Author Topic: CNC Workshop CNC Lathe Build  (Read 15328 times)

Offline kvom

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CNC Workshop CNC Lathe Build
« on: June 13, 2016, 10:25:11 PM »
Over 3 days last week I attended a workshop at which 14 of us converted a Little Machine Shop lathe to operate under CNC control.  While everyone attained axis motions under control of Mach4, there remains a bit of fettling and adjustment to get it working to its utmost.

The design of the mechanical parts was done jointly by Ron Ginger and Joe Katona, while the electronics were designed and produced by CNC4PC.  I believe that drawings and BOM for the mechanical components will be made public at some point, and CNC4PC will sell the electronics, so it should be possible for home builders to obtain the same machine.  Cost for the lathe and everything needed to build it was $2770, which included a Mach4 Hobby license.

We started with a LMS 5100 Hi Torque 7x16 mini lathe:



After uncrating, the first task is to remove everything that wouldn't be needed.  This included the saddle, both leadscrews, and all the change gears and associated mechanism, and the backsplash.  We also removed the compound temporarily to expose and remove the cross slide.

The first operation was then to ream out the hole for the new X-axis ballscrew to 13 mm, and then to drill and tap two holes in the rear of the cross slide to mount a bracket for the ballscrew nut. Here's a photo of the bracket with the nut attached.  A drill jig was used to ensure accurate placement of the holes.



Next the Z ballscrew is attached to a mounting plate, which in turn is secured using the same mounting screw as was used for the saddle.  The right end of the ballscrew is supported by a pillow block containing a pair of flanged bearings.  This pillow block uses the same holes in the frame as the original lead screw.  The left end uses the original pillow block.





The right end of the Z ballscrew is secured with a nyloc nut and a coupling to the shaft of the Z stepper.  The motor itself is attached to a bracket that itself is screwed to the frame.  The bracket itself was used as a drill jig for drilling and tapping the holes.



The X-axis motor mount consists of the Z ballscrew plate, a second parallel plate which also uses a saddle mount screw, plus a pillow block that spans these two plates that carries the near end of the ballscrew.  The motor itself is mounted to a plate that bridge the two support plates.



The ends of the motor shaft and ballscrew have toothed pulleys joined by a belt as seen above.  Tension on the belt is set by the motor mount having three screws, with two in slots, allowing the plate to be rotated slightly.

On the left side of the spindle housing is located an encoder, driven by a belt and two pulleys.  The encoder is mounted to a bracket, which in turn is held to the spindle housed by a screw.  The bracket itself is used as a drill jig for this hole.  A slot in the bracket allows adjusting tension on the belt.  The spindle pulley is placed between the two locknuts.



The control box has estop and power switches on the front, and all connectors on the rear.





With the lathe power cord plugged into the control, starting and stopping the spindle is under control of Mach4.  When I left the workshop, I was missing a cable that will be inserted into the lathe's PC board and thence to the control's VFD socket.  This will allow speed control of the spindle by M4;  until I receive the cable the spindle RPM will need to be set using the stock rotary switch.  The speed control option, when present, will allow M4 to command constant surface speed when desired.  With spindle operation delegated to the control, the oem start/stop switches were removed, and the hole covered with a plate.



Note that in the photos above one can see two homing switched, one for each axis.  These are the orange proximity switches.   For most users these are unlikely to be needed.  However, using repeatable tool mounts such as a gang tool plate they may eventually have some use.

Here's the lathe as it sits:



The major adjustment that it needs now is making the Z ballscrew parallel to the ways.  Presently the left pillow block is not attached to the frame, and securing it with screws as is causes some binding of the ballscrew and limits rapids.  With the pillow block floating free movement of the end is obvious with carriage movement.  Measurement and shimming is needed.


Offline zeeprogrammer

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Re: CNC Workshop CNC Lathe Build
« Reply #1 on: June 14, 2016, 12:25:03 AM »
Nice info kvom. Thanks for the post.

