Posted by SainSmart on

Written by Graham

This is Genmitsu 4040 Reno, the first belt driven CNC router (at least on the XY axes, Z uses a leadscrew) that SainSmart Genmitsu has made and that I have owned. It is a review but also includes how to get started with one, Grbl settings and making a spoilboard.

My initial research leads me to think that this is an entry level CNC router, not quite at the lowest level but still very suitable for someone getting started.


Screws or Belts?

This a belt driven (on the X and Y axes) router, so before I go any further a little discussion on the use of screws or belts to control motion along an axis of a CNC router, while I have 3D printers that use belt drives I have never used them on a CNC router.

I am considering screws to cover both ballscrews and leadscrews with their associated anti backlash nuts and belts to be modern ‘toothed’ belts and pulleys.

It is a difficult subject, as are most CNC topics, to research. A lot of the information applies to ‘professional’ routers, the ones designed to produce 10’ by 4’ signs or precision parts from titanium. I did find though that manufacturers of belt driven CNC routers extol the advantages of belt drive while those who use lead or ball screws extol the advantages of them.

One thing I have missed out is size, as it is not really applicable, the ones I have are all small routers, if you have a 3m long axis you would need a 3m long, unsupported, leadscrew, that is going to cause problems even if you could find one. So it is likely that a belt or rack and pinion drive is used.

I think the reality is it depends, if you want a small router that will cope better with the stresses of rapid cutting, hard materials… and have the budget then go with screws, if not then belts are fine.

There are a few things to consider when choosing a drive system, in no particular order some are:

  • Resolution of movment
  • Repeatability of movment
  • Maximum Force
  • Maximum Travel Speed
  • Wear, Tear and Maintenance
  • Noise
  • Cost

Resolution of movement

Basically what is the minimum distance the drive system can move the axis. This is also going to be affected by the stepper motor, how many microsteps in a revolution…. A screw driven system also uses the pitch of the screw to determine movment, for a belt system the diameter of the pulley has the same effect, a smaller diameter pulley will move along the belt less per step than a larger one.

This router has 160.1 steps per mm for the X and Y axes meaning theoretically each step should move by 1/160.1 = 0.006mm. The screw driven Z axis has 1600 steps per mm so the theoretical movment is 0.0006mm In practice allowing for things like backlash the actual accuracy will be lower. The advertised value is 0.1mm as a running accuracy.

Both belt and screw systems are capapble of a practical 0.1mm (0.004”) or less per step but screws tend to be better especially under higher loads.

Repeatability of movement

This determines the positioning accuracy of the router. Start at 0,0 then whizz around on the axes and return to 0,0. At the end the bit should be in exactly the same position as where it started.

Both belt and screw systems are capapble of a practical 0.1mm (0.004”) positioning accuracy, again screws tend to be better especially under higher loads.

Maximum Force

There are other factors which affect this, the power of the stepper motor for one. To move the bit on a cutting path means pushing it through the stock, the harder the push the more it can cut. While applying the force the resolution and repeatability of movement can be affected by something like a belt stretching or slipping. Screws will not stretch or slip.

Screws win on this one.

Maximum Travel Speed

How fast the axis can be moved affects both positioning speed and cutting speed. Theoretically belt drive should be faster but there are other factors such as how many steps per second can the control board send.

Belts normally win on this one but for small hobby routers the difference may be unnoticeable.

Wear, Tear and Maintenance

Belts can stretch and the ‘teeth’ can wear, screws can still wear but do not stretch!

Screws require lubrication, belts do not although both need keeping free of dust and chips.

Belts also require tensioning correctly, too loose and they can slip, being too tight will promote excessive wear and stretching as well as high stresses and potential deformation of pulleys.

IMHO it’s 50/50.


Belts drives should be quieter in operation than screws.

Belts win.


Pretty much belt drives are cheaper than screw drives.

Belts win.


Belt Tensioner

The belt tensioner for the X axis is not fitted, it can only be installed after the X axis carriage is fitted to the gantry as the belt has to first pass through the gantry upright. The tensioners for the Y axis carriages are factory fitted and pre-adjusted.

The belt is fixed onto the extrusion by a simple clamping plate holding two grubscrews which tighten onto the belt and clamp it to the extrusion.

At the other end there is another clamping plate, this one has plastic grubscrews. I believe this plate is just used to hold the belt during assembly, once the tensioner has been fitted and adjusted this plate is superfluous. The belt then feeds through a hole in the end plate over a pulley and down where it is clamped onto a screw adjusted slider which is also clamped into place by 4 grubscrews.

