Proxxon MF70 mod

[Doxs]

Hi

I am about to make some changes to my MF70.
It is already CNC converted via a kit from usovo.de.

However, after using it for some time I now have huge backlash problems.
I expected to have to do this sooner or later but not this fast. Following the usovo kit instructions I glued the threaded axles directly to the engine axis for each axle.
Thus I have no way to adjust backlash...

To solve this I ordered three SKF ball nuts and a 6x2 axle for the ball nuts.
The plan is to refit the machine and extend the x axis at the same time.

Question 1
The axis I received is not straight, how do I solve that? The plan was to cut the 1m axle in 3 and use one part for each axis.
My initial thought is that any bends should ruin the accuracy and make it useless.
Not to mention cause wear in the ball nuts.

Question 2
I plan on getting the table extended along the x axis.
It has about 16cm travel atm, Id like to extend it to 40-50cm travel.
Extension is to be made by machining a new top for the coordinate table.
Is the original guides going to be able to cope with the increased momentum caused by more table overhang?

Ill post more info and pictures when I get home.

Thanks in advance for any help!

[Leo.D]

Hi,

Are you sure the backlash problem has to do with the nuts and not the axial bearing or play on the guides? Did you try to move the table by pushing / pulling on the x-as motor? If you dont feel that much play as by pulling the table itself, the problem isn't the nut.
I suppose a bended axle has not much influence in accuracy in travel direction. My experience is a bended axle results in extra loads on the guides which will result in play onto these guides!
On my machine I already made twice new spindles, the fist of cheap threaded bars wich were bend to much, the second of grade A2 stainless (the iron store I was didn't had bars in 10.9 or 12.9 quality in stock at that moment) and new nuts from delrin. In the nuts, I made the second thread stage with a regular tool, the final stage with a part rest material of the SS spindle with a grinded cutting side. Result minimum backlash. If there will be to much backlash I know this will come from the nuts because I use preloaded axial needle bearings (see MF70 conversion). New new nuts will be made at minimum costst.
At this moment I only make small parts on my MF70 and use the overhanging weight of the stepper motor to eliminate the play on the guides. As I see how easy play becomes to these guides, I think its not a good idea to increase the travel to 40 or 50 cm.

regards, Leo

[Doxs]

Hi

Yes, I am very certain the slack is in the delrin nuts. Since both ends of the threaded axles are glued at the moment I have no way of adjusting the slack at all. The slides have been adjusted and tightened so there is no slack there.
I also opted for ball nuts to get away from replacing delrin nuts.

Anyway, this is the SKF ball screw I bought:
Buy Ball / Lead Screws and Accessories SCREW SHAFT 6X2 SKF 1101001 online from RS for next day delivery.
(Click dimensional drawing data sheet, its in english)

The deviation is about 1cm when laid against a straight ruler, length is 1 meter.

When it comes to the x axis extension I know 40 cm sound like alot, and it really is for such a small machine. The way I plan to do it is to have a new table machined from aluminium stock to the same dimensions as the current table only longer, then put the motor on one end and a weight on the other end equal to the motor.

That way when its centered there should not be much more stress then it is now, only when I wander off and use the ends of the table. (thus not causing excessive guide wear, I hope...)

I have seen solutions where people bolt two standard tables together, but I really dont think that works in the long run. Creating a completely new table with beefier guides has also crossed my mind, but then I might as well remake the column etc...
I had to draw the line somewhere so I figured it would be better to make the minimun modifications and get the machine back up and running.

Its getting late now so I dont have time to upload pictures, Ill give that a go tomorrow. Im still a bit worried about the ball screw not being dead straight.

This is where I read about the ball screws etc. You can see some pictures there, Im just trying to copy what he did.
Fighting backlash