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

Peter,

Thanks much for the help and I will let you know what I find out as
far as machines. Enco has a mill they state the specs are
0.00078/ft. error for squarness and parallelism. If it is
that 'good', I might not even pull it part for installation of ball
screws and just use the backlash compensation in the software.
The 'only' bad thing about the machine is that it does not have
chromed ways.

Have you ever measured the load (current) on any of the motors during
max acelleration? I would be very interested to see how little /
much the motors are being used. I would like to run steppers and I
am wondering if a 1000 in-oz. or 1250 in-oz. would be large enough.

If I understand you correctly, the pully-head can be turned up to
6krpms? Is the bearings on the pully head larger or the casting
different vs. the vari-speed head? Any idea why the pully type can
spin faster?

What do you mean by dual thrust bearings? One per axis, or two per
axis?

Thanks.

Tom.

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Peter Renolds <prenolds@s...>
wrote:

> 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?
> >
> >
> >
> >
> >