Mud Lathe

[Pete O]

I've been buried in the workshop for the past 3 weeks like a mad inventor, haven't taken the time to post the project before now but the paint is drying and I can relax a little so I'll plot my progress retrospectively.
My eldest daughter has a significant birthday this year so I decided to do something a bit special for her, especially as she has two sisters getting married also this year we thought it best that her birthday didn't go unsung.
She did a pottery course last year and said she would love to do some pottery. The place that ran the classes has a kiln that their students can use for their own work so I thought a potter's wheel to use at home would make a great gift.
I trolled the internet for several weeks, learning as much as I could about potter's wheels so I didn't build something completely useless and getting some ideas of what design to go with. Lots and lots of different types out there but with two main themes- there's the common, modern electric type with either a variable or fixed-speed electric drive contained under a unit that you sit in front of, and there's what is called a 'kick wheel' which has a large flywheel that you spin with your feet, connected by a vertical shaft to the 'throwing head' (which is apparently the proper name for the spinning disc that you put the mud on). There are a few variations on the 'kick wheel' theme, most commonly a treadle of some sort that spins the flywheel by a foot-operated crank and lever. I ummed and ahhed for several weeks over what sort of design to go with. Then I came across this video on Youtube:
https://www.youtube.com/watch?v=uHj_BysG4Ww

I decided to borrow this guy's idea and make a bicycle-powered potter's wheel of my own design. We all know how efficient a 'design-as-you-go' approach is, right?

One of the first tasks was to rescue an old cast-iron tractor seat that used to belong to a glide swing (one of those double-ended playground swings) that was on our property when we bought it, the kids used to play on it all the time when they were little and it got cut up for scrap after they all outgrew it. I've been hanging on to the tractor seats for just such a project. Had to cut the remains of the welded-on swing frame from underneath it, then made my first ever foray into the world of electrolytic rust removal. It went into the tub in the evening looking like it had sat in the weather for decades (which it had), came out the next morning looking like this
   
I was prepared to forgive it the little broken bit.

A trip to the scrap metal yard cleaned up my accumulated metal waste, including a large drum of swarf, and scored me (along with a pair of forklift tynes for other projects) the power unit for the potters wheel
   
and a chunk of heavy-wall pipe perfect for the large crucible i'm going to need to cast the throwing head and the gear blanks. I've looked out for a bit of pipe like this every time I've been to the scrap yard and this time I SCORED!
   
I cut a suitable length in the bandsaw then put it in the magic bath with an anode suspended up the middle of the pipe to de-rust inside and out. Another overnight job, cleaned up nicely.
   
Welded a piece of plate on for a base, some angle iron for a spout and some lugs to lift it by. I'll try and remember to take a photo of the finished crucible. It ended up about as big as my foundry furnace will take and still have room for a bit of fire.

Got to go and feed cows and chooks, I'll post some more on this tonight if I get the chance.

[Pete O]

Took a photo of the crucible while I was outside and i have a few minutes before I have to make pizza.
Finished (and used) crucible.
   
340mm tall. Those with some foundry experience will no doubt foresee the problem I was setting myself up for with this...

Anyway..
I had to refresh all my sand as it had been unused for over a year, it was bone dry. Must make a muller one of these days.
I had to make a new casting flask big enough to cast the throwing head in. The pattern was in about a hundred pieces. First time I rammed it up, the sand fell out of the cope as i lifted it off the drag. second time i rammed it up, when I separated the cope from the drag I found that one of the rim pieces had shifted.
   
Third time went okay except that one of the sections between the spokes collapsed. I decided to go ahead with it and machine out that section after casting.
The large flask holds the throwing head mould, the smaller flask has a slug that will be a blank for one of the bevel gears.
   

More later.

[Pete O]

I fired up the furnace and melted most of a Range Rover transfer case, a coffee pot, the sprues from my last casting, any other chunks of aluminium I could get my hands on...eventually the crucible was full of liquid aluminium so I went and got my wife to come and observe from a safe distance as safety officer. As I lifted the crucible out of the furnace it occurred to me that it was a tad heavy. I put it down and scraped the muck off the top, attached the pouring business and picked it up again. I could not lift the thing up to the trestles where I had the molds!!! Now I see why people make things like pouring trolleys (another future project). I lifted the smaller mould down to the floor and poured it- could just manage it at floor level- then had to get my longsuffering wife to help me lift the big mold down to the floor. Getting my fingers out from under the edge of the mold, it dropped that little way and landed sharply. Oh well, crucible full of just-still-liquid metal, better just pour the thing and see how it goes.
   
