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Bridgeport Milling machine help

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gtrack17/08/2011 08:56:11
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Hi
 
I have recently aquired a Bridgeport Mill and after some `restoration` work was disappointed that the auto table feed unit seemd to be not working. I have since had the motor re conditioned (at ridiculous cost!) but my problem now is....
 
The table only goes fast or faster, I have tried a couple of potentiometers (both 500 Ohm as manual states and 2 Watt, single turn) one was a linear and the other a Guitar audio/log type......but have the same problem with both. I am now at a loss whether it is actually the pot that is the issue?
I have a feeling the bridgeport one may be a special log taper?....but I dont seem to be able to find one anywhere after lots of enquiries (i found a couple in USA, like all things, but they want $75+ to deliver them!)...
 
Has anyone had this problem or know where I can purchase the correct Pot....or any other fixes/advice would be very greatfully received.
 
(I believe it is the 6F type unit. only has a potentiometer feed rate control and rapid traverse button)
 
Many thanks.
Weldsol17/08/2011 12:09:02
74 forum posts
In my Bridgeport manual it shows the pot as
Comp, No, 8500357
Pt, No, ES357
Description Potentiometer 50KL Taper
 
Paul
gtrack17/08/2011 13:03:12
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Paul
 
I think from that the taper is a `one off` type, I have tried to get one everywhere to no avail.
 
But thanks for the info, I didn`t have all of that, might now contact a Pot manufacturer to see if I can get one made
 
James.
Baldric17/08/2011 13:44:28
154 forum posts
11 photos
Paul says it is a 50K Ohm potentiometer, you have fitted a 500 ohm pot, this may be the issue, is it possible to try that?
 
Baldric
Keith Long17/08/2011 13:54:04
844 forum posts
11 photos

Hi James

Have a look at the "Rapid Electronic" website (just a satisfied customer). They have 3 watt rated 10 turn pots between 100 ohm and 100K ohm fro about £8.50 each, you can always install it temporarily on some flying leads - just be careful about insulation and keeping fingers from terminals!

I'd be surprised if Bridgeport had a special pot made rather than use a standard. Also you state you've used a 500 ohm pot and just get fast or faster whereas Weldsol seems to have found reference to a 50K ohm device. I'd get a higher value than you've been using and give it a try, sounds like you haven't got enough resistance in the circuit and all you're getting is the top end of the range.

If you get a linear and you really need a log pot then you should just get the control happening towards one end, but you should get the speed range that you're after, once you've established that you can control it you'll have a much better idea whether you need a linear or a log pot.

Keith


Andrew Johnston17/08/2011 13:55:46
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Hmmmm, just checked on my Bridgeport, where the pot has been changed at sometime in the past. It's definitely not a genuine Bridgeport pot, as it is far too big to fit in the correct position. It has been bodged into a diecast box which is bolted onto the front of the normal panel. The pot is a Colvern wirewound, 50000 ohms, doesn't say anything about taper law, so I'd guess linear.
 
Regards,
 
Andrew
 
PS: Only two wires are connected, so technically the pot is being used as a rheostat

Edited By Andrew Johnston on 17/08/2011 13:57:50

gtrack17/08/2011 14:32:38
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Ah
 
Thanks chaps. All the research I done on it (including wiring diagrams) says to use a 500R ohm pot........but funnily enough the one I originally removed was a strange looking 50K one, I didn`t replace it as was concerned that may be the issue!....I only thought they were a special log taper as that seems to crop up alot in any googled info i have found...I was also a little bit paranoid about everyone fitting larger more resistant pots because the bridgeport ones (and any log type 500ohm pot) seem so hard to find.....but....
 
I wasn`t sure on the higher rating as thought it may be even faster if I done that. or do pots/rheo not work like that? I`m not very clued up on electronics... From what you say it seems I may have got this idea back to front....I did notice on one of the diagrams that it had 3 wires connected but on others only 2, mine only had 2 originally so i`ll stick with that.....i did search Rheos as well but they seem to commonly call the actually unit a potentiometer........
 
