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loply
08-02-2011, 03:41 PM
Hi folks,

Fairly new to this but I've successfully threaded externally on my lathe a handful of times.

I made a new leadscrew for my compound slide succesfully but it has a bit more free play than the original.

I'm about to make one for the cross slide and I want to get it spot on.

Every time I have threaded on the lathe I've struggled with knowing when to 'stop' cutting.

It will be difficult to trial fit this particular one.

Any tips on knowing when to stop? Is this something you can count off on the compound indicator ring?

Lastly, despite a very sharp HSS threading tool the finish I get tends to look like it's been sandblasted. I polish the thread on my bench polisher to smooth it out, but this 'round off' the peaks of my thread. Any ideas how I can help this?

Cheers,
Rich

aboard_epsilon
08-02-2011, 04:25 PM
Have a shaft longer than it has to be ..turn the extended bit to the root depth of the threads..when you scratch this bit ..you are at the right depth...if nut still wont fit, chamfer end of thread and wire brush over threads...watch out for burrs on threads from badly made tool or blunt one .use good neat cutting oil ..like Rocal RTD

ps ..always support end of shaft with tailstock

and make sure the tailstock is in alinement with headstock

pps ...you're brave making a lead screw ..like trying to run before you can walk

all the best...mark

Carld
08-02-2011, 04:41 PM
I will assume the thread your cutting is an acme because most lathe screws are acme. Acme threads are not easy to cut clean and only with experience can that be achieved. Small depth of cuts and clean up passes when near size can produce a clean thread most the time. You have to take light cuts making acme threads or the metal will rip out leaving a rough finish.

You won't or shouldn't have that trouble with a V thread.

When your cutting a thread it is best to machine a relief area at the end of the threading pass. On the compound screw, cross slide screw and lead screw you should see an area where there is no thread that is the minor diameter of the thread and that is what you should do too. That allows you time to release the halfnuts at the end of the thread pass. After you have done a lot of threading you will get real good at releasing the halfnuts even without a relief area.

Gary Gill
08-02-2011, 04:47 PM
Are you asking when to stop the thread depth? There are ways to measure the thread. You might check the thread against the mating nut.

aboard_epsilon
08-02-2011, 04:51 PM
There is a Brilliant South Bend lathe video on how to cut an acme thread ..out there somewhere ..
its on youtube ..but is reduced in size..so not that clear ...but look for the original .

its black and white and made in the 40's

all the best.markj

Paul Alciatore
08-02-2011, 06:13 PM
For precise threading in the lathe I have made up a spindle crank. When doing it by hand you can stop at a very precise spot. The main problem becomes withdrawing the tool at the right rate to make a nice looking end to the groove - a bit of coordination between the left, cranking hand and the right hand on the cross feed.

http://img.photobucket.com/albums/v55/EPAIII/P1010001-1.jpg

I was making a collet holder here and needed a good thread that ran almost up to a shoulder. No problems with overshooting.

rohart
08-02-2011, 06:21 PM
Do you know how to use threading wires ? On a standard screw thread, position two wires on one side of the thread and one on the other and mic across the wires.

A leadscrew thread gets rather large for a mic, so you can try with two wires, and the mic at an angle. You wires should jam in the thread so they stay proud of the major diameter. Just bend a short piece of the correct wire so it springs into place before using the mic.

Find part of the old worn screw that is still a good fit in the nut - usually the end nearest the handle. Put the wires on that and mic up. If your thread is correctly cut - tool geometry correct - then thread till the mic on the wires of the thread you're cutting reads the same. If your geometry is a bit out, you'll probably need to go a touch deeper.

loply
08-02-2011, 06:55 PM
This is a mini lathe and the screws aren't acme, they are 1mm pitch 10mm things. The main leadscrew is an acme thread, but the cross and compound slides aren't.

