'Z' Axis Backlash ?

[andy_ck87028]

Hi all

I've just had a difficult conversation with a very helpful person selling Thomson precision rolled ballscrew rod.

My difficulty was due to his presumption that the role of a 'Z' axis is purely to remove the cutting head from the workpiece and therefore backlash should not be a factor.

My gut feel is that he is wrong and I was trying to articulate that re-positioning after movement when cutting for example thick, hard material would reveal any backlash potential and so I asserted that backlash would be a factor.

I could not justify my argument with authority and so the conversation was left hanging.

Could anyone explain where he might have been coming from and whether I am right/wrong, he is right/wrong and ultimately..... irrespective of that, do I ensure there is no/minimal backlash in the Z axis?

Also, I can either purchase ballscrew with pre-loaded nut or 2 nuts spring loaded.

They can supply pre-loaded nuts with precision ballscrew but they have said the accuracy is agricultural grade and there would be variance in the thread.

I am happy to drive over and talk with their engineers but I need some background knowledge of what I require.

Fyi, my router is positioned on X and Y using GT5 belt with 5 to 1 reduction driven by stepper and Z will be ballscrew / acme driven by 1 : 1 pulley from stepper. The cutting envelope is 3 foot by 4 foot by 6inches. I plan to cut wood, plastic, aluminium.

Thanks

Andy

[Jason Marsha]

Z axis repositioning is important and the perfect example is 3d (2.5d) milling of a design. Repeatability is critical (within reason).

What are the actual specs (accuracy) of an Agricultural Grade ballscrew?

www.Homeshopcnc.com sells rolled ballscrews with preloaded nuts.

Jason

[andy_ck87028]

Hi Jason

Deja Vu ..... see attached link...... http://www.lowcostcncretrofits.com/transmisionAUS.html

It appears identical.... same website content..... both referring to homeshopcnc .... one is in OZ the other is in the US!!! ...also ... cheaper in OZ

The inference in the specs defined in both links are that the same ballscrew is used irrespective of whether a pre-loaded or non pre-loaded nut are used.

In fact, I was told the pre-loaded stock items are of 'agricultural quality' and a whole range of alternatives are available. Does that imply the links are misleading with respect to what you are actually getting for the listed price?

Irrespective, thank you Jason re. highlighting the need for repeatability implying backlash is not to be tolerated.

How fascinating !!!! ... I will delve further....

Andy

[paulC]

When talking routers, the backlash can be overcome by the weight of the router. The more weight the less backlash will factor in. Benny was kind enough to point this out to me in a discussion some time ago. He made the point that no antibacklash at all is required in some circumstances. The weight will force the router to the bottom.
Paul

[andy_ck87028]

That is a good point.

I was wondering that ....... however that point assumedly has less validity when cutting aluminium.

Andy

[paulC]

No not really. Your feed rates are very slow with aluminium so there is time for the router to settle to the bottom of its travel if you are plunge cutting. If not plunging then it has no bearing at all.
See if you can contact Benny I'm fairly sure he is just using a nut on his z. He should be able to give you some idea of how it performs.

Paul

[Jason Marsha]

Two simple nuts can form a very effective AB nut.

Jason

[ger21]

Most likely, the only time you'll notice any backlash would be when plunging straight down, and / or using dull bits or bits not made for plunging. The weight of the router should keep it down, and the bit should be more than capable of cutting whatevers in its way, thus taking out the backlash. Also keep in mind that plunging straight down in wood will create smoke and dull bits, so it's not a good idea.