[acannell]

The springs are 25mm OD , 12.2mm ID and 1.5mm thick. There are 96 of them (same as drawing) and I stacked them the same way as they were when I pulled it apart, 2 one way then 2 the other. Pic from manual doesnt really show what they are stacked like as it only shows a rough representation of the lower and upper few but it does appear to be the same way..

Cant really remember what the orig spindle motor was but think about 8Kw
Hood

wolp heres my back of the cat calculation..i would treat this with extreme suspicion:

based on whats available at mcmaster carr, your spring looks like this one

with a spring constant of 40566 lbs/in (658 lbs / 0.412mm)

so the new springs constant for 96 springs split into 48 groups of 2 in parallel would be Kstack = Kone * (1 / (1/2 * 48)) = 1690 lbs/inch spring constant for the stack. (wikipedia)

this spring stack constant is very linear due to its proportions http://www.mitcalc.com/doc/springs/h...pringERate.gif

friction may account for 3% to 5%..but im too lazy to figure out exactly how to calculate it, although it may be important. more can be seen at the mitcalc page above.

but to know how this all translates into your drawbar force, you have to figure out how much the stack is compressed when a tool is in place. i was able to measure this on my mill by taking all the springs out, and doing measurements of the drawbar displacement with and without a tool, and also how much compression there is when the stack is installed..so you need to know

-stack compression with no tool installed (there is probably some compression otherwise the springs would be loose with no tool...rattling around or whatever..they probably designed it to compress them a little even with no tool)

-additional stack compression with a tool installed (you can do this by pulling up on the drawbar with no springs with a tool installed, then do it again with no tool, and the difference is the _additional_ compression a tool adds)

then you can come up with a number. but you may want to figure out the friction factor on the mitcalc page..

[acannell]

btw Hood, could you help a po' fella out?

at my work there is a "hardinge super precision" lathe..im not totally inexperienced with lathes but almost so..i do all my work on mills..im wondering if i can make some kind of useful bt30 tool holder on the lathe like you made for your machine? is it an advanced thing to do or fairly easy? and what kind of material do i need?

i.e. like an end mill holder (weldon flat style)?

looking at the geometry (tool changer V, nose, etc..) it seems like you would need cnc to do all the angles correctly..or do i just not know what im doing

or what about making pull studs? is that difficult?

[Hood]

Thanks for the calcs, my gauge shows 1600 but sadly my piston is 1/3 square inch area

Its not very easy to measure things as its all internal and no way to get down to measure, well suppose I could remove the motor and maybe get in that way but it would likely be quicker/easier to remove the spindle and measure, refit, put a tool in then remove and measure again. Might do that sometime but with a bit of luck I will get some numbers either from Stefan or Chiron so that I can base my decision on whether things are way out or not.

Regarding the 30 taper holders, if the lathe is CNC then its a pice of cake, if its a manual then again should be relatively easy but getting the taper just right would be the awkward part.
The holders I have made up are mainly just for tester holders or similar and they dont get used too much, so not sure how things would stand up long term. I just used EN19 but not sure what that would relate to in material specs your side of the pond.
I made pullstuds for the Beaver mill, again out of EN19 and they have stood up well. I was going to make them for the Chiron but I found a source in Germany that stocks them for the Chiron and they are only about £5 eacjh so it wasnt worth my while making them, I had made up the code before I found that place so may do some one day.

Hood

[Hood]

I changed the disc springs to be in 3's instead of pairs but as I suspected the travel was too short and it wouldnt grip the pull stud. I stripped it down again and removed half of the discs and put them back to pairs and this time there was enough travel to grip but its just on the edge for releasing. I will keep an eye on it but so far all holders I have release ok. Anyway I shoved the force gauge in and it was now reading 2000 PSI so thats 666.666 lb-f, so was quite a bit better but still lower than I was thinking it should be. I had tried to get info on what it should have been from a few more sources but no one knew, then out of the blue I got a phone call from Chiron. The guy didnt know what the force should be on my machine but he said that the 40 taper machines of the vintage of mine had a force of 4.5kn, that works out at 1000lb-f approx.
I looked up the PDF I had found earlier with usual forces and divided the 4.5kn into the 10.23kn that it said for a 40 taper, I then divided what it said for a 30 by that result and lo and behold the number that popped up was 527lb-f just around what I got when I replaced the discs. So now I am wondering if I should keep it at the higher value or put it back to pairs again, decisions decisions

