# Made a Power Drawbar for the PM728



## GB21 (Mar 24, 2021)

I got tired of the 11mm wrench pretty quick so I took inspiration from around the web and made a ugly prototype drawbar. I am super happy with the results. Next on the list is to machine a nice combo bracket for the the drawbar and encoder and some cable/hose management.


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## Tipton1965 (Mar 24, 2021)

That looks like it works consistently.  Nice


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## davidpbest (Mar 24, 2021)

Looks great if you're comfortable with TSS toolholder for everything.  I like it.


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## GB21 (Mar 30, 2021)

First failure.I was running a 3/8 endmill pretty hard you can see the angle in the bottom of the pocket where the tool was pulling out. Good thing I caught it before I had a catastrophic failure. The working load of the spring washers is 1890lbs. I watched a video saying the tormach machines are running 2700lbs I am going to try to get closer to that and see what happens.


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## davidpbest (Mar 30, 2021)

Not sure if you've seen this, but at 1:40 in the timeline he's dealing with the Belleville washer pull power issue:


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## 7milesup (Mar 30, 2021)

GB21 said:


> I got tired of the 11mm wrench pretty quick so I took inspiration from around the web and made a ugly prototype drawbar. I am super happy with the results. Next on the list is to machine a nice combo bracket for the the drawbar and encoder and some cable/hose management.


What size of air cylinder did you use on that?  I have a number of them lying around...


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## GB21 (Mar 30, 2021)

Fabco 4" 2 stage. It barely fits without modification, I actually had to remove the fan cover from the motor to center the piston. A multi stage cylinder is a must in my opinion. Here is a chart I found useful when trying to figure out what size I needed.


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## GB21 (Apr 10, 2021)

Update, I added one more washer and I have not had any pullout since. These are the thickest ones for a 7/16 bolt mcmaster sells.


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## jlchapman (Apr 22, 2021)

Hi GB21,

Awesome power drawbar.  What is the part number for the washers at McMasterCarr?

I would like to see more pictures of your air cylinder mounts, airlines, switch, etc.

Thanks,

Jerry


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## GB21 (Apr 27, 2021)

Sorry for the late response I have been a little busy. Mcmaster springs # 9712K436.

I used 3/8" b7 threaded rod for the mount with aluminum tube I found in my scrap bin.The shoulder bolts are 5/16" Diameter 1/4"-20 Thread with 3d printed spacers. The other side I used 1/4" allthread with nylon-cap locknuts. For the drawbar I used grade 8 allthread with double high strength locknuts. (Too lazy to get the welder out). For a switch I am using a cheap foot pedal I picked up on amazon for like 18 bucks (tailonz 4F210-08) works great no leaks yet.

 I was trying to keep everything simple in case anything did not work. Turns out everything is working perfect and I have not had to make any changes yet although like I said I will be remaking the shoulder bolt mount a little nicer and to fit a encoder and add more room for the belt.


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## jlchapman (Apr 27, 2021)

Thanks for all the info and pictures.

I see that you removed the spindle cap, c-clip and return spring cup.  Also the spring.  Is the c-clip the only thing holding the quill in place?  I'm assuming the quill drops right out.

Did you put one of the 3d printed spacers in place of c-clip and return spring cup?   I also see in your first photos there is a 3d printed white cap on top of the spindle.  Keeps crap out?

Thanks!


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## GB21 (Apr 28, 2021)

Right now everything is removed and nothing is holding in the spindle except the bolt. Yes I originally had that plastic part to keep the spindle locked into position and prevent any fallout and it also held a gear for the encoder. The plastic piece didn't fit right and I found the any pressure at all against the top of the pulleys generated too much heat in the bearings.. it was a bad design... Temporally If the locking bolt fails the threaded top hat will stop any major disaster of the spindle from just dropping out of the head, it's only 2-3 mm from the plate.  Soon I plan on making some sort of splined sleeve or tube with set screws and a gear to disable the quill and make it safer.


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## ptrotter (May 27, 2021)

Beautiful work.  I am looking at doing something similar on my PM-940V and  I have a couple of questions.

You mention in the post above a "threaded top hat".  Is the top hat threaded onto the top of the spindle?  I have seen some PDBs where the top hat is connected to the spindle so that the washers are compressed between the the top of the spindle and the drawbar nut and others where it is not connected to the spindle and just compresses the Belleville washers between the top hat and the drawbar nut.  I think having the top hat connected to the spindle would provide more downward push on the R8 collet for better release of the tool, but I would have to modify the top of my spindle to do that.

 Also, I see that the red bottom plate rides up on 2 shoulder bolts on the right side, but the left side just sits on the 2 threaded rods.  Why don't you have both sides use the shoulder bolts?  Priest Tools does the same thing on theirs and also I wondered about why he does it that way.  I may be missing something but I would get a straighter pull if both sides used the shoulder bolts.


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## GB21 (May 27, 2021)

The first top hat I made just sat on top of the spindle but the air cylinder just pulled the top hat/washer stack up and would not release the tool from the collet. Adding threads solved the problem but my collet is a little "sticky" YMMV.  

My original design had 4 shoulder bolts but would require drilling and tapping holes in the head or making some kind of bottom plate. The threaded rods simplified the design and I was trying to keep it stupid simple until extensively testing it. I made the holes for the shoulder bolts oversized so everything is just floating around .  Honestly it does not have to pull very straight and precision is not critical here what you don't want is any binding. If I ever get around to making a bottom plate I will use 4 shoulder bolts.


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## ptrotter (May 27, 2021)

Thanks for the reply. I can see where it would be better to have the top hat connected to the spindle.  Priest Tools does not do it on his and it seems to work so I may try it that way first so I don’t have to modify the spindle.


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## ptrotter (May 27, 2021)

Just out of curiosity, was your spindle already threaded at the top or did you have to thread it?  If you did thread it, how hard was it? I assume the spindle is hardened. And how long were the threads?


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## GB21 (May 27, 2021)

The spindle was already threaded M20x1 about a inch of threads if I remember correctly.


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