A Question On Machining

prasad

prasad

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Guys

I started working on a small project of making a small turn table arrangement using an aluminum material. The OD of the disc is close to 6 inches and thickness at the edge is about 0.2 inch. To help me mount it on a stepper motor shaft I thought of having a thicker boss at the center of thickness 0.5 inch. I mounted the rough cut aluminum disc on my 4-jaw chuck and machined one face and got the round boss and finished surface as you can see in the photo below. 20161020_094201.jpg

I then removed it from the chuck and mounted it back on the 4-jaw gripping the boss that I had just made, trued it. and then did the facing operation until I got a decent smooth surface finish. The last operation, I drilled a hole in the center to suit the motor shaft, a small interference fit.
20161020_094108.jpg

Here is my problem. When I turn the motor I find the turntable is not running flat. Its top surface wobbles. Apparently the hole did not go straight. How can that be? Where did I go wrong and what should I do to eliminate the wobble?

Thank you
Prasad
Eastern PA
 
Last edited:
Did you true the front surface or only the center? You would need to make sure the front surface is running flat - 90 degrees- in relation to the drill.
 
When you chucked that part in the chuck did you indicate both the OD and the face ?If you did not indicate the face as being flat, (true), the hole is probably at an angle which will cause the wobble.

"Billy G"
 
You turned it around in the chuck? Your goal is to keep the flat face and the bore concentric correct, face the flat side then bore in the same set up. Flip and turn the back side and the boss as a second operation. Always try to do related features in the same set up.
 
The post by Jim was being done as I was doing mine. At least they are both in agreement.

"Billy G"
 
Assuming you indicated the part so it is running true, then the center of the part should be on the spindle centerline. If the tailstock or drill chuck is not aligned with the spindle centerline then it will drill off and you get wobble. To compound matters, the drill will not drill a truly round or straight hole. If you need a round hole then it needs to be bored or reamed. Since you need it round and straight the best choice would have been to bore the hole.

My suggestion is to make another part, indicate it accurately in the chuck, bore the center hole and face the part. Any inaccuracy from the drill, drill chuck or tailstock is thus eliminated.
 
I indicated it on both - machined side face (which was facing the chuck) and diameter. The non-machined side (face of the disc) was now ready for facing cut. That was how I got truly flat disc. I measured the thickness using my Mitutoyo micrometer and the two faces were truly parallel to less than 0.25 thousandths.

Yesterday I started doubting if the the job held in the chuck may have moved in the chuck when I started the drilling operation. So I once again held it in the same chuck, took extreme care to true it on face and diameter. The indicator showed less than 0.25 division. In this condition I ran the lathe and ran the drilling operation (pushed the same drill bit in). It went in and did not cut any material. That indicated that the job did not get displaced during the drilling operation. However the hole does not appear square to the surface.

I am truly confused.
 
mikey said:
Assuming you indicated the part so it is running true, then the center of the part should be on the spindle centerline. If the tailstock or drill chuck is not aligned with the spindle centerline then it will drill off and you get wobble. To compound matters, the drill will not drill a truly round or straight hole. If you need a round hole then it needs to be bored or reamed. Since you need it round and straight the best choice would have been to bore the hole.

My suggestion is to make another part, indicate it accurately in the chuck, bore the center hole and face the part. Any inaccuracy from the drill, drill chuck or tailstock is thus eliminated.
Click to expand...

Thanks Mikey, that makes some sense. It is possible that my TS or drill chuck held in the TS is not aligned with the spindle. I am not sure if I can bore a 5mm diameter hole. However I do find McMaster selling 5 mm reamer. I will order one. I will also do the reaming by hand first, then mount it in the chuck, indicate it as best as I can before facing. I have to get new stock too.
 
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