Help laying out a 4 spoke wheel.

markknx

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I am stepping out of my comfort zone and want to make a 4 spoke hand wheel. My worry is I am doing it wrong or the hard way. I plan on doing it on a rotary table. I know I have to off set the cut of the spoke off center by 1/2 the spoke. not real sure how to come up with the proper amout of angle to advance the table. Or if I should just mill the circle between the hub and the inside of the wheel then offset the cutter by 1/2 the spoke then advance it till it clears the hub drill the hole. advance thr cutter to the inside of the wheel drill that slot between the two repeat for all four spokes (X8) then center back on the hub and connect the slots.

Here is a sketch of what I want to do. Thanks for any help.
Mark

20150215_143221.jpg
 
My thinking is that the center of the quadrants for your spokes would not be the center of the bore, but where the edge of the two spokes intersect (just past the end of the arrow that reads 1" for the outside dimension of the hub). This would be the center of the rotary table, your diameter for the inside of the rim would be 2.750" adjusted with your x axis. The rotary table would be rotated 90*, your x and y axis would be used to create the sides of the spoke. This method would require 4 setups, but is easier to accomplish without the confusion of tool offsets and such. I hope my answer is clear.
 
My thinking is that the center of the quadrants for your spokes would not be the center of the bore, but where the edge of the two spokes intersect (just past the end of the arrow that reads 1" for the outside dimension of the hub). This would be the center of the rotary table, your diameter for the inside of the rim would be 2.750" adjusted with your x axis. The rotary table would be rotated 90*, your x and y axis would be used to create the sides of the spoke. This method would require 4 setups, but is easier to accomplish without the confusion of tool offsets and such. I hope my answer is clear.
mzayd, I agree. that point of intersection of the spokes is the starting point for the sides of the spokes.( the corner I sketch in by the 1" bore arrow) But if I am understanding you right you are saying center that point on the table (or make it the center of my radius.) this would change axis of the insideof the wheel if I am following you right. Moving theaxis point would cause the iner and outer circle to not be concentric. and it seems then 90 deg would over cut do to the cutter dia.
Am I missing something?
Mark
 
yes, i see what you are saying. the inside radius of the rim would not be concentric with the outside radius. use the bore center as the arc center to maintain concentricity.

1) set the tool to cut starting outside the hub, but on one side of a spoke.
2)use the axis parallel to that spoke to advance to the inside of the rim.
3)rotate the table 70.9* towards the next spoke.
4)use the opposite axis to define the other side of the cutout (or one side of the next spoke).
5)rotate the table 70.9* the opposite direction.

this should complete one pass to complete a cutout.

i got the 70.9* from: (((circumference - total spoke width)/ circumference) * 360)/4

((3.14*3)-2)/3*3.14*360/4

This means that you will only be using 78.78% of the 360* for four spoke areas.
283.6* divided into four arcs is equal to 70.9*
 
I made an 8inch handwheel out of 6025 aluminum this past fall on my RT.
Also, out of my comfort zone.
I cut what I was able to on the lathe and then moved her to the RT.
Be sure you have a good way to dial in center.
I was surprised how much she started to move as I made chips.
Ended up resetting the entire piece twice.
As I learned to use the hold downs better.
I was also surprised by the amount of math involved.
The above recommendations are solid.
Don't forget to think in three dimensions.
What does the profile look like from the front and the back?
Think through the whole thing and each process before you begin.
This is doable!
I learned heaps!!

Daryl
MN
 
I think that mzayd3 uses the approach that I use for rotary table work - but correct me if I am wrong.

I make a CAD layout of the part (I use a free application from SourceForge called "LibreCad, which is similar to AutoCad - no connection with them BTW). After the drawing is complete, it's simple to define the tool paths required to produce the part, just as mzayd3 showed above.

Markknx, we feel your pain concerning this problem, LOL ! We CAN work out the tool paths without using a CAD program, after all, machinists did it for multiple decades. But why, unless you just want to practice your descriptive geometry skills :)

edited again, wish I could spell properly the FIRST time
 
Thanks guys, big help. the good news is thes pokes are going to be recessed in at least .25" so I can take a light cut with the numbers and if I miss just mill some out and start over.
Daryl, I made a center pin that fits MT2 in my RT so that not only help to line it up it also stops center drift so all I shoud have to fear is unwanted rotation. I plan to leave the center bore snug to the pin then bore to final size after. it is only a crank handle for my 4x6 bandsaw. However it is also a test of my skills (and nerves).
Thanks again guys I will post when I get it cut maybe this weak. If not it maybe a few months as I am waiting to hear when I can get my rotator cuff repaired.
Mark
 
Here is the way that I would make the wheel.

Find the center of the rotary table with an edge finder or dial indicator mounted in the spindle. Set your x and y axis to zero at that point. If you do not have a DRO, you will have to account for any backlash in your lead screws. Mount your workpiece on the table so it centered on the rotary table. If you are going machine it all with the rotary table, the positioning is not as critical. I would put a sacrificial piece under the part so I could machine the entire thickness from one side.

You can machine your bore by offsetting the x axis by the 1/2 the diameter of the bore minus 1/2 the diameter of the end mill. Likewise for the inner edge of the rim. You can machine the outer edge of the rim and hub by adding instead of subtracting your end mill diameter. Use the crank on your table to rotate it the full 360 degrees.

To cut the spokes, offset the y by 1/2 the width of the spoke plus 1/2 the width of the 3end mill and cut to intersect the outer edge of the hub and the inner edge of the rim. You will have to do some math here if you don't have CAD. If you have CAD you can pull the dimensions from the drawing. Since your spokes are recessed, you can machine to just short of touching the hub and rim by sight and clean up. This will profile one spoke.

Now rotate the table 90 degrees and cut the second spoke. Use the same x and y dimensions as before. Repeat for the remaining two.

Now cut the arcs between the spokes by rotating the table. Again use some math or a CAD drawing to get the start and finish angles. When cutting the arcs, rotate the table to the start position with the tool path radius set on the x axis and the y axis set at zero. Rotate to the finish position to cut.

You will have to finish the recess on the reverse side by flipping the part and centering on the rotary table. Or you could mount it on a lathe and finish it that way.

Here is a drawing of your part with dimensions Just remember that straight cuts are made with the x feed and curves are made with the rotary table feed. The 15.83 angle is 15 49' 36" and the 36.87 angle is 36 52' 12"Spoked Wheel.JPG
 
IMG_1631.JPG IMG_1630.JPG IMG_1637.JPG I don't pretend to know if this is best practice.
I turned both sides prior to milling on the RT.
Center the RT, then center the work on the RT.
I ended up finding an 8inch piece of pipe. Faced both ends. Then clamped the work to the pipe.
Then she didn't move anymore. Perhaps it was the aluminum, as I was taking light cuts.
Then I centered her back in my 4jaw lathe chuck.
IMG_1631.JPG IMG_1630.JPG
 
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