Jeremy Pullin.
Jeremy Pullin is an experienced operator of additive manufacturing machinery, formerly of Renishaw, currently of Sartorius and a 2019 recipient of the AMUG DINO Award to boot.
As a member of the TCT Expert Advisory Board, we’ve asked Jeremy to tell us a cautionary talk from 3D printing’s day gone by.
It was early April 2010 and at the time I was working for Renishaw PLC in charge of Rapid Manufacturing (as it was regularly called back then).
We had many different types of 3D printing technologies there covering a range of materials including photopolymers, plaster, thermoplastics, sand and metals. We also had a whole range of other kit including various metal cutting plant, laser cutters, vacuum casting, mini-injection moulding, laser welding, chemical etching, a spray-painting booth and a vast array of electronics prototyping equipment. Now this of course was a great place for engineers to work but inevitably it was also a great place for people to do private jobs for home.
I caught people up to all sorts, like spray-painting radiators and a car bonnet (that one was not difficult to spot in the spray booth), machining motorcycle yokes and personalised drink coasters, printing a motorcycle oil tank and endless phone holders. I even managed to amaze a guy one day with my powers of deduction by catching him making something for Valentine’s Day on our laser plotter. “I don’t know what you’re talking about” he said with smug confidence when I confronted him. I took enormous delight in very slowly holding up the piece of acrylic stock that he had carelessly left on the machine in front of his ever-widening eyes. The sheet was completely intact apart from the large love heart shaped hole. Case closed, your honour.
It was, however, absolutely unheard of for the Chairman/CEO Sir David McMurtry to ask for anything that was not very strictly work based. Colour me amazed when he somewhat shyly asked me if I could make something for him to give to his friend Alex Moulton as a 90th birthday gift.
By pure coincidence Dr Alex Moulton was a personal engineering hero of mine as I had gone to school in Bradford on Avon where his bicycle factory is located, and my schoolhouse was named after him. I had also studied some of his work at university, so I was both shocked and delighted in equal measure. I suggested that we make a cut-out model of a radial engine (I’m sure many readers of TCT would have seen this part on various show stands) and emboss "Happy 90th birthday" on the foot section. Sir David agreed and the file, which we had originally been generously given by EOS was scaled down to 80% of its original size and modified with the requisite text. I choose to print it on our SLS machine which was an EOS P385 running PA 3200 GF (glass filled nylon 12).
About turnaround
When Friday arrived the rest of the department had all gone home for their 12:30 finish but I had stayed on as usual. I was surprised when Sir David appeared in the office and asked what was happening with the birthday gift. I assured him that we had put the part on to print and that it had completed. I told him that my SLS operator would break it out from the build cake, clean it and it would be up in his office ready nice and early on Monday morning. “Well, it’s actually his birthday today,” he said, “and I am off to see him now”. I thought he was joking and looking back, my gag about the then 70-year-old Sir David teaming up with his mates to give the 90-year-old Dr Moulton the bumps after jelly and ice cream probably wasn’t the best. He just laughed politely and asked, “Where is it please?” At which point I realised that he was being serious which gave my stomach the sort of sick feeling that you normally only get when you are the passenger in a car being driven by a 19-year-old showing off the speed his badly customised Citroen Saxo can do over a humpback bridge.
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I went with him into the SLS printer room where my pride and joy EOS machine sat, with the man that had given me the money to buy it. He had actually miss-counted the number of zeros on the £300,000 capital request that I had submitted and thought that he was giving me £30,000 when he signed it but I only told him that after the thing was on order. I now had to break the news to this man (who besides being the Chairman/CEO is also an extremely lovely guy) that there was no way he was going to get his gift. I pointed at the powder cake sitting in the breakout chamber as I looked anywhere except in his direction and said with a voice muffled by the baffles of shame, “Ummm well it’s in there David but the powder cake has to cool down before we can do anything with it. That’s going to be a few hours yet I’m afraid”. I did wonder if I should offer to nip down to the nearest Argos and get him a game of Twister to give over as a 90th birthday present instead. Unusually for me however, I kept that one to myself. To my amazement Sir David stuck his finger into the powder cake and cheerfully said “Ah it will be all right, let’s get it out shall we”?
Your cake and heat it
Now the thing is, you should never break parts out of an SLS build cake until they have cooled down. They need to cool down slowly in their own time so that they don’t warp and become damaged as they fully solidify. There is also the slight issue of not wanting to burn your hands of course. We would shove a nice long thermometer right into the cake and wait for it to get to 60 degrees Centigrade or less. If you were in a real hurry you could occasionally really push your luck and go up to 70 degrees. When I looked, the thermometer was sitting there at a steady 80 degrees. I looked at the thermometer then looked back at Sir David’s face and just thought ‘Ah well, mess or glory’ so picked up the paintbrush by the side of the breakout chamber and went in.
I had to keep telling him to keep his hands away as politely as I could as he tried to pick the part out while I was gradually excavating it from the powder cake. After giving it a quick brush, I blew it over as gently as I could with a compressed airline to cool it down a bit. The part actually looked ok although I’m sure that if we had measured the thing it would have been about as straight as a varicose vein on the leg of a 95-year-old cyclist. The partially hollow nature of the part meant that it had a lot of powder inside which was proving tricky to remove as I didn’t want to shock it thermally by blasting it with too much air. Eventually though, there was only a dusting of powder over the part. “Right,” I said, “Let’s get the mechanism moving shall we”?
I tried the turning handle; it was stuck solid. I smiled at a confused looking Sir David in a ‘don’t worry this is normal’ sort of way and wondered if I was too old to start a new career at McDonalds, which was probably the best I could hope for after he had fired me which I was expecting at any moment by now. This time I gave it a good blast and tried wriggling it again but still nothing. It was at this point I realised that we had never actually printed this part at 80% scale, so the clearance gaps carefully optimised by the original designer were no longer big enough. I froze at this point and just thought ‘or Burger king. I do enjoy a Double Whopper after all’. Sir David could see that I was having trouble so casually picked a six-inch steel rule (that he would pretty much always carry around with him) out of his pocket and got stuck in to trying to free the mechanism up.
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The part has some pretty fine and delicate internal details which were even finer at 80% scale and I was convinced that the part was going to be left in the same sort of state as my engineering career was going to be when this was all over. I quickly grabbed my trusty penknife knowing that it had some tools on it which were a lot smaller than the rule Sir David was using but as I tried to take the part back off him he simply said, “Oh good idea” and took the penknife off me. I then stood there helplessly by his side thinking ‘one of the greatest living inventors/engineers is just about to stab himself in the hand with my penknife and it’s all my fault. Forget Burger King I won’t even get a job emptying the fat out of the grill drains of a kebab van after this’.
To my relief and amazement, the thing actually started to move. After a bit more wiggling around, it was all turning freely, in one piece and working exactly as it should. I gave it a light bead blast and final clean and gave it back to him. He smiled at me, generously thanked me and cheerfully wished me a happy weekend as he went off. This is when I realised that he had actually enjoyed the whole episode. I made my way to the toilet and found a pale looking face looking back at me in the mirror. To this day I do not know if my face looked that pale due to the blood draining away from it or because the white SLS powder had stuck to it where I was sweating so much.
I guess the lessons here are always fully understand your timescales, always take post processing into account for your process times and never forget that messing around with parameters, such as scaling, could have disastrous knock on effects.