The belt tension needs to be reasonably tight, tighter than it could be pulled by hand, hence the screw adjusted tensioner. As with any belt drive the correct tension is important, too slack and the belt can slip, too tight and undue stress can be placed on pulleys, shafts, bearings and the belt itself.


Before Assembling

Before starting the assembly, I suggest that you check the carriage alignment on the X and Y axes and belt tension on the Y axis. It’s just slightly easier to do this with them disassembled.


Check Carriage alignment

Each V pulley should be in contact with the extrusion groove, enough so that if the carriage is moved the pulley rotates rather than slides along the groove. The carriage should move freely and easily with no binding.

While mine were basically OK on the Y axis they all needed a little adjustment. On the X axis more adjustment was needed to place each pulley in contact with the extrusion.

How the Carriage adjustment works

To adjust a pulley slacken the side clamping screws and adjuat the tensioning screw underneath!

This is largely irrelevant but I am a curious guy and generally I like to know how things work.

The mechanism is the same for the dual pulley X gantry carriage and the Y carriages, The X is a little wider to take the dual pulleys.

At the top of the carriage the pulleys are simply mounted in bearings on shafts. These are not adjustable, there is no need, the pressure of the pulley onto the extrusion is set by adjusting the bottom pulleys.


At the bottom of the carriage each pulley (or pair) is mounted in a separate carrier. Each carrier is attached to the carriage plates by a solid pivot near the centre of the carriage. Towards the outer edge of the carriage a clamping screw runs in a small vertical slot, when tightened (front and back) these clamp the pulley carriers between the carriage plates preventing vertical movement of the pulleys. The pictures show the screws and bottom plate removed.


Underneath the carriage there is a metal plate with two tensioning screws. Each screw goes through a carrier into a half round nut in a groove at the top of the carrier. The end of the plate rests on two hard ‘rubber’ inserts which stick out from the carrier. When a tensioning screw is tightened as the end of the plate is higher than the screw the outer end of the carrier is pushed up towards the extrusion tightening the bottom pulley which in turn tightens the top pulley. This means the tension of each pulley can be independently adjusted. (But tightening one side does have a slight effect on the other!)


The bolts on both sides of the carriage must be slackened to adjust them which means the cover on the rear of the X gantry and the Z-Frame and the cover on the left Y frame must first be removed. Disconnect any wiring and unscrew the covers leaving the control board or breakout box in place on the cover.

At the base of each carriage slacken the outer clamping screws, the inner pivot ones do not need slackening, to release the clamping force on the outside of the pulley carriers. Not a lot, maybe 1 turn, just enough.

Now check for any play, using a finger or thumb try and rotate a pulley without moving the carriage and adjust the tension till resistance is felt. Tightening a tension screw will tighten the pulley.

Adjust the other side then re-check both. Once adjusted re-tighten the clamping screws, yes I would always recheck the tension after tightening the clamping screws and before re-assembling anything.


  1. Even with the screws slackened the carriers can be a tight fit inside the carriage plates, a little careful tapping of the carriers or prising them up using a large Allen key can help them adjust to the required position.
  2. On the dual pulley X gantry the process is the same but check BOTH pulleys for movement while setting the tension and adjust for the slackest, this shouldnaturally pivot the mounting shaft just enough to adjust both correctly (See note 1).
  3. If you over tighten the pulleys there is a risk of pressing flat spots into them when they are left at rest.

Check Y carriage Belt tensions

What is the correct belt tension? I am guessing, the only exact way to set it would be to apply a known force at a certain point and measure the deflection. A good starting point is to look at the tension of the Y carriage belts; these should have been factory set on the Y carriages.

At the midpoint of the belt between the stepper pulley and a V pulley a gentle finger pressure gives me a deflection of ~2mm (0.1”).

To adjust the belt tension;

  • Slacken the two grubscrews in the clamping plate next to the tensioner to release the belt.
  • Slacken the 4 grubscrews securing the belt tensioner slide.
  • Tension the belt by adjusting the vertical screw to move the slide up and down.

Once the correct tension has been set tighten the four grubscrews which secure the slide.



The manual is sufficient; I am not impressed with it. SainSmart have confirmed that it is being reviewed and an updated version should be published online in the future, check for updates.