I've been looking into a CNC lathe, so this is helpful.

I hadn't thought much about an encoder on spindle.

Where I'm stuck is finding a decent (and free or low-cost) program to convert CAD to g-code for lathe turning.
Lot's of stuff for milling but lathes seem to be scarce.
Carl (aka Zee) Will sometimes respond to 'hey' but never 'hey you'.
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Offline kvom

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Re: CNC Workshop CNC Lathe Build
« Reply #2 on: June 14, 2016, 01:06:36 AM »
Mach4 currently has a set of "wizards" for lathe work that will generate g-code for simple shapes.  CamBam will also do turning generation. 

Offline jschoenly

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Re: CNC Workshop CNC Lathe Build
« Reply #3 on: June 14, 2016, 01:25:03 PM »
I just finished up (wait, are we ever finished?) some upgrades on an HNC Hardinge Lathe with Mach4.  For G-Code, I'm using AutoCAD Fusion 360.  It's licensed for a year at a time for free to enthusiasts and has some pretty nice CAM for Lathe and Mill.  I've posted some code so far and done some edits to the post processor.  Also some hand coding to allow a bar puller to advance the stock I'm using. 

Too much fun!  The lathe looks great and I'm sure you'll be churning out some cool parts soon!  Great info!
Jared
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--- Model Engineering and Home Shop Hobbies in all Forms! ---

Offline RonGinger

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Re: CNC Workshop CNC Lathe Build
« Reply #4 on: June 14, 2016, 02:03:54 PM »
Kvom, thanks for the description, it is a very nice review of the lathe. It was a very interesting project for me, and very satisfying to see 14 guys go away with working machines.

I was a bit surprised at the variability of the machines mechanical fit- some had saddles that slid freely, some had serious binding. I guess its just the way of these low cost machines- they are a nice kit of pre-machined castings for the user to finish.

Mach4 hobby now has a set of 'canned cycles' built in. The release for the class was the first release, these will soon be in the mach installer on the net for all hobby users. A more complete set of wizards, in the style of the NfsMill wizards will be done 'soon'.

We did make up a few extra mechanical kits that will be  offered as soon as I can produce some instructions for assembly. If anyone wants a set let me know. We are considering a couple options for making these more available.

Offline kvom

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Re: CNC Workshop CNC Lathe Build
« Reply #5 on: June 14, 2016, 09:07:03 PM »
First step in aligning Z axis ballscrew is to remove the lathe from the chip pan and mount on the Bridgeport table.  Then a scan of the ways showed that the headstock was .01" low.  After placing a sheet of .01" shim stock under the headstock scanning the ways showed a max variation of .001" over the entire length.  Thus the mill table will provide a height datum for aligning the ballscrew vertically.


Offline kvom

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Re: CNC Workshop CNC Lathe Build
« Reply #6 on: June 15, 2016, 09:30:31 PM »
Next I wanted to measure the vertical variance of the ballscrew over its travel.  A parallel on the screw was held flat by my height gauge, and measured at both the carriage and the right pillow block.  The difference was .002";  at this point I think that can be ignored.

Offline kvom

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Re: CNC Workshop CNC Lathe Build
« Reply #7 on: June 15, 2016, 09:33:32 PM »
To measure the horizontal variation relative to the ways, I employed a 123 block against the screw and used a caliper to measure the space between the block and the side of the way at both ends.  Here the difference was .008".
« Last Edit: June 15, 2016, 09:48:41 PM by kvom »

Offline kvom

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Re: CNC Workshop CNC Lathe Build
« Reply #8 on: June 15, 2016, 09:40:15 PM »
With the power block attachment screws removed, it's possible to slide the left (OEM) pillow block on and off the end of the ballscrew.  It is visible that the tapped holes are several mm too low, and that there is a significant gap between the frame casting and the bottom of the pillow block, probably 20 thou or so.  Therefore the next move is to enlarge the holes in the pillow block to allow it to be screwed to the frame, and for shim stock to be inserted between the block and the frame.

