ACME Threading

[JssParticipant @jss]

Am I doing something wrong here?

I’m in the process of cutting a 10tpi ACME thread for a Potts column type milling spindle. As the work is long and relatively slender I’m using a travelling steady bolted to the top of the cross slide. This should be more rigid than the original Drummond design of steady. The downside is that the fingers have to be withdrawn at the end of each cut and pushed back in contact with the work before the start of the next cut.

The problem that I have is that the process of cutting the thread throws up a significant burr which I’m having to remove with a file every couple of cuts and is spoiling the look of the thread and is time consuming. The fingers of the steady have been fitted with hardened steel tips so, hopefully shouldn’t be suffering too much.

Is there any way I can stop the burr forming?

[JasonBModerator @jasonb]

A traveling steady usually fixes to the saddle and therefore does not need adjusting every time you feed in the tool. If your topslide can rotate to 90deg then you could set it so it is at right angled to the lathe axis and use the topslide screw to put on the cut which won’t alter the position of your cross slide mounted steady.

Looks like you are cutting dry, a good application of cutting fluid such as CT90 or Rocol RTD would also help

[bernard towersParticipant @bernardtowers37738]

Can you not set your compound to 1/2 the angle, you shouldnt get a burr with that method.

[JssParticipant @jss]

Jason, thanks for the reply. The original Drummond design of travelling steady was designed to be bolted to the back of the saddle and had a large slot to allow the cross slide to pass through it, as you may know. I hadn’t thought about turning the top slide through 90 degrees though, I might try that.

The picture was posed, I have been using plenty of cutting fluid.

Hi Bernard, I have done that when cutting standard threads but not sure about ACME threads. Might try that if all else fails.

Thanks, John.

[David George 1Participant @davidgeorge1]

I have just been cutting a new thread for a M Type cross slide and have used the traveling steady as standard for the first time. The thread was extra long and worked well with the traveling steady .

David

 

 

[SillyOldDufferModerator @sillyoldduffer]

What is sort of steel is it?  Burring might indicate an unlucky choice – a steel that doesn’t machine well.

Dave

[Neil LickfoldParticipant @neillickfold44316]

Your insert holder, does not have the correct tapered shim for the helix angle of the thread being cut. The other option is to grind the leading edge of the inset away leaving only a very thin witness of the insert shape and edge. Typically leave less than 0.1mm. Can be removed with diamond file or with diamond wheels with a dremel etc if you don’t have a diamond grinding wheel.

Most industrial threading insert holders have a range of insert shims, and a table of helix angle range that the shims cover. For both internal and external inserts.

To see where the insert is rubbing, coat it with a marker pen or marking blue. Take a light cut and view the area that is rubbing. Use a protractor from the cross slide table, and adjust it to match the helix angle of thread you are cutting . Use the rear side as this will automatically have clearance and be the true angle to measure from. Check that your insert edge is at least 1.5 deg clearance. I like to use 3 deg side clearance.

Mitsubishi turning tools in their threading section of their online catalogue have the data for helix angle and clearance ranges.

you can also change  the compound slide to being 1/2 the angle of the thread form or less. Being less than the back side angle, of the thread form, means that as the tool is advanced, it will take a shaving cut on that edge.

People who make their own HSS tools usually have the leading edge with enough clearance for the helix angle. I like to add some top relief by using a small diamond mounted point and the little relief makes it like a chip breaker and will also give a better surface finish to the thread form. I normally leave about a 0.2mm land of the original shape. Some threading inserts including the Acme forms, have a pressed in chip breaker.

The very smelly Rocol threading oil and compound which has a sulphur compound in it, does give a very good surface finish when threading. So does using cutting oils like the Sunnen honing oil. I have switched to the Blaser cutting oil CSF35 which we also use it work. It does not have any sulphur compounds and does give an improved surface finish over the Rocol and other traditional cutting oils. The Blaser is oil is also a safer product for your health over all. I know also use it for my hone with very good results and stone life. I don’t use their hone oil as it does contain sulpher.  The main task of the oil or coolant is the chip removal from the cutting tool and part being cut.

[David George 1]

On David George 1 Said:

I have just been cutting a new thread for a M Type cross slide and have used the traveling steady as standard for the first time. The thread was extra long and worked well with the traveling steady .

David

 

 

Nice job David! 😉

[David George 1Participant @davidgeorge1]

I use silver steel for these size leadscrews. The material is tough is compatable with cast iron and phosphour bronze and wont wear in use as much as mild steal and it runns true with the steady without having to machine the outside diamiter. The use of a HSS tool gives you more controll over clearances for Helix angle and I use a standard RTD cutting oil to lubricate the tool.

David

[duncan webster 1Participant @duncanwebster1]

In many years of designing machine parts I’ve never called up silver steel for anything that wasn’t going to be hardened (cutting tool or latch block or similar). For a lead screw I’d use En16, which has similar hardness to as bought silver steel, and is easier to get a decent surface finish

[David George 1]

On David George 1 Said:

I use silver steel for these size leadscrews. The material is tough is compatable with cast iron and phosphour bronze and wont wear in use as much as mild steal and it runns true with the steady without having to machine the outside diamiter. The use of a HSS tool gives you more controll over clearances for Helix angle and I use a standard RTD cutting oil to lubricate the tool.

David

Hi David, are the leadscrews ACME threaded or square threaded? I bought the insert (admittedly from EBay) because I didn’t think I could accurately grind a piece of HSS for the job.

