Holding 'thin' parts for off centre hole making.

[PaulR]

I'm sure this has been asked many times but as I couldn't find such a question here on the forum, here goes (I found a few topics elsewhere on the web but they weren't much help).

Lets say I have to make a part from a rectangular piece of metal 6mm thick and it needs a large diameter or very off centre hole. I can't put the part flat against the back face of an outside jaw because I'll end up drilling into it. In the past I've put a piece of suitably sized stock diagonally through the jaws behind the part so I can tap it back to get it perpendicular to the lathe axis, removed the stock then proceeded carefully, using a floating centre to get lined up on a centre punch mark and hoping the part stays 'upright' while I adjust the jaws (checking it's still perpendicular with a sharp point in the toolpost before starting to drill)

So the question is, is there a better way to do this? I thought of glueing or sweating a stub on the back to fill the void but that doesn't seem a very accurate method.


(diagram shows the part right at the front)

[Kim]

Something that I've done is to use a chuck with replaceable soft jaws.  Then you can basically bore a shallow seat in the soft jaws and you are guaranteed that the seat will be perpendicular with the center of the lathe.  Only problem here is that it's a 3-jaw chuck.  Though, if you make the seat a really close size to the piece you're going to hold, it will likely be very concentric also.  But this won't work for your case, as you want an offset hole.

If I have to use a 4-jaw, as you do here, I've put spacers in place, as you describe, and used tape or tywraps, or wire (or something) to hold them in place while you're centering the part in the 4-jaw.  But if you used tape, make sure to take the spacers out!  Otherwise, they're likely to go flying when you turn on the lathe.

You can also use a spider.  Chris has 3D printed various thicknesses of spiders to fit in his chuck.  These chuck spiders kind-of interlock with the jaws to keep it from coming out when it starts spinning.  You'd have to make a spider that has some of the legs off center so that it would work for this situation, since usually, a spider has everything all centered up.  But if you 3D print it, you can put the legs wherever you want!

Just a few thoughts.

Kim

[pirmin]

i would glue the piece on to a larger piece of face turned brass and then u can hold the larger piece in a 4 jaw chuck for positioning. watchmakers often use shellack or Cyanoacrylate glue to workhold small thin parts

[PaulR]

Something that I've done is to use a chuck with replaceable soft jaws.  Then you can basically bore a shallow seat in the soft jaws and you are guaranteed that the seat will be perpendicular with the center of the lathe.  Only problem here is that it's a 3-jaw chuck.  Though, if you make the seat a really close size to the piece you're going to hold, it will likely be very concentric also.  But this won't work for your case, as you want an offset hole.

If I have to use a 4-jaw, as you do here, I've put spacers in place, as you describe, and used tape or tywraps, or wire (or something) to hold them in place while you're centering the part in the 4-jaw.  But if you used tape, make sure to take the spacers out!  Otherwise, they're likely to go flying when you turn on the lathe.

You can also use a spider.  Chris has 3D printed various thicknesses of spiders to fit in his chuck.  These chuck spiders kind-of interlock with the jaws to keep it from coming out when it starts spinning.  You'd have to make a spider that has some of the legs off center so that it would work for this situation, since usually, a spider has everything all centered up.  But if you 3D print it, you can put the legs wherever you want!

Just a few thoughts.

Kim

Thanks Kim, a few times I've thought 'that spacer seems pretty tight, I wonder if it'll stay in place?' but I've never dared try it! So that's what those spider things were for, I noticed them in one of his build posts but didn't read in enough detail to understand what they were doing.

[PaulR]

i would glue the piece on to a larger piece of face turned brass and then u can hold the larger piece in a 4 jaw chuck for positioning. watchmakers often use shellack or Cyanoacrylate glue to workhold small thin parts

Thanks pirmin, my only concern would be that the glue wouldn't hold against the forces of drilling and the part might end up embedded in the ceiling (or me!). I suppose making the part from an entire piece of the same size then either parting off or sawing/facing is the best way to go.

