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Author Topic: Z Axis Not Holding  (Read 6323 times)

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Offline olf20

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Z Axis Not Holding
« on: February 05, 2009, 07:17:45 PM »
Hi all hope everyone is doing well!!
I have a Atlas 9 x 42 knee mill converted to CNC 3 axis, with gecko drives and Mach3.
I mostly do wood products on this machine and it does a great job. Sometimes though
I need to do some milling in metal and I have a problem with the Z axis.
The Z axis is driven with Nema 34 900 oz in stepper with a 2:1 pulley / timing  belt drive.

When the Z does a plunge, as the tooling comes in contact with the metal the stepper will
skip several steps unless the plunge rate is set extremely low. The stepper does not have
enough power to drive the tooling into the metal. When I designed the conversion I talked to Home Shop
CNC and Rick thought that the 900 oz in motor would be plenty big enough. I have 1200 oz in steppers
on the X & Y and they are 1:1.

I was just wondering what size of motor other Bridgeport clones have for their Z drive motors?

Thanks for any help / advise!!
olf20
Mach3, Atlas Knee Mill, 4th Axis, VcarvePro, ESS, Super PID.
Been Heating with corn since 1998

Offline Hood

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Re: Z Axis Not Holding
« Reply #1 on: February 05, 2009, 07:37:00 PM »
That motor should be more than ample, on my manual Bridgeport I had a 316ozin geared 3:1 and it would plunge a 20mm slot drill into 316 no problem. On the BOSS, I have a 916ozin at 2.5:1 and it too has way more than is needed.
 Is it an old motor? could be loosing its magnetism, or maybe just a bad motor.
Hood

Offline Sam

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Re: Z Axis Not Holding
« Reply #2 on: February 05, 2009, 09:17:43 PM »
What type of tooling are you trying to drive into what type metal at what feedrate?
"CONFIDENCE: it's the feeling you experience before you fully understand the situation."
Re: Z Axis Not Holding
« Reply #3 on: February 05, 2009, 10:06:50 PM »
Hi all hope everyone is doing well!!
I have a Atlas 9 x 42 knee mill converted to CNC 3 axis, with gecko drives and Mach3.
I mostly do wood products on this machine and it does a great job. Sometimes though
I need to do some milling in metal and I have a problem with the Z axis.
The Z axis is driven with Nema 34 900 oz in stepper with a 2:1 pulley / timing  belt drive.

When the Z does a plunge, as the tooling comes in contact with the metal the stepper will
skip several steps unless the plunge rate is set extremely low. The stepper does not have
enough power to drive the tooling into the metal. When I designed the conversion I talked to Home Shop
CNC and Rick thought that the 900 oz in motor would be plenty big enough. I have 1200 oz in steppers
on the X & Y and they are 1:1.

I was just wondering what size of motor other Bridgeport clones have for their Z drive motors?

Thanks for any help / advise!!
olf20


Is your Z axis driving the quill or the knee?  That motor should be plenty for the quill, but not nearly enough for the knee.  What pwoer supply voltage are you running?

Regards,
Ray L.
Regards,
Ray L.
Re: Z Axis Not Holding
« Reply #4 on: February 05, 2009, 11:01:55 PM »
Also...if you are driving the quill, are you using a ball screw or are you doing as some and driving the rack and pinion ? The pinion drive would require much more power than a screw.
RC

Offline olf20

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Re: Z Axis Not Holding
« Reply #5 on: February 06, 2009, 06:48:12 AM »
Thanks for the replies!
I'm driving the quill via the rack and pinion and cut aluminum, cold rolled shafting.
Nothing hard.
The power supply is 72 Volts with plenty of power.
The motor was brand new and has done this from the beginning.
The only other thing that I did not think of earlier is the return spring.
I did not unhook it. That I'm sure would help some.
I have a spare 1200 oz in, maybe I'll try it just to see if its the motor.
olf20
« Last Edit: February 06, 2009, 06:51:01 AM by olf20 »
Mach3, Atlas Knee Mill, 4th Axis, VcarvePro, ESS, Super PID.
Been Heating with corn since 1998
Re: Z Axis Not Holding
« Reply #6 on: February 06, 2009, 06:56:19 AM »
That is the most inifficient way to do it and requires a bunch of power. Just compare the torque required to drill with the pinion handle to drilling with the knee crank or the manual worm drive quill feed.
RC
Re: Z Axis Not Holding
« Reply #7 on: February 06, 2009, 02:21:13 PM »
Hi, Using the rack and pinion is a definite no-no. You would probably need at least 10:1 reduction and even then I'm not sure that would be sufficient for all jobs. Having looked at all the ways people have converted their machines whilst building my own, for the quill ballscrews are most common. A clamp bracket is fixed to the quill and the ballscrew is fixed to that at the lower end of the shaft and the ball nut is supported by bearings with a pulley so that  the nut is driven. The shaft of the screw moves up and down in the vertical without rotation. A 2:1 or less reduction would be fine depending on the pitch of the screw.


John     ;D
Re: Z Axis Not Holding
« Reply #8 on: February 06, 2009, 02:32:41 PM »
I haven't been here very long but I have only seen 1 setup where the motor and pulleys were actually put on the manual handfeed worm drive. BP type mill. Probably results in good power through the reduction but I can only imagine the gross amount of backlash.
There is quite a bit of lash in the rack/pinion as well....usually.
RC

Offline olf20

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Re: Z Axis Not Holding
« Reply #9 on: February 07, 2009, 07:50:05 AM »
WOW! The things you learn.
Thanks for the replies. When I set this machine up I looked at as many
posts as I could find on how to do this. Somehow I missed the fact that
people were putting ball screws on the quill. However that being said the
backlash is very minimal.
Great point on the amount of effort need to drill a hole, which brings
another question;
What does the power quill feed use to move the quill. I would think that
this is reasonably accurate, but gets its power from the mechanical drive.
Being a total newbie to the mechanics of the machine, where does the
ball screw fit. Anybody got a picture of a setup???
Again, thanks for the wealth of information!!
olf20

Mach3, Atlas Knee Mill, 4th Axis, VcarvePro, ESS, Super PID.
Been Heating with corn since 1998