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IndustryArena Forum > MetalWorking Machines > Uncategorised MetalWorking Machines > hmm..drawbar force seems to be 245 lbs?
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  1. #1
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    hmm..drawbar force seems to be 245 lbs?

    its a dyna mechatronics DM2900 with a BT30 spindle

    i took apart the drawbar in order to figure out why the tools seemed not to unclamp very easily sometimes. sometimes i had to put on gloves and pull down on the tool while hitting the unclamp button until it finally popped free, rather violently

    so the draw bar is the standard ball bearing gripper type, with a pneumatic actuator

    i did some measurements and calculations, and here is what ive come up with..please comment! i hope i did something wrong because the force seems rather small.

    there are 49 belleville springs. 47 of them are back to back (series), the last two are in the same direction.

    so according to wikipedia, the new spring constant for the stack is 1 / (47 + 1/2) = 0.021. multiple that by the spring constant of a single spring to get the stack constant.

    the springs are 20mm x 10.2mm x 1.55mm carbon steel, as far as I can tell. mcmaster says that one of them has a deflection of 0.337mm at 342 lbs. So that gives a spring constant of 25777 lbs / in for an individual spring. Multiply that by 0.021 from above, and you get 541 lbs / in for this stack as configured.

    When I start tightening the top fitting on the drawbar, I measured the distance the fitting goes from just touching the spring stack, to fully tightened, as 6mm. So thats the compression the stack experiences just sitting there with no tool clamped.

    I then measured the distance the drawbar compresses when it tool is clamped in. This comes out to 5.5mm. (i had to swap the actuator in and out and meausre to the top of the fitting to accomplish this.)

    So it looks like the system as a whole provides 11.5mm of compression to the spring stack when a tool is clamped in. Thats 0.453".

    So that comes out to 0.453" x 541 lbs/in = 245lbs of drawbar force when clamped on a tool. That seems rather low, doesnt it?

    The tech support at Dyna is very helpful but I'm not sure even they know what the drawbar force was exactly supposed to be on this 16 year old machine.

    I could easily change the stack spring material (to CRV) or add more springs in parallel to make the stack stiffer, but I dont want to just redesign whatever the original intent was. But 250lbs seems low. The z axis can pull 400lbs according to the spec so I think that means it could pull the tool out of the drawbar if it were truly stuck in the work, like a big tap that was still threaded by accident during tapping when the Z axis goes to clearance.



  2. #2
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    Seems low to me, unless it has some sort of drawbar lock. Considering a 40 taper machine should be ~1800 LBS, i would expect a 30 taper to be 1/2 that.

  3. #3
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    Quote Originally Posted by underthetire View Post
    Seems low to me, unless it has some sort of drawbar lock. Considering a 40 taper machine should be ~1800 LBS, i would expect a 30 taper to be 1/2 that.
    yeah..this is odd. im trying to rig up something to let me measure the spring constant while its installed..i dont have $820 for a clamprite so its going to be fish scales and a dial indicator

  4. #4
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    The only way you ever going to know for sure is with a drawbar gauge. From what I understand there will be some mechanical advantage at the collet side. At 250 LBS, you should be able to pry the tool away from the spindle QUITE easy. I hope you cleaned those parts as well, they should be coated with Moly grease, and make sure the shaft is clean without grooves in it. Ive had many a drawbar not clamp 100% due to those conditions.

  5. #5
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    Quote Originally Posted by underthetire View Post
    The only way you ever going to know for sure is with a drawbar gauge. From what I understand there will be some mechanical advantage at the collet side. At 250 LBS, you should be able to pry the tool away from the spindle QUITE easy. I hope you cleaned those parts as well, they should be coated with Moly grease, and make sure the shaft is clean without grooves in it. Ive had many a drawbar not clamp 100% due to those conditions.
    welp i tried prying the tool out as you suggested.

    i took a crappy flathead (tip definitely not hardened...hopefully much softer than spindle or toolholder)..and drilled a hole in the handle. then i put my fish scale in the hole. i put the tip of the flathead between the tool holder and spindle, and pulled up on the fish scale until the toolholder started to move. looked like that happens at around 14lbs.

    i roughly estimate the short end of the lever to be 3/16", and the long end (fulcrum to hold in screwdriver) to about 7". so about 37 times leverage. so 14lbs to 37 = 518lbs..hmm..this is pretty rough though. maybe it really started to move at 12lbs, and maybe the leverage was 34 times..so 408 lbs..but lets say ball park 400 to 500 lbs..thats not really what i expected.

    if i could just connect the fish scale directly to the tool holder, and the other end to a bolt screwed into a bracket which is bolted to the casting, and then tighten the screw until the tool holder moves, wouldnt that be a very accurate drawbar force gauge?

    sheesh for $700+ for a bt30 clamprite which is the "economical" version, you would think someone would have come up with a cheaper way..i dont see why this is an expensive measurement. you have BT30 tapers and pullstuds already if you have the machine, isnt it just a matter of pulling down and measuring the force?