The screws aren't actually worn, I have been re-engineering the thing to incorporate bearings and different travels etc. The factory design left quite a bit to be desired, I've improved the compound beyond recognition and I'm on the case with the cross slide at the moment.

aboard_epsilon, that sounds like a very pragmatic solution and would work but for the fact that I don't know what the root depth is, I'm guessing though that it's a standard thing that I can look up (assuming the thread is M10, not just the nearest they felt like getting!). I will try that.

loply
08-02-2011, 07:06 PM
Just been trying to look up the root depth of this thread on the internet, it looks like 10mm thread with a 1mm pitch is not a standard thread? At least, I can't find it in any tables.

I don't have an M10 X 1mm right hand threaded nut to test fit it with, other than the one that's attached to the cross slide, which I'll be using to cut the thread :eek:

May have to resign myself to patiently taking the cross slide off for test fittings, then reassembling for another pass!

aboard_epsilon
08-02-2011, 07:44 PM
This is a mini lathe and the screws aren't acme, they are 1mm pitch 10mm things. The main leadscrew is an acme thread, but the cross and compound slides aren't.

The screws aren't actually worn, I have been re-engineering the thing to incorporate bearings and different travels etc. The factory design left quite a bit to be desired, I've improved the compound beyond recognition and I'm on the case with the cross slide at the moment.

aboard_epsilon, that sounds like a very pragmatic solution and would work but for the fact that I don't know what the root depth is, I'm guessing though that it's a standard thing that I can look up (assuming the thread is M10, not just the nearest they felt like getting!). I will try that.
usually in metric you take the pitch away from the outer diameter and end up with the root ..or core if you like

so its 9mm for tapping

handy site

http://www.gewinde-normen.de/en/iso-fine-thread.html

all the best.markj

Thomas Staubo
08-02-2011, 07:45 PM
Just been trying to look up the root depth of this thread on the internet, it looks like 10mm thread with a 1mm pitch is not a standard thread? At least, I can't find it in any tables. Here are all the thread data you will need, loply :) :

http://mdmetric.com/thddata.htm



.

aboard_epsilon
08-02-2011, 08:14 PM
8.773mm

minor diameter d3

according to Thomas' site

but you never know what the Chinese have done.

still may not thread into your nut

all the best.markj

Don Young
08-02-2011, 10:09 PM
One very important thing you need to realize is that neither the tops nor bottoms of the thread are supposed to touch the mating thread. If they do you will never get a good long lasting fit. Also, neither should be a sharp 'V' but should be slilghtly flat or rounded. That is especially important for the tops of the thread.

Don't depend too much on the existing thread to be 'correct', especially if the fit was loose to begin with.

If your crossslide has a removeable nut you may be able to remove it, lock the crosslide somehow, and cut your thread with only the compound.

Carld
08-02-2011, 10:50 PM
Buy your 10x1.0 mm tap and die here http://www.amazon.com/10mm-High-Speed-Steel-Plug/dp/B0006FI844 it will save you a lot of work.

DATo
08-03-2011, 05:02 AM
One thing you can do is to make a dummy thread to test against the nut. I do this a lot when making o-ring grooves. Cut the thread till you think you should be done and then take it out and test it. If it is too loose try it again with a new piece and stop short of the depth you turned the last time. If it is too large put it back in and pick up the thread for more cutting. If you do this with short pieces of scrap stock you won't waste much material. When you finally get a good fit record the position of your dial and cut the real piece to the same depth.

EDIT: Borrowing a page from Don Young's post: You may have wear on the nut so if you calibrate your new thread based upon the size of the existing screw you will only re-introduce the same amount of play. By making a custom fit, as described above, you can make the fit much closer because you are calibrating to the nut rather than to the screw.

moe1942
08-03-2011, 08:17 AM
Fit it to the cross slide nut.

drmico60
08-03-2011, 10:22 AM
Hi folks,

Fairly new to this but I've successfully threaded externally on my lathe a handful of times.

I made a new leadscrew for my compound slide succesfully but it has a bit more free play than the original.

