From my previous posts we built a simple 4th axis accessory for a CNC Shark HD/Black Diamond based on Bob’s design. This post is to serve as an update.
We stuck to Bob’s principle of “keep it simple”. We did add a couple features. Our version can handle material size up to about 4” OD and 25” length. I carved a pepper mill/grinder using a 3 ½” X 3 ½” X 15” Oak blank to be used to turn two pepper mills. I only turned one this first time.
4th Axis Accessory changes:
I decided to use the free arm design introduced by others. I wanted to have the adjustment for material length. I also decided to keep the two gear twin (right and left) rack and pinion. I’m glad I did as it does add stability and splits the turning torque by driving both ends of the material. We did not see any back-lash issues in our trials.
On a previous post I encountered issues making the larger rack and pinion set. The resolution for this is also discussed in the earlier post.
We added alignment plates to the support arms and this helps a lot to align the machine. The support arms have over sized slots to allow the axis shafts to be adjusted (y and z). We also discovered the right hand shaft axis was skewed. It looks like the Sharks right side gantry support arm is slightly skewed. We inserted a 0.025” piece of plastic shim stock at the top of the alignment plate between the plate and the support arm. This corrected the problem. Our shafts extent about ½ into the material center so alignment was needed to prevent binding. Keep this in mind if you make alignment plates. I drilled mine on a drill press to ensure the shaft would be perpendicular to the plate. The free arm may be adjusted so the shaft ends meet to make it easier to make alignment adjustments.
As for the pepper mill. We are using an Antique Style grinder mechanism available from WoodCraft for our first pepper mill. I should note this isn’t a real robust mechanism but has a nice antique character. We made a simple modification to the mechanism to improve its function. I added a simple set screw to the bevel gear on the vertical shaft. I won’t have any pictures showing this or the mechanism as I have not completed the turning yet. Since I don’t have a lathe I will bore the hole in the grinder on my drill press. I made a fixture and ordered a new forstner bit and extensions for this. Of course this would be easier to do on a lathe. I don’t have one!
We completed the pepper mill turning on the 4th Axis in three steps as follows:
1. Turned the blank to 3” using a simple pocket tool path. It is important to note in my particular machine setup and gear set that one revolution is 10.6”. I used 10.7” for the y dimension to insure a little overlap.
2. Turned the Pepper Mill profile. I designed the unwrapped 3D profile using Sketchup Pro. Note the 2015 version is available for a 30 day free trial. So you can try it before buying. After 30 day the software reverts to the free version and you will lose the export feature.
I used the same 10.7” dimension for y in my SketchUp model. We exported from SketchUp and imported to 3D Machinist because we found that XY were locked on Cut3D. Free 3D Machinist worked just fine for defining the profile tool path. Then we imported the vectors from 3D Machinist to VCarve Pro and made are final adjustments. You can import multiple instances, if you are turning more than one and combine the tool paths in VCarve Pro.
3. Then we Carved 3 instances of a 3D model wrapped around the top section of the Pepper Mill. I used a model from Vector Art 3D named BBQ Guy for my pepper mill. In this case I had to move the zero point or x to the center of the top section and zero on the cylindrical surface. We measured the diameter at this point with a caliper and used this to calculate the amount to resize the y dimension of the model in 3D Machinist.
Calculation notes:
Gear pitch circumference: πD = π x 3.360” = 10.603” where D is the pitch diameter of the gear. So one revolution will always be the same for this setup.
We use the circumference ratio of the material to the gear to calculate the y adjustment in the model. Note the ration is greater than 1 when the material diameter is less than the pitch diameter of the gear and less than one when the material diameter is greater than PD. Note this is the same as the diameter ratio. So you may simply divide PD by the material diameter to get the y adjustment factor for your model.
Y adjustment factor = PD/d = 3.375 / 2.360 = 1.430, in this case we are stretching
We scaled our model by entering the desired x value then multiply the displayed y value times the factor. Then unlock x:y and re-enter the corrected y.
Expect the imported model to appear distorted when displayed in VCarve Pro and position the model where desired after the import. In my case I imported three instants of the distorted model.
I’m attaching a link to a short video of the Pepper Mill profile turning/carve and a picture after the 3D models were carved. Note I left one end of the pepper mill waist uncut to facilitate boring the hole through the mill.
Video:
https://www.youtube.com/watch?v=R0DhoXh ... e=youtu.be
Picture:
I trust this post is helpful for any of you that are trying Bob’s simple 4th Axis.
