CNC Lathe conversionAfter a number of years and several conversions of milling machines I decided it was time to do a lathe conversion. My first plan was to use one of the common 7x12 lathes sold by all the tool importers. I purchased one form Cummins Tool because they were the cheapest.

I started by disassembling, cleaning and deburring the machine. It was fundamentally an OK machine, but somehow I wasn't really into the project, and I diverted at the slightest distraction. It was around the shop for nearly a year with little progress. Finally I reassembled it and sold it to a model boat friend.

At the Cabin fever Expo that year I found an Emco PC5 lathe for sale by one of the vendors for a good price, so I bought it.

The PC 5 turned out to have a bad driver board, so I completely gutted it of electronics and installed all new parts, using Gecko drives, and new stepper motors. Here is a photo of it after the conversion.

This was a nice little machine, but I found it really to small for my needs, so when someone was looking for one on a Yahoo group I offered and sold mine.

I looked at lots of lathes to pick a new one for conversion. After lots of looking I decided on the 8x12 Harbor Freight lathe. From my reading this is exactly the same lathe sold by Lathemaster as an 8x14, for considerably more money.

One of the 9x20 lathes might have been an even better choice, but they have quick change gearboxes and cost nearly double. Since the gearbox would be removed for CNC use it seemed a waste.

This has turned out to be an excellent machine for a mid-size CNC conversion. It is considerably bigger than the 8x12, much more than the 1 inch in the name would suggest. It weighs more than double the 7x machines, and most important has a large, wide and flat cross slide. My machine did not come with the chip shield shown in the photo.

As with all imported machines the first step is to dismantle it, cleaning off all the 'cosmo-goo' its covered with, scraping out the crud from inside and under the castings, etc. As I did this I became quite impressed with the fit and finish of this machine, better than any import I have worked on.

I removed a lot of parts that I would not be using, all the gear train for the lead srew, all the gears and half nut mechanism under the saddle, most of the sheet metal covers, etc.

Here is a photo of my machine after it was finished and in operation. 

On the next page Ill go through the parts step by step.

CNC lathe conversion, page 2Z axis leadscrewThe machine has 2 nice cast iron blocks mounted with bolts and dowel pins to hold the lead screw, so these were obvious choice to use for the new screw. I decided to use the inexpensive rolled ballscrew from McMaster Carr. Ive used this on other machines and find its quite good for the price. It has a spec of .003" per foot, which seems well within my ability to measure.

At the CNC workshop in 2010 Keiling   showed some new imported leadscrew that seemed much higher quality and for not much more money. If I were doing it over Id check that carefully, although I am still pleased with the McMaster Carr product.

One end of the lead screw will require a lot of machining to create a bearing shoulder and a thread to lock the bearing in position. Ive tried to machine this lead screw and it is VERY tough to do well, so my approach is to weld on a cylinder of a more machinable steel. After its welded I chuck the screw into a good collet and turn the extra cylinder as needed.

The right end of the leadscrew needs to be turned down below the root diameter of the screw so that the nut can be threaded on. This is hard, but its a simple turned diameter to a shoulder so its not bad. To make this end fit into the end blok a bronze flange bushing is made to fit the original screw and the turned end of the ball screw.

To mount the stepper motor I made a simple aluminum plate bored to fit the OD of the original bearing block. This is the surface on which part of the original gear train quadrant rides. I mounted the motor on simple stand-off legs and used a no backlash flex coupling between the stepper and the leadscrew.

The photo shows the allen cap screws used to mount the plate to the bearing block, and the ball bearing. Normally this bearing would be pressed fully into a mount, but to keep this short I only pressed it in a shallow counterbore. The double nuts to apply end pressure to the bearings is also visable.

The original carriage is basically a cast iron box. I removed it and all its internal gears and half nuts. I then made two simple aluminum plates to bolt under the carriage slide. The end one is bored to fit the ball nut, which is placed inside the carriage box.

I left the rack gear on the lathe, although it is not needed, The photo shows the corner of the end plate had to be notched to miss the rack. It might have been cleaner to just remove the rack.

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CNC lathe conversion, page 3

X axis, cross slideAdding a ball screw to a cross slide is always hard, there is simply not enough room under it for any reasonable size screw. I have seen a couple conversions that added a ball screw alongside the slide, on the tailstock side. I considered lots of options and finally decided I'd simply use the original leadscrew, at least to start with.

Since this is a new machine the screw is not worn so backlash should be minimal. I also think on a lathe we rarely need to make cuts that involve direction change- we usually cut on the infeed. If backlash becomes a problem I think my first solution will be to use an epoxy nut, like I did for my Sherline Mill

The next issue is how to arrange the drive. I would prefer to have a direct drive, with the stepper right on the end of the screw, but in this case that would have left the motor way out in front of the machine and in the way, so I decided to go with a belt drive. The photo shows the mount for the motor and the belt. The upper pulley was machined exactly like the original handwheel so it fit directly on the screw.

Spindle SensorTo do any threading requires a spindle sensor to give Mach an index position. Mach only needs one pulse per rev of the spindle. Usually such a pickup is added on the spindle, outside the headstock. In this machine the headstock is a big empty cast iron box, so I decided to mount the pickup inside it. This will keep it completely out of the way, and with no way for chips or anything else to hang up.

Instead of a wheel with a hole I made a simple piece of aluminum angle to act as a shutter. I tapped a small hole into the spindle and screwed the angle on. Obviously

this kind of tab would never work outside the headstock- it would grab every chip and wire on the machine.

For a sensor I used an electronic limit switch from Makerbot. When I ordered them they were under $3 each. Similar sensors are sold by PMDX and CNC4PC

I mounted the sensor on a piece of aluminum angle, and used some plastic standoffs to adjust the height of the board to get the shutter positioned correctly to the sensor.

Drive motorThis lathe comes with a single speed AC motor. It has an idler shaft and belts to get 6 speeds. I wanted variable speed and computer control of the speed, so I switched the motor to a 3 phase 1/2hp unit I found at a surplus store. To that I added a VFD from DealersElectric. I've ordered several VFDs from them and always had great service and good prices. this unit is for 120 volt input and 240 volt, 3 phase output. It cost less that $100.

I had to make a new mounting plate for the motor which I made from some 6" channel iron I had in my shop. The motor can swing for changing belts.

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CNC lathe conversion, page 4Tool mountingFor my use I decided to simply use an Aloris type tool holder and manually switch tools. I may someday try to build a tool turret, and Im also thinking about trying some gang tooling. But for now a simple manual switch is all thats needed.

To mount the tool post I made a cast iron block to replace the height taken by the compound slide on the original machine.

This photo shows the underside of the block with the pocket cut for a hex bolt.

 And here is the block mounted with the bolt inplace to receive the Aloris post.

Software and controlMy control box is a simple box hacked from a PC cabinet. I used Keling drives for this machine with a breakout board from CNC4PC. I also used a Smoothstepper. I wont say much about the control box, its pretty straight forward.

I use Mach3   for control and I am currently working on a new screen set that combines full manual control with a set of Turn wizards. My wizards approach a job in the same way a manual machinist would. I will be doing a web page on them when they are finished.

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