It was now about 10:30pm so I left everything to cool for the night while I went to bed to worry about it.
Broke the castings out in the morning and they were better than I could have hoped. I thoughtt the metal might have cooled too much while I was stuffing about to fill the larger mold properly, especially as nothing had appeared at the vent holes. Not so, the cavity was full and both castings were successful.
   
   

[TomG]

Looking good Pete. You have been busy.

Nothing wrong wit the designing as you go. It makes good use of available materials.

Tom

[Vinny]

Design as you go? I do it all the time with fabricating, programming, electronics, cooking, brewing, ... My sister always asks how we're planning to make a certain dish for holiday dinners. My reply is always the same, "I'm making it up as I go". She still asks, you'd think she would know by now.

[Pete O]

Yep, I don't think I've ever started a project knowing quite what it was going to look like when I finished. One of these days I'm going to build something with a set of plans that say 'cut this to X length, join A to B' etc but it's never happened yet.

[Pete O]

Most of the 'home brew' potter's wheels I came across on the internet had a plastic bowl of some sort around the throwing head and a lot of them were broken (like the one in the video at the start of this thread), people tend to rest their forearms on the rim while working so a plastic bowl won't last long. Commercial wheels mostly have a metal tray of some description around the head. I decided to make a steel tray and get it galvanized for long life. Had to at least get the basic design of the frame sorted so I could get the tray made and get it off to the galvanisers. Problem with designing on the go, you never quite know if something is going to work out until you get it made. I had a basic drawing of the layout and had worked out how high the thing would have to be for the pedals to clear the flywheel and the gears. I fabricated this frame out of 40x40x2mm RHS.
   

Setting it up as a mockup, I discovered that when pedalling, the side uprights were too restrictive. Decided they weren't structurally necessary so cut them off and left just the two rear uprights supporting the top frame, also shortened the top horizontal frame back to the first cross-member to mount a P-type bearing block instead of the flange mount. The flange-mount bearing in the photo above sits on the bottom frame and bears the flywheel weight axially.

I don't have any sheet-metal equipment so I had to get a local business to do the folding and rolling for the tray, got them to make a 3-sided tray 400mm wide and 100mm deep, plus roll a 100mm-wide strip to a 400mm radius, and roll an arch out of a length of 16mm round bar which would become the rim of the tray, better arm rest than a sheet-metal edge. Spent a while cutting and shutting and ended up with this
   

I welded four 8mm bolts to the underside so no drilling would be needed once it was galvanised. The two bits of tubing- one for a riser around the main drive shaft and the other tapered piece to put a drain hose on- were salvaged from the bicycle frame.
   

Spent a morning taking it to the galvanizing mob. I was hoping to get a look at the galvanizing process but they are a big operation with lots of things like building frames coming and going, oversize forklifts busy everywhere, I was pleased to get in and out without getting run over.
I phoned them before taking the drive to discuss where I would need holes for hanging and draining. I had made a drain hose spout in the bottom of the tray, with a little more forethought I could have positioned this where it would have worked as a zinc drain for the dipping process. I had to drill a hole in one of the bottom corners and another just under the rim opposite, need to bog these up later. Picked the finished tray up from the galvanizers last Thursday; a bit of distortion but not a problem and I don't reckon it will rust away in a hurry.

I got on with figuring out the drive mechanism in the meantime, the vertical rotation of the pedals is converted to horizontal rotation of the flywheel and throwing head by a pair of bevel gears. I've never cut gears before, I've gathered some equipment in preparation for making a couple of missing change gears for my lathe but have not got around to doing any yet. I was a bit taken aback by just how complicated bevel gears are. To make the drive mechanism with the gears and the bicycle hub work, I had to make the bevel gears a specific diameter (5 1/2") at the large end so that the bicycle hub would clear the vertical drive shaft. Unfortunately, all the calculations for bevel gears are based on the diameter of the small inner end. I spent about 3 hours making calculations and entering them into an online gear calculator until I came up with a 40-tooth 12DP 45 degree bevel gear 1 inch thick which would (a) give me the 5 1/2" large diameter and (b) use an involute gear cutter that I already had. Bevel gears don't even use a sensible gear cutter, you use a cutter for a number of teeth on some theoretical, non-existent gear based on the pitch circle of your bevel gear multiplied by some kind of voodoo spell. Anyway, I had the appropriate cutter so happy days.
I whittled away at the round lump of a casting until I had a gear blank of the appropriate dimensions. Made an arbor to mount the blanks in the lathe and in the dividing head, the two gears have different bores so the arbor is double-ended with a 25mm spigot one end and 37mm (diameter of the appropriate part of the bicycle hub) the other end.
   

I removed the  vertical attachment from the mill- hopefully for the last time as the J-head graft is (I hope) my next project- and got the dividing head set up to 45 degree tilt and centred on the involute cutter
   
   

The 40-tooth gear happily took away one of the dividing tasks as my dividing head is 40:1 so, one revolution of the handle per tooth and back to the same hole.  Bevel gear teeth are cut in 3 passes; one cut down the centre, then the table is stepped across half the pitch distance and the dividing head rotated half the index angle and a second cut is made, the cutter enters the cut through the same gap that it cut previously but then cuts a tapered path toward the heel. For the third pass, the setup is stepped across the same way in the opposite direction and the other side of the tooth gap is cut. This results in the tooth gap having a matching taper to the tooth. Apparently you can delete the first, central cut and do it in two passes but the book I was using recommended three passes. That's 120 cuts per gear.
First cut:
   

Used my poor-man's DRO to step the table across accurately between cuts
   


Final cut (one gear) three hours later:
   

The second gear was cut from a piece of aluminium plate stock of unknown grade that I had lying around, just large enough to have the gear blank hidden inside it. The cast blank was nicer to machine than this piece of stock, end result was much the same though. I sped up the X-feed slightly for the second and third passes on this gear, took only 45 minutes for the 40 cuts rather than an hour. I've already posted a photo of the gears but here they are again
   

Yes that is the throwing head they are sitting on, I've got some photos of that to put up next time I get a chance to post.

[f350ca]

Incredible job on the gears Pete.
So you angle the indexing head to the table axis to make the second and third cut ? Makes sense, how did you measure the angle ?

[Pete O]

(03-08-2018, 06:17 PM)f350ca Wrote: Incredible job on the gears Pete.
So you angle the indexing head to the table axis to make the second and third cut ? Makes sense, how did you measure the angle ?

Thanks for the compliment Greg. The dividing head stays in line with the X-axis, to make the second cut the table is moved across in the Y axis by half of the chordal thickness- which is the thickness of the tooth at the chordal circumference- and then the dividing head is rotated back by 1/4 of the index angle; i.e. one quarter turn of the 40:1 crank handle for my 40-tooth gear. This brings the gap at the small end of the tooth right back to the position in space where the first cut was taken, but moves the big end of the gear to a different position, best way I could understand this was by keeping in mind that the circumference is bigger at the big end, therefore the big end rotates a greater distance when you turn the crank, so moving the table across the same distance that the small end rotated, brings the small end back to where it started but places the big end in a different point in space, so the cutter theoretically doesn't touch the sides as it enters at the small end, but cuts a gradually thicker swathe at it moves along the tooth.
I found this all very difficult to get my head around; I used a book called 'Gears and Gear Cutting' by Ivan Law which presents this complex, difficult to understand information in a complex, difficult to understand format.  I perused a couple of websites and between various sources of info I was able to understand it enough to perform the tasks. I can't claim to properly understand the relationship between the gear surfaces that this creates. This is called a 'parallel depth' bevel, apparently a bit of a compromise tooth form, to get a true bevel tooth form requires specialised equipment where the blank moves during the cut. Maybe I'll try that next time

[f350ca]

Strangely enough Pete that makes sense, but never in my wildest dreams would have thought of it.
Thanks