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Z-axis lead screw length for Dolly Mk2 build

Posted: Wed May 02, 2018 7:42 am
by warren.watts
I am in the early stages of a Dolly Mk2 build - I have the majority of the printed parts printed, and am getting ready to make the first round of purchases from AliExpress. I plan to make the stepper motors, couplers, and lead screws my first purchase. I am going to use T8 trapezoidal lead screws rather than M5 threaded rod, but I have some questions regarding the correct length to purchase.

In the BOM at https://toms3d.org/2017/02/23/building- ... usa-i3-mk2 the length of the M5 threaded rod for the Z-axis is listed as 350mm. However the Z-axis M8 smooth guide rods are only 320mm in length, and they appear longer in the build videos than the threaded rod.

What is the correct length? I suspect it should be 300mm. It seems like 300mm would be more than long enough to extend beyond the top Z-axis mount after being slid into the 5mm-to-8mm couplers attached to the steppers.

Are 300mm lead screws the length I need for this project?

Re: Z-axis lead screw length for Dolly Mk2 build

Posted: Fri May 04, 2018 7:23 pm
by Stefan
Hi Warren,

You are spot on: 300mm for the threaded rods or trapezoidal lead screws. (BTW, make sure you get the right couplers to match the diameter of the lead screws!) The other gotcha will be the nuts for the lead screws. Some will screw directly to the printed parts as is, but some nuts are slightly smaller and will require a printed adapter (which can be found on thingi....)

Keep us posted on your build.

-Stefan

Re: Z-axis lead screw length for Dolly Mk2 build

Posted: Sat May 05, 2018 9:22 am
by warren.watts
Thank you for the clarification, Sir. I really want this build to go smoothly and I want to make sure I have all my ducks in a row. I really don't want to order parts from China that could take a month or more to arrive only to discover that they are unsuitable for my build.

The other area of concern I have is whether the couplers (and thank you for reminding me about the 5mm dia. motor shaft and 8mm dia. lead screw) are going to hit the x-end pieces before the extruder nozzle is low enough to be at the correct distance from the print bed. Has this been an issue for anyone?
The fellow on this blog (https://prusaclone.wordpress.com) apparently had issues and ultimately ended up replacing the stepper/coupler/lead screw combo with stepper motors with integrated lead screws. He also apparently had some issues with wobble introduced by the couplers. I have priced once piece stepper/lead screws and they are considerably more expensive than using a standard stepper with a coupler and separate lead screw.
Is it worth the extra money to buy once piece units and avoid potential clearance and wobble issues, or am I worrying for no reason?

Re: Z-axis lead screw length for Dolly Mk2 build

Posted: Sat May 05, 2018 8:58 pm
by Stefan
Hi Warren,

A picture is worth a thousand words:
IMG_0554.JPG
IMG_0554.JPG (719.28 KiB) Viewed 1114 times
With the nozzle laying on the bed, the couplers have plenty of clearance. I also have no complaints of z-wobble with this set-up. This is my third clone build and the other two had threaded rods instead of lead screws. I like this one best. Also note the adapter for the nuts.

Re: Z-axis lead screw length for Dolly Mk2 build

Posted: Sun May 13, 2018 9:24 pm
by warren.watts
(Slow) progress report

Starting with 2 panels of 1/2" birch plywood, I have clamped, glued, and cut out the frame. I have obtained the needed M8 and M10 threaded rod, cut it into the necessary pieces, and assembled the basic Y-axis frame. The smooth rods and bearings are on their way from China. Hope to order the stepper motors and lead screws soon.

As you can see from the photos, I still have a LOT of sanding to do to smooth out the frame. If I can manage to hide enough of my sloppy jigsaw work with the sander, I was thinking about maybe staining the frame and putting a clear polyurethane on it. Not sure yet. I may just get lazy and paint the damn thing flat back and be done with it. What do you guys think?

ImageImage

Re: Z-axis lead screw length for Dolly Mk2 build

Posted: Sun May 13, 2018 9:41 pm
by Stefan
Looks VERY familiar, Warren!

I painted mine semi-gloss black. Easier to keep clean than a flat. When you mount the frame to the y-axis, the Prusa instructions mention placing it 100mm from the corner plastic to the washer after the counter nut. With a thicker frame, this distance should be less. The original frame is about 6mm. If your frame is about 25mm, that 100mm distance should be shortened by 19mm (new total distance of 81mm). If you do not do this, your x-axis will be too far forward and the hotend will be homed in front of the heatbed.

Hope that makes sense.

-Stefan

Oh, your z-axis motor mounts are upside-down! :P

Re: Z-axis lead screw length for Dolly Mk2 build

Posted: Mon May 14, 2018 4:40 am
by warren.watts
Looks VERY familiar, Warren!
Blue was the only ABS I had, and I printed them before you posted your picture, so it's pure coincidence.
If your frame is about 25mm, that 100mm distance should be shortened by 19mm (new total distance of 81mm). If you do not do this, your x-axis will be too far forward and the hotend will be homed in front of the heatbed.
I was wondering if I needed to maintain the 100mm from the back of the frame, or if it was the front distance that needed to remain constant. You answered my question before I even had a chance to ask it!
Oh, your z-axis motor mounts are upside-down!
I couldn't get them to stand up on their own the other way! ;)