CMT Ursus 250 Repair 2) Lever Balls

[RobinHood]

All the levers on the lathe have the same 30mm (nominal) round, black, hard plastic balls on them. My machine was missing 7. They have a M10x1.5 internal thread, a 4mm counter bore and a flat so that the ball can fit up against a shoulder on the lever stem.

I had some black acetal in stock and decided to make my own replacements. Acetal is softer than to OE plastic, but will do for a replacement.
First op was to cut a blank cylinder on the bandsaw. Then over to the lathe for facing, drilling for tap size, counterboring & tapping. Then I mounted the blank on a lever stem (substitute for an arbor) to turn the cylinder to 1mm oversize on the OD and to final length 30mm.

I do not have a ball turner. But I have a milling machine, boring head and a spindexer. I did a calculation of the angle of inclination of the spindexer axis (about 25*) and mounted the blank in a 5C collet. (This is a prototype - hence the hole in the blank from a previous project).


The boring head was set to 15mm radius with a RH boring tool to cut on the inside. This requires the spindle to be run in reverse while the spindexer is spun. This turned out to be a serious concept mistake on my part - as can be seen here.


I was happily turning the crank handle to form the ball when in a split second the sound changed, and next thing I know the spindexer was almost vertical and one of my fingers was bleeding...

So what happened?

Well, the blank had unscrewed on the arbor and in doing so the depth of cut dramatically increased instantly thus the cutter (turning at 1200 rpm) grabbed the part and flipped the spindexer out of the way, jamming my finger to the mill table.

After a few choice words and a bandaid, it was time to regroup and figure out a way to prevent this from happening. The obvious issue is the cutter trying to unscrew the blank (regardless of spindexer rotation) because it is a standard RH tool cutting on the inside and thus needing to be run in REV. I don’t have any LH boring tools. But then I found a LH lathe tool that fit the boring head. Now we can run the spindle FWD, the cutter is cutting on the inside and will have a tendency to tighten the blank on the arbor.

Here is the prototype ball on the Hi/Lo lever.


A production ball just finished on the mill. The LH lathe cutting tool is visible. The little point is because I need to leave a flat on the back for the shoulder. It gets filed off when I did the final polishing on the lathe.


One of the finished balls.


The 8 production balls (7 required + 1 spare) were made uneventfully.

 

[PeterT]

Neat idea. Glad you didn't loose too much red paint & have all 10 digits. Ya plastics can be interesting sometimes even when the setup is normal. Most machining operations everything goes as expected. And then there are others where it just decides it doesn't want to play that game & make for some shop excitement.

I've often wondered why the lightly recessed groove at the ball equator in many machine knobs. Do you think the original ones were like a molded plastic & related to a parting line?

 

[RobinHood]

I've often wondered why the lightly recessed groove at the ball equator in many machine knobs. Do you think the original ones were like a molded plastic & related to a parting line?

I think that the OE were injection moulded. They have the thread dimensions (M10x1.5) in raised characters at the bottom of the bore. I would agree that the little groove is probably an aesthetic feature to “hide” (machine away) the parting line.

Machining soft material is not without its issues for sure. It is relatively easy on tools and machines, but speeds and feeds are different: I ran the lathe at 1030 rpm with a feed rate of 0.2 mm / rev and a DOC of 2.5 mm to break the chip. 3 passes (5, 5 & 3.6) to get from the 44.6 mm stock size down to 31 mm oversized cylinder OD.



Four new knobs installed - they are the less shiny ones. Since acetal is not as hard as the OE material, it is harder to polish them to the same lustre.

 

[Janger]

I've busted carbide spot drills I was using as a chamfering tool by just brushing a hold down screw the wrong way. The spin action spun out the screw and lifted it into the drill - instant break.

 

[RobinHood]

Oh yes, when it grabs, it is amazing how fast something can unthread. Not even time to hit the E-stop before something breaks (or draws blood as in my case).

 

[YotaBota]

Good to hear you're more or less intact.
Could you use the crazy glue polish technique to put a shine on the balls?

 

[RobinHood]

Could you use the crazy glue polish technique to put a shine on the balls?

That is actually a very good idea - if it works. I don’t know how the cyanoacrylate would bond to the acetal. Not much seems to stick to the stuff - one needs a special primer and the matching glue.

 

[John Conroy]

In this video from CaLem he uses a propane torch to put a shine on a plastic handle he turned in his lathe. It seemed to work well. It's at about the 10 minute 20 second mark. I going to try this next time I make a plastic knob.

 

[RobinHood]

I did notice that when I polished them using a higher rpm on the lathe that I got more of a shiny surface - probably because they got a little warm. Might try a heat gun to see what happens.

Thanks for sharing the video.

 

[PeterT]

I was searching for something else & stumbled on these. I think they make 'plastic' bearing balls in all kinds of materials but $19 for 10 (1" dia) Delrin is pretty reasonable. Chuck in 3K, tap the hole etc.
Aesthetically you have to live with white balls haha

https://www.amazon.ca/25-4mm-Delrin...1&keywords=delrin&qid=1628876091&sr=8-27&th=1

 

[RobinHood]

Thanks for the link. I do have lots of white delrin, but decided against using it on this lathe. I already broke tradition on the Colchester and made the replacement handles white - will change them to black in the future as I am forever trying to keep them clean and white…

 

[PeterT]

Ya if McMaster doesn't stock non-white chances are its going to be more obscure.
https://www.mcmaster.com/plastic-bearing-balls/

I checked some of the offshore suppliers - 99% are white or off white, the odd clear. Nobody cares about bearing color so natural is the way they come. I think turning from solid like you did is the way to go. Only other way is make a mold & use pour-in color urethane. More of a pita unless it was a premium resto project.