[crueby]

As Kim said, I  have had great luck with lathe spiders, since they span between the jaws, they are captive and cannot fly out like a simple bar can. I made mine in several thicknesses so they can be stacked to het different thickness parts out to the end of the jaws. They can be printed in plastic, or milled from aluminum. Very handy!
Here are the type I printed quite a while back - only drawback to plastic ones are that if the part gets hot it will soften the plastic. I also made some that have rings in the center with holes at the middle so I can drill through the part without destroying the spider.

and some aluminum ones that I had laser cut recently, threw them into a batch of other parts I was having cut. These two are different thicknesses - picked the thickness so I could stack two or three to get parts farther out in the jaws.


They dont have to be this complex a shape - even three arms joined at the center will work, just need to span the center hole of the chuck and be long enough so that even when the jaws are far out the spider cannot slide out the side. These, with the slots around the jaws, I think are the best, very little movement possible while spinning.

[john mills]

hold the piece in the chuck looking family close just tighten the chuck enough to lightly hold the piece then use a blunt smooth tool and come against
the piecewhith the spindle turning  and push until it is running true then finally tighten the chuck
john

[90LX_Notch]

A small fixture plate can be held in a four jaw chuck and used as a faceplate.

This particular one measures 1.685 (42.79mm) x 3.000 (76.2mm) as it was made from scrap.  It is tapped with 10-32 holes.


The same idea can be accomplished with a round fixture plate held in a three jaw chuck.


-Bob

[crueby]

Bob, holding the fixture plate is a GREAT  idea!

[PaulR]

Thanks all for the helpful suggestions and photos which make things much easier to understand!

Chris, now I see what your orange spiders were for and yes when I read your description I immediately thought that could be simplified as long as the 'arms' were connected but fitting either side of the jaw is obviously best. This definitely a deluxe solution!

John, that's a good idea, I guess I could even use the square end of a HSS tool. I was doing a similar thing with a point - using the topslide scale to get a reading at a couple of places across the face.

Bob, that's also a great idea although I'd have to come up with something which would allow me to drill right through. Thinking about it there could be some packing between the fixture plate and the part that would do just that or simply a rectangle with a suitably sized hole in the middle and a couple of threaded holes for clamping. I need to have a think about the most versatile simple design.

[Jasonb]

For concentric holes or simply facing the second side of a sawn off disc I use soft jaws a lot. Bumping the outer face won't work if it is sawn and you want it parallel to the side facing the chuck.



If the hole or spigot is off ctr or the part an odd shape and the 4-jaw needs to be used the I tend to slip a parallel or two between chuck face and the work. A bit of tape on the ends keeps them in place while adjusting the 4-jaw then remove before turning.

If it is something like the crank disc on your current engine then use a longer piece of bar which will tend to be lined up by the 4 jaws and put the hole in before sawing/parting off the disc. If you have collet blocks then small round items can be held in those and then the block offset in the 4-jaw. Both will save the jaws damaging a previously turned OD

[PaulR]

Thanks Jason. Yes, your comment about the crank disk is what I was trying to say in reply #4 above - start with an 'entire' piece of stock. I haven't got soft jaws, collets or even parallels yet - my expenditure on the workshop in the last 6 months has been maxed out - but hopefully I'll remember all those suggestions when I do!

[Jasonb]

I found a photo of me doing it while the part is still attached to the bar which you don't need extra tooling for.

[Charles Lamont]

You can often use the front face of the tailstock barrel. With 3-jaw work I grip the job lightly, push it in a bit with the tailstock then tighten.

One day I may get round to making a 'back stop' which fits in the spindle taper and has an adjustable rod through it, to which you can fit a range of different sized flanges. George Thomas came up with a good, if rather elaborate, design.

[PaulR]

I found a photo of me doing it while the part is still attached to the bar which you don't need extra tooling for.

Got it thanks.