Suitable material for workholding.

[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.

 

[unclesyd]

Here are a couple of sites that will give you an idea of whats available. AISI A36 is the more common material. Nearly all companies who make these plates can offer other materials.

http://www.americangrinding.com/divisions/brute/

http://www.interlakesbases.com/plates.php

 

[arunmrao]

Your experience is right. I have manufactured fixtures in alloy steels ,as Mild Steel was not suitable. For some critical parts,Invar was also found suitable.
In all my discussions with those concerned for designing of fixtures, cost was rarely discussed,but reliability and reproducibility was .

_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[sydneyjongleur]

arunmrao-Can you remember what alloy steels these were?

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.

 

[TED7]

O1 or D2. The former for almost everything, the latter for thinner components as it's more stable during heat treatment.

I have found Mild steel is pretty much useless for plates. Okay for pins when case hardened, but I tend to machine from drill blanks for these anyway.

Designer of machine tools - user of modified screws

 

[sydneyjongleur]

I am looking for fixturing up to 1000mm. Does tool steels D2 and O1 come in this size and are they expensive. I would imagine D2 with its high alloy content would be more expensive

 

[TED7]

I'm unsure on the larger end of the scale. I generally work around 400mm maximum, and I'm not the one who does the material sourcing, so I'm not entirely sure.

D2 is more expensive than O1, by how much, again I can't say. Once you know if you can source material in that size. Check with a heat treatment company on how confident they are with hardening O1 with whatever geometry you have. Pay particular attention to stress concentrations and thin areas.

Designer of machine tools - user of modified screws

 

[arunmrao]

sydneyjongleur

It has been some years since I produced. The one steel grade that readily comes to my mind is EN 24 and EN 19 grades. Hope it helps.

_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[unclesyd]

Here are two sources of tool steels mention in the above post. The larger size is is going to be a problem as the there are not any producers around. Beware of foreign steels, a local shop got burned bad a couple of months ago.
Stay with the air hardening material and harden at the secondary hardening temperature. To achieve the highest level of stabilization you can either use the cryo approach of temper 5-6 times
All of our polymer metering pumps are made from D2. The pumps have clearances to 0.0001". The parts hold this tolerance for many years cycling in out of the process. Our biggest pumps are made from 3" thk to 5" thk plate. The gears are made form bar stock. D2 is very good steel for your application, but as stated above there is premium on D2
The Speedy Metals site has cost data

http://www.speedymetals.com/c-8303-round.aspx

http://www.machiningsteel.com/