Member postings for Ajohnw

People are making forges out of nothing more than a steel shell lined with ceramic blanket. It's extremely efficient at keeping the heat in. One of myfordboys video's shows it held in front of his hand with a propane torch heating it to red heat. There is also fluid about for rigidising it. This can be dried out at 90C or just air dried more slowly I suspect. The amount put on determines the depth.

If you want to mix your own refractory it should be easy to obtain a bag of vermiculite from a decent garden centre. It's normally mixed with soil to increase it's ability to hold water. It's mostly air and can also be bought in a board form. It's generally rated at around 1100C

Charcoal - it's capable of melting cast iron with a blower. Not sure about the one posted by Vic but in furnace form a hair drier will do it. The ones with a nozzle type attachment are best for this as they can be kept away from the heat. They just need to blow into a hole in the base of the furnace.

There are details of a burner and forge here

John

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Edited By John W1 on 18/12/2015 09:44:44

I can browse the site now Jason but get the same problem if I click on an attachment. No problem though.

Interesting to see the other method mentioned, charcoal. The design can be very similar to the propane ones. Air is just blown into the hole with a hair drier rather than fitting a propane torch.

I had lost the link to the backyard casting site but like many it's sketchy in places and I feel if people want to the trouble of making one they may as well use the correct materials.

John

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Edited By John W1 on 17/12/2015 23:32:49

Thanks Jason. I'm away for a while so maybe it will work when I get back. We sold our last economy 7 heater a while ago. The buyer wanted the bricks. My impression is that they are a lot heavier than the insulating types.

There used to be lots of info on the web constructing propane furnaces using cat litter, vermiculite and whatever mixed with various things. I suspect it's died out because they don't last very long. I have seen mention that the vermiculite burns out leaving pockets that aid insulation - I'd expect cracks when that happens.

Moulding one as per Myfordboys video is a fair amount of work but as he mentions he wants a tool not something that is just knocked up. Personally I don't see why the same thing couldn't be done with bricks as I don't think they have to be round. If it's important to have the flame circulating around the crucible why not just cast a round part into the bottom of a brick built one. Bricks can be obtained right up to 1700C if needed. Alternatively the sides of the bricks can be angled to obtain a near round shape. Many kilns are built like that.

John

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Edited By John W1 on 14/12/2015 09:50:12

Edited By John W1 on 14/12/2015 09:57:05

I suspect I have been awaiting moderator approval for well over 2 months.

John

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No same thing happens on all links including donate.

John

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I registered a month or so ago Jason, it shows me as logged in at the bottom of the page (ajohn the name I usually use) but any time I try to look at anything I get a message about sections (3 of them) that can't be viewed without donating. I DON'T WANT TO LOOK AT THEM (shouting)

Same thing happened following the link using a browser that is already logged in.

It's a bit like hitting a nag screen. Hope it doesn't catch on.

Edit - thinking further I am not sure if I ever received an activation email.

John

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Edited By John W1 on 13/12/2015 20:50:41

They want MONEY off me Jason for that link. If it's for a for sale add I've put in too much time now to do that. I've looked around at all sorts and the shape and size is generally a problem

One other link I meant to add was these people on ebay. They have been selling this one and another 1400C version for a long time now. Might be of interest for propane or oil.

**LINK**

Not sure how price compares with others.

John

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Thanks Roger. I hadn't found any of them so far. The Kanthal handbook answers a few questions I had. Dudley show more or less what I intend to try. On the heat treatment furnace there is a series of video's showing the construction of a similar unit. They are long but of interest especially the testing which starts at the end of the one before this one.

There is more testing in the one after that. I did talk briefly with Myfordboy - he said forget it and go for propane. He does converse with some one who is trying electric who keeps burning elements out. Trouble is I like a challenge. On the propane front there is an interesting video from a forge person on making a torch. Long and slow with complete instructions rather than "gee look how big a flame I have". His torch looks to be very efficient in terms of heat output and fully burning the gas at least. Interesting little forge too. Myfordboy shows a cheap way of making a needle valve to control a torch and one or two other things as well.

I obtained my wire from here.

**LINK**

Before I do anything though I need to decide on how to make the sides. The bottom will be brick but I'm wondering if I can avoid using it for the sides. Hence in part the earlier question about minimum thickness that will take the heat. Bricks can be cut in half, which I feel would be ok and castables can be cast at any thickness. I'm keen to keep the weight down as well so if at all possible ceramic board would be the best option - if that is possible.

