Very Low Cost "Z Axis" Linear Slide + Dremel®-like Rotary Tool Holder

If you have access to a laser cutter: this Instructable describes an extremely low cost and fairly precise “Z axis” – a linear motion stage and tool holder for use where always loaded in the same direction, as by gravity when vertical. The specific tool holder shown here accepts rotary tools with the classic Dremel® “395” body shape, which includes many clones, and may be adapted to fit other shapes of similar proportion.

Doing Z probes in a CNC application, this Z axis has hit the same micron (±0.8μm) more than 200 times in a row over a span of most of an hour. Practical repeatability appears to be within 25μm (0.001") after warm-up. In other words, it can resolve micron-scale dimensional creep as stepper heat spreads through a structure.

This grew out of a minimal CNC mill/router project described in another ‘ible: “Low(est?) Cost Reproducible 3-axis CNC Mill". That relatively sparse ‘ible refers to a more detailed ‘ible describing how to make a single linear slide: “A Cheap Compact Linear Motion Slide”. This 'ible here also leans on the "slide" 'ible for some details while focusing on what's different about this variation.

By relying on gravity to solve backlash, this slide can use a common type of low cost geared stepper motor (28BYJ-48). In addition to reduced cost, this version has other advantages relative to the previous design. The major benefit is elimination of the counterweight required by the earlier design.

The quick-release tool clamp contributes to the "very low cost" premise by making it practical to use a general purpose rotary tool that isn't captive to a single machine.

CAD

(In this Instructable:

• Some links point to steps within the referenced 'ibles. That doesn't always work. If your browser takes you to top of an 'ible where it seems too general to be relevant, check for a #stepNN tag at the end of the URL.
• Units mix recklessly. Starting with SI bearings and ANSI screws and not getting any better from there. Equivalents appear inconsistently. Not approved for Mars landings.)

Very low cost materials include:

• 1/8" (3mm) hardboard/HDF/Masonite®: about 14" × 10.5" (~350mm × 270mm)
• 6mm linear rod: #s & lengths below
• 6mm linear bearings (LM6UU): three (works) or four (see below)
• 28BYJ-48 geared stepper motor: one (works) or two (anti-SPOF)
• screws 6-32 × 1/4": 55
• glue for hardboard -- I used a PVA wood glue
• POSSIBLY washers #6 small O.D.: 0-2 tbd -- or maybe a file instead
• string: POSSIBLY two kinds
• OPTIONAL limit switch (search "KFC-V-307" or "Camera A15") & two small screws M1.4-M1.6

This 'ible describes the mechanical construction of a thing to use as part of a larger project and assumes that the larger project includes your own choice of electronics, wiring, structure, etc. If you don't already have electronics sorted, this 'ible might help. If you're really starting with just this device by itself, then you can buy motors with bundled ULN2003 modules and ask your favorite search service about "28byj48 uln2003 arduino".

IME the 6mm rod may be the most costly item to purchase, or may sometimes be found in old printers and such.

Many sellers offer the motors; your favorite search service knows. There are 5V and 12V motors. If you don't already know which will suit your application, I'll suggest betting on 5V.

Old desktop/deskside (big case) PCs are often full of 6-32 x 1/4" screws.

More about string later.

Read more about sourcing materials here.

The Cheap Compact Linear Motion Slide ‘ible presents detailed, illustrated, step-by-step guidance for assembling a slide of that design. This 'ible assumes familiarity with that process and concentrates on differences.

If you’re building a “Minamil” CNC and start with the XY table, then I think the rest of this ‘ible will make sense when you get to building this version of the Z axis.

If you’re building this slide for a different purpose, then please read the slide ‘ible up to step 31 for content not repeated here. For the best introduction, I hope you will plan to build the basic example described there. If building that example just as prep for building this, you can skip the leadscrew (steps 6, 15-18).