As Andrew is using a huge ohm pot sucessfully I guess i`ll just get a 50K or whatever one to do the job....it puts my mind at rest that someone else is using one and besides i`m at my wits end so i`ll just fit another one and see what happens!
 
Thanks again..........looks like yet another pot will be on its way to me!
 
 
Keith Long17/08/2011 15:57:14
844 forum posts
11 photos

Hi James

If the pot you've been trying up to now is a 2 watt rating, and isn't getting hot then a 3 watt will be happier still. The rating is a measure of the heat energy that the pot can live with and still be happy.The time to be wary is when you replace a component rated at say 2 watt with one rated at say 1 watt or less - you MIGHT get way with it and for a QUICK test you probably will but the lower rated component will most likely get hot and if given enough time (might be seconds) could fry.

The wattage rating for the pot on the mill will depend on the current that passes through it, the higher the current the warmer the pot gets and the higher it's wattage rating needs to be to avoid problems. Using a 3 watt pot won't give you a problem if a 2 watt is happy, and with modern components you often find that the new component is smaller than the one you're replacing AND has a higher rating - all electronic circuits seem to be shrinking rapidly!

Keith

Keith Long17/08/2011 16:39:13
844 forum posts
11 photos

Hi James

I've just looked at the circuit diagram for your mill, and noticed that on the circuit board the pot that is causing the headaches connects to 2 other pots and a fixed resistor. The 2 pots on the board look as though they are "presets" to set the upper and lower speed for the table feed. It's possible that one or both have been wound to their extreme positions and so rendered the control pot that your playing with somewhat ineffective. It might be worth setting both of those pots (R33 and R34) to about mid range of their travels and see if that has any effect. Also I note that the wiper of the external pot connects to a 100K resistor (R38) it might be worth checking that that is as it should be.

Keith

gtrack17/08/2011 17:29:32
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Keith
 
Thanks. Thats thrown in a curve ball!.....I`m guessing that was on the complicated diagram (that I never understood!).....mine is the series 1 Bridgeport, J type head with 6F........I can see two other pots on there now though (on mine they are labelled Pot 21 and Pot 22..........i`ll have to look at those tomorrow, thats a new one on me....cheers!
 
I`ve ordered up a 50K pot so that should be here soon and hopefully we`ll be ok!...
KWIL17/08/2011 19:17:03
3308 forum posts
63 photos
Could one of you post this circuit diagram?
Keith Long17/08/2011 20:55:26
844 forum posts
11 photos

Hi All

I think these are the relevant circuits:-



Sorry about the quality - it was rather a roundabout route to get them!
Keith


Billy Mills18/08/2011 01:48:53
377 forum posts
Would think that the pot is 500 ohms NOT 50K. Fitting a 50K pot would not allow the 500 and 250 ohm presets to have any effect. The pot is only one of many parts that could cause the problem-it don't have to be the pot! The dissipation rating is not just about internal heating with wirewound pots, the resolution is also a factor. The log taper is extreemly unlikely with a ww pot, that would make it a very rare and expensive part
 
A 50K linear part will work, adding a 510 ohm resistor accross the outer tags will make the part look like a 500 ohm pot so that the limit presets will then work as intended. If the low speed preset has been damaged- i.e. gone open circuit then the speed would be uncontrolable and high, worth checking first.
 
Billy.

Edited By David Clark 1 on 18/08/2011 11:16:01

John McNamara18/08/2011 03:27:29
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1313 forum posts
113 photos
Hi Keith

What kind of motor does the Bridgeport use to drive the feed? Does it have brushes? What voltage does it operate at? You may be able to buy simple a simple plug in speed control unit for drills and routers and replace the old control gear.

Maybe another member will know other sources the UK?
I found this one........
 

I am making a couple of assumptions here that the motor current will not be very high. It should be on the name plate if there is one. It is not an induction motor. And that the Bridgeport does not already have a solid state speed control system. Unlikely if you need a high current pot.

Modern speed controls use the back EMF of the motor to measure the speed. The result will be much better regulation of the speed under load, particularly at low speeds.

