# Removing the Z backlash on my King PDM30  (RF 31 style) round column bench mill



## Canuck75 (Jan 22, 2014)

* Removing the Z backlash on my King PDM30 (RF 31 style) round column bench mill*

Z backlash can be annoying on fine drilling, boring and milling operations and it because of the pinion/rack design of the quill mechanism. There is a reason these little mills are cheap but it doesn't mean they cannot be improved. I found that to get a few thou change it could be problematic if you didn't have the backlash "taken up" correctly with the control handle, the result being that the quill could drop the amount of the backlash when you released the quill lock. The only way I could see to correct this deficiency was to have the return spring acting directly on the quill rather than on the pinion.

I did a lot of measuring and figured that a "windlass" arrangement could be fitted within the head casting to do the job. Moving the pinion spring case to the new "windlass" could provide the same return action as long as I replicated the same "leverage" power that the geared pinion and spring had on the quill. On my machine the pinion rotates 1 2/3 turn for 5.625" of quill travel. Dividing the travel by the turns gives you 3.376. Dividing this again by Pi give you 1.074. Thus a 1" dia windlass with a 1/8" return cable wrapped around it (effective dia of 1 1/8") would experience 1.591 revolutions during full quill travel. This is close enough to the original 1.6 pinion revs to give the return spring, when transferred to the windlass shaft, the same mechanical advantage over the quill as in the original configuration.




 


The top picture shows the windlass in place. The thru shaft is anchored to the windlass by the screw on the left. Yes, there is enough room to come up from the bottom with a long allen key to run the screw into place. The location of the shaft fore/aft was determined by the need for the cable to just miss the casting when exiting the bottom of the head as in picture two. The cover plate had to be slightly modified. Vertical location was simply a matter of where it looked good and for the shaft to miss the internal casting ribs as in picture one. "Squareness" of the shaft in X,Y and Z was a matter of measuring back from the original pinion shaft holes in the head over to the desired location.



 



 

 
A spring cap carrier was turned up out of AL exactly the same as the original and mounted on the head in the desired place. Later it was fitted with a ball bearing. A small ball bearing was mounted on the right side against a shoulder on the shaft and with a suitable end cover cap to hold it in place. The shaft is now "captured" between the bearings.





A block was added to the back of the quill clamp to anchor the return cable.





This shows the spring (and cover) mounted to the end of the windlass shaft and a new "false" cover, turned up on the lathe, to finish off the pinion shaft again.





This shows the finished setup. Someone curious might ask about the "second" cover, but it fits in with the general aesthetics of the mill. It will look better painted which will happen when I am all finished tinkering.


Finally, the quill is now always subjected to a direct upward pull equal to the spring tension, and, as such, eliminates any "backlash" concerns. Also, other than a lot of fiddling and planning, the only cost involved was for the bearings and a bit of cable, about $5 total, the rest came out of the scrap bin.

Hope someone finds this interesting.


Comment are welcome​


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## DAN_IN_MN (Jan 22, 2014)

Nice work!  Well documented!  This should be made into an article.


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## BlueRock (Mar 12, 2014)

A very neat solution to a very annoying problem. Thanks for the post!


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## sdunt (Mar 12, 2014)

Very interesting considering my 1988 Enco 105-1530 (apparently made by Fing in Taiwan) uses two extension springs, NOT a coil spring to retract the quill and those springs act directly on the quill, not through the pinion.

Here is a picture of the clamping plate that attaches to the bottom of the spindle - quill. The Springs run up into the casting and hook onto bolts that were run in from the sides of the casting.. This is the factory design for this unit. It also has a factory installed movable sheave variable speed belt drive on it as well.


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## Canuck75 (Mar 12, 2014)

sdunt said:


> Very interesting considering my 1988 Enco 105-1530 (apparently made by Fing in Taiwan) uses two extension springs, NOT a coil spring to retract the quill and those springs act directly on the quill, not through the pinion.
> 
> Here is a picture of the clamping plate that attaches to the bottom of the spindle - quill. The Springs run up into the casting and hook onto bolts that were run in from the sides of the casting.. This is the factory design for this unit. It also has a factory installed movable sheave variable speed belt drive on it as well.
> 
> View attachment 72341




Now this is a really interesting solution to the problem and much less complicated, I guess I don't think simply enough. One aspect of mine however, is that the spring tension is easily adjustable, whereas yours would require a change of springs unless there is a threaded adjustment from underneath?

I would very much like to see a picture(s) of the variable sheave drive setup.