Benefits relative to previously published design

• No counterweight
• different motor can lift more, and
• won't chatter while easing downward
• Stiffer
• greater bearing separation helps counter loads on long lever at tool tip
• less sensitive to imperfect rod/bearing matching
• Reduced cost
• cheaper motor(s)

Differences

• Stronger geared stepper motor(s): 28BYJ-48
• cheap, torquey, but horribly backlashey
• but loading always in same direction sufficiently reduces backlash
• Conventional arrangement with rods fixed on one side and moving bearings on the other side
• longer, but vertical length doesn't hurt compact footprint
• "Vertical" only
• motor pulls in one direction against ~constant opposing force, like gravity
• assumes upward reactive forces << weight of slide+tool
• slow motors don't risk downward acceleration > 1g
• (or any orientation when working against an opposing force)
• Pulley "winch" and string instead of leadscrew
• Optional second motor/pulley/string for "fail safe" redundancy
• works fine with one motor
• but single points of failure can drop load in freefall
• extreme low cost construction less overbuilt than a helicopter rotor
• second cheap motor is cheap insurance
• Options for rod/bearing configuration
• 3x 100mm rods + 3x bearings like basic slide --or--
• 2x ~200mm[1] rods + 3x or 4x bearings --or--
• 1x 200mm + 1x 100mm + 3x bearings
• Attachment point for limit switch
• limit switches are nice but not essential
• previously hacked on ad hoc -- this time designed in
• fits "KFC-V-307" / "Camera A15" switch

[1] "~200mm" means ≥ 192mm to 195mm depending on material thickness; extra length is harmless so ≥ 195mm is safe. Also 100mm rods may be over-length in this design.

"X" clipart from Vecteezy

The laser cut parts allow for different rod/bearing configurations.

If you've built or collected parts for a Minamil or other use of that slide design, then you may already have three 100mm rods & bearings ready for building this version of the "Z axis".

If you've scavenged 6mm rods from things that worked with office paper, then you may have at least one rod longer than 8.5" (210mm). If you're cutting rods to length from longer stock, then you'd probably prefer to cut fewer pieces.

Options include

• three 100mm rods + three bearings
• two ~200mm rods + three bearings
• two ~200mm rods + four bearings
• one 100mm rod + one ~200mm rod + three bearings

When using two long rods, there is a place for a fourth bearing to stiffen the slide. Three bearings handle the low cutting forces supported by Minamil-like X & Y axes well enough. A different application may see a bigger difference between three or four bearings.

I've found 6mm rods by scavenging old printers and such. The examples above use a couple of over-length rods from a printer that I haven't bothered to cut to size because the excess length causes no trouble in this orientation.

"~200mm" = 192mm to 195mm depending on material thickness; extra length is harmless so >= 195mm is safe

Find some string.

(but it's just a little bit complicated)

This uses string for two different functions and you'll probably end up with two different kinds of string for best results.

Tool clamp

This version uses essentially the same tool clamp as described in the slide 'ible and I'm still using the same stuff:

Use low stretch light line ~2mm or less in diameter. In the photos I've used very light synthetic line because that was what came to hand the first time I tried this idea. It worked. Since then I've continued to use the same stuff to see how long it lasts and how it fails. So far it has lasted and not failed. But if choosing more deliberately I might look for something a little less light.

While "low stretch" helps hold the tool firmly, the slight elasticity of ordinary "low stretch" material maybe makes it easier to adjust the clamping loops to an optimal length for "snap-over" closure. That's not necessary but it speeds clamping up a general purpose rotary tool that mostly lives somewhere else doing other stuff.

Hoist

This version uses a bit of string on a pulley to winch up a load against gravity (or other orientation + opposing force). In early trials I used the same kind of string as used for the tool clamp and that actually worked just fine until I got to chasing tighter repeatability.

Finer cord works better here for reasons. The most firm reason is that this iteration of pulley parts commit to <1mm cord diameter where the cord attaches to the pulley and motor. Also thinner cord allows more wraps around the winch in a single layer which, especially for parts cut from thinner material, keeps the effective winch diameter (i.e. steps/mm) more nearly constant.

