PM-728VT Questions after two-years use

[Bryanaverill]

1) The quill has about .004" of total clearance front to back with the lock off and the quill all the way up (retracted). I know this because I mounted a dial on the table and measured the quill while manually pushing and pulling on the spindle with moderate effort. The clearance goes down to about .001" with the quill lock tight. This make precision spotting of holes a hassle.

2) The spindle drive sleeve has a lot of slop through the splines and rattles at low and medium speed. It's quite annoying and I'm questioning if it leads to tool vibration with deep cuts.

In either case, is it normal and can the slop be reduced?

Thanks
Bryan

 

[pontiac428]

First, I don't have a PM anything, but I used to have a Rong Fu, from which your PM-728VT is directly descended.

In general, quill collars on mills and drill presses have a ring pre-tension adjustment. They are in the form of two pinch bolts, one as a pre-tension adjustment, the other with a lockout handle on it. On the Rong Fu, the tension screw is to the rear of the quill and the lock lever is to the front. My old drill press has two pinch bolts together, so one is a lockout and the other works as a limit for the collar's opening when the quill is unlocked. These are built in for the very purpose of eliminating imprecision as you describe. It may be present on either side of your machine; I can see a few screws in the area on the sides of the head on your PM that may be the tensions. I imagine that the extra cash you paid to have a PM instead of a Rong Fu branded tool gets you a good manual and some of Matt's signature reach-back support; from what I've seen, he'll walk you through it.

I'm not sure how to help with the shaft sleeve slop. If it was old iron, maybe an electroplate buildup would tighten things, but I think you bought PM so you wouldn't have to. Oil helps.

 

[mikey]

Bryan, I own a RF31 with the exact issues you have. In my case, changing the spindle and drive sleeve bearings got me under 0.0001" tir without locking the quill lock.

I wrote it all up in several articles in the Rong Fu section. Take a look and see if you think it might apply to you.

Mikey

 

[Bryanaverill]

Just to clarify, my item #1 is not run-out. The spindle, and any tool I mount in it, runs true as far as I can measure with a .0005" test indicator. The problem is that the bore in the head appears to be slightly larger than the quill. If I deduct .001" for machine flexibility, I'm measuring close to .003" of diametral clearance. I'm not sure if this is normal for this mill. The practical problem is that when I spot holes with quill down feed, I get a small shift of the quill and a small position error. My practice now is to spot holes with the quill locked and feed down with column z-axis. This works but it's somewhat time consuming. I'm just wondering if there is anything I could or should do about it.

 

[Bryanaverill]

First, I don't have a PM anything, but I used to have a Rong Fu, from which your PM-728VT is directly descended.

In general, quill collars on mills and drill presses have a ring pre-tension adjustment. They are in the form of two pinch bolts, one as a pre-tension adjustment, the other with a lockout handle on it. On the Rong Fu, the tension screw is to the rear of the quill and the lock lever is to the front. My old drill press has two pinch bolts together, so one is a lockout and the other works as a limit for the collar's opening when the quill is unlocked. These are built in for the very purpose of eliminating imprecision as you describe. It may be present on either side of your machine; I can see a few screws in the area on the sides of the head on your PM that may be the tensions. I imagine that the extra cash you paid to have a PM instead of a Rong Fu branded tool gets you a good manual and some of Matt's signature reach-back support; from what I've seen, he'll walk you through it.

I'm not sure how to help with the shaft sleeve slop. If it was old iron, maybe an electroplate buildup would tighten things, but I think you bought PM so you wouldn't have to. Oil helps.

I'm not finding any pinch bolts to tighten the fit between the head and the quill. I have looked from the outside but not dug into the matter. Perhaps it's somewhere not easily accessible. I don't see anything in the manual either. It sure looks like the head is simply bored to fit the quill and the manufacturing tolerance is responsible for establishing the running clearance.

 

[pontiac428]

I'm not finding any pinch bolts to tighten the fit between the head and the quill. I have looked from the outside but not dug into the matter. Perhaps it's somewhere not easily accessible. I don't see anything in the manual either. It sure looks like the head is simply bored to fit the quill and the manufacturing tolerance is responsible for establishing the running clearance.

I checked the manual too. Seems to only have a "key screw", but not a collar adjustment. You might could preload the quill feed spur gear against the rack on the quill to brace up any existing play. Maybe if you feel out what moves on you, you can find a way to take up the play. How is your tramming?

 

[Bryanaverill]

I checked the manual too. Seems to only have a "key screw", but not a collar adjustment. You might could preload the quill feed spur gear against the rack on the quill to brace up any existing play. Maybe if you feel out what moves on you, you can find a way to take up the play. How is your tramming?

I have not trammed the machine but tram is not causing me a problem.

I have continued to research this subject and apparently a typical knee mill has the quill fit to very fine clearance to the bore in the head and fine control of cylindricity. I certainly do not have fine clearances on my small column mill.

 

[tcarrington]

FWIW - I have a PM 25 which suffered a similar issue as you describe, maybe worse. I contemplated many approaches but decided to replace the spindle body as an easy low risk effort. The one I received was nearly 0.003" larger in diameter than the original. That did the trick. While you don't need bearings, you might consider it since you will be changing the body, assuming it is similarly made. You will likely need some tooling to allow the removal of the bearing races from the spindle.
My fall back plan in the event that swap failed was to do some very fancy milling on one of the bodies and install shim strips to float the spindle body in the bore on at least three vertical strips. Glad I didn't have to do that.

 

[Bryanaverill]

FWIW - I have a PM 25 which suffered a similar issue as you describe, maybe worse. I contemplated many approaches but decided to replace the spindle body as an easy low risk effort. The one I received was nearly 0.003" larger in diameter than the original. That did the trick. While you don't need bearings, you might consider it since you will be changing the body, assuming it is similarly made. You will likely need some tooling to allow the removal of the bearing races from the spindle.
My fall back plan in the event that swap failed was to do some very fancy milling on one of the bodies and install shim strips to float the spindle body in the bore on at least three vertical strips. Glad I didn't have to do that.

I have thought about this exact thing. By spindle body, I believe you are referring to the quill, which is the part that holds the spindle bearings and slides up and down in the head. I can measure the bore ID and the quill OD and see what the measured clearance is. It may be possible to work with Precision Matthews and "hand select" a quill with a slightly larger diameter and effectively fix my issue. To be confident in what I'm trying to accomplish, I need the measurements first.

 

[qualitymachinetools]

If you are not able to measure your quill OD, you can send the quill in and we can check it for you if needed.

Its a fine line between play or flex, I would call "play" like where you barely touch it and it bumps back and forth. Almost like one finger hardly any pressure kind of thing, if that makes sense.

The "flex" on the other hand would be a little different, that's going to happen. I can move the head .005 or so on our large CNC machining centers by pushing on them. They don't have a quill though so not quite the same thing, there is no moving/sliding part to have play, this would be all flex. And the quill lock did make a difference on yours too

I just measured a couple quills, they are both right around 2.361" (plus or minus .0002) (Just under 60mm if going by Metric) See where you are with yours. The quill bore is 60mm on these mills.

If any issues, please email our tech@precisionmatthews.com I am not on here enough to reply fast (Please put a copy of this in so they know what you are talking about)