PM727V Fine Downfeed Control Handwheel Dial Accuracy

[CKel44]

Hobby-Machinist / PM727V users: Using the fine quill downfeed on my new PM727V I am seeing increasingly imprecise readings on the fine downfeed control handwheel. I double checked the manual for correct operation, actually noticing for usage of the fine downfeed: "For milling operations calling for precise, repeatable control of tool depth..."

Well, it's not very precise. I emailed Precision Matthews and they replied that the fine feed dial inaccuracy is "...not surprising at all. This is why the quill is equipped with a digital DRO as well. Handwheel dials are always going to be approximate."

I've attached images of the DRO and fine feed dial readings. DRO is acceptably pretty darn precise with my indicator, but the handwheel dial shows increasing readings.

For my test measurements I checked 0.080" increments (which should show "0" on my PM727V fine control dial for every test revolution) at 0.080", 0.160", 0.240", 0.320" and 0.400" as follows:

#1: Indicator 0.000", Handwheel 0.0000", DRO 0.0000" (DRO +0.000, Handwheel +0.000")
#2: Indicator 0.080", Handwheel 0.0820", DRO 0.0795" (DRO -0.0005, Handwheel +0.002")
#3: Indicator 0.160", Handwheel 0.1655", DRO 0.1595" (DRO -0.0005, Handwheel +0.005")
#4: Indicator 0.240", Handwheel 0.2470", DRO 0.2400" (DRO +0.0000, Handwheel +0.007")
#5: Indicator 0.320", Handwheel 0.3280", DRO 0.3185" (DRO -0.0015, Handwheel +0.008")
#6: Indicator 0.400", Handwheel 0.4100", DRO 0.3990" (DRO -0.001, Handwheel +0.010")

I've taken these measurements numerous time and the results are extremely repeatable. Do other PM727V users see similar handwheel control dial results? I know Precision Matthews told me this is to be expected, it's just that I didn't expect this. I've used other mills (pre-DRO) whose analog dial readings are much closer than these. Am I asking too much of my PM727V analog fine control? Please correct/advise/reprimand me, THANKS!

Craig

 

[tweinke]

I have a 727M, I found the same as you. I use the DRO for measurement if accuracy is needed. It will be interesting to see what others have to say

 

[benmychree]

Perhaps it is a metric gearing system with an approximate inch graduated handwheel.

 

[jaek]

I think each turn of the wheel lowers the quill 2mm which is actually .0787 inches. The handwheel says .080 inches. Close enough?

 

[RJSakowski]

My RF30 clone has a rack on the quill with a pitch of 8mm and driven by a 10 tooth pinion resulting in 80 mm of quill travel for one complete revolution of the pinion. The fine feed worm drives a 37 tooth gear and has a nominal quill travel of .085"/revolution. In actuality, 37 revolutions of the fine feed advance the quill 80mm or 3.1496". Therefore, one revolution of the fine feed advances the quill 3.1496"/37 or .08512". This is verified by comparison with the DRO.

Gear ratios have to be a combination of whole numbers. Add to that the conversion from metric to inch measure.and it is extremely unlikely that you would have a dial that read inches exactly. If it were a metric mill and the worm gear had 40 teeth, one revolution of the fine feed would be 2.00mm. I suspect that when they converted the mill for Imperial use, they wanted to change as few components as possible, in this case a different worm and worm gear which would still fit in the gear housing.

 

[Driveslayer45]

I contacted PM about the same issue and was told the same thing, unfortunately even with the dro the overall quill accuracy seems weak at best, often slipping after set and locked. Also the clutch bands will often slip when trying to downfeed.

 

[CKel44]

I think each turn of the wheel lowers the quill 2mm which is actually .0787 inches. The handwheel says .080 inches. Close enough?

Well that would certainly explain it. Close enough? I guess it will have to be. THX

 

[mickri]

Part of the problem also lies in the tightening of the fine feed. Virtually impossible to get this the same every time. My RF 30 clone has the same problem. One revolution of the handwheel reads .900 whereas the actual movement varies around .850.