Thread: Straightening lead screw

Hello all,

I am finally biting the bullet, and am intending to straighten my lathes lead screw. The deformed screw causes surface finish issues when I take last couple of though finish cuts. (screw lifts carriage slightly on power feed)The screw has a kink in it which is about 3/16".
Does anyone have any suggestions as to best way of going about this before I get into it with a gas axe/hammer?

Thanks.

Presumably would have to come out of lathe for the job.

Standard procedure is to machine a brass bushing/collet that you can grip in the lathe chuck, marking the collet for Jaw 1. It needs to be slit through for most of its length between the jaws so it can grip the screw as the chuck is fastened.

Mount the screw in the collet and chuck so that the kink is just outside the collet, ensuring that the mark and jaw coincide.

For a shortish/thin shaft, they are normally tapped over with a hide mallet very carefully, with a dial guage run between taps. For the lead screw, I would use a fairly heavy pipe over the free end almost to the kink and minor tweeks to ensure that you dont create a second kink a little way further along. Again let the dial gauge dictate force and direction.

I have seen this done on model aircraft crankshafts, and bent inboard boat prop shafts (8ft x 11/2 in dia SS), both able to get back to within 0.001 fairly easily, so it should work for you.

AHA!

I knew there must be someone with good advice.
Thank you very much. That sounds like a common sence approach to the task. The lead screw is a 3/4" thick 6 tpi jobbie. I have a few lengths of 1" ID heavy walled pipe beside the shed. I don't have any brass at hand to suit the job, but I have plenty of leaded free cutting steel, that is heaps softer than the screw, so that should be ok for the job. I do have a 2 1/2" chunk of brass round, but would be reluctant to carve it up if it isn't neccessary. What does everyone think of using the softer steel for the bush??

Thanks again.

What does everyone think of using the softer steel for the bush??

Anyone???

it would work.

The way I've always straiughtend shafts is to put the shaft in a V block each end and rote by hand while using a dial indicator to locate the high spot. Using a press push the high spot down and check it for straight on the v blocks. I wouldn't use a hammer on a threaded section let alone shaft.

Thanks guys.

The soft free cutting steel should be OK. It will still be a bit tougher than brass, but its a lot softer than hardened chuck jaws.

The guy who taught me this trick is an retired instrument maker who spends about 30 hours a week rebuilding MA engines of all sizes from baby Cox's to chainsaw size. He has one chuck and a set of about 25 collets made just for this work, hasn't tossed a collet since he started, and rarely has to make one these days. With a limited range of suitable bearings, there is a limited range of shaft diameters so the collets are reusable many times.

If possible, try to set up so the collet grips for a couple of inches along the screw, and mount as close to flush with the chuck jaws as possible, or alternatively, leave a thin flange at the end so thet the collet can't get pushed into the chuck when loading everything.

I agree with Echnidna go the way of the vee blocks and the press , leave the hammer in the tool box.
Not only can you damage the lead screw but ,you could also damage the head bearing in the lathe.
I don't think lead screws are made from mild steel either.
I have straightened motorcycle fork tubes this way ,they are bit trickier , because they are hollow .I've also buggered one or two from being over zealous and impatient .

You may have to go past the "bend " with the press because the lead screw will spring back .
Measure the run out with the dial indicator with the lead screw in the vee blocks ,find the highest point and put it at the top , leave the dial indicator on the screw set at zero bring the press down onto the screw and apply some pressure until the screw starts to move when it gets to the reading equal to HALF of the run out then it should be straight , go a bit further say .005" release the pressure and watch the dial indicator and see how much it springs back .
Check the run out again . Keep repeating until the run out is gone. Depending how badly its bent you may have to check in different places along its length. If it's a "long" ( over a couple of feet )bend you may have to place the vee blocks further apart ,this will allow more spring .If a "short" bend then have the vee blocks closer together.
One more thing ,some times cast iron vee blocks will crack down the centre of the vee ,if you apply too much tonnage. Please don't ask how I know this.

Sounds a bit complicated , but its easier to do than explain it in words.
Hope all works out .

Kev.

Well the job is done and it has made a great improvement to the machine.
I managed to get the screw straight to within .003 or so.

Yesterday I installed a new flat belt, and adjusted the headstock bearings.
Today I tested the lathes runout, and it non discernible with a DTI over 5". I love broze bearing lathes.

This afternoon I turned up a cast iron backplate for my new four jaw chuck, out of an old 5lb barbell weight. I hate machining that filthy stuff.
Now all I need to do is get someone to machine a 8tpi thread in the thing, so I can finish it off.
Below are some before and after pics of the backplate. I was wondering were I was going to get a hunk of cast for the job, and then it hit me. I have 300lb's of weights in the shed from the dim dark days when I had the energy and will to lift that stuff!

Anyway, thanks for the help...... the lead screw fix has made a world of difference to my lathe.

Just curious you havent finish machined the OD or faces yet?

Just curious you havent finish machined the OD or faces yet?

Curious is good IMO.

The bore ID is ready for threading at 1 1/2" x 8TPI, and the outside diameter is oversize as well. The register is machined to a touch undersize as well at this stage.

I need to get the thread cut (I don't have the means to machine 1 1/2 x 8TPI), so that I can mount on my lathe spindle for final machining. That way, everything will be spot on concentricity wise.

ie:

1. Get the thread machined.
2. Mount back plate on lathe spindle and machine for back face of chuck.
3. Mark holes and bore slightly oversize holes for mounting studs.
4. Mount chuck and machine down diameoterf back plate to suit OD of chuck.

Be careful with the internal thread. I recently came unstuck with a face plate I purchased a week back. The thread is supposed to be 1.5" x 8 tpi, but will not screw on my lathe spindle.

A learned friend of mine suggested the thread form may not be right. It can either be 55 degrees or 60 degrees.

Just a tiny fly in the ointment.

PS. Does anyone know what the correct thread form is for a Hercus 9" lathe spindle.

(Sorry for hijacking your thread)

Ken

Ken how does it check with a BSW thread gauage,I think it should be 55 degrees,I dont think you will find a standard 1 1/2" x 8 TPI UNC thread. There standard is 6 TPI.

Originally Posted by Fossil

Curious is good IMO.

The bore ID is ready for threading at 1 1/2" x 8TPI, and the outside diameter is oversize as well. The register is machined to a touch undersize as well at this stage.

I need to get the thread cut (I don't have the means to machine 1 1/2 x 8TPI), so that I can mount on my lathe spindle for final machining. That way, everything will be spot on concentricity wise.

ie:

1. Get the thread machined.
2. Mount back plate on lathe spindle and machine for back face of chuck.
3. Mark holes and bore slightly oversize holes for mounting studs.
4. Mount chuck and machine down diameoterf back plate to suit OD of chuck.

Are you looking for someone on here to do it for you,

Hi Ken.

I have an IXL Invicta lathe, which is made in Germany, but sold in the UK as a Brittish manufactured machine, in the late 1940's. Buggered if I know how the Germans made anything post war after the pissling they got in 1945.
Anyway, I am assuming it is BSW (British Standard Whitworth) which of course has a thread pitch of 55 degrees. I assume your Hercus would be the same, given that BSW was and still is, widely used here in Aus.

I will take my other chuck and back plate along to the machine shop for a comparo prior to cutting...... just to be sure. )