Here are two examples that have worked well for me so far:

• heavy sewing thread
• works well
• better repeatability than 3-strand cord I'm using for the tool clamp
• unknown composition “extra strong for buttons, carpets, very heavy fabrics”
• seems vulnerable to progressive wear
• redundant lift is cheap insurance against sudden breakage
• low stretch holds ~constant load at ~constant length
• braided Spectra® (UHMWPE) fishing line
• works great
• I'm using 50lb (~23kgf) strength, 0.38mm diameter (nominal)
• initially appears invulnerable to wear
• redundant lift is still cheap insurance
• practically zero stretch
• retail purchase of branded product = mo$t co$tly component, but generic UHMWPE exists

In the slide 'ible, see Step 1 for more info about material selection and cutting.

If you have not already built slides like that for X & Y axes, then please do go ahead and cut the simple example parts attached to that step to use as consumables for learning how to build this thing from thin soft material and machine screws. (omitting the round thing that doesn't really fit in the layout will save some material)

Briefly from there:

• 1/8" (3mm) "hardboard" ≈ "masonite®" ≈ HDF
• instructions assume single-sided hardboard with one "good" face
• cut with good side down

Choose one of the vector files attached to this step:

• "3.0mm" for 2.5mm ≤ measured material thickness ≤ 3.0mm
• 2.8mm is the thinnest I've actually used, so ?? below that.
• "3.5mm" for 3.0mm < thickness ≤ 3.5mm

Cut from ~14" × 10.5" material (~350mm × 270mm)

I expect that most applications will require modifying the motor(s).

The 28BYJ-48 motor windings have unipolar connections. If that suits your application, then skip this step.

...

If you're not already scrolling for the next step, then you probably plan to use stepper drivers that require bipolar connection to the motor windings. (i.e. not ULN2003 modules)

Several sources describe how to convert the motors to bipolar configuration. Briefly, expect some variation of:

• open the plastic terminal cover
• cut a trace
• cut and discard one of the five wires
• re-order remaining four wires in the connector housing

I've opened one of the plastic terminal covers without completely busting a snap tab or opening the metal case, but I don't remember how. ಠ_ಠ. These motors come from several different sources with minor variations in construction. Sometimes it's relatively easy to pop the metal can open, then the plastic terminal cover can slide out without damage. Randy Glissmann has shared a jig for drilling through the terminal cover in the right spot to sever the right trace; I haven't tried it but it looks like a practical solution. Simply busting the cover off and taping it back on (or not) works.

See the photo for a wire order that should work with some low-end CNC controller boards that use 4-pin 0.1" headers for low-current motor connections, which I expect will be the case for most people looking at this. I think the connectors are JST-XH 2.5mm, but the pin pitch difference over four pins doesn't matter. There are JST-XH terminal pin extraction tools, but I use a pointy thing. Carefully. I don't promise that you won't stab yourself. Occasionally I have to (carefully) bend the locking tab back up a bit after mashing it too much. Alternatively, you could solve pin order when making up a cable extension.

When shopping for motors, note bundled "ULN2003" unipolar driver boards add no value. Unless you have some other use for a Darlington array.

Cheat code (maybe): simply re-ordering wires for bipolar connection to the windings, omitting the common wire, without cutting the common link between the windings works at least to some degree. I've found that sometimes it works well enough, and sometimes not, but sometimes I've later discovered some other reason why it didn't work when it wasn't working, so I've never really proved that it doesn't work. It probably skews microsteps, but that's not really a concern in this case.

tl;dr: skippable backstory

In keeping with the "minimal" theme of the parent project, this design uses a pulley built up from laser cut layers to keep the parts list short and cost down. While this may not be the very best solution, here's the story so far:

I've made the pulley small to hit my target of ~0.025mm (~0.001in) minimum useful increment. In my experience so far, the motors I've used show significant asymmetry between alternating full steps. It may be that it's really the stepper drivers regulating the coil currents unequally for the relatively small current used by these motors. Either way, I see alternating big/small/big/small steps. So the minimum practical increment is two full steps. Given nominal "2048" steps/revolution (infamously inexact), that means 1000ish useful increments per revolution. Then 0.025mm * 1000ish / π ≈ 8mm winch drum diameter.