  6. #6
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    Clamp rite are by far the cheapest. My command set was ~3200. There is more in that clamp rite than you think, it works off a built in hydraulic piston. I made money checking/repairing drawbars for a while, so it was worth it.

    I'm sure if you can find a fish scale that goes high enough, you could rig something up.

  7. #7
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    Quote Originally Posted by underthetire View Post
    Clamp rite are by far the cheapest. My command set was ~3200. There is more in that clamp rite than you think, it works off a built in hydraulic piston. I made money checking/repairing drawbars for a while, so it was worth it.

    I'm sure if you can find a fish scale that goes high enough, you could rig something up.
    okay i just realized why my fish scale method wont work. its essentially pulling the z axis downard by the tool holder..not a good idea obviously. either the z axis will move or it will get hurt by the forces.

    so to make a real gauge..i think i have a starting point. i bet this is how they make the non-hydraulic drawbar force gauges..i.e. the digital ones

    take a BT30 toolholder

    drill a, say, 3/8" hole from and to end

    now mount a strain gage on a piece of material that can fit inside the toolholder. on one side, the strain gage mount should have threads for the pullstud. on the other side, it should somehow connect strongly to the toolholder (many ways to do that).

    tada, now you have the transducer, measuring pull stud "pull" versus the toolholder taper against the spindle. you just need a strain gage amp and a meter of some kind.

    you can get the strain gages cheap from omega. mounting them can be expensive if you buy a professional glue/prep kit, a couple hundred dollars for the chemicals. if someone could find a pre-made, off the shelf, 100lb to 3000lb strain gauge that could reasonably fit inside a BT30 toolholder, that would make life easier for the DIY'er.

    then you can take any cheap-o toolholder, drill it out, and do the modifications to it to mount the strain gage inside

    strain gage amps can be found all over the place..i bet one cheap enough for this job could be had less than $75..and think of all the fun uses you could use it for!

    this seems like a pretty fun project..of course im just rambling on and there could be all kinds of showstoppers here..

    it looks like clamprite makes a good product, but i think they charge so much because they can..i.e. anyone buying one is making plenty of money using it. plus they probably have good QC and its well built.

    but using the above method, I think someone could have an excuse for a fun DIY strain gage project, and make somethign probably accurate to +/- 100lbs...


    actually maybe you could do away with the toolholder completely, and just use a block of steel that gets pulled up against the spindle nose (with cutouts for the drive blocks)..now you just need any kind of strain gage/load cell that will work in tension, that can fit inside the taper..opens up the requirements a bit..just mount the pull stud on a rod in the middle

  8. #8
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    The bar idea won't work correctly. That will give you spring pressure, but not true clamping force. Since all holders are made to standard specs, so are the clamp gauges. If the holder were to go in deeper in the taper, you would loose clamping force, but wouldn't show up with a bar. I've seen this before from taper re grinds. A lot of the price of these is the adaptability. I can do 30-50 taper on both BT and CT.

    My command unit is a digital strain gauge with built in amp and readout. It can certainly be done with a few parts.

  9. #9
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    Quote Originally Posted by underthetire View Post
    The bar idea won't work correctly. That will give you spring pressure, but not true clamping force. Since all holders are made to standard specs, so are the clamp gauges. If the holder were to go in deeper in the taper, you would loose clamping force, but wouldn't show up with a bar. I've seen this before from taper re grinds. A lot of the price of these is the adaptability. I can do 30-50 taper on both BT and CT.

