JET GHB-1340A CNC Conversion (CAD Model link)

[dbsharp]

Let me start by saying I don't expect this to be finished any time soon. For now I am modding the lathe, and making plans for CNC Conversion. I have two other cncs to get up and running before I do any conversion here. Anyways here it is. Got it for 2K, it was a mess, rust and thick grime all over. It came from a metal fab and cut shop so it was covered in grinding dust and old way oil. It came with the stand, DRO, and taper attachment.





I cleaned it, A LOT. Brass wool and wd-40 work wonders, but what a mess! Now that it was clean I started making some cuts. I immediately found problems.

1. The adjustable threading gear lash arm was loose and the gears were making a racket, to make it worse the chinese bolt was manufactured improperly and the allen wrench would not fit. Had to grind one down to fix it.

2. The 3 jaw chuck and faceplate taper were NOT made to the right size, they would not seat against the back of the D1-4 spindle. This made them unstable in cutting and the runout was a joke. There was about a .006" gap. So I made a makeshift tool post grinder out of a pneumatic one, and ground the taper then sanded until they fit snug and would release from the taper with a light tap with a rubber mallet. This was challenging to get the taper right, tenths matter here. One they seated right, I marked the jaws, chuck, and the "master" pinion. Then I ground the jaws inside and out. I'm getting around a repeatable .001 runout with my three jaw with the few size parts I tested, NOT BAD!

3. These guys really screwed up the taper in the barrel for the tailstock. I don't know how it got like it is. There is galling and deep scratches. It looks like they just let things spin in there and jammed chips between the tapers. Suggestions?

This lathe was still sloppy I had about .1" lash in the x axis assembly and about .04 in the z axis. This just wont do. I took the carriage and slide apart cleaned everything. I don't need the taper attachment and it just got in the way, so I made a plate to mount the x axis screw and thrust bearing block to. You can see this in the model.
-After adjusting everything I now have only a few thousandths of lash in the x screw and .0005 in the x GIB. Night and day difference here. I don't think I can do much anything about the Z axis though. It's just all the lash in the gear train from the rack to the handle adding up.
-I adjusted the z axis gib plate, this was a real pain to do a good job. Good job JET.

I removed rust and painted a number of parts, I used enamel spray paint. It takes about a month to fully cure, this can be speed up with an oven. The spray paint enamel did not get hard enough IMO. I will try the Appliance Epoxy spray paint next. If this doesn't work I'm done with cheap paint on machine tools.

MODS:

I am 6'4". I hate bending over to turn handles. I got a scrap generator frame and cut out some .25" formed c channel. A little welding and paint and now this is about the right height!






I don't like how chips can just sit on the ways and contaminate them! WTF? This is an easy fix JET. So I made a couple of chip guards and a felt way wiper. I may make a wiper for the rear, but it's going to be more difficult and I think the guard will do a good job.




My next mod is going to be a chuck guard. I'm thinking some angle iron, and lexan or sheet metal. I would appreciate design suggestions for this or if someone has a chuck guard on a similar machine, measurements and pictures would be real nice.


As far as CNC goes, I have modeled the carriage and x axis and I am working on the details.
I am going to retain the manual turning if possible, I may or may not ditch the threading lead screw and power feed, but I want to keep the handles. The x axis servo will use a x axis acme screw in the stock location. Once I have made the new lead screw and nut, I will sell the full taper attachment kit.

For the z axis, I am thinking of 2 options:
1. Make a new handle with a belt drive pulley and use the rack and pinion already there. It would have .04" slop
2. Ditch the threading lead screw and use that spot for a ball screw. I will make something like a v band clamp so it's easy to disconnect the ball nut from the carriage. Back driving the rack and pinion takes a good bit of force and there is a lot of "inertia". If I can't make something to disengage the manual rack, it will take a strong servo and a large ball screw. Still lots better than .04" lash, and I get to keep manual turning.


That's it for now. Expect my router and boss mill to get finished before this CNC gets started.

As promised here are the CAD files for the stock JET parts and my x axis screw plate.
https://grabcad.com/library/jet-ghb-...-cross-slide-1

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[dbsharp]

Took a couple more pictures this morning.





Also made a coolant system. Pond pump with a oil filter to catch the dirt. Cut a hole in the stand and used a sink drain. I got some foam from mcmaster to act as a pre filter.

[dbsharp]

This build is finally making headway. Parts are rolling in! I have decided to make this a manumatic style cnc where i can operate manually with handwheels if needed. all motors will be contained in one box.

[dbsharp]

For anyone who might be following this build, the design has gotten pretty far, and I expect to build starting this year. I just finished putting a VFD and baldor motor on it. It now has quite a good speed range electronically.

[asuratman]

What size of acme screw on X and Z ? Do you make them ?

[dbsharp]

x will be a 10mm ball screw, it is hard to find one that fits! If it is not stiff enough I will have to go back to ACME but would rather not. z is 25mm ballscrew.

[maxspongebob]

You might think about using MGP's for your manual input instead of disconnecting the ballscrew. If you disconnect the screw, you will not have a digital readout for Z and X. This is what I did on my lathe. I would have liked to have placed the MPG wheels where the manual wheels were, but I was running out of time.

Also, a 10mm screw for X is very small. I was able to fit a 12mm in my saddle which is very similar to yours. If you like I can share my CADD models so you can see my design.

https://www.cnczone.com/forums/verti...-software.html

[maxspongebob]

I was just looking at photos of your lathe. If you are having trouble getting the X screw inside the saddle, you could instead put it on the outside just to the right of the X slide. I would have liked to do this on mine, however my gib adjustments are in the way. You can since your gib is the tapered variety.

[dbsharp]

thanks for the suggestions, I have fit the 10mm ball screw in the saddle fine. I'm not really crazy about having the screw behind the saddle. I'm not disconnecting the ballscrew, and I have glass slides for readout. The feedaback from a manual connection to the screw is what I was going for manual mode. No MPG will give you that, for now, maybe one day someone will make a force feed back mpg.

[ninefinger]

An issue you will run into trying to use the CNC machine in a truly manual way is that the "force feed back" you are looking for will actually back drive the ball screw if you don't keep it held all the time.
I.e. if you are feeding the Z axis, the X axis will back out on you unless you are constantly holding it, and vice versa. Or you need to add locks to each axis so they don't move unintentionally. The benefit of the low friction in a ballscrew is only a benefit for a motorized drive, you actually need friction / resistance to rotation for manual machining.

[dbsharp]

An issue you will run into trying to use the CNC machine in a truly manual way is that the "force feed back" you are looking for will actually back drive the ball screw if you don't keep it held all the time.
I.e. if you are feeding the Z axis, the X axis will back out on you unless you are constantly holding it, and vice versa. Or you need to add locks to each axis so they don't move unintentionally. The benefit of the low friction in a ballscrew is only a benefit for a motorized drive, you actually need friction / resistance to rotation for manual machining.

My plan is use the enable feature of the motors drivers to turn off and on the servo to hold its position.