Converting Rong-Fu 25

[adryan]

Hi all,

first a small introduction
I'm Adryan, studied for mechanical engineer (graduated 8 years ago) and work with my RF-25 mill and a lathe as a hobby. I also fly homebuild model turbine driven airplanes...enough for the introduction

After seeing the site of Jeff Davis, I decided to convert my mill to CNC.
The mill I have is a Rong-Fu 25 with a 1 bhp motor. It's one of the old models with a steel v-beld protection cover. I bought this machine second hand 3 years ago for $650.

I order the drawings and found out they didn't fit to my mill. The sizes of the RF-25 are quite a bit smaller.
I redraw most of the drawings using the drawings from Jeff as a guide and made some minor modifications (e.g. preload to the ballscrews)

Here are some pics of the rebuilding of the mill I'm starting this week.
-I cleaned-up the base and the x-y table
-I lapped the ways.....I know it's not everybody's favorite, but I'm very pleased with the result!
-Next was the priming and painting.
-Installing the x-y ballscrews

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[Runner4404spd]

any chance of getting some more pics. also, i've got an RF-25 and was waiting to disassemble it to measure all the mounts for the ball screw nuts. if you can email me those drawings i would appreciate it. it will save me from disassembling, reassembling, machining, then reassembling and rebuilding.

jczub at hotmail dot com

[adryan]

Here are some more pics of the controller box I have made for it and of the parts for the conversion.
One thing I remarked when I removed the round colum, the were some thin shim plates beneath it. Has anyone seen this before on a RF mill?

[Runner4404spd]

yup, i had the shim plates beneath mine as well. i also had shims where the bearings go for the handles on all three positions.

[Chris D]

The shims are put in to make the colum square to the table. When you re-assemble the machine, you will have to tram the spindle to the table. Make adjustments with shims in the same place.

Couple of questions...

What is that white stuff at the ends of the ball nuts? I realize you are using it for a wiper, but what is it?

You are using DB9 pin connectors for your steppers? If so, are your motors at a low enough current where they won't heat up?

Keep posing the photos and information! It's a great big help to those of us in the planning stages of such a conversion!

Thanks

Chris

[adryan]

Chris,

the white stuf is silicone. I first applied some oil to the screws then put the silicone on it. Let it dry overnight and than you can twist it gently and the "joint" to the screw will break.

The connectors I'm using for the servo's are speakon conenctors (see picture they are rated 20 Amps)
the sub-D 9 pin are for:
-encoder (one each)
-limit switches
-emergency switch
I also have 4 euro connector outputs for mill motor/pump/.../...

Today I worked some more on the mill and installed the limit switches for the y-axis (using microswitches ,IP65 sealed) and shielded cable.
This is also a litlle bit different to the plans from Jeff, but I didn't like having the cables/switched beside the y-slide.
I also made a lubrication connection to the bearings/ballscrews for central lubricating

[adryan]

Time for the next part,

I installed the limit switches for the x-slide (on the inside of the table so you don't see anything on the outside)
You can also see the lube line comming into the table for the bearing and the slides.
I also took apart the head of the mill, so I can grind it and repaint it. The top bearings in the v-belt pully are worn, so I have to replace them. I also think the bearings of the spindle are worn/dry because I can hear them ratling.
Does anyone know how to take the spindle apart? at which side should I start, and what an I expect?

[Runner4404spd]

i've taken my spindle appart plenty of times. you need to loosen the two nuts that are in your last pic on the previous thread. unscrew those and the spindle will pull out. on the bottom of the spindle the collar is screwed on not pressed on. the bearings are tight on the spindle and will probably need to be pressed off. i would tell you what bearings but mine might be different than yours.

[adryan]

spend another 6 hours today on the mill...
I cleaned all the paint and plaster from the head and painted it. You can also see the extra holes I've made for the cables from the encoder and servo.
Also in the front of the head is the enlarged hole for puting the Z-axis servo assembly in.
I took the spindle apart and found there were some fine chips in the lower bearing. After cleaning everything the bearings are still o.k.

[adryan]

some more work done this weekend....
I installed the z-axis servo inside the head, also the microswitch is inside.
I replaced the 3-wire cable for the power supply with a 5-wire so I can use one cable instead of 2. The power supply wire goes to the box on the motor.
..hope do do some sest runs next week.

[ngr1]

Hello, your conversion looks great!

Can you please provide more details regarding you oil/lube configuration for the ways?

I am beginning to see more examples of this but no one has provided any details regarding its design and operation.

Thanks... Norman

[adryan]

Today I did some tests with the mill for checking the backlash.
on the X-axis I had 0.02mm (0.00078") and on the y-axis 0.06mm (0.0023").
With the z-axis I could very much notice there's no preload on the ballnuts. I had a random backlash from 0.05 till 0.1mm.
I tried fitting the spring (see picture) to create a preload on the nut, the backlash changed to a constant 0.05mm (0.0020")
Does anyone know if these values are normal, or can they be improved (tuning the servo's/increasing the preload)??