    My command unit is a digital strain gauge with built in amp and readout. It can certainly be done with a few parts.

    ya that makes sense. okay i have another idea. not that the strain gage idea isnt good, it is, but its more of a project and would take quite a bit of work to get the right strain gage in the right place.

    instead, what about making the hydraulic style like the clamp right?

    this puts it in a machining skills category. no electronics. you just need a pressure gauge that can go up to whatever, 3000psi..and those are common and cheap. you can connect it to the machined/modified toolholder via a hydraulic hose..so forget about it for a second...

    using an oring bore seal, you can make it so the pull stud somehow connected to a piston that compresses some fluid, and then measure the fluid pressure. i believe the hydraulic gauge like the clamprite uses a more sophisticated diaphragm setup, but i would imagine an oring bore seal would also work, and there are calculators galore for oring seals so it should be something that could be definitely determined before machining or design starts...

    so, to lay it out:

    take a bt30 tool holder
    drill a hole through it, say 3/8" dia
    make a rod that will easily fit through the hole. thread it on one side for the pull stud.

    on the other end of the rod, connect it to a piston with an oring bore seal.
    make some kind of bore in something (doesnt have to be inside the tool holder, but does have to butt up against the toolholder and transmit the force from the drawbar to the toolholder and therefore the spindle taper

    make it so when the pull stud is pulled, it pulls the rod, which pulls the piston and compresses some fluid in a cylinder, and then attach that cylinder to your pressure measuring gage

    post the plans on this thread so i can make one lol

    i like this idea..im going to think about this one and how you would actually implement it. i think i have all the parts and tools to make one in my house at this moment so its very tempting

    what about some kind of off the shelf hydraulic piston? just connect the end of it to the pull stud and mount the piston body to the tool holder? then connect the fluid port to the gauge? this is a simplified example..but its got the right logic...now is there one small enough and cheap enough?

  10. #10
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    okay more on this..

    looks like common lip seals are good to 5000psi and are cheap..thats the industrial revolution for you

    so the toolholder becomes just a pass through for the pull stud rod. all the action will happen just under the toolholder, so no worries about fitting everything inside it.

    we just need a short stroke hydraulic piston setup with a port to measure the fluid pressure.

    i think one could be described as:

    a closed metal cylinder.

    inside is a piston with a bore oring seal
    on the top, the pull stud passes into the cylinder through a lip seal, and connects to the piston.
    the upper chamber of the cylinder is filled with a hydraulic fluid.
    the upper chamber has a port which you connect your pressure gauge to.
    do a little arithmetic to convert the cylinder chamber pressure to pull stud force.

    hmm..this actually looks just like my drawbar pneumatic actuator...basically revinventing the wheel here. i wonder if there are any cheap, short stroke hydraulic actuators out there? must be something in some car or heavy machinery that needs something to be pushed or pulled a small distance but with great force?

    hmm...speaking of which maybe it doesnt have to be hydraulic. i suppose the pneumatic actuator is doing the same thing, just in reverse.

    on my mill its about 4" in diameter. so 1000 lbs for instance, gets divided by the surface area of the piston, about 13 sq inches, and comes out to an air pressure of about 80 psi..

    hmmmmm

  11. #11
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    oops..
    pneumatic wont work because it needs to be incompressible or the toolholder will be ejected..derp

    so back to hydraulic

  12. #12
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    I am in the process of building a clamprite style for my 40 taper Haas. Since this is more less a hobby machine I cant justify spending that much on the tool. I am planing to use this as my ram...

    10 Ton Hydraulic Short Body Ram

    coupled to this...

    5000 PSI Filled Gauge

    and turn the threads off a thru coolant pull stud. Weld on a extended shank that will attach to a A frame style clamp that this piston will go in at the bottom of a 1 inch holder. this should work perfectly as I can preload the pull stud by screwing it into the clamp and the flange of the pull stud will be sitting on top of the holder just as it would normally. I will have to post some pics when I finish it.

  13. #13
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    Quote Originally Posted by DaOne View Post
    I am in the process of building a clamprite style for my 40 taper Haas. Since this is more less a hobby machine I cant justify spending that much on the tool. I am planing to use this as my ram...

    10 Ton Hydraulic Short Body Ram

    coupled to this...

