Workholding - 19mm Wheel Blanks

[Wongster]

Hello guys,

Have not been here for a while.  Just started getting back to my little workshop for a small job.  I ran into some problem with holding the work.  Hoping for some suggestion to try out.

This is what I'm to make.  Only 4 at this moment.




4 wheel blanks were given to me for the job:



When I was told about the job, I thought of using super glue to hold the aluminium blanks.  Now that I've the blanks on hand, I realized that I have to drop the idea.  Using heat to remove the blanks will likely melt the plastic ring.

So I came out with this:



The fixture has a protrusion of slightly less than 5mm to fit the wheel's centre hole.  It was tapped M4 and is slightly lower than the hub of the wheel.


Ready to run the gcode.
I ended up with this.  Saw the wheel turning when the spokes were being machined.



The guy who requested me to do this gave me some pix showing how someone in Thailand made his.



Anyone can help me out here?

Wong

[PixMan]

That should be something you could do with a 5C step chuck and a collet block. You'd be getting over 90% contact area on the polymer tire by circular milling a pocket to match the O.D. of the wheels, then get additional security by drilling and tapping the M4x0.7 thread down the middle. Yes, the tapped hole should be a little loose so the step chuck (or emergency collet) doesn't clamp on the screw.

What size cutter, number of flutes, spindle speed and feed rate are you using?

BTW, looks like the Thai guy doesn't have the polymer tires on the wheel blanks. That's important for grip!

[Wongster]

I don't have a 5C step chuck unfortunately.

Spindle speed at 2800
Feeding at 200mm/min
DOC 0.25mm / pass
1mm 2 flute cutter

[DaveH]

Hi Wong, nice to see you  
I have to say I've never had much luck holding anything with just one clamp (screw)  

If it was me I would hold it with two clamps on the perimeter - it should allow you to machine 4 'slots'. Then move the clamps to cut the remaining 4 slots.
When I say move the clamps what I do is clamp the perimeter first with two additional clamps then I remove the other two clamps.
The reason I tend to do it this way is to make sure the part doesn't move whilst removing the clamps.
Bit of a mission but it has worked for me in the past  
 
DaveH

[PixMan]

I would start by cutting that feed rate in HALF. I'm surprised you've not yet broken the little Ø1mm end mill at that chipload per tooth.

Not even a 5C emergency collet? That part is small enough to fit into one. Now is as good a time as any to get one.

[dallen]

you could try mounting them in dope/shellac like people do with gem stones for polishing.

Thai guy looks like he casted them in some sort of polymer to hold em with.

[Wongster]

Guys, thanks for your input.

DaveH, hello hello! Been awhile huh? A friend suggested that to me too. One other way I thought of is to drill and tap a few holes along the outsider edge of the fixture and use the head of the screw to hold the part down, gripping the edge. I was hoping to do the same as what the Thai did. I'll be able to make a set of 4 at one go. If I can't figure that out, I'll still stick to one wheel at a time.

PixMan, I use 1/5 of the feedrate given in BobCadCam thinking that it is a safe figure to use. I'll try 100mm/min as I've only 1 such cutter. After inspecting the failed wheel, I may be going for 2mm cutter as the 0.5mm radius isn't very good looking... I know very little about the emergency collet. Read up a little and wonder if one can be used with my ER32 collet block. Some sites gave the name pot chuck.

dallen, I've not use shellac before. Can the part be removed from it easily and yet provide enough hold?

Guys, pardon me for my ignorance. I'll try to read up as much as I can.

[dallen]

Wongster;

problem with using shellac to hold the parts with would be getting them aligned properly.

one of the modern high strength glues would probably work better, do the parts have to be machined on both sides?.

[Wongster]

Dallen,

Fortunately, only 1 side. I thought of using superglue. But releasing the part would require heat. I was told that most wheels will have the plastic ring and once removed, can't be put back. The blanks came with the job...

[dallen]

if they already have the tires on them, then any method that would require heat is out.