Advice on parallel turning tubes on a lathe.

[Chris123Participant @chris123]

Struggling to get consistent results when turning 25cm tubes on a lathe.

Im using a fixed headstock 90 degree centre and rotating tailstock 90 degree centre. I can get it lined up and make a paswithin the end to end difference in diameter less than 0.01mm, however on the next pass it will be upto 0.1mm difference between ends. I use constant flow of water soluble oil and the part doesn't get noticeable hot, so I don't think its expansion causing it.

Lathe is a Harrison 12×24"

Thanks.

[Chris123Participant @chris123]

[jason udallParticipant @jasonudall57142]

My guess …the work piece is moving…thin walled tube is pushing off away from tool…why now…the workpiece has work hardened or tool “less” sharp…on this second pass…..
See this with tentative /light cuts in stainless steel all the time…seamless ss tube can be a horror to machine

[Chris123Participant @chris123]

Thanks for the reply, it's aluminium HE30 I'm using, taking light cuts <0.25 while trying to get it right!

Wall thickness is 3mm or so.

[jason udallParticipant @jasonudall57142]

I would wonder can you turn this on a mandrel?

[jason udallParticipant @jasonudall57142]

He30 usually well behaved. .but lets say you 25 od 19 mm id tube is square ended…your centers have little to seat on…so can easily wander…maybe more tailatock pressure?…carefully dont want to bell mouth…
Which might explain taper if at ends. .are you measuring under pressure or relaxed?

[JasonBModerator @jasonb]

Are you using the dead centre to friction drive the tube or only part turning with a dog. If its a friction drive is the centre wearing the cut edge of the tube where it makes contact.

J

[Ian PParticipant @ianp]

Are we talking 25cm length? and is the diameter difference over that length?

A few dimension/details would make it easier to diagnose the problem although regardless, of wall thickness you might need to take a last very light cut (say 0.01mm) with an ultra sharp tool to eliminate the job flexing.

Ian P

[Chris123Participant @chris123]

I use a Deburring tool to make it less of a square edge. I've tried quite a high pressure but it's the same.

The measure meets have been checked both on machine and off machine.

Good point about the dog, I'll have to try that!

[Chris123Participant @chris123]

Starting outside diameter is 30mm, being reduced to 25mm, internal diameter is 19mm. So Jason had a very good guess! Length is 25cm.

Looks like the model shops don't sell a dog big enough, so I will drill thro the tube and use a bolt through the tube at 90 degrees, to connect to the face plate.

[Ian PParticipant @ianp]

If you are going to cross drill does that mean you are going to have to shorten the tube after turning or will the hole be part of the design. Why not just grip the driving end in the three jaw?

If you need to face and true both ends I would put a bit of scrap steel bar in the chuck and turn a mandrel (with an undercut) that is tight (hand) push fit. It will have enough grip to let you turn the tube OD even with pressure from the tailstock. You could reduce or eliminate having to apply tailstock pressure by making a close fitting bush with a ballrace inside that sits on a spigot held in the tailstock.

Bear in mind that the tube is unlikely to have a truly circular bore so gripping it on the ID or with a conical centre is a bit hit and miss.

Ian P

[Ian S CParticipant @iansc]

For my hot air engines, I have made hot caps 3" long X 1 3/4" dia(int) (and other sizes), from solid 316 stainless steel. first I bore out the bar to size, and as it is attached to the motor with an internal thread, I cut this. I then mount a bit of scrap steel in the three jaw chuck, turn it to size, and drill a hole down it's length, make sure its a tight fit, and cut a thread on the mandrel to screw the hot cap on, then turn the outside diameter down to the required thickness, I have gone to .005", but for the hot cap of an engine this size .010" to .015" is a bit more robust. Ian S C

[Clive HartlandParticipant @clivehartland94829]

If it is varying in size in both directions I question the Saddle and its fit on the bed. Check the saddle adjustment and then see if it does the same thing.

Also make a plug to fit in both ends of the tube with centers and use the dog and driving plate.

Clive

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