Suitable material for workholding. 2

[sydneyjongleur]

Hi,

I work in the manufacture of aerospace components and I was looking for some advice on stable materials for fixturing these components.

The fixtures can range from 200mm to 1200mm in diameter and between 30mm and 60mm thick.

We currently manufacture from mild steel but are looking to get away from this due to the material movement and wear. We have recenlty tried EN8 with better results and have also considered the cast iron route(although this has an expense attached).

Cost will obviously be a factor in this although quality of the workholding is more important in order to produce these expensive components.

 

[MikeHalloran]

There is an entire industry devoted to the manufacture and sale of mild steel jig and fixture components, providing cost effective solutions thanks to the economies of scale. Their products include replaceable hardened wear points. You haven't provided enough information for us to judge whether those semi-standard components could meet your needs, but you can and should take a hard look at those products, and an honest look at the cost of making your stuff from scratch (as I infer you do, based on zero data).

Hardened contact points, whether custom or standard, should take care of any wear problems. As for things moving around, you haven't made it clear if the movement is a result of machining stress, or differential thermal expansion. Tactics to deal with those factors should be fairly obvious.

If you can do it without giving away the store, it might be helpful to see some photographs and/or drawings of particular fixtures of interest, highlighting the wear points or distortion issues.


Mike Halloran
Pembroke Pines, FL, USA

 

[ornerynorsk]

I use pre-hardened 4140 almost exclusively for critical fixturing and workholding. It is still very machinable in the pre-hard condition ~32Rc, and is very stable, with much greater strength than mild steel. It generally needs to be demagnetized after machining.


It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[sydneyjongleur]

I have attached a picture of a typical base fixture with a part specifc fixture attached and then a component attached to that.

The base fixture ranges from 500-1000mm and is standard on all of our machine tools(cnc lathes and machining centres). This remains on the machine at all times and has various part specific fixtures bolted to it. I was thinking for this design that a cast iron fixture may be the best solution as it provides good thermal stability, wear resistance and compressive characterisitcs. The cost for a pattern will be greater than steel but as these will only be made once for each machine this cost will offset itself in the long term. I am not sure how it reacts to machine induced residual stresses and stresses caused during material manufacture. If anyone has an opinion then I would be grateful to hear.

As we manufacture lots of part specific fixtures(150 approx per year) with varying levels of detail the cast iron route may prove to be costly and design intensive. Does anyone recommend steel grades,or the heat treatments involved ie. harden and temeper and stress relieve, with low thermal expansion properties, wear resistance and stability after production and machining.

If I am way off the mark with this please feel free to correct me.

 

[MikeHalloran]

I might skeletonize that base fixture just to reduce the power demand while the spindle is accelerating it.

Given that it spins, I would NOT make it of cast iron.

I might consider aluminum tooling plate, with steel inserts for threads.


I also might take a look at simplifying the base fixture. I.e., how many of the tapped holes have you actually used over the years? Why keep tapping (or even drilling) the ones you've never used?

Functionally, it's not a whole lot different from a generic lathe faceplate. I see no dowels, steps, pilots, or expanding keys to locate the part specific fixture. The PSF appears to be held on by bolt tension and friction only. Certainly some such fixtures could move by half the bolt clearance under machining forces.



Mike Halloran
Pembroke Pines, FL, USA

 

[sydneyjongleur]

There are bushed 20mm holes at 100mm pitches- this is what we use as location. We then bolt down with capscrews. The size of the plate matters as the next fixture could be twice the size. We make use of most of the hole pitches.

Why do you say that you wouldnt use cast iron as its spinning? Is this due to its low tensile strength.

My fear with aluminum is that it will damage and wear easily. Part speciifc fixtures get loaded/unloaded on a daily basis.

 

[MikeHalloran]

Cast iron, especially in custom castings, contains hidden flaws.
Sometimes they reveal themselves under stress.



Mike Halloran
Pembroke Pines, FL, USA

 

[sydneyjongleur]

ornerynorsk- what size range was the fixturing that you utilised in 4140. Was it large or small?

 

[Screwman1]

I would worry about movement in cast iron as it goes through it's initial cycling and aging process. We always let critical castings sit for several months because they sometimes seem to have a fair amount of residual stress and move prior to machining.
I would go with the pre-hard material and hardened inserts in wear locations. I have always been of the opinion that it is best to spend the money needed for first class part holding because it prevents so many problems down the road.

 

[ornerynorsk]

Sydney, most everything I've worked with has fallen within a work envelope of 16 x 20 inches (400 x 500) and in plate thicknesses of (usually) an inch, 25mm. However, we have built many press brake and tube bending dies out of the same material with cross sectional dimensions greater than 6 x 8 inches (150 x 200). I have also used 4140 pre-hard in injection molds, rubber compression/transfer molds, and as baseplates for stamping dies. I have found it to be a very good material for many applications.

As a plus, it can also be selectively flamehardened in areas that might experience more severe wear or usage.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.