Spindle Runout Issues on PM-728VT

[pontiac428]

If I had those numbers I'd be very happy. I don't think I'd want to see more than the .0005 at 1 1/4, given the impact to tooling in that sort of sweet spot range where most of your cutters will be, but even double the 12" number (.002) at 12" would probably be fine. .

Having those numbers represents a higher purchase price than Precision Matthews is offering. It indicates a different class of machine, which is not to say that you aren't entitled to quality; I think for the price paid that you should get a QC sheet, but it is telling that you didn't.

For my mill, the manufacturer won't release it unless it's below 0.0008 at 12" down on the test bar. This is not my QC sheet, it's an example that came with the manual (they all should), so don't snicker at the 1975 date, it's beside the point.

 

[netmagi]

Having those numbers represents a higher purchase price than Precision Matthews is offering. It indicates a different class of machine, which is not to say that you aren't entitled to quality; I think for the price paid that you should get a QC sheet, but it is telling that you didn't.

For my mill, the manufacturer won't release it unless it's below 0.0008 at 12" down on the test bar. This is not my QC sheet, it's an example that came with the manual (they all should), so don't snicker at the 1975 date, it's beside the point.

View attachment 446417 View attachment 446416

Agreed, particularly the number at 12" down. I don't expect .001 at 12", let alone .0008 That's impressive!

 

[pontiac428]

Agreed, particularly the number at 12" down. I don't expect .001 at 12", let alone .0008 That's impressive!

It's Lagun, so they used German bearing specs. I don't think I've ever run a tool that long, but when you're spinning carbide mills fast, runout will shatter tools. At the very best it will impact your finish and leave you adding fudge factors to tool diameters.

There are some tricks that go into pressing races onto spindles. Your bearings should have been indexed for runout. I don't recall if it was covered in this thread, but when you are measuring up the spindle in vee blocks, you index the high spots and use the witness marks on the bearings to take advantage of systems of misalignment. You stack the error on the bearing to counter the error in the spindle. This is the kind of topic you see discussed in old machine shop texts. A milling machine is not an AR-15, you don't just buy the parts you like and bolt it together. Precision parts must be fitted, which takes time and skill, and always adds a sizeable cost due to the high value of skilled human labor in manufacturing.

 

[netmagi]

Yeh, I don't even think I'd have 12" to work with on the 728 with a vise on the table, but since axial runout will increase the further away from the nose you are, it speaks to how minimal that axial runout is. I've been limited to very short tooling in this mill thus far for exactly the reasons you stated.

 

[pontiac428]

In your shoes, I'd be sharing your concern. .003 is too much at tool length. When you swapped spindles, were the bearing races marked for index?

Also, is that radial runout or total?

 

[netmagi]

In your shoes, I'd be sharing your concern. .003 is too much at tool length. When you swapped spindles, were the bearing races marked for index?

Also, is that radial runout or total?

TIR

I don't believe they were, but I can check the pics I took. There's 2 bearings near the bottom, one angular contact P5, and a std. quality ball bearing right above it, and then another std. quality ball bearing in a small size further up. All were C & U brand (China).

 

[netmagi]

In your shoes, I'd be sharing your concern. .003 is too much at tool length.

PM markets this machine as "The Highest Level of Quality and Precision in a Bench Top Mill Available Anywhere", and goes on to state "Ultra Precision Ground Spindle, with Triple Bearing Support". I realize that's just marketing speak when it's not backed up by any real error tolerance figures, but what precision *should* one realistically expect from a machine in this "class"?

 

[DavidR8]

My 1978 Long Chang RF30 clone with original bearings had .0003 runout at the spindle nose and .0004 at the end of a 1/4" gauge pin chucked in an MT3 collet.
Edit: my LC30 was also Taiwanese.

 

[pontiac428]

My experience matches David's. My 1996 RF31 (Taiwan) was great at passing tests.

Hmmm. I suppose with ABEC 5, it could go either way, but typically you see it when bearings are paired. Only one precision bearing on the spindle, not two angular contacts in an opposed pair? I don't know what to think about that. If I were to spend retail on that machine, I'd hope at least $500 of that goes to good bearings in the head. It's a hell of a tricky place to cut corners, the mill head. I'm glad the dealer is working with you, but I'm giving side eyes to anyone thinking about purchasing one of these machines. You can't skimp on the parts that make the mill what it is.

 

[netmagi]

My experience matches David's. My 1996 RF31 (Taiwan) was great at passing tests.

Hmmm. I suppose with ABEC 5, it could go either way, but typically you see it when bearings are paired. Only one precision bearing on the spindle, not two angular contacts in an opposed pair? I don't know what to think about that. If I were to spend retail on that machine, I'd hope at least $500 of that goes to good bearings in the head. It's a hell of a tricky place to cut corners, the mill head. I'm glad the dealer is working with you, but I'm giving side eyes to anyone thinking about purchasing one of these machines. You can't skimp on the parts that make the mill what it is.

I actually took video showing each of the bearings when I disassembled the quill thinking the part numbers might be helpful to someone in the future. Sure looks like a single angular contact bearing to me.

Just uploaded the vid as "unlisted" on my channel: