Mini Mite clone - slow build thread

I've been considering building a compact little muller for kicks. A friend dropped off the 450mm dia x 12mm thick steel plate. Already have a collection of cast wheels pictured. The big one is a little over the top but the smaller ones definitely show potential. Alternatively I have a slug of 5" steel I can bore and fit for bearings. I also had an old leaf spring to make the wipers with.

Not pictured is some 3mm steel sheet I plan to use for the tub walls. As well as motors and reduction worm gear boxes aplenty.

That is where the project is up to while I'm working on some others.

 

Let's see some progress now!

I've come to the conclusion muller geometry is not rocket science, just stir, baby!

 

And while you are stirring, please include some crushing from a wheel rolling over the sand to break up lumps that make up a large part of sand once it has been used.

Denis

 

I only had half a Saturday this week and the darn CNC router is at the top of the list at the moment.

Don't know what the geometry is to stir babies but sand looks straight forward.

Here is where I am a little torn on the design decision. I could go with a straight vertical roller. But the Mini Mite has the wheel in an incline plane, rolling and 'smearing' at the same time. A larger heavier vertical wheel will have the effect of smearing too but there is a point where you are just compacting it more to some extent. Hence the scraper/wiper. An inclined wheel gets around both issues without the need for the added weight.

Either way... I can cut weld and machine it to suit either design if one proves less desirable.

 

A vertical wheel will do the job. Scrapers on the wall and scraping the floor will turn the sand over like a plow. Together the scraper/plows and the wheel will do the job. A scraper on the wheel is optional. I have not felt a need for one though sometimes some sand adheres loosely—-I just scrape it off with a trowel.

Denis

Edit: I neglected to put the "not"in the last sentence previously. Ooops.

 

Decided to tackle the skirt on the drum/tub for my sand muller. Also got tired of this 3mm steel sheet pieces sitting in the way. First I made a bending bar. Basically a length of 1" SCH40 pipe just longer than the strip of sheet steel is wide. Weld some scrap angle section to the ends to give it something to clamp to. Then double clamp both sides to the workbench with a 5mm gap between the edge of the workbench and the pipe. The rest is all brutish inelegant application of leverage and excess body weight. Using the workbench as a backstop and the pipe as a fulcrum. A larger pipe is not a bad idea to avoid sharp kinks when bending though it is more likely to slip when the diameter becomes smaller.


What I like to do is bend a little every 5mm, little at a time, adjusting the curvature evenly. Too small or large of a radius is easier to fix than an uneven spot that is either bent to sharp or not at all. This is easier said than done but if you stick to the rules it actually come out looking so. Then I tweak the ends till the join just right. If you run your hand over the joint you can follow the curve. If there's a bump or a dip I go back and adjust it before clamping and welding a few spots. Then I continue to run it through and even it out where I can before removing the clamps.


Those tack welds broke while I was tweaking the overall roundness by leaning against it on the ground. But it looks good enough to knock the rust off and weld it on properly. A decent sheet roll with 3mm capacity would cost more in time to find than it did to roll this by hand. Though I do feel a little tender in places where I used to have muscles.


Thus finishing another installment in the snail pace progress of this project!

 

Good idea forming that steel. A ring is always a major pain in the ass in the home garage. I have a tree in the front yard that makes a mean 16" diameter.

 

One thing I recently realized about my muller that probably is generally true: the diameter of the wheel determines the maximum depth the sand allowed prior to the wheel stopping its rolling action and essentially becoming a plow. Maybe it is too obvious to everyone else besides me to even mention, but once the sand depth in the muller is such that the wheel is encountering sand over it entire leading surface there is no reason for it to turn. It will only turn if there is more sand friction on the lower half the wheel. Sand fluffs up as it gains green strength and its depth in the drum increases so that in my case 60 pounds of dry sand is no problem at all for my muller as it is just a few inches deep. But, when I add some water to it and it starts to gain green strength it’s depth more or less doubles and is just shallow enough to allow continued rolling.

In my case by pure dumb luck my wheel diameter happens to be just right as more sand would start to tumble out of the drum when I reach stall depth. I will have to measure next time I’m out at the foundry , but I think my wheel diameter is about 2/3 the drum depth. Also the wheel “suspension” should have some freedom in it to allow the wheel to move up and down a couple inches so it can roll over clumps of sand and thickening sand rather than stalling tub rotation.

Denis

 

Good observation about wheel diameter, Denis. As large as will fit in the muller is probably not too large.

Guster: Great job rolling a ring. In industry lots of large heavy vessels are bumped like you did instead of being rolled. Almost done in a big brake. Then after long seam welding we would send them back for rerolling. They can't bump or roll right to the ends so you weld it and put it back into whatever it was made on to make the curve right past the weld. You did just about the same thing.

 

Thanks, I use this a bit so I thought I'd share it. I make wood forms/bucks for other things all the time. But a bit overkill for this job. 3mm is also overkill but just what I had already.

Yeah I think rolling resistance is a ratio to sine of the radius if I recall. Where the ratio is determined as a factor of compressability of the medium. Hence why the larger the radius, the lower the rolling resistance in relation to the depth of tread or object being cleared. Always marveled that it was a similar equation to calculating the RMS value of an AC power supply and that it takes very little to stop a skateboard wheel.

I don't want too large a muller either. I'm eye'ing up a 5" slug of roundbar for the roller at the moment because it is a little wider and thus has a bit more weight for the diameter. Unless I feel like machining a bit off the large cast iron ones I have. Else I have plenty of large bearings to run them

Thanks Andy. Yeah, I have seen that done which is how I figured I'd do it at home without expensive machines that take up needed space. I often use larger pipe if I need to match a radius and just clamp it across the length using some angle bar. This was a little larger again.