I know the urge is always to keep the machine original however this is an area where the old technology is not very good. You can always leave to old stuff in place for posterity.

Cheers
John

Edited By John McNamara on 18/08/2011 03:34:58

Edited By John McNamara on 18/08/2011 03:39:02

Edited By David Clark 1 on 18/08/2011 11:16:39

John McNamara18/08/2011 06:33:06
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1313 forum posts
113 photos
Hi All

I should have read the previous thread….It looks like it is a shunt wound DC motor. And it has a solid state controller. Thank you Keith Long.

A different speed control is needed.
Or fix the existing one.

Cheers
John

Edited By David Clark 1 on 18/08/2011 11:17:03

gtrack18/08/2011 10:18:02
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I`m hoping that this is a follow on from my thread, which I seem unsble to view the last posts on.....apologies if it is not.
 
Keith, that diagram is the one I have, following on from the threads on the original post, thats why I got confused on the 500 Ohm issue, It looked (looks?) like it should be a 500 Ohm pot but as many have suggested (and use) it should be a 50K one.....thats the one I have coming anyway.
 
As for the log, I was told that by an american shop that sells bridgeport parts, how reliable it is I don`t know, obviously there are far more on here that have hands on experience of the problem......but I was told it is a one-off that only they and another company can supply (my sceptical mind is saying `convienient`) and thats why it costs over $80!!........needless to say thats was the reason for my initial enquiry on here!
 
Thanks

Edited By David Clark 1 on 18/08/2011 11:17:39

Keith Long18/08/2011 10:50:00
844 forum posts
11 photos

At least we've both got the same circuit diagram to look at - but from one of your earlier posts I'm not sure that it's the diagram for your machine - sorry about that!

My reasons for saying that are that you removed a pot from the machine and that was 50K, and the pots that you've found on your board are labeled R21 and R22 rather than R33 and 34. Also other posters have said their machine have 50K pots for the speed control so it sounds to me as though there are a least 2 versions of the board out there.

If the pot that was originally fitted to yours was 50K then my instinct would be to fit a 50K and see if it works. Using a multi turn should allow you to control the speed even if the original pot had a strange resistance track - you might just have to do a bit more dial twisting to get the speed you want.

I'm wondering now if Bridgeport changed the board design to get away from using a custom made pot to a standard and at the same time brought the control circuit up-to-date as it was them - could possibly explain the variations in component values and resistor numbering. The manual that I downloaded last night only has the circuit diagrams that I posted, if anyone has later versions could they please let us know what the differences are.

Keith

Edited By David Clark 1 on 18/08/2011 11:18:14

Andrew Johnston18/08/2011 11:12:48
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5635 forum posts
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I worried that I might have made a boo-boo on the pot value, so I've just been out to measure it. Definitely 50000 ohms not 500. And it only has two wires connected. The control unit was made by Erskine Laboratories in Scarborough. My Bridgeport is an Adcock & Shipley clone, but I'm surprised they used a UK company for the controller. One would have assumed that the parent company would have just supplied their existing, US made, controllers.
 
The box has a cover over it, but I can just see one of the those old-fashioned Mullard multi-coloured capacitors; haven't seen those since the 1970s! Mind you, my power feed works at the moment, so I'm not going to poke any deeper, as the wiring looks a little 'delicate'.
 
Addendum: Much to my surprise Erskine is still doing power electronics in Scarborough, albeit now part of much larger group.
 
Regards,
 
Andrew

Edited By David Clark 1 on 18/08/2011 11:18:40

David Clark 118/08/2011 11:20:08
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3357 forum posts
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Hi There
Thread has been fixed and all relevant posts have been moved to this original thread.
regards David
 
David Clark 118/08/2011 12:36:24
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3357 forum posts
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Hi David
I don't know if there is a bug in Word.
The last couple of problems look like the posting is a website that has been pasted in, both were basically the same.
We now have facilities to move to another thread but only one message at a time.
It would cost several £0,000 to change the editor that is used so we will just have to live with it.
regards David
 

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