Hood

[acannell]

I changed the disc springs to be in 3's instead of pairs but as I suspected the travel was too short and it wouldnt grip the pull stud. I stripped it down again and removed half of the discs and put them back to pairs and this time there was enough travel to grip but its just on the edge for releasing. I will keep an eye on it but so far all holders I have release ok. Anyway I shoved the force gauge in and it was now reading 2000 PSI so thats 666.666 lb-f, so was quite a bit better but still lower than I was thinking it should be. I had tried to get info on what it should have been from a few more sources but no one knew, then out of the blue I got a phone call from Chiron. The guy didnt know what the force should be on my machine but he said that the 40 taper machines of the vintage of mine had a force of 4.5kn, that works out at 1000lb-f approx.
I looked up the PDF I had found earlier with usual forces and divided the 4.5kn into the 10.23kn that it said for a 40 taper, I then divided what it said for a 30 by that result and lo and behold the number that popped up was 527lb-f just around what I got when I replaced the discs. So now I am wondering if I should keep it at the higher value or put it back to pairs again, decisions decisions

Hood

very interesting..now i feel double good about the 500 to 600 lbs number for bt30 since your source says the same thing that dyna told me....

remember it may take some calculating to figure out the actual new force when trying out new spring combinations or number of springs..i.e. changing from 2 in parallel groups to 3 in parallel groups might make the stack have a higher spring constant, but the reduction in stack length means less compression, so you have to account for both to get to the overall effect.

if it wasnt gripping a tool thats probably because the springs werent compressed at all due to a shorter stack height and there was a gap which allowed the drawbar to move and not grip anything. or are you saying you added springs to make in 3 in parallel? im just mumbling now

[Hood]

It didnt have enough travel to grip the pull stud. With the original paired discs I had 24mm travel when fully compressed (96 springs/2 =48 with 0.5mm per pair) but putting them to 3's would reduce the travel to 16mm. I did add an extra few springs to take the stack to the same length as previously so think I ended up with about 17 mm travel but it wasnt enough, the springs were fully compressed before it was down far enough to let the balls in the gripper past the stud.
When I removed half and put back to pairs that gave me 20.5mm of travel (I had an extra pair in) and it gave me enough travel to grip the stud and it it is just enough to push the tool out but its close.
The way the setup is on the drawbar there is no slack when fully up as there is a plug that screws on top of the drawbar and compresses the discs slightly.

Hood

[Hood]

Actually just been thinking about this, the set up in the Chiron drawbar has some balls up at the top, they sit in a groove in the drawbar when its up and there is a sleeve over them so they can not move out and thus the drawbar is held captive. So this is a kind of self locking device, when the drawbar is up and gripping a tool the balls will be in the groove and the sleeve will be up over the balls thus not allowing any movement in the drawbar.
I will now have to have a look at it again with a tool in it, if I remove it I may actually find the drawbar isnt quite far enough up and thus the sleeve is still down and the self locking function of the balls is non-existent and the stacking in 3's then 2's might actually be worse than having the lower force.
Have attached a pic that kind of shows the setup but the pic posted before of the drawbar and the complete spindle shows the groove and the sleeve I am talking about.
Hood

[Hood]

Well it was 1:37 am when I wrote the above post and when I thought about it this morning I realised that no matter what I had done to the springs it would not affect the height it raised to, so I know the sleeve will be up.
The part about it locking however is correct and I realise now that is why Chiron can get away with a lesser drawbar force than other similar sized/power machines with a 30 taper. This has eased my mind and I think I will change it back to the pairs of discs rather than half 3's and half pairs.
First however I will do a test, I will try and see if I can get a lever in between the spindle nose and the flange of a tool and see if can move it, if not the discs are going back to the way they were originally
Hood

[acannell]

check out the spindle dynamometer torque idea thread for the next fun project!
:wee:

[num1bryanp]

Hello, Any updates to the DIY drawbar force gauge.

I have a new to me 1989 Bridgeport 720 I just changed the spring washers.

The drawbar seems to be working okay but it would be nice to know what the pulling force is, or at least in the neighborhood. That way I can check in the future and watch for changes.

Thanks, Bryan