Norman,

I just grinded a z-shaped slit in the ways as can be seen in the pictures. The slot is about 2 mm wide and 0.5mm depth. I drilled several holes through the table base to get the possibility to connect it to a central lube-tube.

[Chris D]

[QUOTE=adryan]Today I did some tests with the mill for checking the backlash.
on the X-axis I had 0.02mm (0.00078") and on the y-axis 0.06mm (0.0023").
With the z-axis I could very much notice there's no preload on the ballnuts. I had a random backlash from 0.05 till 0.1mm.
I tried fitting the spring (see picture) to create a preload on the nut, the backlash changed to a constant 0.05mm (0.0020")
Does anyone know if these values are normal, or can they be improved (tuning the servo's/increasing the preload)??
QUOTE]


It appears as though you have double ball nuts on each screw. You will need to adjust that to get rid of your backlash. Take the time now to do it right and don't rely on the backlash comp. feature in the CNC software to do it for you - you won't be happy with the results. Now, remember, to get rid of the back lash, you have to create some "binding" between the two nuts. There is a good article on a website somewhere and I will be darned if I can remember where it is. It also appears as though you are using rolled balled screws which may not have a constant pitch over their full length. This means you may have to adjust your ball nuts so that the backlash is compensated for in the "tightest area". Be prepared to spend a lot of time on this, the results will be well worth every minute of effort!

Chris

[adryan]

Chris,

on the z-axis is only one nut, because of the limited space, so I think I'll have to live with that.
Can the backlash on the X and Y (they are preloaded) really be brought back to zero? or will their always be some?

[Chris D]

The Z axis is generally not as critical as the X and Y axis - but there are times when it is a problem.

You cannot get zero backlash - especially witha rolled screw. However, you do want it as minimal as possible and I don't know how close you can get it with a rolled screw. Even precision ground screws, with pre-loaded nuts etc., will have some backlash - .0002" or so. If there is truly zero back lash, then you are basically either in a "Bind" or very close to binding. If I were you, and I will be there later this winter I hope, I would do my darndest to get that backlash down to .0005" or less if possible.

CHris

[adryan]

I've reasembled the mill and found one small problem in the y-axis setup. I didn't screw the ballnut entirely to the aluminum bracket because I wanted to have the tubes of the ballnut on the top. To fix the ballnut in the bracket I used a small setscrew. Looking at it closely the ballnut could move a bit in the bracket when pushing hard. I now fitted some shims between the ballnut and the bracket so it can be screwed tight together. (see picture)

but now the real problem
I have the following setup:

encoder 250cpr
belt reduction servo 12 tooth - shaft 24 tooth
Thomson Precision Rolled Ballscew Stock- .631 x .2 Pitch
so if everything is correct:

250cpr => 1000ppr
i (reduction)=1:2
=> 2000ppr (pulse per shaft revolution)
pitch=0.2"=5.08mm
=> 2000 pulse/5.08mm =>393.70079 pulse/mm (entered as pulses/unit, and units are set up as mm)

In Mach2 I've set this up under motor tuning.
I've tried the setup again today and entered 0.1000 step and step mode in Mach2.
When moving forward/backwards the slide moves 0.15mm instead of 0.1mm (I've tried several times in same and in oposite direction)
When setting up 0.6000 the slide moves 0.90mm....

.....but when seting up 50.0000 step the slide moves exactly 50mm!!

I've tried another Gecko..another servo, but nothing makes a difference.

Am I doing something wrong????

[Chris D]

It has been about a year since I setup MACH with my stepper motors. I remember getting myself pretty twisted around and messed up, so I wouldn't want to walk you through it with servos and encoders. You may want to post this on their yahoo group...

http://groups.yahoo.com/group/mach1m...guid=175280003

Chris

[Halfnutz]

Nice job, congratulations! I like how you went the extra mile and set up a lube system, and buried your switches out of the way.

You should be able to eliminate almost all of the backlash with those double nuts, at least all that your capable of seeing with that machine.

I dont think you will notice the Z axis on most jobs, I never have on any of my machines. As long as everything else is tight it will be fine.

Nice work!!

(The above oppinions are the authors and do not reflect those of CNCZone or its management.)

[adryan]

Finally got the backlash problem for the x and y slide solved.
Backlash is now 0.01mm (0.000393") for both, wich I think is very good....
I fitted a cover over the front part to prevent dust/particles coming onto the ballscrew.

I took the mill head apart again and measured the ballnut for the z-axis. There is about 0.1mm (0.039") play on it. The balls are 3.144mm so I'm going to try replace them with 1/8" balls (3.175mm) and see if I can get the backlash down a bit. Anyone tried this before on the Thomson ballnuts?

[adryan]

Just got the replacement balls of 1/8" (steel ones, 62Hrc)
What a difference! they fittet o.k. and when turning there is absolutely no slob! ...at least I can't feel any
So I'm going to try them on the z-axis.
Today I started working on a bench for the mill.
Basic parts are almost ready, just have to fit a pipe for letting the coolant out.

I'm thinking of using a pump of the central house heating for the coolant. Anyone tried this before?