Draw Bar Order of Operations

[bug_hunter]

Hey Guys,

Looking to turn a one piece draw bar for my mill. A drawing (.pdf) is attached. Suggestions for an order of operations to ensure concentricity. Thanks,

Greg

 

[projectnut]

It appears the section of your drawbar between the handle and the collet is solid. This section of a drawbar is usually a hollow tube. That allows you to put a workpiece in the collet that's longer than the collet itself. The longer workpiece can extend into the drawbar tube, so only the portion that needs to be worked on extends over the bed.

 

[bug_hunter]

It appears the section of your drawbar between the handle and the collet is solid. T section of a drawbar is usually a hollow tube. That allows you to put a workpiece in the collet that's longer than the collet itself. The longer workpiece can extend into the drawbar tube, so only the portion that needs to be worked on extends over the bed.

Understood, but I will only be holding endmills in there, and most, if not all of mine do not extend beyond the length of the collet.

 

[GrifterGuru]

Materials can become out of square (out of concentricity) as material is removed and stress is relieved from within the material itself, so that is something to be aware of and take into consideration.

I would also consider using over-length stock to account for chuck jaw depth, use a 4J independant chuck as well as a steady rest and tailstock live-centre so I could dial in for maximum initial concentricity.

Having done something similar (not a drawbar, but requiring concentricity), if it were me and I stress, if it was me machining this, I would use an undersize drill to start the bore, then use a boring bar to bring the hole to size for threading (either using a tap or single point threading), then turn the diameters smallest to largest.

Also, @projectnut is correct, drawbars have a through hole. Consider machining a through hole now, so you do not regret not doing it at a later point in time.

Another consideration, do not remove the work from the lathe once you start otherwise you will lose the concentricity you will have gained to the point you remove the work. Once you start, leave the stock in place until you finish the work. If that means leaving it alone in the machine whilst you walk away from it, so be it.

 

[bug_hunter]

Having done something similar (not a drawbar, but requiring concentricity), if it were me and I stress, if it was me machining this, I would use an undersize drill to start the bore, then use a boring bar to bring the hole to size for threading (either using a tap or single point threading), then turn the diameters smallest to largest.

Thanks,

If I do as you suggested, can I assume that the live center would never be used? I was considering setting the over-length piece of stock in the 4J, drilling a center, and cutting the stock to the largest finished diameter plus some, then set up a steady rest and removing the live center to undersize drill, boring bar bore, and single point thread. I could then cut a short, 60 degree taper to set a live center again, and remove the steady rest and insert the live center and cut, smallest to largest.

 

[GrifterGuru]

I, personally, would get the bore and thread machined first, then use a tailstock live centre during the rest of the machining. With the live-centre located on and within that bore, everything **should** then be centred to it.

Literally, I, personally, would bore and tap the thread, use a live centre and **then** machine the diameters.

Put another way.. If you do all the diameter machining, then machine the bore for the thread, there **may** be a chance of it being slightly off-centre.

The trick is to avoid disturbing the setup as much as is possible to maintain concentricity.

Another trick.. Machine down to within, for example, ten thou' of final size and take light cuts thereafter, maybe a thou' at a time, to get down to final size. It puts less stress on the tool and material. Yes, it will take longer, but you are gaining accuracy, avoiding the stress of "was that cut too deep", potential for deflection in the material but also gaining a good finish.

Have you given yourself a tolerance? say +/- 2 thou'?

 

[RJSakowski]

The part is short enough that neither a steady rest nor a live center is needed . I would mount over length stock in a 3 jaw or a 4 jaw chuck (if the 1.5" diameter is the diameter of the stock). I question what the purpose of the .490" diameter. Why not .740" all the way to the 1.5" diameter. Order of operations: face threaded end, drill, bore and thread the socket; turn .740" diameter up to 1.5" diameter. If stock is larger in diameter than 1.5", turn 1.5" diameter, Part the work off, leaving enough stock for facing the a.5" diameter . Reverse the part and face.

 

[GrifterGuru]

@RJSakaowski..

As you will note, I said that was how I, personally, would do it. I prefer to make things as rigid as possible and avoid deflection, even in a 4" long part.

 

[bug_hunter]

Have you given yourself a tolerance? say +/- 2 thou'?

No tolerances

The part is short enough that neither a steady rest nor a live center is needed .

3.5-inch of sickout seems like a lot to me, but I am a self taught rookie.

I question what the purpose of the .490" diameter. Why not .740" all the way to the 1.5" diameter. Order of operations: face threaded end, drill, bore and thread the socket; turn .740" diameter up to 1.5" diameter.

I am making the draw tube as the one I have is ~0.740 the whole length. When I use it, I am instilling runout on the endmill. I am hoping to make a draw tube that allows for the collet socket to self center and not be interfered with by the draw tube. Might work, might not.

 

[RJSakowski]

No tolerances


3.5-inch of sickout seems like a lot to me, but I am a self taught rookie.


I am making the draw tube as the one I have is ~0.740 the whole length. When I use it, I am instilling runout on the endmill. I am hoping to make a draw tube that allows for the collet socket to self center and not be interfered with by the draw tube. Might work, might not.

No problem then. Reduce the diameter of that section as a last operation. Concentricity of that surface won't be critical. Or to preserve concentricity, you can turn that surface prior to parting and facing the 1/5" diameter end.