Measuring concentricity errors

I am making Clickspring's finger plate. I have drilled the 10mm hole in the base, and bored the 16mm hole concentric with it. I then made the bushing holder on the lathe with a 16mm OD, and a 10mm spigot.

The 16mm bushing holder is a good close fit in the 16mm hole. the 10mm spigot fits well in the 10mm hole (from the underside). However the two pieces don't fit. The two holes, or the two diameters of the bushing holder are not concentric.

Taking 0.1mm off the 10mm spigot makes it fit, but I would like to work out whether the issue is lack of concentricity of the holes or the spigot.

Suggestions as to how to measure the concentricity of holes and spigots would be gratefully received

typo

Edited By Peter Cook 6 on 01/07/2023 22:01:55

If the 16mm OD and the 10mm spigot were turned at the same setting the problem lies with the drilled hole and the bored 16mm hole in the plate.

Emgee

When needing concentric holes you should bore both diameters as drilling can wander a small amount unless you have bored the hole near to size and finnished it to  size eith a drill on the same setup and without moving the parts.

David

Edited By David George 1 on 01/07/2023 23:09:45

Did you start with 16mm stock and turn down the 10mm? if so then if held in a 3 jaw chuck you may not have concentric parts. Better to use next size up or clock 16mm material true in a 4-jaw before turning the 10mm as a 3-jaw can be a bit off.

As for the holes it would have been better to drill say 9mm and then bore both 10mm and 16mm as drills can wander

As for measuring the errors, the holes can be done by setting the plate up below a mill or drill spindle. Put a dti in the chuck and rotate the spindle by hand measuring the TIR of each hole, any difference between the two readings will be your eccentricity (double the offset)

Not sure how much you have to get hold of but for the round part put that in the lathe chuck and do similar measuring the TIR of the two diameters and compare the results. May be hard to set the part running axially true if it has now been cut off the bar.

Edited By JasonB on 02/07/2023 07:37:14

Thanks for the replies. The 16mm was turned down from 18mm, and then down to 10 for the spigot, so I am fairly sure they are concentric. They are still on the stock, so I can clock them in the 4 jaw and just check.

The 10mm hole was drilled through the baseplate, and then bored out to 16mm without moving the mill table. My little SX1LP will not drill 16mm in steel, so I had to use the boring head.

Next time I will do as Jason suggests and bore out both holes. I will see if I can get the holes measured with the DTI on the mill, but I will need to centre (one of) the holes as a starting point. I was hoping someone had a neat trick before I start doing that.

It's not actually an issue as the 16mm is the location fit, but I would just like to see where I went wrong - this time!

Thanks again - the finger plate actually seems very useful.

Edited By Peter Cook 6 on 02/07/2023 11:12:04

It's not critical to have one hole spot on ctr, you are just measuring a difference between the two so long as it is not massively off it will work OK.

If you do want to get one spot on then it is just a case of rotating a dti in the chuck or collet and adjusting X & Y so the needle gives the same reading (does not deviate) in all positions. This is quite a common way to locate holes and the outside of round parts on the mill, There are things like co-axial finders or holders that make it easier to read the dial as the spindle is rotated but not essential.

From what I can see concentricity and size are immaterial. The hole is merely there to allow the pin to clamp a bar into the vee, so position is generated from the vee and the bar within it with the hole in the pin acting as a clamp on the bar.