John

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Kanthal is a brand name. Most of the wire that is around seems to be described as kanthal like now. The same sort of alloy. People do phone suppliers and ask for elements to be made and they will do that but it can result in more grief when in real terms there is no easy way of determining what wattage will be needed. If I did that I would go for a lower wattage than I think I need that could be shortened to increase it if needed without upsetting the surface loading figures to an unacceptable level. Those set how long the element is likely to last.

Anyway i have bought the wire which cost me £11 and am thinking in terms of 2kw. The sums suggest that is around 30% more than needed. I could reduce that by extending the length of the element using the same wire. The wired came in the form of 5 10m spools and each spool should be ok for 1kw with 120v across it so 2 should be ok for 240v and plenty left to add more if needed. It was a buy 4 and get one free deal. I have seen the real thing at £1.79 per m but don't want to spend much until I know it will work.

The lower temperature range low density bricks seem to have better thermal insulation properties but kanthal needs one grade up from those really. These are usually listed as 1400 to 1500C, or grade 26

It pays to look around for suppliers of the basic materials. Prices vary significantly. Vitcas is one of the main suppliers in the UK. They do have an on line shop. They have lots of things not shown in the shop. I managed to get some high temperature ceramic blanket off ebay, left overs from some one who made a pizza oven using materials that were a lot better than needed really.

For propane furnaces the forge people may have a simpler route than casting refractory. They just line a barrel with ceramic blanket and replace it now and again. Maybe just add a cast base for smelting aluminium. There is info about on the web. The higher temperature range blanket might be needed. The sub 1300C stuff might not stand the heat. It can be made more rigid on the surface if needed but so far I have no idea what the liquid used to do that can stand. The same sort of thing can be done to board but there is even less info about.

John

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Myfordboy on youtube has some pretty comprehensive video's on making furnaces including oil burning. He also shows a method for vibrating the casting which the materials used need to get rid of air pockets etc and assure it's all well compacted.

Things may have changed in respect to small electric kilns/furnaces. There seems to be some about now that heat up very quickly. There are calculations about but they assume a certain rate of heat loss against temperature. This sets the power needed. I'd guess that the calcs are based round a single layer of ordinary insulating brick and very little else so if I add more that also has less thermal mass the elements may burn out due to overheating.

One person seems have got round the overheating problem by using a cheap triac controller ahead of the main temperature controller so just turns a knob to limit the max power the controller can provide. It sounds like it should work but I'm inclined to try and make it easy to fit more elements in series to reduce the max power or start with too many and remove as needed.

From the electric "cigarette" brigade it seems that kanthal can be annealed by heating to orange heat. There is some info about on winding furnace elements that use a turn or so round a reel to provide the tension but I suspect that will harden the stuff again. That method is a bit like wrapping a turn of electronic hook up or copper wire round a terminal screwdriver and pulling it to straighten out kinks and bends. I have wondered about winding it around a mandrel, keeping it tight and then heating the lot up with a propane torch. Tension via blocks of wood might be a decent idea.

As to calcs there isn't much about and zero on constructing small commercial ones that heat quickly. Potters can't be keen on saving the planet. One kiln is being sold on ebay at the moment - outside got so hot it started a fire. This is all I have managed to find on the sums for normal types. I suspect it would be a good idea to check the actual resistance of dubious kanthal wire.

Interesting thought at the end of the Russian offering.

There is plenty of talk about not using the cheap elements from China off ebay. It might be possible by buying ones that state 2 or 3 times the power needed and using them in series. The surface loadings are far to high used as they are listed.

John

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Edited By John W1 on 12/12/2015 18:05:09

I thoughtfully linked to the cheapest thermostatic one OB as suspect that is what you have and that you wouldn't like the manual types.

John

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I've been working through the design of a small electric furnace. It's looking reasonable for a chamber with an 8" square base that is circa 10" tall. The cheap kanthal wire I have found looks a bit dodgy at 1300C but should be ok a bit lower. Two questions hoping some one else has done something like this.

I'm going to wind my own elements and intend to do it on a lathe and wonder about tensioning as it seems to be pretty springy stuff. I intend to screw cut a mandrel to the pitch I need or use all thread and then stretch as needed.

