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centec 2a halfnuts

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john anderson 522/03/2019 17:07:24
30 forum posts

hi everyone can anyone help with advice on making half nut for my mill, i,m hoping that i could get away with replacing the half nuts to reduce the backlash, and the leadscrew may be ok , i,m also wandering if i can use the milling machine ok with a dro if this compensates for backlash,

peak422/03/2019 17:25:28
1671 forum posts
175 photos

Somewhere recently I've seen a thread on a chap in Germany who makes half nuts to order, but can't currently find it.
Essentially someone UK was offering to act as g-between as the German chap didn't speak English. I think it was mentioned in a thread about someone restoring a Centec; sorry, best I can remember for now.


Dave Halford22/03/2019 17:25:40
2001 forum posts
23 photos

How much lash is there?

Slack off the gibs a little and check that the backlash is constant or nearly so along the length of the table travel, if so the leadscrew is OK.If it varies or the handle gets stiffer to turn with the gib a bit loose then it's the lead screw.

I have used an old Heidenhain system on my 2A which has huge scales and it's fine. However lash will still be an issue if you end up climb milling.

john anderson 522/03/2019 17:34:06
30 forum posts

thanks peak4 i had seen that but its really expensive and i don,t want to send what i,ve got to germany

john anderson 522/03/2019 17:37:35
30 forum posts

hi dave i really am a newbie when i seen your advice , very good sugestions i,m going to start renovating my machine and i,ll check what you said, i think if ive measured right theres about 20thou back lash

Howard Lewis22/03/2019 17:51:35
6004 forum posts
14 photos

At the risk being thought complacent, my 25 year old Chinese Mill/Drill has 20 thou backlash in the X and Y traverses, and has had for as long as i can remember. You will never be free of backlash. As someone else said recently on another post, No backlash would mean No movement, ie effectively seizure.

The backlash will not desperately affect accuracy. If youn overshoot and have to back off, you back off by more than the backlash, say half a turn, and in returning to where you wanted to be, you take out the backlash again.

DRO will not eliminate backlash, but will tell you how far you have moved the table (which is what the dials also do.)

A DRO can read incorrectly, if there is any stiction. You set the depth of cut, as you think, and find as you machine, that the reading has crept up by a thou or two, because of the vibration! What do you believe, the analogue dial or the DRO? Then you measure with a Mic or a Depth Gauge.

Where ever possible, arrange the work so that you can measure the dimension, independant of the dials or DRO.

If you are a newbie, you are brave (Much braver than me ) to start stripping a machine to rebuild it!


Edited By Howard Lewis on 22/03/2019 17:56:53

john anderson 522/03/2019 18:09:33
30 forum posts

thanks howard what you said is a great help to me i,m not going to touch this mill if i can get away with it. its a matter of knowing what your doing i,m not there yet ,give me woodwork machines and i,m in my comfort zone , thanks again john

Howard Lewis22/03/2019 18:22:34
6004 forum posts
14 photos

Don't know where you are, but, as a newcomer to metal machining, as i was MANY years ago, try to find a local Model Engineering Society and join. It will be surprising if you do not find folk prepared to give advice, help and demonstrate.

On at least two occasions, when looking for a means to produce an unusual thread, fellow club members have said "I've got a tap for that. Come round and borrow it" Or on another occasion, "Come round and do it on my machine"

You will climb the learning curve quite rapidly with their help.


john anderson 522/03/2019 19:01:29
30 forum posts

hi howard i,m in richmond yorkshire . i was thinking of joining a club i would like to see what other people are doing with there machines

T.B22/03/2019 19:25:00
54 forum posts
21 photos


I don't know what facilities you have but if you have a lathe and a mill then its very viable to refurbish a worn Centec half nut.

I have done it a few times by plunging an endmill down the length of the threaded part of the nut taking off just enough material to remove the worn threads. i think i used a 20mm cutter.

Then turn and bore a bronze cylinder the same O/D as the end mill you used and screw cut the I/D to the same thread as your centec feedscrew, probably 10tpi acme or square form.

This cylinder can then be soldered into your old half nut and the top half machined off.