On a 5mm shaft, that doesn't leave a lot of extra material for anchoring the hoist cord.

And: I doubt these motors are designed to carry much of a radial load out at the end of the output shaft. So I want to keep the load close to the motor body -- especially near full extension (unwound) where I expect most work to happen. Admittedly it's only a few mm difference between one side of the pulley to the other, but that still makes a difference in the cantilever load on the stub shaft.

And: after burning some time chasing repeatability, I wanted to route the cord as directly as possible to something harder than hardboard for an anchor.

So: the cord goes through the inner (motor side) cheek of the pulley to anchor around the metal shaft.

But: between the 8mm pulley and 5mm shaft, the cord has to cross the sharp punched steel edge of the motor case around the shaft exit (scary red Edge of Slicing).

So: a simple slot in the pulley cheek would let the cord go in at the 8mm diameter on one side and come out the other side inside the sharp edge.

But: that would let the thin cord work like a wire saw to cut between the cheek and middle piece. And since the hoist cord anchors through the cheek which is not keyed to the shaft but relies on adhesion to the adjacent keyed part, cutting those layers apart would drop the slide.

So: a little hook in the slot holds the tangential load at the 8mm diameter surface of the pulley on one side while the radial tail of the slot lets the cord cross the scary red Edge of Slicing to come out the other side safely close to the shaft.

The hoist pulley is a stack of three pieces glued together. One of the glue interfaces will be stressed. Doing this before you need it will give something like PVA wood glue time to fully cure.

Light sanding will lightly rough up the smooth sides and knock loose fibers from the rough sides for stronger bonds, I think. Also break the edge of the good sides of the larger pieces.

See photo for the orientation of each piece. Especially the hooked slot, and the smooth sides facing inward.

Test fit parts on the motor shaft. They are dimensioned slightly smaller than the shaft to compensate for some kerf. Ideally they will fit with some friction. If they don't fit at all, tweak minimally for friction fit.

I've been doing the glue-up like this:

• use motor shaft for alignment
• place the slotted piece good side up
• butter both sides of the small disk with PVA wood glue
• place the small disk good side up
• place the remaining large piece good side down
• clamp
• might require e.g. a clamp on each side of the shaft if the stack is not thicker than the shaft length
• wipe up glue squeeze-out around small disk before glue sets
• leave clamped on shaft for 15-30ish minutes for initial bond
• carefully pry up under the bottom piece to push the stack up off the shaft before full cure
• clamp, ensuring perpendicular clamping force
• leave for full cure time per directions on glue bottle

I don't know how hard PVA would stick on the shaft if left to cure. So far I have avoided finding out.

Glue squeezed into the shaft space while the glue initially sets up will help conform the finished fit to the shaft. I get finished fits that are easy enough to press firmly onto the shaft, maybe possible to pull off by hand -- gripping the bottom (motor side) disk only, or maybe requiring some prying with a flat screwdriver or such between the motor and pulley to move the pulley up far enough to get more finger meat underneath it.

If you have already built a slide from that 'ible, you should be able to continue with the relatively general guidance here.

If you have not, then hopefully you have already cut parts for the simple example attached to Step 1. For the best orientation to content not repeated here, follow that 'ible up to up to Step 31 to build the basic example described there. If building that example just as prep for building this, you can skip the leadscrew (steps 6, 15-18).

The more conventional configuration of this version -- with fixed rods on one side and moving bearings on the other side -- allows more simple assembly sequence, so this 'ible includes less sequential detail.

The slide 'ible covers details including how to compress and secure the rod/bearing clamps, part "good side" orientation, and screw torque for soft material. Etc. This isn't a complete summary.

See the slide 'ible for details of how to do this.

