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View Full Version : Restoring old lathes, backlash in leadscrews



irving2008
24-08-2009, 11:13 AM
Hi all,

This is one for the experts, JohnS, Robin, etc. I suspect.

I need to fix the backlash on the cross- and top-slide on the lathe (the 85y old one)

It currently has 8tpi ACME (or something like that, square topped) threaded screws working onto a tapped cast-iron block which is the slide itself. In other words there is little opportunity to make another part. I'll get some pics/drawings done in due course but I'm guessing the wear is in the cast-iron block (nut) and not the screw as I can move the slide back and forth 2mm on the screw.

What are my options if I:

want to retain the existing screws
fit new metric trapezoidal screws, say TR10x2
A couple of ideas I had:

bore out the cast iron block (with my new boring head on the mill), assuming there is space and fit a bronze/delrin/whatever sleeve.
mill out a section of the block and drop in a new 'nut'

I think the advantage of the second approach is I can trial different nuts and maybe get some form of anti-backlash approach in there...

BillTodd
24-08-2009, 01:10 PM
The Cross-slide is likely to be a left-hand thread. Are they really 8tpi? surely both are more likely 10tpi (How are the dials scaled?).

If you have room, make two new lead screw nut for each slide thread them on the OD. drill the blocks and thread it to take the OD of two new bronze nuts. Use the OD thread to take up the backlash.

Alternatively, again if room allows, a new nut could be screwed on to the existing screw and fixed to the existing block.

Pictures would help :)

Ross77
24-08-2009, 07:30 PM
Like you said one for the experts. :smile:

I'll be watching this one tho. Need to fix mine at some point.

So lots of pics please.

irving2008
24-08-2009, 10:59 PM
OK, a closer inspection and the application of some cleaning cloths showed I was wrong in assuming the SuperRelm was like its smaller brother. Where the RelMac has the leadscrews working directly into the cast iron, the SuerRelm is more sophisticated and I found, after moving the tool carrier and cleaning 85 years of gunk off, a screw head. Further inspection revealed a bronze/brass nut held in by a screw - see pics.. this is the topslide, but the cross-slide also has a screw head only visible once the topslide is removed.

It is 8tpi LH x 3/8", I measured it accurately with the digital calipers. RDG do a 3/8 x 10tpi LH tap for 10 but not an 8tpi :sad:, then again i dont want to be making a new screw if I can help it...

I can move the nut back and forth about 1.0mm on the screw. And the screw's endfloat in the brass bearing/mount is about 0.3mm but thats easily rectified with a washer under the handle

How do I measure wear on the screw - is it likely to wear or can I assume the wear is all in the nut?

BillTodd
24-08-2009, 11:15 PM
Very similar to my Southbend. I sawed across the nut, down toward the mount (right-angle to bore), a thread or two from the back edge. Then drilled and tapped two small holes either side of the lead screw at the split end, so that two screws will push the split apart and reduce the backlash (I'll try to get a picture tomorrow).

Alternatively, you may be able to cut the nut completely in two and screw one part against the other (see attached pictures of my old mill's y nut). You'll have to find some way of fixing the two parts together to stop the adjuster nut turning.


[edit]


How do I measure wear on the screw - is it likely to wear or can I assume the wear is all in the nut?
Both will be worn. You should be able to detect the difference between the middle and the less-used ends of the thread. That said, the adjuster suggested above works quite well on my 72 y.o. SB 415 - it goes between tight, loose and tight over the whole travel, but the backlash is workable (perhaps ~10-20 thou at worst)

John S
24-08-2009, 11:33 PM
Quite a few ways to take up wear in a nut but if the screw is worn there isn't really a lot of use.
Square thread of 8 tpi means it has crests and spaces of 1/16" equally, second pic shows ? the spaces wider to the left of the nut ?
Hard to tell could be camera angle.

One alternative is to replace the screw and nut with a Myford ML7 part which is also 3/8" but 10 tpi acme to give 100 thou per rev

irving2008
25-08-2009, 12:57 AM
Think its just the camera angle John, tho its hard to tell... The calipers are in the garage now and can't be assed to go get them, but it looks fairly even - cetrtainly not 1.5mm wear's worth!

Can't find a 3/8 x 8tpi LH tap anywhere... and quoted $201 to have one made! So thats not going to happen then...

The threads inside the brass block, if they were ACME square top originally are very much triangular now... so much so I wonder if they were ever ACME - well after 85 years a bodge or two wouldnt surprise me... but as you say John, maybe not worth bothering with, althopugh cutting the block 1/3 way downwould not be hard to do

Replacing them with 3/8 x 10tpi LH as per Myford had occured to me, I can get a tap from RDGTools as mentioned, but where can I get a die?

John S
25-08-2009, 01:01 AM
Why do you need a die ? Just buy a spare screw from Myford or Jim Marshall who breaks Myfords.

http://www.homeworkshop.org.uk/

irving2008
25-08-2009, 08:42 AM
Why do you need a die ? Just buy a spare screw from Myford or Jim Marshall who breaks Myfords.

http://www.homeworkshop.org.uk

I could, but thats 30 or more, plus if I am going that route I'd rather replace with something metric as its more useful. A 150mm length of TR10x2D and a bronze nut would be cheaper...

