Importance of ball screw pitch size

[Goemon]

I had been working on the assumption that a faster pitch is best for a CNC router because that is what I read here before buying my screws but I just read that a corse pitch reduces the "resolution". Is this correct?

If it is, to what extent does it matter on a diy benchtop CNC machine controlled with steppers and a Gecko G540? i.e. Can you even notice the difference in "resolution" between machines with 5mm pitch screws and ones with 20mm pitch screws with such a set-up?

I will only have approx 31" x 16" x 8" of travel / work space. If I upgrade the ball screws on my build, is it better to look for a corse or a fine pitch? There seems to be some good deals around on ground C5 ball screws with a 20mm pitch but, should I be looking for a finer pitch instead?

Does the grade of ball screw matter when choosing the pitch? I.e. Are the gains in "resolution" on a 5mm pitch screw negated by the loss of accuracy from choosing C7 grade instead of C5 or C3?

Does it matter what you are cutting when selecting the pitch size / speed? If I will be cutting aluminum, do you need a slower pitch than you would for machines that cut softer materials?

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[ger21]

With stepper motors, you are dealing with a fixed resolution (2000 steps/revolution).
Because of this fixed resolution, and the limited rpm range of steppers, you are left with a tradeoff. The faster you go, the less resolution you have.

A good tradeoff would be 10-20mm of travel, per stepper revolution. If you want better resolution, go with 10mm pitch. With a 20mm pitch, you can always use a 2:1 belt reduction, which would effectively be the same as a 10mm pitch screw.
Will you notice the difference? Maybe? It's hard to say, because there are many other things that also can factor into the finish of your parts.

[Goemon]

With stepper motors, you are dealing with a fixed resolution (2000 steps/revolution).
Because of this fixed resolution, and the limited rpm range of steppers, you are left with a tradeoff. The faster you go, the less resolution you have.

A good tradeoff would be 10-20mm of travel, per stepper revolution. If you want better resolution, go with 10mm pitch. With a 20mm pitch, you can always use a 2:1 belt reduction, which would effectively be the same as a 10mm pitch screw.
Will you notice the difference? Maybe? It's hard to say, because there are many other things that also can factor into the finish of your parts.

I'm not sure I fully understand the term "resolution" in the context of CNC machines. On a flatscreen tv it obviously relates to the number of pixels so you would expect to be able to see less detail and a less precise image on a low resolution display. With screens though, you have the resolution of the content to determine what resolution of screen you could benefit from.

Is it the same with CNC machines I.e. that the finer the detail you are engraving, the finer the screw pitch you need?

[Jim Dawson]

Resolution in this case means the smallest possible movement of the drive system. Assuming a 1:1 ratio and 2000 steps/revolution, a 5mm ball screw would give you a resolution of 0.0025mm (about 0.0001 inch) (5/2000) and a 20mm ball screw would give a resolution of 0.01mm (about 0.0005 inch). You have to decide what minimum resolution is acceptable.

[Heavymetalcnc]

If you are milling a circle. The finer the resolution, the more round it will be. The courser the resolution, you will start to notice facets in place of roundness.

[louieatienza]

I'm not sure I fully understand the term "resolution" in the context of CNC machines. On a flatscreen tv it obviously relates to the number of pixels so you would expect to be able to see less detail and a less precise image on a low resolution display. With screens though, you have the resolution of the content to determine what resolution of screen you could benefit from.

Is it the same with CNC machines I.e. that the finer the detail you are engraving, the finer the screw pitch you need?

I would say to a certain extent, yes. Of course, just because the controller commands the smallest possible distance, doesn't mean the machine will move that distance. Then again, you can resolve a lot of detail with just 200 steps per inch with microstepping. Magazine print is only something like 300ppi.

[Goemon]

I would say to a certain extent, yes. Of course, just because the controller commands the smallest possible distance, doesn't mean the machine will move that distance. Then again, you can resolve a lot of detail with just 200 steps per inch with microstepping. Magazine print is only something like 300ppi.

From how you describe it, it sounds like the sort of problem that would not be noticeable to your average person unless you were engraving fine detail with a small diameter end mill. Is this correct?

If, for example, you were cutting a wooden rifle stock with a 1/4" end mill, would you expect problems as a result of using 20mm pitch screws instead of 10mm or 5mm? I guess "problems" in this case would mean a rougher finish and more hand finishing being required.

[Jim Dawson]

I would say you would need a microscope to see the difference. The lead screws on my machine are 40mm and 25mm pitch and it is perfectly capable of making very smooth finishes.

[Goemon]

I would say you would need a microscope to see the difference. The lead screws on my machine are 40mm and 25mm pitch and it is perfectly capable of making very smooth finishes.

Thank you. That answers my question.

With what you said plus the tip from Ger21 on using a gear reduction (if needed), I feel better about the deal on some 20mm pitch ground ball screws that has been made available to me.

It's all coming together. I kinda wish I could pass some of this info back to myself at the beginning of the process. It seems like the largest potential cost in a diy build is changing your mind on the design and having to buy the same parts twice... I'll literally have enough parts for a second build when the first is finished....

[louieatienza]

The only caveat is if you go the coarser screw and gear reduction route, you have to take into account the accuracy rating and backlash rating of the gear reducer. It may be a different story if you are using servos, and you need to gear the motor down to a more useable speed. And if you think you can start by driving the gear reducer with a stepper then switch to servo, it's not that easy A NEMA23/24 stepper has a different bolt hole pattern and pilot size than a 60mm servo, and a NEMA23/24 shaft is usually 1/4", 3/8", or 8mm while a 60mm servo shaft is 14mm.

