# Push- Pull Drawbar For 8520 Mill



## dbassing (Dec 10, 2015)

Hi all,
I remember seeing a post of a design for a push- pull drawbar for the 8520 mill and I have not been able to find it. Could someone point me in the right direction. My current drawbar is the two nut style with an acorn top nut that one hits to release the tool from the spindle. The top of the splined quill is threaded for 3/4-16. It would be nice to be able to release the tools without hitting. Any thoughts or comments always welcome.
Thanks,
David


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## Terrywerm (Dec 12, 2015)

David, there are two components that you need, one being the drawbar and the other being the top cap that threads onto the top of the spindle spline. Your current drawbar will probably not work with the top cap. I was fortunate enough to obtain a drawbar from another member, then made my own top cap. It works pretty well, but there are still times when I have to hit the top of the drawbar to pop out a tough collet or other MT2 item such as a drill chuck, but I don't usually have to hit it very hard as I 'preload' it by trying to push it out with the drawbar first. .

Give me a little time and I will upload the drawings that I have here for those parts. I may not have a drawing for the drawbar, but will take the time to make that drawing and post it here also.


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## Terrywerm (Dec 12, 2015)

The two attached drawings should get you what you need. You can make the drawbar from a single piece of stock if you wish, but you could make it in two pieces with the top of the drawbar brazed to the shank. I would recommend hardening and tempering the top of the drawbar as per the notes on the drawing. The only problem with doing that is that it would require making the drawbar from a single piece of stock. You could leave it unhardened, but if you choose to do so do not be tempted to use an open end wrench on the drawbar, as it will round over the hex way to easily. I use a socket style wrench on mine that I made from an old 1/4" drive six point 3/8" socket with a piece of round stock welded to it for a handle.

There are two drawings, one for the drawbar and one for the cap.


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## dbassing (Dec 12, 2015)

Thank you Terry,
I had a feeling you may be the one to have a good answer. I will review the drawings and begin getting the material.

Thanks again. Hope all is well there,
David


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## Terrywerm (Dec 12, 2015)

You are most welcome, David.  The drawing for the drawbar calls for O-1 or W-1 tool steel, but you could make it out of 1018 or 4041 if you wish. You would not be able to harden the latter two, but it would still work. The drawbar that I have is not hardened and tempered but I sometimes wish it was. Be sure to yell if you have any questions or concerns. I will be out of town all of next week for work, but will certainly answer on the weekend when I return.


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## 34_40 (Jan 1, 2016)

Certainly not as nice as Terry's drawings but I did take some measures of mine and youcan find them here, about halfway down.  http://www.hobby-machinist.com/threads/8520-8525.23152/

Hope it can help.


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## Terrywerm (Jan 1, 2016)

For some reason I had the drawing open for the top cap and realized that there is a minor error on the drawing. Normally, a 3/4"-16 thread calls for a hole diameter of 11/16" or 0.6875" but one must remember that this will only deliver approximately a 75% thread. If you look at the drawing, it calls out to bore to 0.690" and then thread 3/4"-16. This is an error, and quite frankly I don't know why I put 0.690". It should obviously be 0.6875" to be correct.  At another point on the drawing, it calls out a 0.642" minor diameter for the thread. Naturally, this is calculated for a 100% thread. So, now you can all see the discrepancy.

When I made mine, I bored to 0.650" then cut the increased diameter of 0.750" further in, but left it at 0.740".  This created a relief area for purposes of cutting the threads. I then single pointed the threads as I did not have a 3/4"-16 tap available. Once the threading tool started to cut a mark in the 0.740" ID my thread cutting was complete and I finished up by increasing the 0.740" ID to 0.750" as per the drawing.

I apologize for the error, but since I realized it was there I felt I should point it out to avoid any confusion.


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