View Full Version : how to convert a mill to cnc?

21-10-2010, 07:38 PM
Im thinking about converting a fairly big mill that ive got with lead screws (i would say that the xyz axis are fairley easy to move). It has quite a lot of backlash at the moment so am going to see if i can reduse it but is good for its age (i realise it wont be super accurate). I have used hass mills before but know very little about converting mills. I have been looking at this system 4 kit http://www.diycnc.co.uk/html/cnc_systems.html and woundered if it would have enough power with 3nm steppers geared down 2:1. so i just want to know everything i need to know and if its worth doing.


Robin Hewitt
21-10-2010, 08:20 PM
A Newton is about 100 grams and motors are usually rated by holding torque rather than pull in torque. You need to see a torque against speed graph to know if you are in the right ball park. If the mill is of any size then you'd want NEMA 34 steppers or servo's for speed.

Suggest you wind some kind of tape around one of the handles and then see what weight you have to lift on the end of it before you feel the kind of torque you reckon would make a cut.

22-10-2010, 05:34 PM
Ok, so can i get a simular system for nema 34s without costing a fourtune. I do not particually want it to be very fast, so dont mind if its a bit slow.

23-10-2010, 04:48 PM
I tryed pulling on the handels and measuring the weight and at about 2" away from the centre it took 4 kilograms and at 6" it to under 1 with no load to move the table. like i said i would say it has very low friction and have used smaller mills with more friction. so dont know how much power i need?


Robin Hewitt
23-10-2010, 07:47 PM
Interesting choice of units :smile:

I calculate 1 kgf inch = 0.25 Nm or thereabouts.

4kgf at 2 inches = 8kgf inches = 2Nm
1kgf at 6 inches = 6kgf inches = 1.5Nm

Table's kind of stiff if that's at no load.

You can get more Nm out of a motor with a belt reduction but that doesn't do your top speed any favours.

Q1: Can you free the table up?
Q2: Does it matter if it's kind of slow?
Q3: Could your budget stretch to servo motors?



24-10-2010, 07:53 PM
Ok, i dont mind if its slow, dont think i can free up the table anymore. I dont know how much a servo system would cost, so dont know about budget. I thourght they were a lot more so dont know anything about them.


26-10-2010, 04:09 PM
What type of Miller?


26-10-2010, 06:53 PM
Hi, ive got a thiel 158 tollroom mill. Ive seen this kit http://www.rhonmac-cnc.co.uk/index.htm and woundered if the one with the 5.8nm steppers would be enough to power it geared down 2:1, but dont quite understand about the driver, as it appears to me that it can run 4.2 amp but the motor takes 7?


27-10-2010, 01:23 PM
I used to have a couple of Thiel 158's, excellent machines. I remember the X axis being nice and free flowing, but the Y axis was always stiff. The Y axis is created by an overarm coming forward and backward with the spindle is attached, this all added to the weight and stiffness. Have you thought how you will get the stepper/servo drive to the Y axis? I am assuming you have a fixed table and not the XY fancy one.



28-10-2010, 03:20 PM
Hi, i have the fixed one i think, its the same as the one that you attached a pic of. The y axis is surprisingly free moving with little backlash but can see how it could be hard to move. My x axis is the one that is harder to move and also has a bit of backlash so i need to find a way to reduce that first.


30-10-2010, 06:09 PM
I have now reduced the backlash in my mill to 5 thou from 10, but dont know what an aceptable level is, so is this too much or will it do?


Robin Hewitt
30-10-2010, 11:16 PM
The acceptable level is whatever you are prepared to accept :smile:

If the sloppy slide is tight enough to resist the cutting pressure you can then correct backlash with software.

07-11-2010, 08:37 PM
thanks for info, the table should not move under cutting pressure so backlash compensation should help. been looking at the nema 34 kits from zapp and woundered if they were powerful enough but dont know what size motors i need

08-11-2010, 08:17 AM
Hello Speedhound,
It looks like a nice machine .
And it will take a serious effort to make it run .
But nothing that a few hours in your workshop cant fix
Is it correct to assume you have changed the axis to ballscrews (there are zero backlash preloaded ballscrews).
If so which kind of bearings you use on the end (the rigid side).
You will need bearings that get rid of the radial and axial play.
you can do that whit one bearing ore a combination of more then one.
On the opposite side they need to be supported whit a needle bearing ore a ball bearing .
In a way they can compensate expansion and shrinkage of the ballscrew.
For the motor it will depend on how fast you want to go whit the feeds.
They have told me for a mill like yours you need about 8 NM on the screw
So if you have 3NM motor it need a reduction of 3 to accomplish your goal.
Enjoy your build.
Kind regards,

Experience is the sum of stupidities you have done.

09-11-2010, 04:05 PM
Dont forget a 3Nm motor will not give you 3Nm, unless it is at zero speed.
The 3Nm is holding torque and the actual torque you will get is dependant on the driver, RPM and Voltage.
You need to look at the torque curve of the motor to determine the torque.
as an example a 3nm holding torque motor will give you between 0.4Nm to 1.8Nm depending on the voltage and speed.

09-11-2010, 06:58 PM
Thanks for replys, it doesnt have ballscrews which is why i was concerned about backlash but hopefully backlash compensation would help out with that. 8nm is interesting, this is the kind of thing i need to know as i dont know what motors to use. I understand that the motors are rated on holding torque. What size motors do you think i would get away with(would i need 12nm), i dont mind if its a bit slow but want it to be useable, so i could gear it down 2:1 or something but i dont know how fast it would be.


Robin Hewitt
09-11-2010, 09:16 PM
Are you sure that leaving 1.5Nm of torque eating gunge in the slideway so that the table will hold position for your backlash correction is the best way to go? Surely it is eating the nut :eek:

10-11-2010, 09:13 AM
I assume you will need a central lubrication system to keep everything running and then you will have possibly less then 1,5NM on the slideways .
If you set it at intervals of 20 min and lasting approximately 10 sec on each slideway.
Kind regards,

Experience is the sum of stupidities you have done.

10-11-2010, 08:07 PM
I dont think that it is gunge causing the 1.5nm of torque, its just a very heavy table, and that is the torque that is needed to get the table moving. Im not planning to use it loads, but keeping it well oiled like you say should help with the friction.