I'm about to make one for the cross slide and I want to get it spot on.

Every time I have threaded on the lathe I've struggled with knowing when to 'stop' cutting.

It will be difficult to trial fit this particular one.

Any tips on knowing when to stop? Is this something you can count off on the compound indicator ring?

Lastly, despite a very sharp HSS threading tool the finish I get tends to look like it's been sandblasted. I polish the thread on my bench polisher to smooth it out, but this 'round off' the peaks of my thread. Any ideas how I can help this?

Cheers,
Rich

Hi Rich,
I had the same problem when I made the feedscrew for my minilathe cross-slide. I solved the problem by making the feedscrew longer than needed then cutting off the end and using it to make a tap so that I could then make an exactly matching nut. The cut off piece was fluted and then case hardened to make the tap, see
http://mikesworkshop.weebly.com/cross-slide-extended-travel.html
After I made the new nut I made a longitudinal saw cut along its length. When the nut wears then it can be removed and squeezed to close up the saw cut a little to compensate for the wear.
I hope his is useful.
Mike

loply
08-04-2011, 07:15 AM
Hi folks,

Thanks for all the advice thus far.

Actually I have now sidestepped the issue - I was aware for some time about a technique posted on here by Evan for making close fitting nuts by melting acetal.

I tried this in the past with little success, but tried again last night and got it to work perfectly.

I therefore plan to ditch the original nut and melt an acetal one instead, meaning the thread form is not overly relevant any more.

Thanks for the useful advice though, I learned quite a bit from this.

drmico60
08-04-2011, 07:56 AM
Hi folks,

Thanks for all the advice thus far.

Actually I have now sidestepped the issue - I was aware for some time about a technique posted on here by Evan for making close fitting nuts by melting acetal.

I tried this in the past with little success, but tried again last night and got it to work perfectly.

I therefore plan to ditch the original nut and melt an acetal one instead, meaning the thread form is not overly relevant any more.

Thanks for the useful advice though, I learned quite a bit from this.
Hi Rich,
I will be very interested to hear how you get on with your acetal nut. I have also contemplated replacing the nut with an acetal one. However, my concern has been that the two small screws that hold the nut to the cross-slide may not grip sufficiently in acetal to make a secure mounting. Please advise of progress.
Mike

loply
09-12-2011, 06:50 AM
Hi Rich,
I will be very interested to hear how you get on with your acetal nut. I have also contemplated replacing the nut with an acetal one. However, my concern has been that the two small screws that hold the nut to the cross-slide may not grip sufficiently in acetal to make a secure mounting. Please advise of progress.
Mike

Hi Mike,

Sorry for the slow reply.

The acetal nut worked really well. What I actually did was to make a metal holder, similar in shape but larger than the original nut, which I bored out to accept the acetal nut. I turned the acetal down till it had a wall thickness of around 2mm, then superglued it into the holder.

Thus it was the steel holder which I tapped to accept the holding screws, not the acetal.

It has less backlash than the original cast iron nut but I made two mistakes which made it less than perfect - firstly my leadscrew was cut with a taper due to the tailstock being broken (meaning I have more backlash at one end than the other!), and secondly when I chased the acetal to loosen it up I took too much off and somehow chewed up the threads in places.

In places I have zero backlash, in other places I have a thou or so.

I had to adapt Evan's original technique because the 10mm conventional thread did not transmit enough heat into the acetal to melt it.

What I did was to turn the acetal down till it had about 4mm wall thickness, then I found a steel tube which it fitted into nicely, and I cut the tube in half lengthwise. I then cut the acetal in half and placed each half inside one of the steel half pipes.

I then melted the acetal directly with a heat gun before placing the leadscrew ontop of one of them, and then placing the other half ontop of that, then clamping it in my vice. This ensured fully melted acetal so that it conformed perfectly to the thread.

I will try to get a pic if you want.

Cheers,
Rich