John

[old martParticipant @oldmart]

When I was working on the X axis leadscrew of the Tom Senior light vertical,the model with the longer bed, I wanted to have antibacklash nuts. The design would allow this, but the nut halves have to be about 3″ apart, one fixed and the other adjustable. To make things awkward, there was some wear in the central part of the thread and the thread was not long enough, although the leadscrew was. The travelling steady for the Smart & Brown model A had two pieces of brass angle attached to the fingers to spread the contact over an inch of the leadscrew. I had to check the adjustment of the fingers frequently as the brass wore from a line contact to about 2mm wide by the finish. As Jason mentions, travelling steadys are attached to the saddle which makes things easier. I cut 4 inches of new 3/4 x 5 ACME using an insert bought from APT and also had problems with burrs which had to be removed with a file after each pass to stop them interfering with the steady. To minimise the wear in the centre section, I cleaned up that part and made the rest of the thread match it. Since the leadscrew is over 3 feet long, the thread had to be done in sections, which sounds bad, but it is very easy to pick up a 5tpi thread. The lathe is only 20″ between centres and as the leadscrew was 3/4″ it would pass through the spindle. The nuts were leaded gunmetal made to fit the finished leadscrew. Not having a DRO, there is no easy way to check the accuracy of the leadscrew, but is probably better than 0.002″ out in the 34 odd inches of length, good enough for government work as the saying goes.

[David George 1Participant @davidgeorge1]

Hi John. The thread I have cut is a square thread as standard on Drummond  lathes but if you need acme you could grind a HSS tool just the same slightly easier than a square thread as if you have made it a bit narrow you can grind of the side to make it wider as the taper on the shape allows that but if you have ground a square tool too narrow you have to start again. Also you can travers sideways to make it wider if it is a bit tight on an acme thread.

David

[old martParticipant @oldmart]

With the ACME leadscrews on the Atlas 12 x 24 lathe cross slide and compound, I could get new stock from the USA for making two longer ones for the cross slide, but the compound is the opposite hand. Also, There was a nut with the American length which I used in conjunction with the old nut or antibacklash. The compound leadscrew was not so easy, I had to make a new one, but 1/2 X 5 ACME was too small for me to singlepoint nuts, the core is pretty small. The answer was to shoehorn in a 3/4 x 5 ACME as I could make the nuts with a cut down tool. The picture shows the leadscrews.

I made a two start ACME male thread for a special toolpost for the Smart & Brown model A and the helix angle was so steep that I made a pair of shims to tilt the er16 X 8 ACME toolholder.

The other fun I had was buying an ACME tap and trying it out on aluminium before making nuts out of leaded gunmetal and not wanting to risk breaking the tap as the forces were extreme. I made a one off holder with a smaller pitch ACME insert and used it to partially lathe cut the thread before finishing with the tap.

 

 

[JssParticipant @jss]

Well that took some doing. The first attempt turned out to be a failure, despite attacking the insert with a diamond file to increase the side clearance and turning the top slide over to 14.5 degrees and using that to put the cut on. I changed the cutting oil for some Moly Paul Easy Cut which seemed to reduce the burr a little but the work piece still needed attacking with a file after each cut. As the cut approached the correct depth I kept checking against the female part, but the thread went from far too tight to far too slack. I don’t think the insert liked taking fine cuts.

In the end I decided to grind my own HSS tool as David George suggested (I also want to make an extended screw for the top slide). This worked much better than the insert as I ground it with a slight top rake and it didn’t produce a burr. Unfortunately I don’t have an ACME gauge to check the width at the tip so purposely made it slightly narrow than it should have been as I had bought a suitable die to clean the thread up with. However I don’t think the tap that I used to form the female thread matches the die as even with the gap in the die fully closed up the thread is still somewhat tight. Hopefully it will ease with use. Next job is sourcing or making a small hand wheel.

[David George 1Participant @davidgeorge1]

Hi John I have made a replacement handwheel for the crossslide on my M type and the castings are available from Rob Gough if that is suitable to use for your Acme screw.

 

 

David

 

 

 

[Andrew CrowParticipant @andrewcrow91475]

Hi John, a quick question, where did you find the plans/castings for the Potts milling spindle?

Andy

[David George 1Participant @davidgeorge1]

The castings are supplied by Rob Gough.  He has control of the paterns for many parts including the milling spindle. email him rob.gough@thermicsolutions.com they are suplied at cost of castings plus delivery.

David

[Andrew CrowParticipant @andrewcrow91475]

Thanks David 👍

[JssParticipant @jss]

Thanks David, I hadn’t thought of using one of those handwheels from Rob. I might go for that if nothing else suitable turns up.

John.

[Nigel McBurney 1Participant @nigelmcburney1]

regarding material, about 15 years ago I replaced the cross slide screw on my Colchester master,Instead of using a tough material,My thought was to use leaded mild steel as a very good finish can be achieved,and this finish would would not cause any wear to the bronze nut and it has worked and remains accurate ,checked occasionally with a long travel dial gauge,I cut the thread on a S7 and the pitch of the thread was very accurate,I have had the Myford from new and rarely use the lead screw for feed ,leadscrews are meant for threading not feeding,I think it is better to have have a good thread finish than a tough material with a poor finish which will only wear the nut.The Colchester screw was badly corroded and not worn,the lathe was ex government and purchased direct from a government auction,it may have been used with a more corrosive cutting fluid which got trapped in the cast trough where the cross slide runs in the saddle with possibly out of use for some time. At the company where i served my apprenticeship screw cutting tools were in those days hand ground though we did have the advantage of an optical projector to check the thread angle though I still hand grind my threading tools from HSS and use the moore and wright thread gauge.I certainly would not use silver steel.

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