It is simple and quick to assemble, but I advise taking your time and double checking everything rather than trying to beat the clock. There are not many assembly photos here, the illustrations in the manual are clearer! Follow the manual, it’s not great but it will get you there. These are just notes and hints.

IMPORTANT NOTE: The belts make the carriages much easier to move manually than screw driven ones. When the carriages and so the stepper motors are moved by hand, or gravity when moving the router, they will act as dynamos! If they are plugged in to the control board and moved too quickly sufficient reverse current can be generated to burn out a control board. If moving the carriages by hand, or gravity, do it slowly!!

X Gantry to Y axis uprights (Step 1)

  • First check the carriage pulley alignment as described above.
  • The belt sticks out at both sides, on the left just a little and fits into the gantry upright slot.
  • At the other end the belt passes through the gantry slot and will be fitted to the tensioner later.
  • Fit the left side first, leaving the 4 M5x25 round head screws loose.
  • Then thread the belt through the slot in the right gantry upright making sure it is not twisted, loosely fit with 3 M5x25 round head screws, and at the top right an M5x18 small socket cap head screw which will fit under the slot for the belt tensioner.
  • There is a tightening order for the screws shown in the manual, I can see no reason not to follow it.

Front and rear X profiles (Step 2)

These profiles keep the Y carriages square and also provide a frame to which the bed is attached.

The orientation of the extrusions is important, the flat edges go to the top and outside of the router.

You are going to have to turn the partially assembled frame on its side or over in order to get at the screws underneath the Y extrusions.

At the front and back of each Y carriage there is a screw in a captive T nut at the bottom of the extrusion. Captive does not mean fixed! these will freely slide along the extrusion under gravity and seek out the most inconvenient places to rest. They can be easily moved though using the end of a small Allen key but it’s best to take out and re-insert the screw when the extrusion is flat to prevent gravity taking hold, or place something into the extrusion slot to prevent it from sliding too far. I used a crumpled-up piece of kitchen roll.

One at a time trap the captive T nut, remove the Cap head screw, insert it through the hole in the extrusion and into the T nut, repeat at the other end. Then slide the extrusion up to the end plate and loosely tighten the Cap head screws

The end of each extrusion is also screwed into the end plate with 2 M5 x 25mm round head screws. Tighten these loosely.

Place the router the right way up on a flat surface. The final alignment of the frame is performed using the X gantry to keep the Y side frames parallel, so move the X gantry slowly all the way to the front of the router, then tighten the bolts through the end plates. Move the X gantry slowly all the way to the rear and tighten the back screws. Finally lift each side of the router and tighten the bolts from underneath into the captive T nuts.

When fitting the rubber feet the retaining screws go into the small holes underneath the X extrusions.

Check the frame is Square (Not in the manual)

The frame (defined by the Y extrusions and the front and rear X profiles must be square. Each side being parallel to the opposite and all the corner angles being 90°

Another way of checking is to measure the diagonals between the corners made by the Y and X extrusions they should be the same.


NOTE: I suggest changing the order of steps 4, 5 and 6 in the manual they are pretty much independent of each other but I found my order easier.

  • Step 5. Install the bed first! It doesn’t make any difference but I want to check the alignment of the XZ Axis assembly to the bed during installation and measure the heights. I can’t do that until the bed is installed.
  • Step 6. Install the belt tensioner and set the X axis belt tension, this is slightly easier without the Z frame installed.
  • Step 4. Install the Z axis frame, steps 5 and 6 are slightly easier if this is done last.

Bed (Step 5)

As the bed is bolted down in two halves, along the centre each half will flex independently. While attaching the bed to the frame I strongly suggest applying wood glue between the two halves, this greatly improves the rigidity of the bed especially alongside the centreline.

Tighten the screws holding it down securely but do not over-tighten, the MDF will compress under the screw head!

X axis Belt tensioner (Step 6)

Start by looking at the Y axis tensioners, these are pre-assembled and tensioned so look like the finished article should, including the way the belt is tucked in and clamped.

How it works:

  • The tensioner base is screwed into the cutout on the gantry upright.
  • The slider which runs up and down in the base is locked in place by 4 grubscrews, 2 on each side.
  • The long screw running vertically when tightened will pull the slider down so tightening the belt, slackening it will reduce the belt tension.
  • The belt comes through the slot on the gantry upright just above the tensioner.
  • It passes over the ‘Isolation column’ this provides a curve for the belt to follow preventing the belt from making a sharp angle into the tensioner.
  • The ‘Isolation column’ is held in place by the end of the long vertical screw, the sides of the tensioner base and the belt.
  • The belt is then fed into the slot on the slider under the belt clamp. It is designed so that the belt is doubled back on itself under the clamp, if not the clamp will not hold it securely.

