Quill downfeed on bench mill

D

Driveslayer45

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Hello All,

I've had a PM727V for several months and loving it, except for one part. The down feed of the quill seems to be inconsistent. as i feed down the quill will stop dropping then will 'drop' to catch up. i have contacted PM and they indicate this is normal as the quill is 'dropping' to take up backlash in the worm gear. intellectually I understand this response and it makes sense. practically its driving me nuts. This is my first mill as I am coming off of an old import 3 in 1, but that 3 in 1 doesn't exhibit this behavior. I have been told to snug the quill lock (on the left) to prevent or minimize this.. maybe I just cannot get the feel for it.

What is everyone else doing, are we just living with it, is there an adjustment to make, how snug are you making the lock ..

Any input is appreciated as sometime I have to work the quill back and forth to get it to the depth I want. Also I assume this isn't a problem when I am actually cutting as the pressure of the material will take up the backlash ..

Thanks much
 
I don't have a PM but what you are describing doesn't seem right. My mill drill has a return spring which always keeps the quill at the upper position. Gravity should not be sufficient to pull the quill down, only user applied downward force. Spring tension is adjusted until this is so.
 
So the return spring is where I started, I tightened it up, a lot, and still had the problem but in doing so I introduced another; sometimes, with the spring tight, the quill would stop going down when i turned the feed dial.. i would have to loosen the engagement knob and retighten it.. effectively the clutch was slipping.

I was advised to loosen the spring till it would lift the quill, but no more. i loosened it which cleared up the issue i caused, but i still have the problem that caused me to tighten the spring in the first place.
 
Driveslayer45 said:
So the return spring is where I started, I tightened it up, a lot, and still had the problem but in doing so I introduced another; sometimes, with the spring tight, the quill would stop going down when i turned the feed dial.. i would have to loosen the engagement knob and retighten it.. effectively the clutch was slipping.

I was advised to loosen the spring till it would lift the quill, but no more. i loosened it which cleared up the issue i caused, but i still have the problem that caused me to tighten the spring in the first place.
Click to expand...
OK Driveslayer, I owe you an apology. I went down to look at my mill and found that I had made a modification, likely to cure the same problem that you are experiencing. I made the mod more than thirty years ago and frankly don't remember it. But the spring isn't in the parts diagram so I must have.

The Quill return spring is a constant force spring which provides the restoring force to the pinion shaft. It does not account for back;lash in the pinion gear/rack. In that regard , the quill will take up the lash in the downward position. As you start to move the quill down, the quill will hang up until the lash is removed and then start to move. If everything is loose enough, gravity will pull it down agaqin taking up the lash. This the the erratic motion you are observing. There are to ways to eliminate this.

One is to loosen the quill up so that gravity will always pull it down, the extent being controlled by the pinion gear. This is not desirable as when end milling the are additional forces pushing the end mill and therefore the quill up.

The second way is to slightly tighten the quill lock. This provides additional friction which counteracts the gravitational pull on the quill and the quill only responds to the force provided by the pinion gear. You will still have backlash and to that end all final quill adjustments should be downward.

A third way is to mod the machine. I attached a heavy extension spring to the back of my quill with the other end anchored high inside my head. The spring is strong enough to counteract the gravitational force Thought the travel range of the quill. Checking my quill, I have smooth travel over the range of the quill. There is about .011" of backlash but since I make all my quill adjustments downward, it only comes into play when I am retracting the quill.
Quill Assist .JPG
 
RJ that is beautiful, thank you for sharing that and taking the time to look at your own setup. It is comforting to know that this is a common symptom, now i just have to figure out the best way to fix it for me. like you i prefer to make all my adjustments during the down feed as well. thank you.
 
My Rong Fu does what you describe. I cut the amount of backlash by 2/3 by loosening the mounting bolts for the quill feed R&P and adjusting the tooth engagement using the oversized slop in the bolt holes. When the play was almost gone, I tightened everything back up and have been appreciating the difference ever since. Else, there are some ideas out there for using a gas strut or a weight and pulley to force the quill to one end of its backlash. The remaining backlash that I do have I work past by finding my number and locking the quill tight.
 
I had that problem with my PM25, I instaled a brass button backed by a stiff compression spring inside the quill clamp. By feeling where the spring is totally compressed, then backing off a bit, I put pressure on the quill to keep it from dropping.

Edit: correct typo.
 
I have (or had) that problem with my LMS 5500 (Sieg SX2.7). I discovered it when trying to plunge mill some blind holes to a specific depth. I used the fine feed to feed to the depth I wanted, but I didn't stop the spindle. In a couple of seconds, the EM had plunged a few thousandths lower than when I stopped feeding it. I fought that shortcoming for awhile by s-l-o-w-l-y downfeeding when I got close to my target depth. Then I added a quill stop which I can preset at my target depth. No more problem.

Tom

Edit: The same problem was also discussed in this thread: https://www.hobby-machinist.com/threads/perplexed-a-little-help-needed.79163/
 
Thanks for the feedback everyone.
Pontiac - taking out the excess backlash is something i am interested in tryin to figure out how to do on this machine.

T B - i was thinking of doing something similar but i think the 727 uses a squeezing clamp to lock the quill. I am looking for a stiff spring that will go over the shaft of my quill lock so i can use it to keep some tension on my quill, then still be able to tighten down to lock the quill, fully collapsing the spring.

H - i have experienced the same thing and it is very frustrating. I've ruined a few parts before i figured out what exactly was happening.
 
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