Backplate fitting

[Anders BeParticipant @andersbe]

[Anders BeParticipant @andersbe]

Dear all!

Working with a lathe, cortini H105 (swing 105 mm). The chuck is mounted with a 1-1/2"-8 tpi thread. The chuck is 125 mm in diameter and say 80 mm long including back plate.

If I push the chuck (or a bar mounted in the chuck) forth and back introducing a rather low bending moment on the chuck, the radial movment at the outer end of the chuck is say 0.02 to 0.03 mm. However as close as possible to the bearings I don't see any movment. I suspect this depends on movmens in the thread fixing the backplate to the spindle.

Could I be right?
If yes, is this what I can expect from a mounting with thread and a stright register?

If not, what can the problem be?

Bad axial fiting? Could be, but I have verify it with mark color.

Fiting of the register? Spindle is 37.97 and backplate 38.00 mm in diameter giving a gap of 0.03 mm.

Bad fitting in the thread? The fitting is not tight so to say.

I combination of all?
Any other suggestion or thaught? Bearing problems anyway?

Thank you for your reading.
Best regards
Anders B

[Ady1Participant @ady1]

As long as the thread can tighten up properly, it's fine

The key part is the SPINDLE REGISTER

If the thread is too tight it may even interfere with the proper fitting on the register, so a bit too loose is better

2cents

Its the nice smooth bit behind the thread and it has to be very accurate

Edited By Ady1 on 19/11/2020 23:20:13

[Anders BeParticipant @andersbe]

Many thanks for your answer. What would be the ideal diameter of the register in relation to spindle diameter?

Best regards

Anders B

[Ady1Participant @ady1]

Your machine has specifications for that number, a few pictures of your setup and the parts involved would make it easier to explain/help, and we could see what your register/bearing setup looks like

That picture btw comes from a useful post in here

Lathe Spindle/Chuck Concentricity

Technique Required to Make Accurate Backplate

Edited By Ady1 on 19/11/2020 23:32:37

[Howard LewisParticipant @howardlewis46836]

If the chuck can be moved, it is not seating properly.

Either the backplate on the register, or the chuck on the backplate. And, are the fixings tight with all surfaces clean, and free from dirt or damage.

Off course, it could be that jaws are a poor fit in the chuck body?

I would suggest starting checks with the register / backplate, then backplate / chuck body, until you find the cause.

Howard

[old martParticipant @oldmart]

Smart & Brown use a 1 3/4" X 8 thread with a 1 3/4" register. We have about 8 chucks and faceplates with clearance between the spindle register and the bore of the backplate from 0.0005" (0.0125mm), to 0.020"(0.508mm). They all tighten up and repeat perfectly for concentricity and face flatness. The important thing is that the rear face of the plate is in tight contact with the corresponding face on the spindle. The most difficult chuck to attach to the lathe spindle is the one with the tightest fit.

[Tony Pratt 1Participant @tonypratt1]

Posted by old mart on 20/11/2020 17:26:18:

Smart & Brown use a 1 3/4" X 8 thread with a 1 3/4" register. We have about 8 chucks and faceplates with clearance between the spindle register and the bore of the backplate from 0.0005" (0.0125mm), to 0.020"(0.508mm). They all tighten up and repeat perfectly for concentricity and face flatness. The important thing is that the rear face of the plate is in tight contact with the corresponding face on the spindle. The most difficult chuck to attach to the lathe spindle is the one with the tightest fit.

OK I will be the first to ask, with .020" clearance on diameter how do they repeat on concentricity ?

Tony

[old martParticipant @oldmart]

It has to be the threads centring each time, there is no other explanation. I would usually check a backplate on its own when the chuck is off being cleaned, and have never had to correct any runouts. The only threaded spindles that totally rely on that register are Raglan, I believe they use square threads. I have had to remachine the square end of backplates and that is when the front face would need a skim.

[Anders BeParticipant @andersbe]

Many thank!  Ady1. Have only briefly glanced through the first thread, the measured gap is much smaller, say 0,004 mm rather than 0,04 that I have. Have to read them carefully …
Best regards
Anders

Edited By Anders Be on 20/11/2020 22:34:52

[old martParticipant @oldmart]

How would you measure a gap of 0.004mm, my feeler gauges only go to 0.001" which is 0.025mm.

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