• Drive screws through the four motor mounting holes now & remove them. To pre-form threads so you can attach motors later without excessive force on the assembled slide.
• Match the V & clamp parts with three big holes with the end that has space for three bearing supports. This is the top end of the sliding part.
• Use bearing supports under bearings (not obvious in photo).
• Assemble the ends of the slider with two bearings in the bottom end and one or two in the top end, located for your chosen configuration.

See the slide 'ible for details of how to do this.

• Loosely assemble the rod clamps on the fixed base.
• The middle "end screw" will be inaccessible. Set it to hold the middle clamp & "V" plates close but not so tight that the clamp can't slide open/closed.
• Leave clamps open (slightly raised) to allow free clearance for rods.

You've read the side 'ible by now, right?

• Align the moving and fixed parts
• Insert rods through an end clamp, then carefully through the corresponding bearing.
• go slow without force to avoid catching and stripping bearing balls out of a bearing race.
• Align and insert rod ends into the middle clamp.
• For long rods through two bearings, continue carefully through the second bearing and into the third clamp.
• Align the rod ends to final position and firmly compress and secure the two end clamps.
• Push the slide away from the end you're working on to compress the clamps without stressing the assembly.
• Support under the middle clamp as shown:
• Something in the gap between the top of the clamp and the moving slider - several sheets of paper make shim of adjustable thickness.
• Something hard, flat, thicker than the screw heads, and narrow enough to fit between the two rows of screws.
• Compress & secure the middle clamp.

When assembling a slide with three short rods, it may help to secure the top end of the single upper rod first while you can still reach both ends of it to feel for equal position in both clamps. Then insert the lower two rods.

Photos here and following show the three "feet" attached to the base plate. That's not necessary. I've stuck them on with bits of double-stick tape for my own convenience because I'm assembling/disassembling this stuff often enough to warrant the effort.

Attach the blue and two magenta-ish parts in diagram, including small perpendicular brace.

Attach the "pulley guard" (blue) with the good side facing inward, so that it will bias outward (toward shaft centers) if the edge isn't perfectly square.

The main purpose of the hoist anchor is to provide a point(s) for the slide hoist(s) to hang from that is:

• rigid
• directly above the tangent surface of the hoist pulley, to minimize change of angle of the hoist cord over the range of travel
• green line in diagram
• above the "wound-up" side of the pulley because that's where it will be when closest to the anchor with greatest effect on angle

The same part does a few other things, because it's there:

• anchor the hoist cord
• includes cleat(s) for tying off the hoist cord(s)
• keep load path and tie-off points in the plane of a single flat part
• block pulleys from walking off the ends of the motor shafts
• cyan bar in diagram
• in combination with lower extension that serves only this "pulley guard" purpose
• the edge of the anchor part is broken up, but the high spots are all at the same level with gaps smaller than pulley diameter
• this does not need to be a smooth running surface; the pulleys should never touch these parts; if they do that indicates a problem to fix; I haven't seen a pulley move on a shaft at all and wouldn't miss this feature if it weren't there but it seemed prudent to block a potential free-fall failure
• mount optional limit switch
• place and pilot holes for fixing switch in correct working position

• Cut one or two lengths of the string you found in Step 4, about 18" or 50cm. That's generous; it will be easier to trim excess than to stretch short strings later.
• Tie a slip noose in one end of each. Like a backwards slipped overhand knot. It's not super clear in the first/left photo that the bitter end is on the left and the working part goes off to the right. The bitter end is more clearly on the right in the second/right photo.
• Tie close to the bitter end or clip the end short -- leaving just a few millimeters.

This knot has been holding for me even in slippery UHMWPE, and can be cleared from thread-like small string like this by "scraping" it off the end between your fingernails.

• Feed the hoist cord through the side of the pulley with the loop to the outside
• Draw the loop of the hoist cord tightly around the full round part of the motor shaft against the face of the motor
• Slide the pulley up to the motor and fit it onto the shaft
• Pull the cord through the pulley so there is no excess under the pulley
• Make sure the loop stays down around the round part of the shaft
• Press the pulley down tight against the motor.
• Pull the hoist cord firmly in the direction shown to set the knot around the shaft under the pulley
• Ensure the loose end is short enough that it cannot reach around the side of the pulley and get under the working part of the hoist cord.

