Draw bar setting/cleaning/adjusting?

[Brian L]

I was working on modifying an endmill holder today, boring it to 20mm and shortening it as much as possible to mount a 3D taster (Chinese copy of a Haimer). I have numerous 3/8" holders and so one willingly gave it's life to be augmented....

This is where the fun begins, after I spent considerable time drilling through the case hardening on the side to install a new set screw much deeper in the holder, and cut the front end off with an abrasive saw.... I put it in the spindle and find when the tool is supposed to be tight, you can turn the tool holder by hand and clickety-clack it against the drive lug.

I thought, well, it's a bunch of used tooling I found locally... tried others, everything grips just fine except for one brand. Then I tried several different brands of pull studs, thinking maybe I had a bad pull stud.... nope, all the other holders and any combination of studs work in my other holders, but not in this brand.

Now, I could be on the ragged edge of clamping with the others, so now I figure I better double check. I found mention of changing bellevilles in past posts, but couldn't get a handle on how the draw bar is assembled in the spindle, and how it comes out, and if there is any adjustment available to adjust the amount of draw back it has.

Anybody have the process, done it, know if there is some specific setting you do to make sure your gripper is pulling back enough?

[Brian L]

OK, found more old posts about the drawbar and a picture, are all the drawbars the same with the pancake air cylinder on top? I don't quite have a grip on how the springs are held onto the assembly and where this set screw and such is... which part is threaded on, the large nut looking part on top, with a brass bushing under it?

Pictures and more info in this thread:

http://www.cnczone.com/forums/milltr...retention.html

[jpawelk]

If you are qusetioning the drawbar force, you may want to get ahold of a dynomometer. You may also just try and tigthen the nut at the top of the drawbar as well. Overtightening will sacrifice the life of the washers.

[underthetire]

Put a good holder in, see if you can GENTLY pry the holder down with a pry bar. If it moves at all, you got problems. The dynamometer is always a good option, but they ain't cheap, even from clampright. Belleville washers are available really cheap from key Belleville. Orings, scored shaft, rust, and broken springs are the main drawbar problems.

[Brian L]

OK, I will have to get help from friends to pull the thing (I'm in a wheelchair and it's way out of my reach), but I'm wondering where the draw bar rests inside the spindle, on the shoulder that appears to be a washer under the Bellevilles? I can kind of understand how the tightening at the top gives more pressure, you tighten the stack and tension the washers, but what controls the final length of the gripper head?

I had a thought to check the amount of grip I'm getting on some of the other holders, ones that seem tight, I'll back the draw stud out .010 (use a feeler gage to check the gap) and see if it still grips, if good, then .020 and so on, how much grip distance. i.e. pull back should I have?

Out to do some tests and will report back....

[Brian L]

OK, I went out and did some testing.... with a brand new HPI holder that 'grabs" I had to back the draw stud out .020" for the holder to spin in the spindle (I have the drive lugs off the machine at the moment).

I have multiple holders of another brand and they are all loose, maybe only a few thousandths, but they are loose.

On the BT holders I couldn't get anything between the holder and the spindle, they are pretty close, but the CAT holder had more space (don't have a tool changer so I use either one), so I was able to get a couple screwdrivers in the space and tried to pry the holder down.... no movement that I could feel or see.

Does anybody have a parts list that shows the way the parts assemble in the head? My parts list shows a variable speed head (which I don't have) and no real info on the draw bar, gripper and washers.

Don't know if I can link to the previous picture, but I'm just trying to get a handle on how this all works.... is the gripper assembly screwed on to the draw bar at varying heights to establish the correct about of pull back? The posts seem to say it's screwed on completely.... or wait... there appears to be a setscrew in the middle of it, is it what you turn another 4 turns or whatever after it gets finger tight?

As you can see, I'm confused....

http://www.cnczone.com/forums/attach...p;d=1200336422

[sportybob]

You did not list your machie type, I suspect that it is aRH bedmill. If your manual does not have the RH head drawings, you shold contact Milltronics service and ask for a set. Milltronics tech support is 952-442-1410
or email service@Milltronics.net

The drawbar is held in by the ball lock cylinder on the bottom and the nut on the top. Removing the nut on top will allow the drawbar to slid out the bottom of the spindle. Watch for the 4 small 1/4" balls, hold your hand under the spindle to catch them. The bellville will stay inside the spindle. You can fish them out using some welding rod with a small hook on the end. If you have broken washers use a magnet.

If the drawbar rod is gouged, replace it. If the ball lock cylinder "ball holes" are enlongated replace it. Replace the bellville washers anyway.

For more info PM me.

Sportybob

[Brian L]

Thanks Bob,

I did find a drawing and can now see how the whole system works, am rather surprised that there is only .020" of draw back, so to speak according the the Milltronics info. Oh, machine is actually a Partner 4, 1992 vintage.