine up a dedicated taper for this so welding it together wont matter as I will not have to take it apart again (hopefully )

    Hood

  10. #10
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    Quote Originally Posted by Hood View Post
    Yes it should act as an intensifier much in the same way as the air over oil intensifiers work on some drawbars.
    It is just 10mm square stainless but I am going to thread and then tig weld both sides so hopefully it will be strong enough, if not I will just make a larger one. I had originally intended to drill a hole through the cylinder and use a bit of 16mm dia bar but the bit Alu I had just wasnt quite long enough to do that so ended up opting for the square section bolted.
    I am going to machine up a dedicated taper for this so welding it together wont matter as I will not have to take it apart again (hopefully )

    Hood
    sounds great! youve got guts to make it un-disassembleable! fwiw the pass through toolholder im using for mine works well and it comes apart so fast that i dont mind not having it all welded up. what type of toolholder does your machine use? got any pics of your machine?

  11. #11
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    Actually just thinking about it I can make it come apart. As I will be machining a bespoke holder for this I can make the bar through the centre basically any diameter I want (within reason of course) so I will make the end of the bar have an internal thread and I can screw the pullstud into it. Now that I am thinking along these lines I really need to make the centre cross bar wider but I think instead of machining the slot in the piston wider I will just throw it in the lathe and bore out a round in the centre and then mill a cross bar like the pic below.


    My Chiron takes 30taper tooling but they are non standard due to the way the tool changer works. Here is a video of the first job I did on it after the retrofit and also below is a pic of the Chiron when I purchased it.
    Chiron First Parts - YouTube
    Hood

  12. #12
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    Quote Originally Posted by Hood View Post
    Actually just thinking about it I can make it come apart. As I will be machining a bespoke holder for this I can make the bar through the centre basically any diameter I want (within reason of course) so I will make the end of the bar have an internal thread and I can screw the pullstud into it. Now that I am thinking along these lines I really need to make the centre cross bar wider but I think instead of machining the slot in the piston wider I will just throw it in the lathe and bore out a round in the centre and then mill a cross bar like the pic below.


    My Chiron takes 30taper tooling but they are non standard due to the way the tool changer works. Here is a video of the first job I did on it after the retrofit and also below is a pic of the Chiron when I purchased it.
    Chiron First Parts - YouTube
    Hood
    that tool changer looks like something out of an alien abduction. how fast is that? 1/3 second? dear me! somebody really wanted to engineer that machine differently from the looks of the tool changer and bed layout. that must have been a real crowd pleaser on the trade show floor back in the day! what do people say when you show them that thing? is it in your house?

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