Pultra model P

[david bennett 8Participant @davidbennett8]

Mark, have a re-think. Look at your original leadscrew, and measure the wear (measuring the diameter of the middle of the screw and the end should tell you enough) Can you make one better thaan it is?  Try your home made nuts un the original leadscrew Are they satisfactory? Would splitting them get rid of enough play for you. It sounds like you do not yet have enough skill to improve on the original worn leadscrew.

dave8

[david bennett 8Participant @davidbennett8]

Mark, p.s. While you are set up for nickel plating, have you considered plating the original leadscrew? and maybe the nut?

dave8

[mark smith 20Participant @marksmith20]

Dave8, yes plating them did come to mind whilst  i was  plating those nuts.

I have tried the home made nuts on the original screws and they seem a big improvement without any back lash , wiggle or back and forth movement. Same goes for the nuts on the homemade screws. So i dont really see the problem ,i dont intend altering any of the original parts, so if im not happy i can just put them back on and try to sort out the wear particularly on the cross slide .

The original  nuts on the original screws wiggle like mad and have a lot of back and forth movement which i can easily feel in my hand.

At least i’m learning a bit  about these things and im not claiming to be an experienced machinist .

[david bennett 8Participant @davidbennett8]

We’ve all learned a bit. The expert advice is that the bronze nuts get impurities imbedded, then act as laps, wearing the leadscrew. You have shown it is the nuts that wear.

dave8

[david bennett 8Participant @davidbennett8]

(My post has vanished. here it is again)

We’ve all learned a little. The expert advice is that the bronze nuts get impurities imbedded in them and act as laps, wearing the leadscrews. You have shown it is the nuts that wear.

dave8

[mark smith 20Participant @marksmith20]

Dave8, managed to drill a hole in one of the nuts for the cross slide, looked dead on centre . Tried it and it almost immediately  locked up. A bit of head scratching and examined the original .

Turns out the original is a mile off centre ,as is the hole going through the casting !!!! I wasnt expecting that. So the hole right through the casting is centralised at the front and gradually wavers off towards the back.

[david bennett 8Participant @davidbennett8]

You could try using the casting as a drill guide. Clamp a new nut in the hole (with a thin washer to maintain clearance) Using a long drill should give you a replica. You may need to make a thin-walled sleeve for the drill, to . match the hole. Ideally, tap it while still clamped.

dave8

[mark smith 20Participant @marksmith20]

Good idea thanks,ill try it tomorrow .

[mark smith 20Participant @marksmith20]

Ive had a rethink , ive carefully examined the screws,turns out they are the same diameter all along ,doesnt appear to be any real wear to the screws, So all the wear is in the nuts.  This seems to be the opposite of what most people say that the bronze nut wears the steel screw ,partyicularly in the most used section..

I have the compound sorted with one replacement nut .

The cross slide i thought i had it sorted but after the second attempt making a new   anti backlash nut  for the front,(which i had tried and it worked perfectly ) but when tapping the  second hole for the two tiny grub screws ive broke the tap. Any tips how to get this small 6 BA tap out??

[Peter Cook 6Participant @petercook6]

Dissolve it out in a warm alum solution. That’s the way clockmakers get steel out of brass.

See thread here

 

[mark smith 20Participant @marksmith20]

Thanks i’ll have to give it a try . How long does it take? Does it definitely not attack the bronze?

[david bennett 8Participant @davidbennett8]

Bronze is notoriously sticky, especially for small taps. Use a tapping drill at least one size larger than the usual.

dave8

[mark smith 20Participant @marksmith20]

Yes it is, i got almost all the way through ,then it  suddenly broke.

[david bennett 8Participant @davidbennett8]

Good luck with the alum. If it doesn’t work, here’ a desperate measure that might – although it doesn’t need splitting into 2 pieces,as discussed earlier, if you split it with a jewellers saw by hand, you could turn one half off the tap. leaving enough of the tap to get a grip on for the other half. You may need to sacrifice a blade or two sawing round the tap. Then clean up the sawn surfaces.

dave8

[david bennett 8Participant @davidbennett8]

Hold that thought. I had forgotten that this is the older nut form which I do not have. It looks like my idea will not work – the different pitch threads are never going to line up. if, as you say the thread is nearly complete, perhaps it would work if you split it near the end, and have a shorter nut.

dave8

[DellParticipant @dell]

I have used Alum to dissolve steel , the solution must be very strong and preferably warm but it’s not a quick job and it will not affect the brass or bronze.

Dell

[mark smith 20Participant @marksmith20]

Well, the part has been sitting in alum in a jam jar ,in an old baby bottle warmer for hours,lots of bubbling but not much has dissolved yet .

In the mean time i decided to make another outer backlash nut, ,temporarily glued the original onto the new one so i could quite accurately get the four outer holes lined up . ,this time the two tapped holes went well with a slightly oversized tapping drill.  fitted it and its great  no movement in cross slide and smooth movement with the hand dials . Compound also works great . Overall a big improvement over what they were like before.

I’ll still leave the other part in the alum to see how long it takes and whether it damages the lead screw thread .

Just noticed the two alignment pins at the end of the casting for the handle bracket are missing on the cross slide.

 

[DellParticipant @dell]

I have only used it to remove small broken clock screws and it is a very slow job but it will do it eventually.

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