The cord must not completely unwind before the slide stops at its lowest point. but must make at least part of a turn around the pulley. On the other hand, extra string on the hoist pulleys doesn't help because:

• repeatability seems better with fewer wraps
• limited number of wraps fit in a single layer
• may or may not be enough for full travel before wrapping second layer changes pulley radius
• which isn't awful assuming position matters more at lower end and top end is mainly to lift the tool out of the way

So ideally the pulley(s) should be rotated so that the hoist cord(s) make about 1/4 turn around the pulley when attaching the motors, like the last picture. This will become the bottom end of the range of motion. Turning a pulley requires steady firm torque because these motors do not "back drive" easily.

Photos here and following show a single or first motor installed in the lower position before filling the upper position. The lower position is slightly less offset from the load on the slide, but I doubt the difference is actually significant apart from following along with these photos.

Pass the free end of the(a) hoist cord(s) through one of the pulley clearance holes, then fix the motor in place with two screws. If you pre-formed threads in the material in Step 10 then you won't have to squash the assembly to drive the screws. The threads in hardboard are soft, so start by holding the screw perpendicular with slightest pressure and turn backwards until you feel the screw thread "drop" into the hole thread.

Check clearance where the hoist cord passes by motor mount screws. Depending on the thickness of your material, the screws may interfere. In the top half of the comparison photo above, the string passes close by two of the motor mount screws. What's not obvious is that it just barely clears the lower(far) screw while the sharp edge of the upper(near) screw slightly interferes.

To keep screw ends out of the path of the hoist cord, you may have to shorten one or more motor mount screws. In the top-view picture of the motor, you can just barely see a washer under one of the screw heads to effectively shorten that screw. The outside diameter of that washer is smaller than a common #6 flat washer. It looks like those are called "small OD" or "small O.D." washers. Alternatively, you can file the end of a screw to make it actually shorter.

The lower half of the comparison photo shows how the string can get hung up on screw threads even after backing out the interfering screw enough to get it out of the way. That won't happen in normal operation, but note that it can happen whenever the axis slide is lifted manually which lets the hoist cords go slack.

Repeat if using two motors.

Photos show tie-offs for one motor or two.

This can be tedious. It should be easy, but stuff gets in the way. So far I haven't come up with a more comfortably convenient arrangement that doesn't clash with some other constraint.

The offset motor locations line up with the two sides of the hoist anchor, so one hoist cord runs up each side to the point where it turns through the part.

To tie off, I’ve been making a few turns around the cleat base and then a locking hitch on each horn. A crossover and single locked hitch seems too easy to loosen, at least when using UHMWPE cord. There's probably a better way.

The end of the slide 'ible from Step 56 on deals with attaching a rotary tool.

Step 57 describes assembling the clamp, which is similar. This version attaches more simply with a screw in each corner, but does require aligning the slide with holes in the base for access to the screws.

Notably:

Beware the clamp is very fragile until attached to the slide; when driving screws make sure to directly hold the part that you will press into because the slide will fold up easily if you press on one side while simply holding the clamp in your hand

In the slide 'ible, Steps 58 and 59 describe rigging and adjusting the tool clamp.

The clamp saddles fit the classic "395" style of DremelⓇ rotary tools and many generic clones.

The saddles may be modified to fit other tools, whether by direct physical hacking to fit, by modifying the shape in the vector file for laser cutting, or modifying in CAD to generate a new cut file. When modifying to fit a different shape, simply omit the middle saddle. It is non-structural and serves only as an axial positioning guide for the "dremel" tool shape.

The hoist anchor has an extension with two small pilot holes for screws to attach a limit switch.

Small switches sold as "KFC-V-307" or "Camera A15" fit in this location.

The screws I've used probably came from an old laptop, camera or CD/DVD drive. The 1.6mm diameter screws, presumably M1.6, have a slight interference fit through the switch body, which works fine. M1.4 should work too, at cost of somewhat less strong attachment.