    5000 PSI Filled Gauge

    and turn the threads off a thru coolant pull stud. Weld on a extended shank that will attach to a A frame style clamp that this piston will go in at the bottom of a 1 inch holder. this should work perfectly as I can preload the pull stud by screwing it into the clamp and the flange of the pull stud will be sitting on top of the holder just as it would normally. I will have to post some pics when I finish it.
    AH-HA! a short stroke hydraulic actuator! and the gauge!!! nice find!!

    so the drawbar pulling up will compress the ram via the frame/clamp you will be making right? can you link to a pic of what the clamp will look like? i cant seem to find one. crap i think i may go buy those parts and give this a shot...

    i dont think this will be a big issue, but how do you fill the ram and the gauge with hydraulic fluid and bleed it of air? maybe the bleeding isnt needed because the preload can compress the air to nothing?

    hmm i think you should start a separate thread dedicated to this project..alot of people will probably follow in your footsteps..including me if it looks like it will work lol

    btw some good news on the strain gages for anyone interested in that:

    from what ive read online, some strain gages can be bonded using just super glue. it depends on the backing material and the strain required. and Omega offers a $10 kit with bonding agent..so its not hundreds of dollars. there are also youtube videos showing strain gage bonding procedures..so this is all very inexpensive and accessible. omegas adhesives are here:

    http://www.omega.com/pptst/Strain_Gage_Adhesives.html

    and, i found lots of inexpensive strain gages on ebay, including a 45 degree rosette that will allow torsion and shear stress measurements, and its only $7.50 shipped for a single one! the average price for linear singles and rosettes is about $40 shipped for qty 5..


    Budd Strain Gage Rosettes C12 111 R3A 120 Ohm Gage Factor 2 01 NOS Qty of 1 | eBay

    the 45 degree one is really not meant for tension measurements..in fact i think its specfically designed to null out non torsional/shear forces. but it was so cheap i figure if i can get it to work then ill by the tension style ones. like this 2 gage rosette 5-pack for $35 shipped!

    5 New Vishay mm WK 00 250TB 10c Precision Strain Gage 1000? ±0 4 w Opt W | eBay

    so i bought the 45 degree rosette and im going to try bonding it to something and getting some kind of meaningful measurement, maybe some kind of torque wrench verifier lol. i have a differential amp with a 7 segment display from a different project so I think I should be able to measure the output, although I may need to make a bridge of some kind.

    there is a very small chance that i may end up attempting to use it in a drawbar force gauge somehow..but it looks like DaOne's items up there would be perfect.

  14. #14
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    Here is a photo of what I mean. Basically 3 parts required to be made and if you own a mill it should be easy to make.
    Attached Thumbnails Attached Thumbnails Tool Assembly 2.jpg   Tool Assembly.jpg  

  15. #15
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    Oh and on the oil and air part. I was going to use a lightweight oil and actuate the piston to bleed out as much air as I can. Then assemble the piston to the gauge while its submerged in the oil.

  16. #16
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    Quote Originally Posted by DaOne View Post
    Oh and on the oil and air part. I was going to use a lightweight oil and actuate the piston to bleed out as much air as I can. Then assemble the piston to the gauge while its submerged in the oil.
    okay i think i get your textual description of the device but i cant seem to make it jive with the drawing...

    the idea is that the hydraulic ram gets squished by the pulling force of the draw bar, via the pull stud, right?

    so the pull stud force is transmitted through the toolholder via the rod, which is connected to the _far_ side of the hydraulic ram, so that its a compressive force (far side of ram being crushed against toolholder).

    but i dont see how thats happening in the drawing, the square shaped object seems to be free floating?

    can you describe it a bit more plz

  17. #17
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    The ram rests against this surface. The square loop is pulled against the other side of the ram squishing it. The pull stud rod is threaded into the square loop so the whole thing can be preloaded by screwing the pull stud down.

  18. #18
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    Maybe this pic will help. The square loop and pull stud are connected. The spacer is connected to the holder. When force is applied to the pull stud the ram is squeezed between the loop and the spacer.
    Attached Thumbnails Attached Thumbnails Tool Assembly4.JPG  

  19. #19
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    Quote Originally Posted by DaOne View Post
    Maybe this pic will help. The square loop and pull stud are connected. The spacer is connected to the holder. When force is applied to the pull stud the ram is squeezed between the loop and the spacer.
    ahh i see now..i thought the ram piston or possibly housing was in the drawing..now i see that the drawing is the custom parts you will be making..this is very nice..i would really like to see how this turns out..

    id like to make this..i need to get a coolant pass through BT30 like you mentioned...

  20. #20
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    Well the coolant pass pull stud is just used to line up the threaded extension. Sure it can be done without one.I was just gonna turn dow that rod to fit in the hole of the stud to hold it in place while I tig weld it to the pull stud.

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