How to Build Part 3 - Carriage






carriage_described.jpg


While it might seem more logical to the headstock and spindle at this time, we will first build the carriage and cross slide and temporally add a auto wheel hub to the head stock casting. This can serve as a "temporary" lathe that can be used to machine the cast aluminum parts needed for the spindle assembly.


The importance of aligning the carriage with the ways is all important. If the carriage does not fit perfectly it will flex and the cutting tool will "chatter" that most dreaded of all machine tool diseases.












Our original carriage design used a frame like this (shown below). The design seemed like a good idea at the time because it used only standard shapes of steel that should have been available as scrap almost anywhere. It was adjustable and only required 2 short welds. The flat surfaces contacting the round ways would wear more quickly but the ways could easily be turned to unworn areas. In a perfect world this might have worked but in a Developing World condition, problems became apparent.
  • Most welds warp,
  • Most shops do not have a truly flat work surface.
  • Most construction grade steel is not flat.
  • Most machine builders will not have accurate measuring tools.
  • Very few workers will have the skills needed to do a highly accurate alignment job in these conditions and without a perfectly aligned carriage, the machine makes a good boat anchor.


frame_top.jpg


A Yeomans like solution

Lucien Yeomans' lathe carriages were much more complex than ours because they were made to produce cannon shell at the highest speed.

Our answer to all these problems was build the frame in two pieces, clamp the pieces to the ways and then pour concrete to connect the sides. They should come out "pre-aligned". Any distortion from concrete shrinkage can be eliminated by placing shims behind the 25mm wide wearstrips at the end of the "shoes" (have to call them something).

The shoes are made from a split piece of pipe that will probably have an ID about 12mm larger than the way OD. Wearstrips (split bushings) make up the difference in pipe sizes. In ideal conditions the pipe should be split on a milling machine but in the real world the pipe will be cut with a torch and the edges ground smooth (hopefully, since they will really show). Distortion caused by welding the 12 to 18mm re-bar or pipe to the sides will probably not matter because that part of the shoe should be well separated from the way. Any distortion caused by welding on the tabs for the way clamps will show up when the wear strips are inserted and can be corrected with a file.


round_shoes_8.29.jpg

form plans go here


A form is next made and concrete poured



concrete.jpg
Carriage concrete casting shown without the shoes


bottom_a.jpg
Shown as the carriage would be after the forms are removed



















Cylinder head bolts welded to 2 steel cross bars and embedded in the carriage concrete can be used to attach the cross slide. Don't use common all-thread for this because it is usually made of very poor quality steel.







Shown without the shoes




The carriage is of course cast in place. Before the top sets up the greased base of the cross slide should be temporarily put in place over the bolts and leveled. It will be later grouted in with non-shrinking grout.







carriage_clamps.jpgbottom_b_with_clamp.jpg

Carriage clamps are bolted to the tabs on the shoes. In an ideal world these would not be needed because this is a weighted type carriage. This is an almost 200 year old idea that was also used by Yeomans. The Yeomans design was easy to scale down 95% but scaling it down 97% may have reduced the weight of the carriage so much that the the carriage will try to lift up in some circumstances. This will probably not be a problem because the steel cross slide is heavy and cutting forces usually try to push the carriage down and back. A less than expert operator, cutting with a dull and miss aligned cutting tool can cause immense forces in dangerous directions and these clamps could be needed.


carriage_base.jpg
The carriage base should be moved as far forward as possible so that it will be easier to work close to the chuck





cross_slide_rear_a.jpg
The slide anchor bolts should be located in the thickest part of the concrete and closer together than shown.























epoxied_blocks.jpg
Since the contact area of the slide base is small, the base should be additionally supported by 2 100mm long steel blocks epoxied in place inthe areas shown by the squares


Support is also needed in these areas because of the force exerted by the thread follower.


OK, time to make chips!


We make a quick lathe and a milling machine

The remaining part of the carriage (except for the cross slide) is the the drive device.

Only 4 simple parts are used here due to the fact that the lead screw does not turn. A nut with a handwheel on it just screws up and down the lead screw, moving the carriage.







handwheel_top.jpg
The clamps, the 2 jaws that engage the handwheel nut should be the same width as the slot in the all-thread coupling nut.
handwheel_rear.jpg

The clamp mechanism is quite simple. Unlike most lathes that have complex "aprons" with many parts, this one just has 5 simple parts that can be built using just a drill, hacksaw and file. The handwheel can be replaced by a bicycle sprocket that later can be linked to another sprocket in an easier to reach location. Or, to get the lathe up and running in a hurry (so it can make it's own parts), just make the mounting plate, add a nut that can be turned by a wrench to move the carriage forward and heavy springs to pull it back. The clamp parts, grooved nut

and handwheel adapter could be easily made at this stage.

  • To make construction easier this first version of the lathe does not include a compound slide but room has been left to add one later.


topslide.jpg
topslide.jpg

The drawing got a little distorted in the 3D to 2d conversion but it shows a great 80 year idea by J.V.Romigexternal image pdf.png machine_tool_slides.pdf. Usually adjusters to take up for wear are difficult to make but this one takes just a hacksaw cut and a tapped hole for the adjusting screw. The brass wear strip is optional.
To better keep cuttings away from cross feed lead screw, the sides of the cross slide should be reversed and some kind of a chip deflector added. The leadscrew side clamp piece bolt holes could be slightly oblong so that the clamp can be adjusted.
It is usual practice to build a machine slide so that it is one and a half to two times as long as it is wide. This is a time tested way to keep the slide form "cocking".

Note: Using a cross slide like the one shown above will require a wider piece of steel than the one below it and the base will be narrower still. Adding a compound cross slide will probably require a thicker topslide because of the pivot as shown below.

Compound slides, mostly used to cut tapers, are mounted on top of cross slides. One reason for adding the compound slide at a later date is that the lathe can be used to bore the large mounting hole in the top of the cross slide.










Flattening the cross slide parts
When grinding an optical (or other kind of flat), three disks are used, let's call them "A", "B" and "C". Put "A" on "B" with some fine grinding compound.Grind until a frosted finish is seen on both surfaces., now do the same with "B" on "C", now repeat with "C" on "A" until the surfaces have 100% contact, repeat until it takes little (or no) work to get 100% contacting all three combinations. The surfaces will be very flat. It works on steel as well as on glass.How does it work? A on B results in a spherical surface, B on C results in a less spherical (closer to flat) surface, C on A results in a closer to flat surface after grinding. Each pass results in flatter spheres (if A is concave. B is convex and C is concave.) When A and C are ground to each other, they the high points first, now either A or C is concave and the other is convex. Grinding both against B results in the flats being averaged. Eventually they are flat enough. Gravestones and monuments are often VERY flat. They are good layout tables!


The plate edges are also important since the clamps are screwed to them. Edges of hot rolled steel plate are not flat and this has to be corrected since clamp parts are screwed to them. Carefully file the edges flat while constantly checking with a square. Keep flipping plates over and end for end while checking them side by side until you get them filed to identical widths, parallel sides and with flat edges.


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