Delrin Cross Slide Nut

No matter the material or method used to make a lead screw nut, it cannot correct for poor quality or uneven wear in the lead screw itself that I know of. Getting the right feel from one end of travel to the other is an elusive goal and ultimately a personal choice.
Molding a polymer nut on the lead screw is an interesting approach that I hope to try eventually if the situation warrants. For now the metal split nut currently riding the lead screw is adjustable so backlash can be adjusted to my liking.

Turcite is a plastic which combines acetal and silicone and used on wear surfaces in many applications, including lathe and mill ways. Delrin is a trade name for an acetal polymer. Delrin AF contains Teflon and is used for similar applications as Turcite

I have used both acetal and Delrin and can't honestly say that I noticed any difference in performance. For the purposes described above, either should work. They tend tend to have self lubricating properties which is why it is used for bearing surfaces. Delrin has a yield strength of between 6.5 and 9.6 Kpsi or about 30 to 45% that of bearing bronze. Delrin and acetal a common choices for gears. When I built the reversing banjo for my 602, I used acetal gears. Given the contact surface for a lead screw nut, I would expect good to excellent performance under conditions normally seen in a small lathe.

I have done a bit of thermal machining of Delrin. In one instance, we were looking at making a .009" hole in a 1" block. I used a length of wire connected to a power supply to "cut through the plastic much like a weighted string on a block of ice. The Delrin melted around the wire and the wire slowly moved to the center of the block with melted Delrin filling the path and freezing. The result was a wire embedded in a solid block of Delrin. When everything had cooled down I pulled the wire, leaving a .009" hole.

If using the above described method to form a lead screw, the Delrin will shrink as it solidifies which will result in an interference fit. Typically, a clearance fit in a screw /nut is obtained by modifying the basic pitch diameter. With the Delrin thermoformed nut, it will also be changing the thread pitch. This will amount to a preload which can be a good thing if it isn't so much as to prevent movement.

One word of caution when heating Delrin. It gives off some very obnoxious fumes if heated to too high a temperature which attack the eyes and mucus membranes so do so in a well ventilated room

R J,

An excellent write up for Delrin and comparison to Turcite. And yes, it gives off some obnoxious fumes when machining the stuff, too!

Ken

ddickey said:

Would I need to make a new lead screw or just mold the nut onto my current one?

Click to expand...

You will likely need a new lead screw also. The screw and nut wear together and change geometry depending on which part of the screw gets used the most. You can measure the thickness of the threads where it wears the most and compare to the ends to see if you have excessive wear. I just replaced my cross feed screw and (bronze) nut on a SB Fourteen, and the new parts makes a world of difference. Solid, predictable tool movement, much better finish, a great improvement. Brian at Miller Fabrication and machine made it and delivered in a few days time. My lead screw is fine, but pretty soon will do the same thing with the compound screw.

Glenn

RJSakowski said:

If using the above described method to form a lead screw, the Delrin will shrink as it solidifies which will result in an interference fit. Typically, a clearance fit in a screw /nut is obtained by modifying the basic pitch diameter. With the Delrin thermoformed nut, it will also be changing the thread pitch. This will amount to a preload which can be a good thing if it isn't so much as to prevent movement.

Click to expand...

A good point. In my case I prefer to have the least amount of drag between lead screw and nut even at the expense of a bit of clearance. Bear in mind that another source of backlash is the lead screw to carriage connection so we have to live with backlash in the system anyways you look at it.

What is important to me is that I can feel only the drag in the dovetails when setting the gibs and when operating from then on. The technique that Phil mentioned in post 38 is a reflex for me.

Add to the backlash equation, the clearance in the lead screw thrust bearing and "lost motion" caused by flex of the "solid" parts when force is applied. The latter, almost counter intuitively, increases as you tighten things up.

Regarding the thermoformed nut, A makeshift tap can be made from a section of lead screw to cut clearance in the nut. Simply grind a cutting edge in the fashion of a tap in an old screw thread. For cutting Delrin, it needn't be hardened but remember that Delrin likes a sharp edge. If you don't have an old screw laying around one can be made fairly easily by single point threading.

Glenn Brooks said:

You will likely need a new lead screw also. The screw and nut wear together and change geometry depending on which part of the screw gets used the most. You can measure the thickness of the threads where it wears the most and compare to the ends to see if you have excessive wear. I just replaced my cross feed screw and (bronze) nut on a SB Fourteen, and the new parts makes a world of difference. Solid, predictable tool movement, much better finish, a great improvement. Brian at Miller Fabrication and machine made it and delivered in a few days time. My lead screw is fine, but pretty soon will do the same thing with the compound screw.

Glenn

Click to expand...

Glenn,
You replaced just the nut?

I took my compound and cross slide apart a few days ago just to clean and inspect. I was not certain about taking the top part of, I guess it would be the carriage off. Anyway two things I noticed. when I put the cross slide gib back in it stuck out more than before. I really can't even get the wiper to fit properly. Also I didn't realize the small thrust bearings on the handle can also be a source of backlash. You can't tighten them down real hard or you won't be able to move the handle.

