Holding Small Jobs

Hi guys,

After sorting my QCTP out I can now begin on the task in hand.

As an exercise, I want to turn down a piece of brass bar (square cross section) so all sides are parallel.

This is what I have come up with so far:

Will this set up be sufficient to hold the work? Using some old tool bits as spacers. (The other jaws will obviously be clamped to the work too).

Are the 'faces' of the jaws (what the rear of the brass is sat against) flat enough to turn faces parallel?

Is machining each face in turn sufficient? I don't want to end up with a parallelogram sort of cross section.

Sorry for so many questions!

Cheers,

Kane

Hi Kane,

I really would not want to spin that set up as any slight movement of the job would have the tools flying out at you. You would do well to wire them to the chuck somehow. Toolsteel is heavy!

The steps on the jaws are machined flat...and that is your problem. If you need that job truly square then this set up will only mirror the back face of the job on each cut and just give you a smaller version of what you have got already.

I have not done this particular job but the old books I have (Internet Archive is a wonderful resource for out of copyright books) state one way as using a faceplate and angleplate.

Mount the job on the angleplate true and cut the first side as you have it mounted now.

Use the cut face as a datum and mount to cut the next sides. Finally, cut the ends using the flat sides as data.

A second way, and one I would prefer, is mounting the job on a vertical slide or on the saddle and use a flycutter in the mandrel. Then take the same steps as above but mirror-imaged (the tool spins and the job is fixed), ending up with cutting the ends. This would also give you a better finish.

Ready to be shot down in flames...

Andy

Hi Kane,

I have used that technique many times with success. I would use smaller, softer spacers - toolsteel is to hard (even though you are gripping the shanks) and could slip out. The machining forces for brass will be modest.

You should use the jaws at top and bottom of your picture to support the work as well, even though it isn't resting on them. That gives it nowhere to go and so it's much less likely to move.

The result should be square enough to look square, or at least parallel sided, although it may not be perfect.

Neil

HI kane & Guys
The two jaws that are clamping at the moment turn them round Then there would be more length
To hold the job square .As Neil says use much shorter protection say 1/8" ally then all 4 jaws wont mark the job. Ady's idea is good as well but you would need a nice square vice on the vertical slide . .otherwise clamps would get in the way . I will wear my flack jacket awaiting responce
Have you a mill drill ?
Nobby

Thank you everyone, some interesting ideas.

Nobby, the reason I turned those jaws round was to have a flat surface against which the brass could be held. But as Andy states (and which makes sense now), this will only reproduce the surfaces I already have and so I wouldn't necessarily get truly perpendicular faces.

Andy, I like the sound of the angle plate idea, but I'm having a hard time picturing how I would clamp the brass to the angle.

Would it be as simple as using some G-clamps? Will these hold up to the forces involved?

Am I right in thinking, to get two initial faces perpendicular they would each in turn need to be held flat against the angle plate, and not against the faceplate? (But resting on the faceplate to provide support).

Thanks again,

Kane

Hi Kane and everyone,

I scanned this pair of photos from Tubal Cain's "Workholding in the Lathe" (well worth buying for this very reason) to show a mounting as it was easier than setting up for a new photo:

What set me to this idea was that you said that your block had to be perfectly square.

Mount your job on the angleplate and, like you say, rest it against the faceplate until you have a flat datum surface to work to (assuming your faceplate is true).

Mounting on the cross slide for flycutting would just be the same, clamp the job to the cross slide using parallels to set your height (I assume that you have T slots), or mount your angleplate on a vertical slide.

I hope that this helps.

Andy

Hi Kane, the method Andy has shown is a safe way of holding your block. Whatever way you do it, don't be tempted to use G-clamps or anything else that can escape during turning should the set up become loose.

Regards Nick.

HI Kane
Using the angle plate system you still have to set the first face you machined square to machine the next face how would you do that . Are the long faces of your chuck jaws out of square . ?

Nobby

If you need geniunely flat surfaces without any hassle and it's worth your while buy a small shaper

Hobby Lathes can be a bit hit and miss for "flat"

Hobby shapers are designed for flat

HI

As Andy says You can get true flat faces on a small shaper
Nobby

Edited By Nobby on 30/03/2013 12:59:36

Edited By Nobby on 30/03/2013 13:00:50