Canuck75


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## JimDawson (Mar 12, 2014)

Very nice job!  I am doing about the same thing to my mill.  Rather than a the coil spring, I am using an air cylinder attached to the cable to provide the spring.


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## Canuck75 (Mar 12, 2014)

JimDawson said:


> Very nice job!  I am doing about the same thing to my mill.  Rather than a the coil spring, I am using an air cylinder attached to the cable to provide the spring.




Lots of different ideas. Post a picture when you are done.

Canuck75


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## sdunt (Mar 13, 2014)

Canuck75 said:


> I would very much like to see a picture(s) of the variable sheave drive setup.
> 
> Canuck75






The large 'dial' at the front, setting on top of the casting, when rotated pushes on a rod, that by cam action depresses the TOP sheave of the center large belt pulley and the rear pulley is spring loaded so it changes size in response. There is a manual 2 speed adjustment possible with the pulleys on top of the motor in addition.

Right now its kind of a Mute point as this is a 3 phase motor and  I had to add a VFD in my shop to be able to drive it.. The VFD does a much nicer job of variable speed than the belts can do..  http://monarch14.blogspot.com/2014/03/3-phase-power-in-home-shop.html


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## Canuck75 (Mar 13, 2014)

sdunt said:


> View attachment 72402
> 
> 
> The large 'dial' at the front, setting on top of the casting, when rotated pushes on a rod, that by cam action depresses the TOP sheave of the center large belt pulley and the rear pulley is spring loaded so it changes size in response. There is a manual 2 speed adjustment possible with the pulleys on top of the motor in addition.
> ...




Very interesting and complicated. Looking at this picture makes me ask how the quill control handle works, because it appears like there is no through shaft behind the quill on the left hand side of the head casting?. Any chance of a shot of the right hand side of the of the mill? I had also asked if there was an adjustment on the bottom of those two quill return springs since you said they just hook on two bolts up inside the top of the head casting?

Thanks
Canuck75


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## sdunt (Mar 14, 2014)

Canuck75 said:


> Very interesting and complicated. Looking at this picture makes me ask how the quill control handle works, because it appears like there is no through shaft behind the quill on the left hand side of the head casting?. Any chance of a shot of the right hand side of the of the mill? I had also asked if there was an adjustment on the bottom of those two quill return springs since you said they just hook on two bolts up inside the top of the head casting?
> 
> Thanks
> Canuck75


Sorry, nope no adjustments on the springs.. Just cut to length I guess if thats an issue.

The quill shaft, the one with the pinion gear on it that runs the 'rack' on the quill does NOT come all of the way through the casting, since there is no coil spring attachment on the left side.

Here is the right hand side:




No power down feed and your normal 'fine feed' worm drive assembly.  Also here you can see that the column clamping is in FRONT of the column. This casting is NOT split and squeezed together. There are two blocks on each side of the column that press in on the side and front of the column to lock it.  I've no clue if this is better or worse that other designs. On my todo list is a complete tear down and clean up. I got it used from a factory and there is old dried gunk on the table ways and the column has gunk on it. I need to get the engine hoist - Cherry picker back from my buddy to lift the head and column off.  

(Gray rectangles in that picture near the base are plywood that has spots on the other side for R8 shanked tools. Whoever built these "plugged" them into the 2 rear mounting holes and you can swing them in and out to make space around the mill when working.. )


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## Canuck75 (Mar 15, 2014)

sdunt said:


> Sorry, nope no adjustments on the springs.. Just cut to length I guess if thats an issue.
> 
> The quill shaft, the one with the pinion gear on it that runs the 'rack' on the quill does NOT come all of the way through the casting, since there is no coil spring attachment on the left side.
> 
> ...





Thanks for the extra information and the picture. I don't see the fixed length return springs as a problem either but just thought there might have been an adjustment mechanism available. Regards the column clamping arrangement I don't know which is better either. Mine has a split casting and is clamped from the rear. I do know from experience that this can, at times, torque the head a bit when tightening and I always had to reset my X after any head movement. Actually, that issue of keeping the X is what resulted in me pinning the rack to the column and bolting tight fitting guide blocks to the head to ride along the rack to keep the X. I don't worry about raising or lowering the head during operations now as this is no longer an issue for me.

With regard to removing the head, on mine I had unclamped it and tried lifting it up the column as a unit only to realize just how heavy it was. I didn't have a hoist either so my solution was to strip the head totally in situ since I was going to tear it apart anyways. After that it was quite easy to lift the bare head casting off by hand and afterwards, to reassemble it in reverse order - for what it is worth.

Anyways, nice machine.
Canuck75


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