Member postings for david bennett 8

Howard, your instructor had good eyes.Thats about the maximum resolution of a good steel rule (without using a microscope )

dave8

Edited By david bennett 8 on 04/10/2021 00:45:15

I assume that was tongue-in -cheek. Very good -but could you use a rule (of the measuring type) to measure that 0.001" ?

dave8

Edited By david bennett 8 on 03/10/2021 06:07:50

Pease do not ink me with Nicholas Wheeters comments. I had no intention of joining in, or being rude.

Dave8

No, I did not miss that post, but that was not what I asked. How woul I turn a piece to 18mm diam. within 0.03mm ?

This is about precision. Numbers are precise by definition Rules are not.

Dave8

These exotic fractional inches may be accurate and precise, but of no practical use to model engineers until they are "translated" into practical measurements such as mm. How would you turn a piece to 90/27"  diameter and how would you measure it?

Edited By david bennett 8 on 01/10/2021 02:34:25

Edited By david bennett 8 on 01/10/2021 02:38:08

Edited By david bennett 8 on 01/10/2021 02:50:16

Edited By david bennett 8 on 01/10/2021 02:50:41

Ian, here is a version of what you are suggesting, which I posted on another site.**LINK** look towards the end for a post by dave-b (my alias there)

Edited By david bennett 8 on 19/07/2021 23:32:55

Edited By david bennett 8 on 19/07/2021 23:45:34

Derek, a bit of practical advice here **LINK**

I think you will find my post was timed BEFORE yours.

I do not agree. The curved surface will contact both ends of the curved profile at the same time. Then the curve will force the sliding cotter around the bar and into a wedging action at the bottom.

What makes you think it was addressed to you? It was a general comment on the thread.

There seems to be a bit of overthinking this wedging problem. If you accept that wedging can happen, the only place it can happen is where the clamping action is less than about 7 degrees. Eliminate that and solve the problem. It only needs a few microns of movement to cause wedging on a shallow angle and have no effect on the greater angles. The clearance on the sliding cotter/clamp bolt will allow this to happen.

Edited By david bennett 8 on 25/06/2021 23:39:30

Edited By david bennett 8 on 25/06/2021 23:41:43

Wow ! something new to learn every day.

Edited By david bennett 8 on 09/06/2021 20:31:51

There is something dramatically wrong with your spindle. The link that MichaelG posted is exactly like yours should be (except for size) There should be double angle opposed bearing at each end of the spindle. Does the taper bearing at the rear on yours slide off? It is tapered the wrong way.

Edited By david bennett 8 on 09/06/2021 02:23:54

Edited By david bennett 8 on 09/06/2021 02:24:34

I should have clarified. Of course I meant the height of cut in relation to the centre of rotation of the work. The error is very small as the OP surmised,but the shape of the error is important. I think all of GHT's calculations were based on the centre height being the same at each end.

Yes, a 10 thou error would not matter much if it was constant along the length. In the case of a high tailstock the height of cut varies along the length, causing non-flat sides. I believe this would matter in a morse taper fitting, but the OP just wanted to visualise the problem.

Yes, conic sections do come into it. Imagine a perfect cylinder. Now picture cutting a section out of it as would be your travel i.e from half diameter at the headstock end (on centre) to below centre at the other end. When viewed from above, the section will be parabolic. That represents what you have cut.

Edited By david bennett 8 on 07/06/2021 17:32:10

Edited By david bennett 8 on 07/06/2021 17:54:55

Just for the record, I checked some spare 1750/70 bearings. They also have timber inserts.

The slots do contain some kind of plastic/ Paxolin? I believe its to allow a limited flexibility to the bearings when shims are adjusted.

Edited By david bennett 8 on 26/05/2021 14:38:43

Edited By david bennett 8 on 26/05/2021 14:44:36

Ah, thanks for clarifying - I couldn't read that before . I was maybe hoping it was the elusive clearance!

Dave.

Hmm, what do you read the 38.453mm dimension as (on the w20 nose) ?

Dave.