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

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

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.

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 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

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

Andy

Thanks David 👍

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.