Fixing A Bad CNC Conversion Kit K<solved>

[RJS100]

A couple years ago, I purchased a CNC conversion kit for my PM833-TV. Just after purchasing the kit, the owner passed away and closed the business. The kit I purchased was defective in many ways, but I managed with the help of a few online buddies to make it workable. Now with a tad of ability to make parts, I would like to fix this conversion once and for all. I am starting with the z-axis:

Z-axis Main Issues:
1. The bracket that holds the angular contact bearings that supports the z axis has two bearing pockets divided by a piece of aluminum that is only 1mm thick. This is causing excessive backlash and is a very poor design (see images). If I make this section thicker, I will run out of threads on the ball screw. My plan is to reduce the thickness of the two 9/16 x 18 jam nuts by .040 each and increase the thickness of this 1mm section a bit. The combination of these to changes will result in an increase from 1mm (.039 inches) to .185 inches total. I also redesigned the shape of the bracket to make it sturdier.

I spoke with the Dave from Arizona CNC kits who now make a nice conversion kit for the PM833. He said that the section between the bearing pockets on his designs are between .200 and .280 thick, so I think I am getting in the range. Also, the bearings for this conversion have a 32mm diameter. The bearing pockets are 32.2 mm. Dave suggested making the bottom pocket a bit loose to stop things from breaking.

Two questions:

1. Is there anything else you would do with respect to modifying this bracket?
2. Do you agree with making the bottom pocket looser? If so, how big would you make it?

[suntravel]

I would use a FK15 bearing block with two angular bearings like this one:

https://www.sorotec.de/shop/Flange-fixe ... -Line.html

If a bearing plate is done right, no need to make the bore for the bearings out of tolerance.

Uwe

[Muzzer]

I'm guessing that there was a reason Dave suggested opening out the bearing bore. Unless you manage to measure and correctly implement the various machine dimensions with deadly accuracy, there will most likely be a tolerance or misalignment between the ballnut on the ballscrew and the stationary bearing. When you run the ballnut up close to the bearing, any such misalignment will require the ballscrew to move out of square, which it won't want to do. You may be surprised how stiff the ballscrew rotation will become, despite there being ball bearings on all the joints.

The pragmatic solution is to allow some radial movement of the bearing. You can then nip up the bearing bracket once it is correctly aligned, particularly at the end of travel where the ballnut is right up against the bearing.

Incidentally, I doubt a 1mm thick shoulder would deflect at all in this application. If you are seeing backlash, it's more likely to be caused by poor preload adjustment of the angular contact bearings and/or the ballnut-ballscrew itself.

[suntravel]

Jep if it won't fit, let it wobble

That is why I do not buy conversion kits, but for a non-educated machinist sometimes the only option to get started.

ok quality FK 15 are cheap and you can use ballscrews with default end machining from TBI or HiWin.

Uwe

[RJS100]

Thanks for your thoughts, and sorry for the delay in getting back to you. It seems that for every minute I spend in the shop...my wife is on Youtube planning trips. I am currently in Albania for the next 3 weeks with limited Wi-Fi.

Uwe, I love the idea of the bearing block but there are two obstacles. 1. It will raise the ball screw requiring me to raise the servo mount to raise the pulley height to align with the raised pulley on the ball screw. 2. I don't know the manufacture of the ball screw so I will have to research the dimensions a bit further to see if the bearing block will play well with the ball screw. All I know is that it is a 25mm ball screw and it is ground, not rolled. Just curious, the description of the bearing block says the bearings are in a "O" configuration. I am only familiar with back to back and face to face. What is an O configuration?

Tom, you are correct with your assumption as to why Dave suggested making the bottom bore a bit bigger. He said he does this so "things won't break". The good news is that due to the geometry of the mill, when the z axis is in the highest position, the ball nut is still something like 3 to 5 inches away from the bearings, so the forces due to misalignment are not as bad as they could be.

I will have to give this some real thought to figure out how to proceed. Modifying the current design would be very easy to make and take about 20 minutes to install. The other option is a bit more involved.... But not impossible.

Considering that the x and y axis design are similar, and they have less than a half thou of backlash, it is tempting to see how well a modified version will fair.

With respect to backlash, I did some testing long ago and don't remember what I learned. Dave from Arizona CNC disassembled the x and y ball nuts and re shimmed them as they needed adjustment. The Z ball nut may need the same treatment, but it is a pain in the ass to remove so this will be my last option.

One last question on the Z axis:

Most of the conversion kits sold at the time used a large Nema 42 open loop stepper. They said it was probably needed to have enough detent torque to stop the head from crashing downward when the power was shut off. The head weight is a bit over 200 lbs. I thought this was over kill and used a 1700 ounce Nema 34 closed loop stepper. Just to be on the safe side to stop the head from crashing, I added a gas strut to off load about 80 lbs.

It work perfectly fine and the z axis zips up an down with little effort. I also tested it without the gas strut and all is well. I have heard that the gas struts don't last long and cause issues. With a 25mm ball screw, 200+ lb. head and 1700 ounce stepper, would you dump the gas strut or keep it to minimize wear on the ball nut and ball screw?

Ok...No more questions for now.... Richard

[RJS100]

SOLVED - All axis redesigned. Less than 1 thou backlash and a direct drive z axis.

THANKS FOR ALL YOUR HELP!

Richard

[cnckeith]

nice!

[RJS100]

Thanks Keith. Most of what I have learned comes from this forum and the Fusion forum. Loads of fun!