Re: [CAD_CAM_EDM_DRO] Re: What is the weak link in machine precision - Knee Mills

Hi Tom,

My servo motors (with 1000 line encoders) are OEM versions of a Baldor
MT-4090-BLYCE - they are 880 oz-in torque at 2460 RPM and 17.5 Amps.
Peak torque is around 2500 oz-in. They are bigger than required but
were priced well on eBay a few years back. I'm using the same size
motor on all 3 axes. Some of the other comments about BP clone mills
are very important - the range of quality when you start looking at the
imports is all over the place. As mentioned, many of the Taiwanese
units are very good quality - the two Chinese units I looked at were
very poor quality - not to say they won't do good (accurate) work, just
that the fit and finish were sloppy and that's on the parts I could see
- who knows about what you can't see... One VMC rep I spoke to
recently said that he knew of a company that bought a bunch of chinese
knee mills (several years back) and when they went to mount things to
them, they found that there were "gobs" (his words) of bondo all over
the castings to hide major casting defects. I've drilled a couple dozen
holes in my machine (all over) and have never found more than a thin
layer of filler. That's a relief!

There are good Taiwanse knee mills out there, and maybe some not so good
ones - you have to do the reasearch. I looked at many (including a
couple of BP's) before buying and ended up with excellent quality. Mine
is a First brand (in Canada) but is Sharp brand in the US. Unlike the
original BP's and some of the copies, mine came with dual thrust
bearings on X & Y - many just have dual axial bearings. If you want to
maintain rigidity, a pair of pre-loaded thrust bearings is the way to
go. All ways are hardened and chromed and after a couple years of heavy
CNC use, they have taken on a bright shine. Mine also has some patented
feature in the way you tram the head but I'm not sure what specifically
is different to a BP design.

One thing I would suggest is that if you plan to do a lot of CNC work at
higher RPM's go for the pulley head rather than the varispeed head.
Then use a VFD to vary your speed. You'll only need to change pulleys
to a low ratio for very slow high torque work. I don't do much with
bigger than a .75" end mill, but have to slow the speed for drilling and
very occaisional boring. I have done in a set of varispeed bearings as
I run most of my stuff at 4500 rpms and now have about 2000 hours on
the spindle. Even though I run it from high to low etc. every day, most
running is at high speed. I know of one industrial vendor that runs
pulley head knee mills at up to 6000 rpm's - I think the varispeed head
would shake apart at that speed!

The reason I initially installed the gas springs at the front of the
knee (see my pics in CCED - Retro of BP Clone CNC) was specifically to
overcome a vibration of the knee when I was running the knee fast (i.e.
rapid mode) - with the acme screw, before I fitted the ballscrew. I had
planned to add a brake to the Z motor because with a Z axis ball screw,
the knee just falls... Anyway with the gas springs, it holds position
just fine with the motor power off so I never bothered to fit the brake.

Both gas struts were from McMaster Carr P/N 9416K23 - 150lb Force
You'll also need four Ball Sockets P/N 9416K75 and four Ball Studs P/N
9512K83

You'll also need to drill and tap the four 5/16"-18 threads - two on the
sides of the knee and two at the front of the base. Make sure your
positions allow the spring to be vertical and not foul on any thing. It
took me a while to figure it out but it has worked out great!

Cheers, Peter



zipdrive2k wrote:

>Peter,
>
>Thanks for you input. What is the size of the motor you are using
>for the knee? Are you using a stepper with encoder feedback, or a
>servo system? Did you have to have the ways chromed after you bought
>it or did they come chromed? Do you have any other kind of supports
>to the table (air shocks, etc.) to help remove some of the static
>weight from the knee?
>
>
>
>
>