Pm727 quill accuracy

[tweinke]

might sound a bit silly but is the read head loose on its mounting?

 

[Driveslayer45]

It was when I first checked it. I remounted it and tightened it up.

 

[Rex Walters]

In many cases the difference like that is it may have a metric screw and the error is in the conversion to the handwheel imperial dial.

Sorry to revive an old thread, but I just discovered EXACTLY the same error on my Grizzly G0762 square-column benchtop mill.

The dial has one hundred divisions that are supposed to represent 0.001", but one revolution of the fine feed handle moves the quill axially 2.5mm (0.0984"), not 0.100". That's a ~1.6% error which makes it hard to hit dimensions to the nearest thou (generally my goal when milling).

This is a pretty big deal on benchtop mills because we can't move the table in Z. On a knee mill, I think one usually raises the table for accurate Z adjustments and tends to leave the quill locked with minimal stick-out for rigidity when milling — the quill dial is only for boring holes. On a benchtop mill, head movements in Z are harder to do precisely (lowering vs. raising, usually with no dial or DRO for the Z-location of the head itself), so we pretty much always use the quill to adjust cutter depth (re-locking for each pass).

This explains more than a few of my blown dimensions, and explains why I always had to creep up on things with constant mic'ing instead of depending on my dials. If I hadn't just installed a DRO, I'm not sure I'd have discovered the error. I've been blaming lack of rigidity and (when nobody is around) lack of skill!

This is a big enough deal that I plan to start another thread in the Asian mill section to warn people. I'll show how to use an indicator and gauge pins (or end mill shanks) to check for errors in the dial markings. In hindsight, I think adding a cheap LCD battery powered DRO to these Asian mills should be a priority.

 

[wrmiller]

On my 935 knee mill, I always brought the table up to close to the part to be milled, unlocked the quill, brought the quill down (gently) till just touching the part and locked it. Then I zeroed my z-axis on the dro (or zero the dial if no dro), and 'bumped' the z-axis handle bringing the knee down just slightly. I then moved the cutter off the part, dialed in the depth of cut, and moved into the part (side cutting).

I did the exact same thing with my square column bench mills (I've had several) except I moved the head up to get the cutter off the part instead of moving the part/table down. Worked for me, but YMMV of course.

NOTE: This method doesn't work for pocket milling.

 

[Rex Walters]

Yup. That's pretty close to how I do it on my mill, except I'll use a pin or a feeler gauge on top of the part to do the touching/feeling. Then enter that diameter/thickness instead of zero.

I then moved the cutter off the part, dialed in the depth of cut

And right there is when I was blowing my dimensions ... because I believed my dial.

I often move Z in 0.100" increments when milling. But I was always a few thou away because my dial was lying to me.

 

[wrmiller]

Yup. That's pretty close to how I do it on my mill, except I'll use a pin or a feeler gauge on top of the part to do the touching/feeling. Then enter that diameter/thickness instead of zero.

And right there is when I was blowing my dimensions ... because I believed my dial.

I often move Z in 0.100" increments when milling. But I was always a few thou away because my dial was lying to me.

Sorry, just re-read your post above and saw (this time) the 'Imperial is not equal to Metric' line. Ouch. Are the screws metric?

 

[Rex Walters]

Sorry, just re-read your post above and saw (this time) the 'Imperial is not equal to Metric' line. Ouch. Are the screws metric?

Yup. I installed a DRO on X and Y the day I got the mill, but had been relying on the dial for Z. I've never even looked at the dials on X and Y, but I'm fairly certain they have the same issue.

There is a special place in hell for people that make machinist tools that effectively believe that "25mm equals one inch".

For my sins, I buy cheap collets. I've lost count of the number of fractional-inch labeled but actually metric sized collets I've acquired. For 5C collets it's particularly egregious because you really do want to be within a thou or two for it to hold accurately.

 

[wrmiller]

Yup. I installed a DRO on X and Y the day I got the mill, but had been relying on the dial for Z. I've never even looked at the dials on X and Y, but I'm fairly certain they have the same issue.

There is a special place in hell for people that make machinist tools that effectively believe that "25mm equals one inch".

For my sins, I buy cheap collets. I've lost count of the number of fractional-inch labeled but actually metric sized collets I've acquired. For 5C collets it's particularly egregious because you really do want to be within a thou or two for it to hold accurately.

I had a very early PM25 (Weiss) that I put a 3-axis DroPro DRO on, with the Z-axis on the column. Worked well enough for my pistolsmithing projects.