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Lost Foam Casting: a Crankcase in Zinc Alloy

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awemawson:
Thanks Steve.

I suppose this stems from your RC modelling activities, an arena I've not been involved with. I'd have thought with careful construction that wouldn't do too badly, after all you're not looking for +/- 1 thou are you  ?

Eugene:
Steve,

Thank you, that's been a great help. I've already dropped the power (and hence temp of the wire) considerably and got a much better result just working freehand; time for some sheet ally now.

Your set up isn't "quite crude", this is crude ... !



The only excuse is that I made it yesterday from leftovers from the roller shutter cupboards which were themselves made from left overs; like you I hate waste. The only difference between it and a thousand others might be in the power supply which is from an old PC.



I just made a contact block to take the various power leads.Switching them and moving the position of the crocodile clip on the wire gives pretty fair control; something I did after reading your reply. The thing that looks like the end of a safety pin between the green and black ground wire is actually the end of a safety pin between the green and black ground wire; it just provides a load to switch the unit on.

Today I scrounged an old (Polish!) propane bottle which is just the right size for a charcoal smelter, got the brass tap out and filled it with water. It'll stay that way till Friday when I'll be back int t'hut.

Again thanks,

Eug

vtsteam:
Andrew, sorry, you're right it is quite accurate enough for wings since movements are relative and we tend to sand things, cover them etc. I just wanted to warn exacting machinists that the accuracy isn't what they would expect from ball slides and a rigid gantry, etc for a CNC mill. and metal work.  And for pattern work, we usually expect surface finish to be at least foam texture, and likely having a machining tolerance as well.

A hot wire has a variable kerf width also depending on the direction of cut. Because the wire is cutting by radiant heat (properly) rather than contact, and the melted foam gasses travel upwards an upward vertical cut is wider kerfed than downward, and horizontal varies with angle from horizontal, etc. Also depending on software, speed of cut tends to vary around curves, somewhat -- naturally the wire is in an area of foam longer when doing a tight radius and can actually melt out a section even though the linear speed stays the same. etc.

Some software compensates for this (somewhat) or some people play the heat control manually. Still others accept the slight variation (yours truly!).

All this tends to make for even more wasted material when you suddenly lose a section that on paper (in CAD, I mean) it looked good.

When you work manually, the knob between your shoulders does some amazing compensation at times after a little practice, seemingly without any calculation, just by the appearance of the cut. And quickly adjusts placement of a part to best utilize remaining scrap after a botched piece -- something that's difficult to re-program into a CNC machine as quickly.

vtsteam:
Eugene, that looks great!!! Much better than my vertical wire. Now I'm ashamed and no need to post mine.

Also, if you want variable heat control with your computer supply DC output -- assuming you aren't "attached" to your alligator clip method (we have alligators here, crocs very rare and confined to the Keys) try one of these:

http://www.ebay.com/itm/DC-12V-24V-Single-Color-Adjust-For-5050-3528-LED-Light-Strips-Dimmer-Controller-/171104731465?pt=US_Lighting_Parts_and_Accessories&hash=item27d6a30149

awemawson:
A comment from someone who knows nowt about hot wire cutters:

Why are you bothering with DC power supplies - I'd have thought a suitable low voltage transformer (transformer not wall wart) powered  by one of the very cheap phase angle  'power controllers' available on ebay would have given you full control as needed and mains isolation.

http://www.ebay.co.uk/itm/High-torque-50V-220V-10A-AC-Motor-Speed-SCR-Controller-Power-2000W-UK-Seller-/281387453688?pt=UK_BOI_Industrial_Automation_Control_ET&hash=item4183ffccf8

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