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  1. #1
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    Dec 2012
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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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    Mar 2004
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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

  7. #7
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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?

  13. #13
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    I think chip to chip is specified as 1.2 seconds but I actually have the tool changer speed turned down so think its more like 2 seconds. It must have been amazing right enough, its 1987 vintage and I suppose the normal toolchange speed in these days was 15 or 20 seconds for a fast machine. Chiron have always had basket toolchangers like this and they still make them, almost the same, even today.

    It is in my workshop and yes the layout is also not the norm. It is great however as the table is always right at the front, so no having to move to load and also weight is not an issue, you can put as much weight on the table as you like.
    Hood

  14. #14
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    well i tried to replace my old/unknown/weak drawbar springs with new ones. unfortunately it looks like i didnt buy enough new ones. i need about 12 more to make the stack long enough to be compressed. so for the moment, i just replaced 36 of the 48 with new ones, and re-used 12 of the old ones.

    then i tested out the drawbar force with the deali-hoo

    its now about 244 lbs! hmm thats definitely a big improvement but still wayyyy lower than i would expect. once the additional springs get here hopefully their higher spring constant and height will add up to a force 500+ lbs

  15. #15
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    Do you have info on your particular machines standard drawbar force?
    I looked up TAC Rockfords site and they suggest around 1200 lb-f for a 30 taper, which is what I presume yours is.

    I put some water in my contraption a wee while ago and squeezed the piston in with my hand and I got the gauge to show at 200 PSI, so that would be 66.66 PSI if all is well, not sure if that sounds about right or not.
    Might not make it to the workshop tomorrow so likely will be some time during the week before I can get the rest of the parts made up.
    Hood

  16. #16
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    Quote Originally Posted by Hood View Post
    Do you have info on your particular machines standard drawbar force?
    I looked up TAC Rockfords site and they suggest around 1200 lb-f for a 30 taper, which is what I presume yours is.

    I put some water in my contraption a wee while ago and squeezed the piston in with my hand and I got the gauge to show at 200 PSI, so that would be 66.66 PSI if all is well, not sure if that sounds about right or not.
    Might not make it to the workshop tomorrow so likely will be some time during the week before I can get the rest of the parts made up.
    Hood
    dyna mech asked their engineer in taiwan or china or whatever and they said at least 300kg..so about 660 lbs. problem is, the number if springs i found in there was 50 instead of the 48 their drawing says. also, i carefully measured the springs, and the stack compression with a tool in it, and the numbers dont add up anywhere near 660 lbs..more like 240. and the gauge showed about 100 lbs!

    if i can get it up to more than 500 lbs i think ill be happy. once i replace all the springs with known springs, hopefully the force gauge readout and the calculations come close and i can call it a day..

  17. #17
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    Well I made it down to the workshop and got the other parts machined. I decided to go for the cross piece as per pic in previous post. I tried to fill the oil via the hole I had bored but it was too small really so I resorted to unscrewing the gauge and filling with oil then screwing the gauge back in. Obviously this put a reading on the gauge, actually about 3000lbs which was the number I was hoping to see for the drawbar. Anyway the filler screw suddenly became a bleed screw I was able to slacken it slightly to release the pressure so the gauge read zero again. Great I thought now to the testing, put it in the spindle and big dissapointment, only 600PSI showing which equates to 200 PSI if my thoughts on the 1/3 inch area pison is correct.
    I thought it may have been the gauge but I robbed one off a welding regulator and got similar results, so that now means either the method of measuring is not working as anticipated or my drawbar force is very low. If I get a chance I will make up an adapter so I can put it into the Beaver Mill, I put in new Bellevilles last year so it should be good.
    I now have to try and find info on how the Chirons drawbar works, I know its air that operates it but how it actually is constructed I have no idea as it is inside the spindle and I dont have any drawings.
    Below are a couple of pics of the tester.

    Hood

  18. #18
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    Quote Originally Posted by Hood View Post
    Well I made it down to the workshop and got the other parts machined. I decided to go for the cross piece as per pic in previous post. I tried to fill the oil via the hole I had bored but it was too small really so I resorted to unscrewing the gauge and filling with oil then screwing the gauge back in. Obviously this put a reading on the gauge, actually about 3000lbs which was the number I was hoping to see for the drawbar. Anyway the filler screw suddenly became a bleed screw I was able to slacken it slightly to release the pressure so the gauge read zero again. Great I thought now to the testing, put it in the spindle and big dissapointment, only 600PSI showing which equates to 200 PSI if my thoughts on the 1/3 inch area pison is correct.
    I thought it may have been the gauge but I robbed one off a welding regulator and got similar results, so that now means either the method of measuring is not working as anticipated or my drawbar force is very low. If I get a chance I will make up an adapter so I can put it into the Beaver Mill, I put in new Bellevilles last year so it should be good.
    I now have to try and find info on how the Chirons drawbar works, I know its air that operates it but how it actually is constructed I have no idea as it is inside the spindle and I dont have any drawings.
    Below are a couple of pics of the tester.

    Hood
    very interesting! your device looks really nice. (thats what she said) my gauge has also shown me numbers that seem significantly low, but without a way to verify it all we are sort of in the dark i think. i suppose there is also the possibility that the devices are working perfectly and both our drawbars were dangerously low on tension! but im definitely still suspicious of it.

    do you have any heavy weights laying around, to where you could rig up something to put a known tension on the force gauge somehow? perhaps something to grab the pull stud, mounted to the ceiling or what have you (make sure to use drywall anchors lol jk) and then some thing heavy to pull down on the taper, in order to act like it was in a spindle? hmmm maybe i should try to figure some way to do this too

    some things to check:

    -did you tighten up the pullstud/rod so that there was absolutely no play, and maybe even the gauge just started to move? i think that is needed in order to lessen the effects of air pockets and/or slack
    -can you tell if the pullstud is being pulled out of the taper at all while its installed? definitely want to minimize its displacement

    maybe if we can find a patent on drawbar force gauges we can confirm that this design is proper (or improper)

  19. #19
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    uh oh


    Hydraulic Force

    why is the rod diameter (not just the piston diameter) involved in the pulling force equations for hydraulic cylinders?

    it cant be something this dumb right? im suspending my disbelief. a 1000 lbs pulling tension does not depend on the cross section of the member in tension does it? wtf

  20. #20
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    oh i get it..they are just subtracting the diameter of the rod from the cross section of the cylinder since its a double acting piston..phew..felt like i was in the twilight zone for a minute..all is well again

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