DIY CNC Interval 2A: Leadscrew, Leadnut & Bearing Selection
In the first part of this Diy CNC Series I had given a brief introduction on my way of building a CNC. Here I talk on one of the most important part of CNC viz.: leadscrews(& its support bearings) and leadnuts.
Leadscrew(or simply precision threaded rod) is ‘the’ part that converts rotational motion of stepper/servo motors into linear translation. The Leadnut is similar to a nut which moves back and forth as the leadscrew rotates. The pitch of the Lead Screw and the number of thread starts decide the accuracy of a CNC and also the maximum linear speed aka Rapids. Pitch is simply the distance between the center of 2 threads on the Lead Screw. When the Lead Screw makes 1 complete rotation it makes the Lead Nut travel a distance which is equal to its Pitch. In My case I have used a standard Metric Lead Screw of Diameter 10mm and pitch 1.5mm. When it rotates once the lead nut mounted on it travels an exact distance of 1.5mm in the respective direction.
Leadscrews come in a variety of standards and types. Amongst them most famous and widely used in DIY CNC are ACME and Metric M-x(where x is the diameter) thread type leadscrews. ACME is as per the imperial standard and most widely in west. Metric M-x is as per the Metric Standard. Lead screws having ACME threading are generally precision threaded rods. Metric M-x thread type is generally used in fastners hence the leadscrews may have high tolerences hence lower precision. But Precision Metric M-x leadscrews also do exist.
You can find more information on Leadscrews at these links:
- www.cnczone.com/forums/linear_rotary_motion/20745-screw_threads_101_a-5.html
- www.roton.com/acme-lead-screws-nuts.aspx?line=Acme
- www.roton.com/trapezoidal-lead-screws.aspx?line=Trapezoidal
- www.cncroutersource.com/acme-lead-screw.html
Initially I had thought of using ACME 10TPI(2.5mm pitch) threaded Leadscrews but ran into a bit of stock issues since at my place we follow Metric Standard .. hence ACME components are not easily available while Metric components are very cheap and readily available. The counter part to ACME Leadscrew in Metric system is Trapezoidal Leadscrews. Trapezoidal Leadscrews are too not readily available at my location despite of the fact that its a Metric component! Hopefully I found a manufacturer of Leadscrew who could custom make Leadscrews as per my requirement be it ACME or Trapezoidal. Now the problem next was the availability of Trapezoidal nuts and Leadnuts. Trapezoidal nuts are costly .. I could have got them but that would mean more time waste. Since this is the frist time I am making a CNC I thought of using readily available components to make a sort of boot-strap CNC after I could make newer version using ACME leadscrews.
I finally decided to go with M-10 Leadscrew which I got manufactured from a manufacturer. M-10 Leadscrew has 10mm diameter and a pitch of 1.5mm. The advantage of lower pitch is more fine resolution but at the cost of low feed rates. Advantage of having higher pitch is higher feed rates but a bit lower resolution. But the fact is : even in the case of higher pitch the resolution is still very good since stepper have a step angle of 1.8 Degrees and they can be run in micro-stepping too!
M-10 Precision Leadscrew (Mild Steel):
Now , coming to Leadnuts. I ordered 4x M-10 Anti-backlash Leadnuts from www.dumpstercnc.com. The Leadnuts are indeed as per the specs and of top notch quality. When I tried them on M-10 Leadscrew the backlash was 0!
Anti-backlash(AB) Leadnuts:
AB Leadnut mounted on Leadscrew:
To support the Leadscrews at both the ends I also got some flange type and normal ball bearings. I’ll talk about how I have used these bearings for the Leadscrew assembly in detail in next my next post.
Bearings:
Now the last thing remaining was to decide on bearings for the Lead Screw end support which will secure Lead Screw in place. I purchased some normal bearings with ID = 10mm and OD = 19mm and similar Flange type bearing with flange diameter around 21mm. Using these bearings with a 20mm Shaft end support I made my own DIY Lead Screw End Support and later on used Thrust bearings to lock the Lead Screw in place.