Hello Guest it is October 17, 2019, 10:14:42 PM

Author Topic: Prima Lathe conversion  (Read 28294 times)

0 Members and 1 Guest are viewing this topic.

Re: Prima Lathe conversion
« Reply #50 on: March 10, 2014, 03:59:53 AM »
My z axis ball screw is almost 900mm. I am using deep groove ball bearing at the free end. The x axis screw has no end support. It is just 250mm long. You can also try the rotating nut mechanism similar to the one used in early Bridgport series I CNC mills.

Offline vre

*
  •  20 20
    • View Profile
Re: Prima Lathe conversion
« Reply #51 on: March 24, 2014, 01:22:00 AM »
thank you!
I have checked the rotating nut mechanism it can give better column strength and better critical speed with the
caveat that the Z axis servomotor must be fixed to carriage and move together with carriage.
(I also found that some high end machines use hollow shaft servos for direct driving the ballnut )
My Z axis servo is 8kg so it is too much to move x & z axes motors with carriage.

So I have 2 solutions remaing
1) Bigger diameter Z axis ballscrew (40mm) to overcome column strength and critical speed.
I have a dilemma if i would use 32mm or 40mm diameter ballscrew because both ballscrews fulfill the critical speed but the 32mm has about 2900kg column strength and my servo can give force about 3500kg the 40mm fulfills both critical speed and column strength force but has about triple inertia of 32mm ballscrew and this will degrade much the performance.
 
2) Do not use a ballscrew but the rack pinion mechanism that lathe has for semi-auto feeds.
This would be inaccurate and will have a lot of backlash but it will not have the the problems of critical speed & column strength
From that i can see this lathe which is very long uses this method http://www.youtube.com/watch?v=E-F5vgQnKVg
(or uses a ballscrew ? i can't see clearly)

Your Z axis ballscrew is 900mm and 10mm pitch but what diameter ? 20mm 25mm or 32mm ?
Re: Prima Lathe conversion
« Reply #52 on: March 24, 2014, 05:44:40 AM »
I guess the most cost effective solution would be to go with the 40mm screw. Mine is 32mm. Extra inertia will result in slower accel and decel rates. Rack and pinion would be ok if you use a linear scale on the z-axis and use servo drives that can handle two encoder inputs (like Delta A2 series servos). You'll have to mount the servo with a gear box to the slide carriage for rack and pinion mechanism.

Zafar

Offline Hood

*
  •  25,855 25,855
  • Carnoustie, Scotland
    • View Profile
Re: Prima Lathe conversion
« Reply #53 on: March 24, 2014, 06:41:42 AM »
JFYI my lathe has a 2m long, 40mm dia 5mm pitch ballscrew on Z. It is supported both ends and I have my Velocity set to 10m/min and accel 800mm/s/s. I previously ran at 15m/min with no problem but reduced to 10m/min as I thought that fast enough.

Hood

Offline vre

*
  •  20 20
    • View Profile
Re: Prima Lathe conversion
« Reply #54 on: March 24, 2014, 11:07:33 AM »
Hood
From this calculator http://www.nookindustries.com/EngineeringCalculator/MetricCriticalSpeed with 2000mm length 4005 ballscrew (36mm root diameter)
with type c end support the critical speed is 1600rpm so 1600rpm x 5mm = 8000mm/min.
How you get 10m/min and 15m/min rapids without whipping problem ? Maybe the calculator is wrong and calculates with big safety factor ?

Offline Hood

*
  •  25,855 25,855
  • Carnoustie, Scotland
    • View Profile
Re: Prima Lathe conversion
« Reply #55 on: March 24, 2014, 12:52:49 PM »
The actual length of screw free of the bearings is approx 1700mm on my lathe and I certainly dont have issues with whipping.

The length of the screw on my Chiron FZ12S is approx 900- 1000mm outside of the bearings, its only supported at 1 end and spins at 2000rpm (10mm pitch and 20M/min rapids), the calc says about 1200-1500rpm.
 I think Chiron will know what they are doing seeing as their machines are top end and as the machine is 27 years old I think it would have had issues before now if they were wrong.

Hood

Offline vre

*
  •  20 20
    • View Profile
Re: Prima Lathe conversion
« Reply #56 on: March 24, 2014, 03:46:45 PM »
I measured again and the shortest length of screw i can put between bearings in my lathe and is 1750mm.
I will put 10mm pitch ballscrew but i have not decided for diameter 32mm or 40mm.
32mm has lower column strength but has lower inertia so better acceleration also is cheaper.
40mm has better column strength (almost double) but much higher inertia (almost tripple) so worse acceleration also is more expensive.
Critical speed is ok in both cases.
In Z axis i will use 1.8kw servo 3000rpm with reduction 1:3 so i will have max 1000rpm to ballscrew with 10mm pitch rapids will be 10m/min.

If it was your lathe what diameter would you install 32mm or 40mm ?
Because this size ballscrew has some cost i dont want to make a false selection.
I want just your opinion 3210 or 4010 ballscrew ?
thank tou! 

Offline Hood

*
  •  25,855 25,855
  • Carnoustie, Scotland
    • View Profile
Re: Prima Lathe conversion
« Reply #57 on: March 24, 2014, 04:46:36 PM »
I dont know what size your lathe is, mine is fairly big, the carriage, I suspect, (with the turret) would weigh in about 6-800Kg.
Hood

Offline vre

*
  •  20 20
    • View Profile
Re: Prima Lathe conversion
« Reply #58 on: March 24, 2014, 06:04:15 PM »
My lathe is the same with this http://images.ua.prom.st/28306498_w640_h640_27032013431.jpg
but has 400mm more z axis length mine is 1400mm between centers
and the lathe in photo is 1000mm between centers but all the others are the same
My lathe has total weigth 1250kg

(to correct my previous post 40mm ballscrew has x1.6 inertia of 32mm ballscrew not tripple)
« Last Edit: March 24, 2014, 06:18:13 PM by vre »

Offline Hood

*
  •  25,855 25,855
  • Carnoustie, Scotland
    • View Profile
Re: Prima Lathe conversion
« Reply #59 on: March 24, 2014, 06:25:33 PM »
That lathe looks a similar size to  a Colchester Triumph and the saddle looks to be relatively light in comparison to the one on my lathe.
Below is a pic of my lathes original front toolpost with a Colchester Triumphs toolpost sitting on top to give an idea of scale. The boring bar in the toolpost is 32mm dia.
I would think the 32mm dia should be enough for a lathe your size if 40mm is on mine and works fine.
You may be able to get the length of the ballscrew down even shorter if you can mount the ball nut to the front of the carriage, that way you could have the end bracket in a bit from the end of the lathe bed.

Hood