I have 2 options on the element carrying sides. Cut up insulating bricks or make them from castable refractory. A lot of the heat up time in these is caused by the bricks so I wonder just how thin they could be if the bricks were cut down or made from the castable. I intend to use ceramic blanket for most of the insulation.

Another option for these might be ceramic board, RFC. It's semi fluffy stuff but is seems that it can be routed or cut with a knife. I believe it can be hardened using a silicone colloid but can't really find any info on what that involves in practice. Sounds better than other options as it's thermal mass is lower. Pricey though.

Yeh I know propane is easier and quicker for smelting aluminium. And also that the furnace would be much easier to build.

John

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From Mark's link it sounds like I should have a play again.

One option for a high current power supply is those used in servers that are plugged in as one unit. Usually described as redundant power supplies as the server has more than one and will switch automatically allowing a replacement to be plugged in. Model aircraft etc people use them to make battery chargers so there is some info about on what pins need shorting out to get them to power up. They crop up very cheaply on ebay at times and can even be new. Stuck with 12v though but some people have disconnected the internal grounding and used 2 in series for things like induction heating.

The motors are rated in power and revs per volt. So say it's a 600 k motor that means 7,200 rpm at 12v. The lower revs per volt ones tend to be aimed at helicopters. 24v will loosely mean 14,400 rpm, more of a problem as I see it. I looked at using timing pulleys to reduce the speed - probably quieter than gears and they can transmit a lot of torque.

I feel that there is a lot of hype kicking about in some areas concerning brushless motors. Comparing apples with apples eg and a conventional universal brushed motor they can be more efficient as would a could a permanent magnet brushed motor - the sewing machine claims for them are correct. Low speed improvement though is more dubious and of course as always actual power drops of with speed reduction as the torque has to remain at similar levels. Belt's, gears etc increase torque when they are used to reduce speed. Brushless motors do have no brushes though which will be a gain at some point if the rest of the motor lasts long enough. There is no reason why the electronics driving a brushless motor should be any more reliable than brushed, same for the motor.

There are industrial sewing machine motors and controllers about. Pricey and they usually spec input power.

There is info about on the web concerning building your own brushless motor. Some of the parts can be bought.

One thing about the model aircraft motors is that it looks like replacing bearings is a manageable job. Replacement shafts are available for them as well.

The web effect as I call it might up the price of redundant power supplies now I have posted that.

Do remember that lots of current will be drawn so cables to the motor need to be short and or rather hefty. The model stuff uses a silicone rubber coated wire as it is likely to get hot at the sizes typically supplied.

John

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It seems to be 5hp 20-1600 rpm and universal if it comes with the overarrm etc and head. A typical cooking milling machine as far as I am concerned. The sort of thing I have seen lots of in several large toolrooms. A toolroom might have something smaller and faster about if they need it. I have seen Thiels and Deckels around and more recently a larger high speed machine who's name I unfortunately can't remember - not that I would ever have space for one. It was exceedingly heavily built and capable of high speeds.

I understand a 2 speed motor Bridgeport can reach getting on for 5,000 rpm. As far as low diameter carbide cutters go that just means a higher feed rate can be used. Carbide cutters do not have to be run at maximum speeds. I have seen one used for heavy work, a drop forge die sinker working from "home". He was for ever scraping the horizontal slide that carried the head to retain the machines accuracy and getting a bit cheesed off about it.

John

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Edited By John W1 on 12/12/2015 10:48:23

If you read what I stated Ketan I used the word THINK. Reason - one of the lathes they do has changed the type of motor. Which one I don't know. I noticed it recently so use the word THINK.

John

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What - lime scale in Brum - no way but that sort of thing depends where people live.

John

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This web site may help sort out use of legacy and errors on pages. It's the one Opera use.

**LINK**

John

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If I'm buying something like that I start at Screwfix. They are sometimes cheapest but once I have sorted a model I look to see what's on the web. Even amazon and ebay.

The 1st thermostatic one from screwfix set for lowest price 1st is this one

**LINK**

We have high pressure hot water now but have used Triton in the past.

John

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Arc do some rotary tables that are somewhat different to the usual offerings. Might help, not sure.

**LINK**

John

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The same technique is used to initially grind telescope mirrors and lenses. Convex or concave. I think it also slightly relates to how snooker balls are made.

John

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