Ddicky, no, sorry wasn't clear, replaced both nut and cross slide screw. Both were worn, not badly, but caused a lot of erratic cutting action and very inconsistent and undependable depth of cut. Rock solid now, thanks to Miller Fabrication and machine!

Also, re: your last post. Look for swarf or chips that may have accidentally dropped into your ways or cross slide dovetail as a possible reason for not fitting back together as it should. Generally when I have cleaned my equipment of of old oil and gunk things loosen up more, not less than before - due to increased clearance caused by removing old dried oil etc from the mating surfaces. Once in awhile chips sneak into the cleaned area and get snared by the fresh oil, obstructing the fit you expect.

Also sometimes the wipers are not symmetrical. Try fitting them to opposite ends to see if the screw holes line up better.

Indeed the thrust bearings and gib work together, sort of, to control unwanted movement of the cross slide. I found first, take slack out of the screw with the thrust bearing (use a dial indicator to reduce overall cross slide movement to as close to .000" as you can - however to binding on the wheel- back off a tiny amount to get the torque load on the bearing just right - finger smooth movement is the goal. Then tighten up the gib to stop side to side 'rocking' movement of the compound base on the apron. Then when it binds, slackmoff a nudge to get easy fluid one finger control of the cross slide. It turns out to be a delicate adjustment on both parts....

BTW, Iam also replacing my gib and the rear thrust bearings on my SB Fourteen in hopes of having more control over the adjustments of the cross slide .

Cheers, it will all work out!

Glenn

RJSakowski said:

Turcite is a plastic which combines acetal and silicone and used on wear surfaces in many applications, including lathe and mill ways. Delrin is a trade name for an acetal polymer. Delrin AF contains Teflon and is used for similar applications as Turcite

I have used both acetal and Delrin and can't honestly say that I noticed any difference in performance. For the purposes described above, either should work. They tend tend to have self lubricating properties which is why it is used for bearing surfaces. Delrin has a yield strength of between 6.5 and 9.6 Kpsi or about 30 to 45% that of bearing bronze. Delrin and acetal a common choices for gears. When I built the reversing banjo for my 602, I used acetal gears. Given the contact surface for a lead screw nut, I would expect good to excellent performance under conditions normally seen in a small lathe.

I have done a bit of thermal machining of Delrin. In one instance, we were looking at making a .009" hole in a 1" block. I used a length of wire connected to a power supply to "cut through the plastic much like a weighted string on a block of ice. The Delrin melted around the wire and the wire slowly moved to the center of the block with melted Delrin filling the path and freezing. The result was a wire embedded in a solid block of Delrin. When everything had cooled down I pulled the wire, leaving a .009" hole.

If using the above described method to form a lead screw, the Delrin will shrink as it solidifies which will result in an interference fit. Typically, a clearance fit in a screw /nut is obtained by modifying the basic pitch diameter. With the Delrin thermoformed nut, it will also be changing the thread pitch. This will amount to a preload which can be a good thing if it isn't so much as to prevent movement.

One word of caution when heating Delrin. It gives off some very obnoxious fumes if heated to too high a temperature which attack the eyes and mucus membranes so do so in a well ventilated room

Click to expand...

Nice one RJ
Re :- The fumes , it also catches alight with a nigh on invisible flame & it sticks to your pinkies very well , even the tiniest burning splodge is agony ... guess how I found out ?

In the earlier post I mentioned using cheap digital thermometers fixed to the bar at the ends & a free one to check the middle to ensure you get an even quality melt .

If you heat the threaded bar to the melt point ( do a test on some scrap & scrap thread to find you thermoplastics melt point ) and use a square former (made from two sections of angle iron ) you can make melt nuts or other shaped parts for that matter of fact . I've been considering making a smooth aluminium cylinder with an aluminiun piston at each end in which I've taken out enough metal to make a cast form put in sufficent plastic ( by volume in water .... Eureka I've found it ) then use the heat method I've described & a vice to squeeze things up to produce repeatable parts Using my Dremel to grind /remove metal so as to get the forms I need in the soft aluminium piston faces.



Reading the post about turning a single point thread , I've seen a set of quality Acme taps available at the cost of an arm & a leg too .
Would either way work well on a 3/8 internal acme screw thread ?

I can see it would be difficult for me to turn one up with my limited skills in such things , especially getting the tool cutting angles correct for an Acme thread .
However I've learnt to turn up a very accurate rectangular square block in the four jaw chuck out of all sorts irregular shaped rubbish /scrap . Perhaps putting a thermoformed nut on an acme thread & then turning it to a round rod between centers should allow me to use the cross slide to put a decent mounting flat along the cylindrical face & have it reasonably accurate when aligning / fitting things up .

Another thing might be buying a fairly thin walled acme phosphor bronze sleeve , knurl it a bit then thermo-form / embed it in the bigger part of the assembly ,

Ooooh , I'm spoilt for options & have so little time to play at doing them . Oh for a decent precision mill, lots of attachments & the space to put it .