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7x12 mini lathe in fetching blue!

7x12 mini-lathe and micromill - Guidelines on principles of use

As I have said elsewhere on this website, buying cheap imported chinese machine tools is a bit like buying a car. The manual for your car will not tell you how to drive the car - it doesn't describe the operation of the clutch (note for American readers, clutch is the third pedal in a manual transmission car, and makes driving much more fun) or the brakes - all this is assumed knowledge.

As such, you are assumed to know how to operate your lathe. It is up to you to work out how to use the lathe to complete the jobs you want to do.

If your job consists of making a nicely finished end on a bit of round stock, then, it is nor difficult. However, if you want to make some bearing blocks out of 4"x1" flat bar to take 62mm outer diameter bearings, and have the two blocks concentric, a little more cunning must be employed.

The most fun part of this hobby is thinking of inventive ways to use your limited tools to make something that should really need a bunch of fancy tools. The design of projects, or, more specifically, the design of the solutions you are going to make based on requirements is principally driven by the tools available, and what you can get away with. However hard you try, you are not going to turn a 12 inch diameter disk on the 7x12 mini-lathe, so don't allow such items to form part of your design.

At the same time, don't think "I can't do that" - at first sight that pair of bearing blocks might need a 6 inch 4 jaw chuck. I don't have one of those. But, if you think for a awhile, you'll often come up with a tooling solution that doesn't involve buying anything.

You have to work from both ends: You design is driven by the capacity of the available tools, but, the project can force you into thinking of new and inventive ways to use your existing tools. Somewhere in the middle you get a solution.

"Transfer of precision" is a critical part of planning. This is all about straight lines being relative to other straight lines. When making things in metal that need to fit and work together, you have to make things fairly precise. For example, if I take a bit of aluminium, whack it with a centre punch, and then drill a hole in it using my power drill by hand several things will be true: The line of the hole will not be perpendicular to the plate.. the hole will not be round, and, it is unlikely to be exactly where I punched the hole, because drills wander across the surface and bend.

When designing your parts and your plans, you must think about what aspects of the part must be precise - and with respect to what other parts? If the hole in the plate doesn't' need to be straight, then by all means drill it by hand. Once you have a clear idea of where you need to be careful, you must then use the precision inherent in your machinery to get the result you need.

This is a lot easier than it sounds. There are only a limited number of things that can be relied upon to be precise. This is only true after you have check and adjusted: eg tramming the mill head.

  • The lathe rotates around. The tools is moved precisely with respect to the rotational axis of the lathe. eg a bit of stock faced in the lathe is guaranteed to be truly flat.
  • The bed of the mill is flat. The mill head will being tools to bear exactly perpendicular to the bed.
  • THe bed can be moved back and forward, and side to side at right angles.
  • Your engineers' parallels are flat on both sides.
However:
  • A bit of 1/2 inch aluminium plate is *not* guaranteed to have any two sides parallel until you've faced it.
  • Simply putting the milling vice on the bed and bolting it down does *not* mean that the vice jaws are aligned with the XY action of the table.
  • Gripping a bit of stock in the vice jaws without it contacting the bottom of the vice does *not* mean the job is parallel to the mill bed
  • Putting a bit of round stock in a 3 jaw self-centring chuck does *not* mean that the stock is exactly aligned with the rotational axis of the lathe.

Let us go back to my two bearing blocks. The requirements are simple:

  • There must be two bearing blocks. The exact size is not important. Roughly 4" by 4" by 1".
  • They need to have bore which is a gentle press for existing 62mm outer diameter bearings. The bore must have shoulder inside to hold the bearing.
  • The bottom edge must be parallel to the rotational axis of the bearing.
  • The gap between the bottom of the block and the centre of the bearing bore much be EXACTLY the same in each block.

Bearing blocks

First the rough stock is cut on the 6x4 bandsaw. Two pieces are cut off some 1.25"x4inch flat bar. This gives two rough blocks of aluminium.

Next, I clamped each block in turn to the mill bed using parallels to raise it up. I then drilled holes in the block that match my faceplate on the lathe.

I don't have a real faceplate: I have a bit of 1/2 inch aluminium stock which is drilled and tapped so I can remove the lathe chuck and attach this plate. A quick facing pass and the front face of this plate is precise. This plate also has some holes drilled in it, so I can attach other stuff to it.

On the face of the large blocks, I used the correct sized transfer punch to mark holes to drill out that match the job mounting holes on my faceplate. I then drill these out on the mill using a 4.2mm drill after starting with a centre drill. I also use the mill to tap these holes M5. THe mill doesn't like running the tap all the way through, but so long as i start the tap using the mill, I can finish it by hand.

Now I can bolt my faceplate to the spindle of the lathe. I give it another facing cut and carefully clean off all the swarfe and dirt with spirit. I then bolt the big block to the faceplate.

The first lathe operation is to knock the corners off the block. Having very sharp corners flying around at high speed is bad for your health. So, I use a right hand cutting tool and the lathe power feed to reduce the "outer diameter" of the blocks by about 1/2 an inch.

Then I am free to face the end of the block. Once the outer face is smooth I can unbolt it from the faceplate, and bolt it back on again, the other way around, making sure to remove swarfe and burrs.

Then I face the other end of the block. Once this is done, I have the two large faces of the block smooth and parallel.

Repeat on the second block.

Next, bolt the two faced blocks together to make one lump of metal. Bolt this to the faceplate. Indicating on the front face of the outer block should give <0.001" error - if not, you have done something wrong.

Now I can drill and bore the bolted-together blocks as one item.

Finally, I can remove the two blocks from the lathe (keeping them bolted to each other) and mount them in the milling vise. I can use an end mill to clean off the four sides. Each side will be at right angles to the face. Each face is perpendicular to the bore. Therefore the sides will be parallel to the bore. I hope!


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