Before I weld on the skirt I really to make the port/gate in the bottom for 'draining' the sand out.

 

I'd be interested in seeing the formula derivation for a wheel traveling through a compressible inelastic medium---sand. Though physics was my major for undergrad, I do not recall addressing that sort of problem. I did find this article which talks a bit about the problem but offers no calculated solution.


https://www.plantengineering.com/ar...esistance-a-safer-move-for-material-handling/

Not based on any formula, just intuition, I'm thinking 5" may be a bit on the small side for the tub you are making. I am guessing it is 18" diameter and maybe 14" deep. So, I would suggest a wheel 10" diameter or so and maybe 4" wide. If the wheel you have or aquire in a larger size is on the light side it could be spring loaded or weighted in a variety of ways to make up for its lack of mass---not that you had not already thought of that. 5 inches will limit you to about 5" depth of sand and so reduce the capacity of your muller by half. Myself, I prefer a muller of the greatest capacity I can come up with. The larger size equals less fussing loading and unloading. Just my opinion...

I really liked the simple way you rolled you sheet metal. Looks like it is very smoothly formed!

Denis

 

I'd have to go look it up myself. The Unis in our area used to run a recumbent pedal trike race among engineering departments and the course sometimes included an oddball obstacle like cutting across a park with deep soft grass or a hard turn on a downhill dirt section. Considered an applied practical learning component of the curriculum which is a nice way of sponsoring a bit of competitive learning. The part of the team I was in took on the front wheels, steering and brakes. Also had bad luck blowing out the spokes on BMX wheels 3x years in a row during fast turns due to high lateral loads they weren't designed for. Long story short... it became part of applied physics to do some wheel analysis. It was interesting until they started designing the optimal wheel and had no way to make or build it. I changed major to computer science a bit later since the engineering market was saturated.

Thanks Denis. There are some subtle irregularities in places but it's good enough for what it needs to do.

My dimensions are 450mm dia x 250mm high which translates to 18" dia x 10". Just double checked the 3" round bar drop is actually 6" dia and the large cast wheels are 9"dia x 3" wide which have enough meat to machine down to 8" but leaves less vertical travel room and needs to be run closer to the middle of the tub to clear the walls. More so if there is any lead to allow for pivot to give any vertical travel.

If I were casting every weekend I'd be game for a larger muller too. But its just another machine in an already crowded workshop and I'd be happy if I was casting 12x a year. This one may live under the house a fair bit.

 

An inch or 1.5” of vertical wheel travel should be enough. I’ve learned to remove my hard lumpy sand from the used flask and spread it on the concrete floor. Then I walk on it some and turn it with a shovel smacking the larger lumps. THEN I shovel it into the muller. That minute of pretreatment saves five minutes mulling time per batch and means that my wheel rarely travels and inch or so. For quite a while I simply transferred the lumpy sand to the muller without reducing the chunks. I guess I thought it was the muller’s preordained job to handle the sand by itself without help from me. It did. But at the cost of wasted time. ;-)

Denis

 

Handy tip. I've also made a sand fluffer(aerator) already that does a decent job of breaking the lumps too. Back on post 45 of my intro:
http://forums.thehomefoundry.org/index.php?threads/another-kiwi.661/page-3#post-16008
Only change I've made is to reduce the 6 prongs on the impeller down to 3 as 6 was enough to support the sand and form a plug. Knowing that I'd use smaller diameter rods for the impeller. Though it works well enough now.

 

Kelly,

One advantage of such a setup is that wheel stalls due to large sand loads might be reduced since the “wave front” of the pushed up sand would tend to fall down the tub wall across the front of the wheel. But the back of the wheel would tend to stay clear.

Also noteworthy is the relatively small size of the wheel.

Denis

 

The slickness of the bottom really makes a difference to the sliding action too. Greensand doesn't pose much of a problem in my rotating drum muller but petrobond certainly can. I'm considering grinding in some score lines on my floor to minimize the sliding.

Pete

 

Interesting! Never thought of that possibility. I guess an angle grinder would make quick work of cutting in some radially oriented grooves or maybe cross hatching on the drum bottom plate.

Denis

 

Hi Kelly, that was my original intent as well. I based initial ideas on the photos on this forum thread: http://forums.thehomefoundry.org/index.php?threads/new-to-me-mini-mite-muller.513/
It looked like a decent way to run a smaller compact muller with less torque needed for that mechanism.

Still planning to run a static drum so I can have a gate in the base to drain it. Idea of digging sand out of the muller seems counter intuitive and I don't mind a bit of steel fab work.

Also a little less torque needed. The wheel is both a plow and a roller because of the angle it runs at with the leading edge of the wheel pitched down in relation to the contact patch. Part of it is rolling and some is smearing/plowing. The pressure also appears nicely balanced between the wiper and wheel acting in almost the same plane.

 

First job was to weld the skirt onto the muller tub. But first I need to cut a slot and mill a shoulder for a knife gate in the base plate. For the Gate I planned on using some 8mm x 100mm flatbar I found in my stock.


With that now done I could weld the base in. Still have to open a recess in the skirt for the flat bar to slide in.


Showing my muller wheel options and some strange distortion by the camera lens. The large wheel is perhaps going to be more trouble than it is worth. I would prefer to put a little lead on the wheel bracket so the wheel is pulled with a few degrees toe in to help it steer the circle. Not forgetting some room for rise and fall "suspension". Perhaps if I take a little off it will just provide the clearance for all of that.


Next, I want to reinforce the rim with some flat bar with some locking pins or perhaps just some threaded bolt holes. Then make a lid with a nice large inlet and start planning the drive mechanism and some brackets.