New Machine Build- Backlash and Axial Play Relationship

[joprinz]

Thank You Guys for all the Help U guys had given me while I was building my first CNC Machine. Its complete and works well within my design specs. Now I'm trying to Build an upgraded version of the same and this time I'am planning on using ballscrews. (ok thats a Lil too much excitment... But come on...U see I have read soo much about Ballscrews and their advantages but this is the first time I'm actually laying my hands on them...)

I was going through my Ballscrew spec sheet and found a column named AXIAL PLAY. No Backlash rating was given in the spec sheet though. I read a lot of posts out here explaining backlash but somehow I'm confused if Axial play and Backlash has a relationship or are they both the same???

Hope someone can clear this out for me.

Also, the ballscrew Axial Play is rated as 0.1mm. It is a 20mm Diameter 5mm Lead Rolled BallScrew from PMI.

Here is my Machine Spec:

• 2' x 2' x 6" CNC Mill
• For working with wood, aluminium and probably some mild steel
• Fixed Gantry Design with Two 25mm Supported Shaft and 4 Linear Bearings per Axis.
• Steppers are all 400 Oz.In Microstepping mode and running on 50V 10A PS.
• Basic construction is in Welded steel and 1" Aluminum for moving axis parts to reduce load on steppers.

I was wondering if the above mentioned ballscrew is sufficient for my design. If it is, then can someone help me calculate the maximum accuracy i can acheive with this ballscrew.

Please Advice!

Thank You!

[ger21]

They are similar. but not exactly the same. Backlash can be the result of other factors as well as the screws axial play.

[andy_ck87028]

I'm no engineer but conceptually it would appear that axial play on a ballscrew is totally irrelevant if the linear guides are doing their job properly.

Therefore, assuming that there is no axial play from the linear guides, the only error potential left is backlash and if the ballscrew has zero backlash, then you've got a fully functional solution.

Removing axial play from the linear guides is a separate problem to this thread and will have certainly have been covered elsewhere in threads that talk about skew and torsional forces, etc.

I'd personally ignore the ballscrew axial play as it should be engineered out via linear guide design / construction.

Any engineers out there able to add to this opinion?

Andy

[gerryv]

Originally Posted by andy_ck87028 Removing axial play from the linear guides is a separate problem to this thread and will have certainly have been covered elsewhere in threads that talk about skew and torsional forces, etc. Andy

Isn't axial play exactly what was brought up by the author of this thread

I'd like to know what it is and how it differs too, if that's okay.

[LeeWay]

I think axial play is the slop when a ball nut has pressure to one side, then pressure is applied to the opposite side. That space between the screw, the balls, and the nut races. Backlash is the play in the other direction with the screw when it changes from one direction to the other. They would almost have to be related to each other. If you have some axial play in a ball screw, it likely has some amount of backlash.
As stated, rigid linear ways nullify axial play, but they have little to no effect on backlash.
Backlash can be in the nut itself or can also be from poor mounting blocks. Preloading or double nuts and double bearings are typically used to acheive better results in backlash. Preloading the nuts would help with axial play, whereas double nut probably wouldn't.

[andy_ck87028]

The axial play originally referenced was on the spec sheet for the ballscrew.

My understanding of the term axial play is loosely referred to as slop.

For example if you put a clamp on the ballscrew nut so that it is 90 degrees to the line of the ballscrew thread, then axial play (or slop) would be when the clamp can be moved to an angle that is less than or greater than 90 degrees.

My assertion is that axial play on the ballscrew is irrelevant if there is no axial play on the linear guides.

[CarbideBob]

Axial play in a screw is for all practical purposes a spec of the backlash in the screw/nut assembly.

It is only one part of the backlash in a system and only one of the sources of errors in a screw.

Backlash can be compensated to some extent by software.

It is impossible to calculate the accuracy of your design from just the screw parameters as the are many sources of error in a machine tool.
From your limited description a wild guess would be about 0.2-0.5 mm at best. Repeatability, which is much different than accuracy, would undoubtedly be much better than this.

Most machines are very repeatable. Few are highly accurate with the possible exception of mega-buck inspection machines.
Generally speaking you cut one part then make size adjustments (program changes or tool comp changes) to bring the part into print.

People make +/-.02 mm parts on machines that are 0.2 to 0.5 mm accurate everyday.
Bob

[ger21]

Originally Posted by andy_ck87028 I'm no engineer but conceptually it would appear that axial play on a ballscrew is totally irrelevant if the linear guides are doing their job properly.

I believe your thinking of radial play, which would be side to side. Axial is along the screw.

[joprinz]

So shall I summarize it as the following?

• Axial Play will cause some level of Backlash if unchecked
  
• Pre-loading the nuts and using rigid linear guides can reduce/nullify the effects of Axial Play
  
• Keep other reasons for Backlash (by using angular bearings etc) as minimum as possible.
  
• Deal with whatever backlash that is left using Software.

Correct???

Considering I do ALL of the above, Is it safe to say I will have a reasonably accurate machine within DIY standards? (Like Bob stated - "within about 0.2-0.5 mm at best"???)

1. Also on a related question, can someone tell me How effective Software Backlash compensation Is?
   
   
2. And is there a particular Control software which is better at such build error compensations or do they all offer similar features and results?

I'm Currently working on Mach3 most of the time but do occasionally use EMC2.

Please share your experience in compensating for Mechanical Build Errors using software.

Thank You!

J

[joprinz]

Please share your experience in compensating for Mechanical Build Errors using software.

Thank You.

Joe!

[ger21]

Originally Posted by joprinz Also on a related question, can someone tell me How effective Software Backlash compensation Is?

I believe that it's more effective at some things than others.

Most will tell you that it's far better to remove the backlash than try to compensate for it in software.

Keep in mind that it will impact performance, due to the fact that to compensate, the axis will need to reverse direction at times to take up the slack.

[joprinz]

Thanks for the advice Gerry

I'm planning to start Building my second project in 2 weeks time. I'l do my best to minimize Build errors. Thank You.

Joe!