John S
25-08-2009, 12:27 PM
You will never cut an accurate thread for a leadscrew with a die be it imperial or metric.

irving2008
25-08-2009, 12:50 PM
You will never cut an accurate thread for a leadscrew with a die be it imperial or metric.

Good point... but then you need a lathe and thread-cutting ability to make one. Do you know of a source of 3/8 x 10tpi ACME threaded rod other than buying a Myford leadscrew?

BillTodd
25-08-2009, 02:11 PM
You will never cut an accurate thread for a leadscrew with a die be it imperial or metric.
Quite correct. You'd be better off just bodging something with a bit of threaded rod!

Irving;
I've just been looking at Tony's lathe site: If the lathe has any historical value I suggest you do not cut the nut as I suggested, it would devalue the lathe considerable.

I'd offer to make you a LH square threaded nut (sounds like a challenge :)) , but my Hardinge will not cut 8 tpi (a major PIA).

I think the best thing to do would be to replace the whole handle, screw and nut. Perhaps using the TR10x2 trapezoid with a metric dial, although I agree with John, adapting a part from another lathe could save a lot of work.

irving2008
25-08-2009, 04:53 PM
Quite correct. You'd be better off just bodging something with a bit of threaded rod!

Irving;
I've just been looking at Tony's lathe site: If the lathe has any historical value I suggest you do not cut the nut as I suggested, it would devalue the lathe considerable.

I'd offer to make you a LH square threaded nut (sounds like a challenge :)) , but my Hardinge will not cut 8 tpi (a major PIA).

I think the best thing to do would be to replace the whole handle, screw and nut. Perhaps using the TR10x2 trapezoid with a metric dial, although I agree with John, adapting a part from another lathe could save a lot of work.

I think you are right, tho what historical value the lathe has is moot.

I can get a 18" of 3/8 x 10 LH ACME (enough for both top and cross-slides) for $20 inc shipping and a LH tap from RDG for 10. A brass block off eBay (enough for 5 nuts) is 6, and some steel stock to make the shaft I have already.

Plan would be to turn a new shaft (basically 3/8" dia rod with a 7/16" dia x 1/8" wide section which locates in the base of the brass support. The end of the shaft turned down to 4mm. Centre drill the new screw 4mm dia and loctite/pin the screw to the shaft.

Mill a block of brass to size with a lug on top, turn the lug round on the lathe. Drill out the block and tap (remembering to turn it backwards!)

How hard can that be? :whistling::nope:

Later I may remake the brass support to incorporate a dial (but then I'd have to make a 4th axis indexer to engrave it on the mill... hmmm small chuck, a bearing and a 50tooth gear would give me something I could engrave 50marks, equal to 2 thou steps...)

BillTodd
25-08-2009, 05:53 PM
Is the compound/topslide LHT?

If you are going to the extent of fitting new screws then adding dials is a must, surely?

And , if you're going to make your own nut, it'd be worth adding some kind of backlash adjustment to it (you can't adjust a tap, the way you can a split die :() .

irving2008
25-08-2009, 06:48 PM
Is the compound/topslide LHT?

If you are going to the extent of fitting new screws then adding dials is a must, surely?

And , if you're going to make your own nut, it'd be worth adding some kind of backlash adjustment to it (you can't adjust a tap, the way you can a split die :() .

Yes they are both LH threads. And you are right, it makes sense to add the dials, its not that much more work (!)

As regards backlash adjustment, what do you suggest? the current nut is about 20mm square so I was thinking that a 20mm cylindrical section screwed into the block and locked in place with a small grub screw. It would need to be a fine thread, maybe 20tpi. Not sure I could do that with the changewheels I have today.

Here is a rough pic...

BillTodd
25-08-2009, 07:01 PM
As regards backlash adjustment, what do you suggest? the current nut is about 20mm square so I was thinking that a 20mm cylindrical section screwed into the block and locked in place with a small grub screw. It would need to be a fine thread, maybe 20tpi. Not sure I could do that with the changewheels I have today.

Sounds like a good plan to me :)

Can you get at to adjust it in place? (I can just reach the SB adjuster from the back of the crosslide)

An alternative (used on the hardinge) is a brass bar (with the end threaded in place) pressing down on the screw via the nut mounting hole (I'll add a picture later).

irving2008
26-08-2009, 09:26 AM
Neat approach there Bill, but sadly the lug on mine goes into a blind hole in the slide and it would involve modification of the slide itself.

I actually measured the screw accurately last night and its actually 7/16" not 3/8", although if I am remanufacturing it makes little difference. What matters is finding some leadscrew of around that size and a matching tap for the nut at a reasonable price.