You can see the excess solder and seam between the old nut and new threads in these pics.



john anderson 522/03/2019 19:44:28
30 forum posts

hi tb that is really clever, to be honest i,m just starting off ,i have a myford ml7r and the usual piller drill ect, i just bought a centec and didn,t know what to watch out for when i checked it over, so i thought when i,m good enough i,ll have a crack at making the half nut . your method is a great idea, i,ll just have to brush up on silver soldering , i did a few bandsaw blade with silver solder and they didn,t snap so fingers crossed . thanks for advice

T.B22/03/2019 19:52:23
54 forum posts
21 photos

The nut shown was done with an old ML7 and the centec that needed the nut !

If the screw cutting is daunting you can buy acme taps from the usual model engineering suppliers for not too much money , although square form maybe harder to find.

It wasn't even silver soldered , my old plumbers blow torch and solder , not text book i'm sure but it works just fine !

john anderson 522/03/2019 20:30:23
30 forum posts

i,m getting excited about trying this method out, i haven,t taken it out yet,but is there a opposite half to the nut or just one half that swivells up and locks into place on the leadscrew . on mine it seems to be locked in place permenantly and i think the guy made a power feed that could be disingaged

Georgineer22/03/2019 21:47:12
563 forum posts
32 photos

There was an interesting thread on making leadscrew nuts from acetal on Home Shop Machinist a while back. It's a long thread and some of the early posts are missing, but all the information can be found. I've no idea if it would be suitable for the Centec 2A.


duncan webster23/03/2019 00:17:32
3919 forum posts
61 photos
Posted by john anderson 5 on 22/03/2019 20:30:23:

i,m getting excited about trying this method out, i haven,t taken it out yet,but is there a opposite half to the nut or just one half that swivells up and locks into place on the leadscrew . on mine it seems to be locked in place permenantly and i think the guy made a power feed that could be disingaged

The standard set up is just one half nut applied from underneath. This can be swung out of the way when you want to use the capstan for x travel. I've taken the capstan off mine it was in the way, and the half nut stays permanently engaged. There is very little space between the top of the screw and the underside of the table, I've made half hearted attempts to measure it in the past and failed, if anyone knows what it is i|'d be grateful for the information

john anderson 523/03/2019 16:51:10
30 forum posts

hi duncan thanks for advise if i take my table off i.ll make sure i measure the clearance between the screw and table. on my 2a the lever that does the faster traverse has been taken off , and the nut is engaged permanently there is a small power feed that can be disconnected by a small lever, also there is another lever taken off the vertical head

john anderson 523/03/2019 16:53:15
30 forum posts

thanks georgineer i had a look at the acetol type half nuts but i think it would be difficult to put on mine

not done it yet23/03/2019 22:02:01
6716 forum posts
20 photos

My 2B had the fast traverse bits removed a long time ago. It now drives through a fixed feed nut fitted at the end of the table. Not sure if the same can be done with a 2A. Gary wooding is your man for these sort of mods - he is a veritable expert on Centecs.

Winding the table back and forth with the hand wheel might be slower than the fast traverse (when engaged), but I have not found it a too-onerous task.

The acetal feed nuts are a good idea for a lathe, but I'm not so sure for a mill. Climb milling might be too much for plastics. I wouldn't even attempt it with half nuts.

I used the 'heat and squeeze' method to make a feed screw nut for my old lathe. Easy to do but I never fitted it to the machine as I had a spare which got me running again in about half an hour (after I found the spare!). Acetal nuts are a tight fit unless 'eased off' a tad.

Edited By not done it yet on 23/03/2019 22:02:36

john anderson 524/03/2019 09:19:28
30 forum posts

thanks not done it yet, did you take the half nut of completely and replace with a fixed circular nut

not done it yet24/03/2019 11:16:24
6716 forum posts
20 photos
Posted by john anderson 5 on 24/03/2019 09:19:28:

... did you take the half nut of completely and replace with a fixed circular nut

Nope. Done a long time before I took ownership. As I said, the rapid long traverse parts were removed, so it only has the fixed feed screw nut.

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