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Strange effect when turning

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DiogenesII25/05/2021 13:20:58
859 forum posts
268 photos

blush

..ah, yes - wondered when someone would spot that - look on it as kind of Quality Assurance check..

I must say that was quite a kind response..

larry phelan 126/05/2021 13:15:09
1346 forum posts
15 photos

I am happy to be able to report that due to careful shimming, I have reduced the error to almost nothing, something like 0,002mm as far as I can see, over 150mm. At that point, I decided to leave well enough alone [like a Mother-in-law ]

My thanks to everyone who helped out. I shall now go away and play [until next time ! ]

PS Stick with it Robin, you will get there in the end !

Howard Lewis26/05/2021 14:26:50
7227 forum posts
21 photos

First check should be for twist in the bed of the lathe.

Check as per Ian Bradley "The Amateur's Lathe", or his "Myford Series 7 Manual"

This advocates what has already been suggested, or turning a "dumbell" and adjusting or shimming the Tailstock end of the lathe unrtil both ends of the dumbell are the same diameter.

Read through the thread posted by Robin, the method will probably be described there. The same problem is present.

Either that or use an Alignment bar, or even try making one.

Use Silver steel, of the largest diameter that will pass into the mandrel, (ideally 1" or 25 mm so that it is stiff ),

Ideally, mount in a 4 jaw chuck and clock to centre as accurately as possible.

If only a 3 jaw available, take a very light cut over a slightly longer length.

DO NOT CHANGE THE CROSS SLIDE SETTING. Preferably, lock it.

Mount with only a short length protruding from the chuck and centre drill. Repeat the light cut

Mark in line with a chuck jaw.

Reverse the Silver steel, end for end, with the mark in line with the chuck jaw, and centre drill.

Turn a longer length to the diameter of the first end.

Grip this longer end in the chuck,again with the mark in line with the chosen chuck jaw.

Set a clock on centre height and set to Zero on the turned section at the Headstock end.

Move it to the Tailstock end, on the turned section.

Shim, or adjust the Tailstock end of the bed until both readings are the same.

You have now taken the twist out of the bed, and can check the Tailstock.

Mount the bar between centres, and set the clock on centre height and Zero at the Headstock end. Adjust the Tailstock (there should be a screw, low down and facing you, and another at the far side of the Tailstock.

Adjust these screws until the clock reads Zero at both ends.

The Tailstock should now be aligned with the Headstock, in the horizontal plane.

Hopefully, it should be aligned in the vertical plane. To check, bring the Tailstock and Headstock centres so close that they grip a thick feeler gauge, or a steel rule. It should be held vertical by the centres..

Hopefully, the lathe will now turn parallel between centres or without.

But remember that long slender work will deflect under cutting forces, unless supported by a travelling steady.

HTH

Howard.

Robin26/05/2021 15:56:44
avatar
678 forum posts
Posted by larry phelan 1 on 26/05/2021 13:15:09:

PS Stick with it Robin, you will get there in the end !

Well done that man. If my M14 stub turnbuckle arrives on time I should have it all back together over the Bank Holiday.

Howard Lewis26/05/2021 17:33:11
7227 forum posts
21 photos

My lathe (A BL 12-24 ) is a very similar machine (Typical Taiwanese clone ) to the Craftsman and despite its 300 Kg weight, the bed will twist. Mine sits on six 1/2 UNF setscrews, between the lathe and the bench, which provides a convenient means of fine adjustment to remove any twist.

Previously, I have mounted lathes on box section, as riser blocks, with M8 studs for adjustment.

Howard

John Reese07/06/2021 02:52:51
avatar
1071 forum posts

I strongly suggest that you use HSS tools instead of carbide insert tooling. If you don't want to learn to grind HSS I recommend a tangential tool holder similar to what Eccentric Engineering sells. If you insist on using carbide get inserts made specifically made for aluminum. The object is to minimize the pressure the tool exerts on the work in order to minimize deflection of the work.

It becomes more difficult to track down alignment issues when your test pieces are deflected due to tool pressure.

Howard Lewis07/06/2021 10:35:24
7227 forum posts
21 photos

Well Larry, Put us out of our suspense!

How have you got on?

Hope that you have not fiddled with the Headstock alignment!

Did you find any twist in the bed?

Have you corrected it? (By shims or adjustment of the mounting bolts at the Tailstock end.)

If YES, has the taper situation improved?

Once you have the twist out, you can start checking that the Tailstock is correctly aligned.

For this, you use the two grubscrews low down on front and back sides of the Tailstock, so that a DTI gives the same reading at Headstock and Tailstock end of an Alignment Bar held between centres.

If you are unsure, PM me and I'll take a picture of mine. I can also tell you how to make up your own Alignment Bar from a piece of precision ground stock, or silver steel, if you want.

Howard

Michael Gilligan07/06/2021 12:14:01
avatar
23121 forum posts
1360 photos
Posted by larry phelan 1 on 26/05/2021 13:15:09:

.

I am happy to be able to report that due to careful shimming, I have reduced the error to almost nothing, something like 0,002mm as far as I can see, over 150mm. […]

.

Howard ... for info. ^^^

MichaelG.

Howard Lewis07/06/2021 14:31:49
7227 forum posts
21 photos

Thank You.

Good to know a successful conclusion has been reached

Onwards and upwards!.

Howard

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