I have decided that the screwed in section I previously suggested would be hard to manufacture as I have no means to thread it at the moment. So I am going with the split nut and screw idea previously discussed

ptjw7uk
26-08-2009, 10:38 AM
Have you considered casting a nut thread using moglice not any idea how much it costs but an idea.
http://www.moglice.com/articles/replication_techniques/wrotethebook.html
(see figure 14)
I suspect you would need a new length of threaded rod to replace the screw.


Peter

irving2008
26-08-2009, 10:13 PM
Have you considered casting a nut thread using moglice not any idea how much it costs but an idea.
http://www.moglice.com/articles/replication_techniques/wrotethebook.html
(see figure 14)
I suspect you would need a new length of threaded rod to replace the screw.


PeterPeter, I've looked at this stuff before, but its hard to get in small quantites. I did wonder about using plastic epoxy metal and packing the nuts, oiling the screw, screwing it in and leave to set... might contain the backlash for a while - worth a try maybe

irving2008
26-08-2009, 10:42 PM
Took the other leadscrew (cross-slide) off today. Its identical to the topslide one, although at some time in the past somone has lopped 7/16" off the brass bearing and inserted a (poorly machined) micrometer dial and attempted to solder a pointer (now missing) to the brass sleeve.

I now have everything ordered I need to do this except some 7/16-10 LH leadscrew, wanted to find a UK source as the US source is 10 days shipping and was hoping to make a start this weekend. Kingston Engineering don't go down to 7/16, their smallest is 1/2 but this won't fit through the holes in the slides :sad:

Anyone know of a UK source for 400mm or so 7/16-10 leadscrew? I wanted to go metric but although MD have 10x2D LH at 99p/100mm I cant get a 10x2D LH tap at reasonable cost unless someone has one they can lend me...

John S
26-08-2009, 11:53 PM
Order a bit extra and flute it and get it cyanide hardened and use that for a tap.

.

irving2008
27-08-2009, 12:19 AM
Order a bit extra and flute it and get it cyanide hardened and use that for a tap.

.
Interesting, I assume that would be OK for brass, being a lot softer (I dont suppose it would tap brass without hardening?)

Where and how much roughly?

irving2008
27-08-2009, 01:13 PM
Hmmm..got a couple of 'approximate' quotes for hardening a 150mm length of 10mm - 80! and that depends on the actual steel used.

Maybe I need to learn how to do hardening at home... I have the book...

John S
27-08-2009, 01:48 PM
Hmmm..got a couple of 'approximate' quotes for hardening a 150mm length of 10mm - 80! and that depends on the actual steel used.

Maybe I need to learn how to do hardening at home... I have the book...

Not easily done just by the heating and quenching as the screw will almost certainly be low carbon steel.
The only easy way to do this is by pack hardening in something like Kasenite which makes a mess of fine details like a tap, hence the reference to molten cyanide.

If you fancy going this route, buy an extra 150 to 200 mm length, send it up and I'll flute it and get it hardened in with the next batch of gear that goes over.
I'll make two, you get one back, I keep one for payment.

irving2008
27-08-2009, 02:05 PM
Not easily done just by the heating and quenching as the screw will almost certainly be low carbon steel.
The only easy way to do this is by pack hardening in something like Kasenite which makes a mess of fine details like a tap, hence the reference to molten cyanide.

If you fancy going this route, buy an extra 150 to 200 mm length, send it up and I'll flute it and get it hardened in with the next batch of gear that goes over.
I'll make two, you get one back, I keep one for payment.
Now thats a deal John!

I was looking at Kasenite and similar stuff. It does say in the instructions that its ideal for one-off, low-use tools such as home made taps... but I can see that the fine detail might be problematic...

The company I talked to re cyanide hardening needed to know exactly what steel was being used and its exact composition else they couldnt harden it... I wonder if MD have that data? I'll email them...

irving2008
27-08-2009, 04:36 PM
Found these guys...http://www.ondrives.com don't recall seeing them mentioned here before

They do 10x2 LH similar price to MD (11.34 for 1m) and steel nuts in LH for 5.34 each. They advise steel nuts OK for hand operation and given the level of use I dont see this being a problem (comments?). The nut is 17mm sq and 15mm long so would easily fit inside a (brass?) carrier with a locking screw.

Is this a viable option?

irving2008
08-09-2009, 12:51 AM
Ok, this is what I plan to do... the top shows the existing screw, very simple, very crude.

The lower two diagrams show the proposed 10x2G replacement, with a pair of 8x16x5 thrust bearings retained by a circlip and 12mm washer inboard and an M8X1.0 nut outboard to adjust endfloat. I'll probably extend the outboard shaft to allow me to fit a zeroable micrometre dial with a backing dial mounted on the bearing block.

Comments?

John S
08-09-2009, 12:57 AM
Swap the screw to 20 tpi and use one of the C3 digital displays.
Instant imperial / metric change over, zero at any point and no counting turns ?

irving2008
08-09-2009, 01:05 AM
Swap the screw to 20 tpi and use one of the C3 digital displays.
Instant imperial / metric change over, zero at any point and no counting turns ?75 is a tad outside my budget John. An adapted 6" vernier off eBay at 10 would be a cheaper DRO!