Metric thread - pitch

[Fredxxx]

All through the Screwfix and Toolsatan website there are many examples of metric bolts and nuts but they never seem to specify the pitch.

Wikipedia article:

suggests there are course and fine threads.

The Tapping sets sold by Screwfix often contain, for example, M6 x 0.75 and M6 x 1.0

Please spare my confusion which would be considered "standard", fine or course?

[Clive Arthur]

Coarse is standard. Fine is special. I work in electronics and almost never see metric fine threads.

Cheers

[Clive George]

The article to which you refer confirms the answer which my namesake gave you. It's in the Designation section (and it's there twice...)

"If the pitch is the normally used "coarse" pitch listed in ISO 261 or ISO 262, it can be omitted (e.g., M8)"

"If, for instance, only M20 is given then it is coarse pitch thread."

But thank you for asking the question - it made me look it up and learn something myself :-)

[Fredxxx]

Many thanks to Clives for clearing this up.

If I had read the wikipedia article a more carefully ........

[T i m]

On the subject of thread pitches ... I had to re-establish where we were on our 3D printer recently when updating the firmware and re-checking the stepper motor steps to distance_moved figures.

The motors are .9 degree per step and the printer Z (vertical) axis is directly driven using two lengths of M8 stud. The 'default' pitch for M8 is 1.25mm and so each complete rev of the motor would raise the head 1.25mm, therefore 320 steps [1] would raise it 1mm (so that was the 'steps / mm' value) and therefore the minimum vertical increment was 1 step = 0.003 mm. ;-)

Given that the head tends to be moved up ~.3mm at a time (each printed layer), that means it takes 100 whole steps to do so, potentially making such movement pretty accurate. However, because it's just 'studding' and so really designed to bolt things together, I'm not sure the resultant accuracy (linearity) is actually that good (but not bad). ;-)

I understand the preferred mechanism for such a role is engineering leadscrew and whilst that is a much coarser pitch, it's supposed to be more accurate (linear) and still much better than the X or Y axis that are around only 10 steps / mm (so .1mm / step 'resolution') and rely on toothed belts and pulleys.

Cheers, T i m

[1] The electronics sub divides a single motor step (in this case) by 16x (using current / phase control or some such) so for the firmware and the Z axis that's 5120 steps / mm!

p.s. Because our printer has (typically for such machines) no movement feedback / servo mech it can only assume that an instruction to step say 10000 steps results in 10000 actual steps made. That's why I am, even nearly 2 years after building and running the printer pretty regularly, still fascinated / amazed that it can all stay in sync, even after running a print job for ~6 hours! How many million steps have been made over all 3 directions?!

p.p.s. I'm also amazed (as in 'Standing on the shoulders of giants') how you can add things like live filament diameter measurement when you tell the firmware the distance between the measurement device and the extruder ('print head') and it 'considers' any change in filament diameter as it would be by the time (distance) it travels the extruder and adjusts the filament speed accordingly to compensate.

And you can print / build such things for yourself. ;-)

[Andy Burns]

not a ballscrew?

[T i m]

No, I don't think so Andy, or not that I'm aware of anyway.

The reasons I have seen are that they are not generally good VFM for

3D printing because they are often 'chunkier' than the straight leadscrew / stud type mechanism and because of that, they also carry more inertia and so put more strain on the stepper motors etc.

They are also 'too accurate' in that they would completely outstrip the abilities of laying melted plastic filament (and wouldn't be fast enough to use on the X and Y axis).

Am also not sure if ballscrews 'like' stepper motors and so are typically used in closed loop mode with std servo motors?

I think they would use belts on the Z axis of these d-i-y level 3D printers but they wouldn't enjoy the head dropping onto the print table when the power was turned off. ;-)

I understand ballscrews really come into their own when you need to push say a CNC cutter into the workpiece, not really something required on a 3D printer.

Are there any diy / small 3D printers that use ballscrews that you use or are aware of Andy?

Cheers, T i m

p.s. I'm probably going to buy the 3D printer off my mate soon that he bought on the understanding that 'we' built it (he's moving and doesn't have the room) and I did consider getting some leadscrews to 'upgrade' it. However, the existing M8 ss studs and twin (one spring against the other) nuts seem to be doing the job ok so I might not bother. ;-)

[Andy Burns]

I've seen some homebuilt CNC routers (with steppers) that use them, and assumed they would have the same advantage for 3D printers, i.e. lower friction, better accuracy, it might be that since you need little force for 3D printing they're not required ...

[T i m]

I think that's the thing Andy, the things that they would be good for say (CNC) milling wouldn't be a specific advantage for a 3D printer.

Not that they wouldn't or couldn't be used etc, especially on some of the bigger machines or those printers also doing milling etc.

Cheers, T i m

[Andy Burns]

If the positioning of the print head is 100x more precise than a printed layer, is it possible to e.g. print in a continuous spiral up and around the side of a cylinder, rather than having discrete layers?

[newshound]

Perhaps you need a variant on the system once used for ruling diffraction gratings. This has a fine pitch screw and a *long* nut lined, iirc, with cork so that pitch errors average out. Can't for the life of me remember the name of it. Standard Physics A-level in the

1960s. I guess photo litho methods replaced ruling soon after. Can't persuade Google to divulge the name.

[dennis]

You can its commonly called vase mode.

[T i m]

e.g.

I might give it a go for the S&G's. ;-)

Today I printed a new case and lid for the electronics (Arduino Mega + RAMPS). I did the body in black and the lid in translucent so I could see the output indicators doing their thing.

I also replaced the components for the vernier Z axis optical endstop carrier for the 'beefed up' ones I re-designed and printed the other day. I've now re-re-designed the end cap that holds it to the Z smooth rod as a fastening nut and bolt just touches it (I could just file the corner off but it's easier to print a new one). ;-)

Cheers, T i m

[newshound]

The brain kicked in eventually, it is a Merton Nut

[T i m]

That's a clever idea. ;-)

I think the 'bottom line' (outside of 'don't fix what's not broken'?) is that some lead screws could both speed up the z axis (quicker homing etc) whilst potentially providing better linearity.

I also understand that stainless steel nuts running on ss threads might 'gall' more easily that non stainless steel nuts / threads? This is good when it's being used as a fastener but not if you want a freely running joint?

This is quite interesting:

Cheers, T i m

[T i m]

Interesting, thanks. ;-)

Cheers, T i m