Imagine the fun of familiarizing yourself with the different gear reduction manufacturers, deciphering their nomenclature, and making a determination as to condition just by looking at a few photos?! You thought ballscrews were fun...

[CaptainVee]

Unless you are a super experienced metal worker when you start the first cnc, I think no one will notice the difference in resolution when you choose 16 mm pitch over 5 mm pitch.

But you will notice the difference in acceleration and top speed. (higher pitch is higher top speed and acceleraration)
Which you probably do not need at first because it will scare you stiff but when you get used to working with your cnc, speed is nice to have,

So I'd say, go with the bigger pitches.

[ger21]

I would not use a gearbox to go from motor to ballscrew. Belt drive will typically have less backlash, and be much cheaper. And it's much more commonly used.

[Goemon]

Does it matter if you use a different pitch size for each of the axis?

It's a lot easier to find good deals if you don't need to find three identical spec ball screws.

[Al_The_Man]

I had been working on the assumption that a faster pitch is best for a CNC router because that is what I read here before buying my screws but I just read that a corse pitch reduces the "resolution". Is this correct?

It is called the Least Input Increment.
See Post #35 here http://www.cnczone.com/forums/servo-...ne-closed.html
With steppers it depends on the steps/distance.
Al.

[louieatienza]

Does it matter if you use a different pitch size for each of the axis?

It's a lot easier to find good deals if you don't need to find three identical spec ball screws.

Not really, because you adjust everything at the control. However I do like to have the same mechanical resolution at each axis.

[louieatienza]

Unless you are a super experienced metal worker when you start the first cnc, I think no one will notice the difference in resolution when you choose 16 mm pitch over 5 mm pitch.

But you will notice the difference in acceleration and top speed. (higher pitch is higher top speed and acceleraration)
Which you probably do not need at first because it will scare you stiff but when you get used to working with your cnc, speed is nice to have,

So I'd say, go with the bigger pitches.

That is a misnomer; if you have enough motor speed to generate the same linear speed with either screw, then the finer pitched screw will give you better acceleration because the inertia of the load decreases by the square of the gear ratio.

Of course, as there is a limit in available torque at speed for steppers, one must refer to the speed/torque charts to determine what is the sweet spot.

[CaptainVee]

I wrote what I think is right because of the second part of your post.

[Goemon]

Not really, because you adjust everything at the control. However I do like to have the same mechanical resolution at each axis.

How come you prefer matching screws (aside from the neatness of the purchase)?

I can see that for someone who specifically needs a fine pitch for micro engraving that it might be a problem if one of the axis has a corse pitch but.... assuming that a fine pitch is not required, what concerns would you have from mixing different pitches?

I got a good deal on a C3 16mm diameter 5mm pitch THK ground ball screw with a double nut for my Z axis and a C3 20mm NSK ground ball screw with a 20mm pitch for my long axis. I am waiting to hear on another deal for a C5 15mm NSK ball screw with a 10mm pitch for my moving table axis. That would be 3 different sizes and pitches for my screws. Will I be ok with these choices or should I keep an eye out for a 5mm pitch screw for the long axis too?

There is another potential deal on a c3 5mm pitch screw with 31" of travel coming up. I didn't wait for it because I thought the long axis would benefit more from a faster pitch and larger diamater.

[louieatienza]

How come you prefer matching screws (aside from the neatness of the purchase)?

I can see that for someone who specifically needs a fine pitch for micro engraving that it might be a problem if one of the axis has a corse pitch but.... assuming that a fine pitch is not required, what concerns would you have from mixing different pitches?

I got a good deal on a C3 16mm diameter 5mm pitch THK ground ball screw with a double nut for my Z axis and a C3 20mm NSK ground ball screw with a 20mm pitch for my long axis. I am waiting to hear on another deal for a C5 15mm NSK ball screw with a 10mm pitch for my moving table axis. That would be 3 different sizes and pitches for my screws. Will I be ok with these choices or should I keep an eye out for a 5mm pitch screw for the long axis too?

There is another potential deal on a c3 5mm pitch screw with 31" of travel coming up. I didn't wait for it because I thought the long axis would benefit more from a faster pitch and larger diamater.

Different pitches mean potentially different dynamics for each axis. Using a coarser pitch may necessitate using a larger motor to compensate for the decreased "gear reduction." Also you machine will only be as fast as your finest pitch screw when it comes time to 3D machining. And speed is not your primary concern, so why not have more available torque and mechanical resolution?

[Goemon]

Different pitches mean potentially different dynamics for each axis. Using a coarser pitch may necessitate using a larger motor to compensate for the decreased "gear reduction." Also you machine will only be as fast as your finest pitch screw when it comes time to 3D machining. And speed is not your primary concern, so why not have more available torque and mechanical resolution?

My choice was largely based on the availability of good deals on quality ball screws. I also figured that the larger diameter wouldn't hurt on a longer axis and the one that carries the most weight.

I guess I'll have to see if it causes problems for me. If it does then I'll have to replace it with a fiber pitch screw and use the old one on another build.

You are right that speed is of little concern to me unless it causes a cutting problem. My main goal is to be able to cut a mold cavity in 6061 and getting it right first time. 6061 is not super expensive but I can't afford to be screwing up 1.5" x 8" x 36" slabs on a regular basis.

It would probably be cheaper to upgrade a stepper motor than to buy a new ball screw though. When I bought my stepper motors I was building a different machine with slightly different cutting goals and a lighter spindle. They were on my list of components to revisit to see if there were opportunities to improve anyway.

I keep meaning to read up more on why metal cutting machines use much larger stepper (or servo) motors to see if I was missing something. I can't shake the nagging thought that more holding torque would be helpful for cutting metal.