At that point I can decide if I need to shim the right pillow block to bring the screw more in parallel to the ways.

Offline zeeprogrammer

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Re: CNC Workshop CNC Lathe Build
« Reply #9 on: June 16, 2016, 01:32:35 AM »
Mach4 currently has a set of "wizards" for lathe work that will generate g-code for simple shapes.  CamBam will also do turning generation.

Thanks. Good to know.

Now I'm wondering, when it comes to lathe turning, what constitutes 'simple' shapes versus 'complex'.  :thinking:
Carl (aka Zee) Will sometimes respond to 'hey' but never 'hey you'.
"To work. To work."
Zee-Another Thread Trasher.

Offline kvom

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Re: CNC Workshop CNC Lathe Build
« Reply #10 on: June 16, 2016, 11:04:03 PM »
Before taking the lathe off the mill, I measured the runout of both the chuck body and a ground round clamped in the chuck.  About .001 or less TIR on the outer surface of the chuck, and .002" on the rod.  This measurement was taken when manually rotating the spindle.

Offline kvom

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Re: CNC Workshop CNC Lathe Build
« Reply #11 on: June 16, 2016, 11:12:57 PM »
I removed the left Z-axis pillow block and milled out the mounting holes as slots.  Then added some shims until it "seemed" in the right position.  Also added a .005" shim to the right pillow block.  Then after some tweaking of torque on the mounting screws, I managed to get 65 ipm rapids across the length of the bed without losing steps.  I might get more with more fettling but for now I'm going to leave it.  I did reduce the acceleration from 8 to 6 since it's less important for most lathe ops compared to mill.

The mounted the DTI to measure backlash (4" rapid away followed by 50 ipm return).  Backlash was consistently less than .001".  I intend to reverse the setup and measure closer to the headstock next time out, and also measure X backlash.

Offline kvom

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Re: CNC Workshop CNC Lathe Build
« Reply #12 on: June 23, 2016, 08:53:26 PM »
In an effort to get rapids faster than 55 ipm I've tried various tweaks without success, including milling the end of the frame square to the ways intending to make the motor shaft parallel to them.



Coming to the conclusion that 55 ipm rapids will have to do, and having received a set of index lathe tools from Shars, I wanted to determine if the 4-way tool holder would be useable, or whether I'd need to fork out for a QCTP setup.  First I chucked a length of ground rod, measured it with the mic, and then use the height gauge to find the distance from the ways to the top.  Subtracting the radius of the rod gave the height to the center of the spindle.



Then I measured the height from the ways to the tip of the turning tool, and determined that it was .049" higher than the center.



Milled the bottom of the tool holder in several increments.



Finally made my first cut in anger - facing some brass.   ;)



I had placed the various Shars tools on the surface plate, and the cutting edges are all very close to the same height, so hopefully this holder can work for all of them.  Time for more experimental cuts.

Online Dave Otto

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Re: CNC Workshop CNC Lathe Build
« Reply #13 on: June 24, 2016, 01:53:06 AM »
Hi Kirk

Looks like good progress; have you considered gang tooling? I run a Sharp CNC Tool room lathe at work and we have it set up with gang tooling; it is relatively easy to configure and teach all the tools for the job. With a gang set up you can also put your drills reamers etc. in ER holders. If you are running repetitive parts you don't want to be having to change tools for every operation. Once your tools are set up you also don't' have to worry about errors created by your quick change tool post. Have you given any thought how you are going to drill holes?

If I were to have a small CNC lathe at home this is probably the way I would go.

Dave

Offline Flyboy Jim

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Re: CNC Workshop CNC Lathe Build
« Reply #14 on: June 24, 2016, 02:56:07 AM »
This is an interesting project.

If you get the time, it would be interesting to hear some background on just how you intend to use this setup.

Jim
Sherline 4400 Lathe
Sherline 5400 Mill
"You can do small things on big machines, but you can do small things on small machines".

 

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