My new lathe a Warco 918

I edited the previous post as you replied so I will repeat what I put in in case you missed it. The diodes on the first diagram are the wrong way round.

Martin C

Thinking about the diodes for the circuit I drew, put positive on terminal 1, negative on terminal 2 then the diodes should be as follows:

The top switch should have the triangle pointing to the left, bar on the left.

The bottom switch should have the triangle pointing to the right, bar on the right.

If the motor runs in reverse to what you expect reverse the motor connections as it is easier than having to swap the diodes around if you change the input to the switch.

The bar on the diode symbol matches the band found on most diodes.

Martin C

Thanks yet again Martin, will let you know how I get on once the parts arrive

Ron

I thought that at first, but looking again they have a smart purpose.

If the limit switches are hit they open and stop the motor. Now the clever bit! When the current is reversed with the DPDT, the diode bypasses the open switch and allows the motor to back away from the limit. The motor can't drive past the limit but it can always reverse away after being stopped.

Ron's first circuit is wunderbar because it recovers automatically from a crash. The diodes are needed and they're shown the right way round. Martin's circuit is OK but it requires manual resets after bumps.

Dave

Dave, you are correct, the original circuit works as drawn. It does require start up current to flow through the diodes though so heavy duty diodes would be required. I wonder what would be specified. Don't think my version needs manual resets. I've drawn the switches open in the operated position when they should be closed (nc contact) which may be why you think it needs resetting. 

Martin C

Edited By Martin Connelly on 02/07/2020 15:22:03

Morning Martin,

I have sketched the circuit adding the diodes etc can you check I have it correct. Also, I have shown the switch to how you suspect it will be and I have assumed it will be cross wired 1a to 2b and 2a to 1b..?

Thanks

Ron

Big current diodes aren't my thing but many automotive types look good, this MBR10 from Farnell does 10A (surge 150A) up to 100V for under a quid. I doubt Ron needs a heatsink for this application because the diodes only have to work until the switch takes over again - milliseconds, unless there's a jamb.

I thought your circuit needed resetting because I didn't think it through! Slapdash, sorry...

Dave

The lathe came with a number of accessories, one of which is a vertical milling slide (picture below) I have never used it as I have the mill but this morning was the first time I have taken a good look at it. It looks to be quite decent quality, certainly robust and with a very smooth action on the lead screw with no play or backlash. The vice is good to for its type and I think the swivel arrangement on the mounting plate is well thought out.

I wondered if anyone can recognise it as shop bought or homemade, just interested.

Sorry double post why I keep getting this I have no idea.

Edited By Ron Laden on 04/07/2020 12:38:41

The improved sketch looks right to me. If you have been following the discussion Dave and I have had the original circuit would also work. Since a diode that failed short circuit in either circuit would bypass the switch it is across rendering it ineffective you may chose to use my offering but omit the diodes, your choice. You can add them if there is a problem in the future but with the current and voltage you are using I think it is unlikely.

Martin C

Hi Martin,

I havnt received the DPDT switch or diodes yet but I have the micro switches and a small test motor, I also found a matching pair of SPDT switches so sandwiched them together to make a DPDT for a bench test.

Works a treat as I,m sure you knew it would, I think I will leave the diodes out to start with.

Thanks for all your help, very much appreciated.

Ron

Regarding the vertical slide, there are some features that look like they would need a mill to produce. Specifically the insertion slot in the base for fitting the tee bolts into the slot. If you have a mill for this would you make a vertical slide for a mill? Examination of this feature may show if it was milled or hand made.

Martin C

Dave, I think I was equally slapdash in assuming the first circuit's diodes were for spark suppression and not thinking through beyond that.

Martin C

The vertical slide is an early Warco one. This was designed with two hold-down bolts to fit the Myford cross-slide, either facing the spindle or at 90deg to it. The steel plate was an adapter plate, sold with the vertical slide, to allow it to fit the Warco 920 lathe and "allow rotation through 360deg". The three part vice was a standard fitting that came with the slide.  It cost £90 new and was advertised in ME around 1992.

Added:  I just noticed that someone has modified it by putting two spacers in to hold the end plate away from the slide - a darn good idea because one fault with the slide is you can only load the t-bolts from the "bottom" of the slide which is a right pain!  Note to self - make two spacers for my own slide!!

Edited By Bizibilder on 05/07/2020 20:13:49

Edited By Bizibilder on 05/07/2020 20:18:20

Edited By Bizibilder on 05/07/2020 20:22:14

Made up a mount for the cross slide control and added a small enclosure, seems the obvious place for the control. This will have an on/off, fwd/rev and a speed/feed control. Just waiting on components to finalise the wiring.

I finally got the electrical parts together to allow me to wire the cross feed drive and get it finished off. I mounted the end stop micro switches onto a thin walled piece of alu angle which also serves as a swarf guard. I fitted the mount to the r/h side of the saddle with a couple of 4mm spacers to stand it off the slide. I also milled a slot in the mount to allow the switch activation pin to protrude through, the pin is a piece of M3 studding which is screwed into the side of the slide. The mount covers access to the slide gib adjusting screws but should I need to make any adjustments just 2 screws and the mount is removed. Overall travel front to rear of the slide is 85mm which I think will be fine, a couple of pics below.

Edited By Ron Laden on 23/07/2020 15:10:35

Edited By Ron Laden on 23/07/2020 15:14:19

Tis finished, the cross slide drive is all wired and working well, really pleased with it how it turned out and good fun to do. I think I mentioned before that I have gone with the easy option, as you can see in the video below the motor/gearbox/dog clutch sit a fair way back but I am ok with that. I dont have a rear splash guard so I could get away with it, the lathe didnt come with guard so I just use pieces of heavy cardboard or whatever and change when needed.

With a rear guard there are a couple of options, you could cut a horizontal slot in the guard just enough to allow the motor to pass through. Other than that the motor/clutch would have to be lowered to beneath the rear of the cross slide and brought forward but that would get more complicated as it would need a belt or gear drive up to the leadscrew. I did consider a drive on the front of the saddle but it would be seriously complicated and need some major surgery and as much as I wanted a cross drive I didnt want to go that far.

The variable speed I think is a nice feature from dead slow to quite fast, too fast at the top end for cutting but good for bringing the tool back from a cut which on larger work could be quite handy. The torque from the motor/gearbox is quite impressive considering they are not that big a unit, thats why the end limit switches are a must as an over-run could do some damage.

Cost wise about £75 in total for the motor/box, speed controller, DC power supply, switches/electrical bits, I think that comes out as pretty cheap in adding a driven crossfeed from scratch to a lathe without one. So for any 920 owners if you fancy a driven cross slide it is doable and I,m sure there are more ways to skin a cat this was just my approach.

I forgot to say there is one job left to do and that is to add a support bearing to the rear end of the leadscrew, it will need a pocket milling in the leadscrew channel to accept a mounted ballrace. The end of the leadscrew is unsupported rear of the nut and it doesnt run dead true which is not normally a problem you would never know but connected to the clutch which has some clearance between the parts it creates a bit of out of true running. It works fine but would be better centered and supported in a bearing.

Ron

Edited By Ron Laden on 26/07/2020 20:20:33

I thought I had better test the drive with a heavyish load to check for any problems so took a 100 thou face cut in steel (spec unknown). No problems at all, not a hint of hesitation so pleased with that, you can see the fast retrieve from the variable drive which is quite handy.

Very nice video.

Next must be a parting off video.

That is where wheat and chaff are sorted.

Can we have some newer information on ideal motor torque,rpm and gearing?

I want it on both axes.

Thank You for experimenting and finding the way.