The switch placement works when the upper bearing(s) is(are) positioned with the lower/innner edge of the snap ring groove even with the upper/outer surface of the "V" plate holding the bearings. Then the slide will stop in between tripping the switch and smashing into the switch body.

Congratulations on building your new thing! Now it really wants to be attached to another thing. Because:

• It really wants to be part of something bigger that itself
• The base is not nearly stiff enough by itself, but depends on secure attachment to something more rigid

To attach to something else:

• See the first photo for six mounting screw locations
• Use the three "feet" as spacers for solid attachment around the screw heads
• While some photos here show the "feet" attached to the base, that's not necessary

Here be numbers.

I haven't tried to work out limits or optimal values, and I've read an inconsistent mix of specs and recommendations for these motors. Fortunately, I haven't had to push them motors very hard to hoist a rotary tool with a ~4mm pulley radius.

I've mostly used A4988 stepper driver modules with an 18V motor supply.

The 5V motors I have measure a little under 45Ω for each full winding.

I have the driver current limit set for what should be ~160mA according to the math and measured reference voltage. But I haven't measured actual delivered current. With constant power, the motors get hot enough to make me want to let go after several seconds of holding one in a firm squeeze for good thermal contact. I think that's not too hot.

I don't know the exact gear ratio for the motors I have, and people report some variation. 2038 steps/revolution, approximately, seems to be a popular number. For an 8mm diameter pulley the numbers say that's about 81 full steps/mm, which is really close to 0.025mm or 0.001" per two full steps, which I figure is the practical minimum useful increment. But the actual effective diameter won't be exactly 8mm because, aside from general imprecision, subtract kerf and add some fraction of cord diameter. I think it's fair to start with 80 multiplied by your microstepping factor for steps/mm to get in the ballpark, then measure actual travel over intervals that you care about and adjust.

I've tried 1/1, 1/2, 1/4, and 1/8 microstepping. All work and I haven't tried to determine which works best. 1/8 microsteps do actually move the slide -- as indicated by repeated Z probe values drifting by 1.6 micron steps. I think that's kinda neat, but not practical for more reasons that I'm going to fit in this paragraph. One reason is that the length of those microns varies by quite a lot over a ~25 micron, two step cycle. I figure coarse stepping is more likely to provoke resonance issues but haven't assessed that for real. On the other hand, gratuitously high microstep rates can consume a nontrivial fraction of Grbl's finite step rate limit. At the moment I'm using half steps for no great reason.

More values that I'm currently using because they're working well enough to not make me look any closer:

steps/mm = 162.75 (calibrated pretty close but not best effort)

max rate = 1000 mm/min

acceleration = 50 mm/sec^2

These rates work for one or two motors.

maximum travel = 65 mm (could be a little more but less than 70)

If you built your Z slide with two motors:

You may have tied off the hoist cords close enough to equal tension to look and work at least ok, and one motor can do the job (for the example use case) but since you have two, let's let them share the load. That will also help hold position better if one of the cords, pulleys, or whatever fails.

To equalize hoist cord tension:

• attach the expected working load e.g. your rotary tool
• move the slide to slightly above the bottom of its range, or wherever you expect to have it doing most of its work
• disable motor power, disconnect one motor, re-enable motor power
• make small adjustments with the powered motor while feeling for equal tension in the two hoist cords
• they won't sound the same when plucked, because they are not the same length
• instead feel for similar resistance to deflection
• disable motor power, reconnect idled motor, re-enable motor power
• verify smooth operation through range of motion
• move to a significantly different position e.g. from near bottom to near top of range
• check that tensions are at least pretty close to same balance
• two handcrufted pulleys are not likely to be exactly identical
• but shouldn't be very much different either

oxen art CC0 freesvg.org

It's just one axis and not very stiff when not secured to a more rigid surface, but if you've read this far you probably have in mind some idea of combining this with a few other things to make another thing that will do something. Thank you for reading and please drop a comment about where you plan to go from here!