To fit:

The instructions may look very complex, the installation and adjustment are not difficult, just a little tricky in places, my instructions are designed to simplify the installation and adjustment.

  • Slacken the four grubscrews to release the slider.
  • Totally remove the long vertical screw, this releases the slider from the base exposing the screw holes.
  • Attach the tensioner base to the upright with 3 M3 x 12mm countersunk head screws.
  • Re-install the slider and tighten the vertical screw through it so the end of the screw is level with the top edge of the slider.
  • Move the slider up to the top of its movement and clamp it in place using one of the grubscrews (not necessary but makes it easier as it won’t slip down)
  • Slacken both the screws on the clamp and remove one completely leaving the clamp to pivot around the remaining screw. The belt needs carefully tucking it underneath the clamp in the next steps. I found leaving a screw in rather than removing both only needs 2 hands rather than 3.
  • Place the end of the belt underneath where the belt exits from the gantry upright slot and fold it back onto itself with the toothed side in.NOTE: I found the belt stuck out too far from the gantry. You need 9-10cm, longer is ok but will leave a big loop. I trimmed the belt so that only 10cm stuck out.
  • Drop the ‘Isolation column’ into the slot formed between the slider and the base.
  • Slide the folded belt into the slot under slider clamp with the slider at the top of its movement.
  • Rotate the slider clamp to cover the belt, re-insert the screw and tighten the clamp with the belt hand taught and the slider at the top of its travel.

NOTE: The belt MUST be folded, the slot and clamp on the slider will not secure just a single thickness of belt. And it also presents a flat surface to slide over the ‘isolation column’.

Setting the belt tension

After installation of the belt tensioner the manual says “adjust the belt to the proper tension”.

If the belt tension is too loose the belt can slip over the drive pulley, too tight and unnecessary stress is placed on the belt, drive and guide pulleys which can cause the carriage assembly to bind.

As the Y axis modules both have belts pre-fitted and pre-tensioned, examine the tension on these by deflecting the belt midway between the drive pulley and the V pulley. The deflection, with gentle finger pressure in the middle, should be ~2mm (0.1”).

  • Slacken the grubscrews in the clamping plate at the top of the extrusion, next to the tensioner.
  • Slacken the grubscrews holding the tensioner slider in place.
  • Tighten the vertical screw to increase the belt tension while checking the deflection between the drive pulley and a V pulley.
  • Tighten the four grubscrews locking the tensioner slider in place.
  • I did not tighten the two grubscrews in the clamping plate at the top of the extrusion, they are plastic and just slipped. I cannot see these really being needed, the belt should be held in place by the tensioner.

Z Axis frame (Step 4)

The rear of the Z frame fits into a slot on the X axis carriage. This aligns the spindle motor vertically over the X axis.

There are four notches cut out which fit over the base plate screws giving two vertical mounting positions, one 30mm (1.18“) higher than the other.

The high position allows the use of thicker stock, maximum of 86mm (3.39“), the lower position allows 56mm (2.2“) This is the clearance between the bottom of the Z frame and the bed and has nothing to do with Z axis travel. Fitting a spoilboard will reduce this by the thickness of the spoilboard.

I suggest mounting it low. If you only plan to use thin stock, depending on the bit you may have problems reaching the bed in the high position. It is easy to adjust.

Wiring (Step 7)

There is little to connect, the stepper motors need plugging in, and at the top of the Z axis the spindle motor and the Z axis limit switches need connecting.

NOTE: If you have purchased the WiFi module then install this first as some cables need to be removed from the control board to fit it.

When installing the 24 pin XYZ cable between the control board and the breakout box I really, really, strongly suggest that you use tie wraps to provide strain relief for this cable’s connectors. The cable will move and flex as the Y axis moves putting a great deal of stress on the connectors.

 On the rear cover of the Gantry there is a mounted cable holder, use one of the larger tie wraps.

 On the Y axis side plate there are two pairs of slots through which tie wraps can be threaded. Use both! Small pliers make inserting the cable ties easy.

This is really worth doing, if not the natural movement of the cable will pass all the stresses of the movement onto the board connectors at each end which they are not designed to withstand.

At the top of the Z axis there is also room for some strain relief on the spindle motor and Z limit switch cables.


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