Boring and threading a back plate - confusion about matching the spindle threads

[benmychree]

It does not want to be bored for a tight fit on the spindle, to do so invites galling of the spindle and backplate, it is the threads that center up the backplate.

 

[SLK001]

Ok, wasn't sure what that feature was called. What's your recommendation for boring the clearance counterbore. Before or after the single point threading?

If you bore it last, you'll have that area as a fail-safe to getting the threads correct (if they are off track, you can stop and fix the problem), but at the cost of needing to clean up the beginning of the threads once you finally counterbore (and they are now recessed inside of the back plate). If you bore it first, you give up the fail-safe region, but now you can give the start of the threads a proper chamfer. A skilled turner will use the CB first method. A beginner will usually use the CB last method.

I will CB first, then thread.

 

[WobblyHand]

HanTnot want to be bored for a tight fit on the spindle, to do so invites galling of the spindle and backplate, it is the threads that center up the backplate.

Thanks.

A spare replacement plate was 0.010 larger than the spindle. Of course, it is hard to tell best practice, from a single example. I didn't know if the back plate I had was representative or not. Didn't want to copy a bad example, which is exactly why I asked.

 

[WobblyHand]

If you bore it last, you'll have that area as a fail-safe to getting the threads correct (if they are off track, you can stop and fix the problem), but at the cost of needing to clean up the beginning of the threads once you finally counterbore (and they are now recessed inside of the back plate). If you bore it first, you give up the fail-safe region, but now you can give the start of the threads a proper chamfer. A skilled turner will use the CB first method. A beginner will usually use the CB last method.

I will CB first, then thread.

Thanks for your recommendations.

I'm hardly a skilled turner. Haven't had trouble getting the threads to start on the mark in a quite a while, so not too worried. I have a loaner 3/4" LH threading tool, so I'm going to try threading away from the chuck. My chuck back plate has spindle retaining clamps, so with light threading cuts I hope things will be ok.

 

[WobblyHand]

The thread bore is 1.6270", calling that good.

 

[benmychree]

I would rough out the CB first to avoid cutting more thread length than necessary, then after completing the thread, I'd finish the CB, and take a finish cut on the seating face and machine the chamfer.

 

[WobblyHand]

I would rough out the CB first to avoid cutting more thread length than necessary, then after completing the thread, I'd finish the CB, and take a finish cut on the seating face and machine the chamfer.

That's my plan Thanks for your help!

 

[RJSakowski]

It is my understanding that threads should never be used as a registration surface. Radial runout is controlled by the unthreaded portion of the spindle neck while angular runout is controlled by the face of the shoulder.

On my G0602, I measured my spindle diameter, the OEM faceplate, and the three Grizzly back plates. Bore diameters were measured with a bore gage and spindle diameter with a micrometer. There is a light interference fit when threading the 5C chuck on the spindle. The other three plates thread on with no resistance. Just engaging the threads, there is .020" of radial play.

• spindle diameter: 1.8107"
• faceplate: 1.8107"
• 3 jaw back plate: 1.8107"
• 4 jaw back plate: 1.8106"
• 5C chuck back plate: 1.8103"

 

[pdentrem]

On my back plates, they all fit to the register. Here is my post on making mine.
Pierre

Making a couple of threaded spindle backing plates for my lathe

When I sold my Atlas 10F24, I held on to my Bison 5” 3J, as it was fairly new. Naturally I needed to make a new backing plate to mount it onto the new lathe. The old plate was 1 1/2” X 8 while the new spindle is 2 1/4” X 8, so I needed new metal to do this. I recently purchased 2 backing plates...

www.hobby-machinist.com

 

[WobblyHand]

From what I have read, this is almost a religious argument. There are folks in both camps. For what it is worth, my Grizzly back plates are not a very tight fit in ID. I have three I can measure right now, one is in use on the spindle. Have no trouble just spinning them on.

My measured spindle OD is 1.8101", obviously measured with a micrometer. The IDs I can measure with a telescoping gauge and the same micrometer used to measure the OD. Be back in a bit. My repeatability is +/- 0.0001".

New spare 5" back plate, never been used. 1.8162" <-- purchased last month
5" back plate on 5" 3J chuck, quite used. 1.8155" <-- came with lathe
Face plate, old but hardly ever used. 1.8147" <-- came with lathe
6" back plate, on the lathe <-- came with lathe

So not a sloppy fit, but nowhere near the tight fit that @RJSakowski shows.