cant get angle of thread cut correct

Hello all, I am trying to cut a thread on my myford super 7. I know the profile should be 60deg as in top dia, no matter what I do I get a flat sided profile (bottom dia). I have tool ground to 60 deg, topslide set at 30 deg (also tried 25, 27, 29, 35, 40 all same result) tool at 90 deg to work. Any ideas why it cuts this flat sided thread??? I have used more than one 60 deg tool, same results.

Do you have the topslide set 29degrees from the lathe axis or 29degrees from a line front to back, if the former then this will give a 1 sides thread.

If you look down on the lathe the handle of the topslide shold be at about 5 O'clock not 3.30 O'clock

EIt Have a look at this thread for the angle of topslide

Edited By JasonB on 11/05/2016 14:27:10

dia of profile

Hello Jason, yes I have topslide at 29 degrees, ie 5 oclock, as per the link, also tried the other angles mentioned. same faltened results.

Photo of setup and results?

Although you have the topslide turned to 29*, the tool requires to be set at 90* /\, so set the topslide (if you must) to the 29* angle, then put the tool in place and set it up with a thread angle gauge. I only use the angled topslide on large coarse threads, on (say) a 5/8" fine thread and less, I just go straight in with a HHS tool, I tend to confuse myself when using any other method, not enough practice.

Ian S C

Hello Ian, yes tool is at 90* & yes I used the fishtail gauge. Will upload pics shortly.

dw,

I believe you may have the tool also set to halve the thread angle, you need the tool set at 90 degrees to the lathe centre line.

Emgee

Ian SC beat me to it !!!

Edited By Emgee on 11/05/2016 15:10:05

Both the tools in red are 60deg but only the top one will give the correct profile

I think Jason hit it first time. If you are setting according to the scale facing you you need the compliment of the half angle which is (90-30) = 60 degrees.

Martin

I can't see how that could happen with a 60-degree tool at 90-degrees to the work, however you applied the cut.

All I can think of is the tool has insufficient relief below the cutting edge. and the vertical edge at the front is cutting back the thread.

Neil

I kind of agree with you Neil, I think he is a little out with the description of the straight flank. I bet it's not as straight as he thinks. But if he has set the topslide from the front setting line at 30 deg the tip of the tool is going to cut the trailing flank as shown. I think it's more like 60 degrees one side and 30 the other

here are some pics

I thought I had ground the angle (60*) wrong so re-did it several times, also used 60* indexable tip got same lop sided results.

Topslide is wrong as I said, set it to read 61deg on the scale.

Look at the photo in that link I posted, the ones with the red lines and angles drawn on

Your topslide should be lined up with the middle blue line shown below

Edited By JasonB on 11/05/2016 16:37:23

doing that NOW it reads 29 on the circular topslide scale at the mo

Isnt knowledge a wonderful thing! Thanks all for your time/help. Its now WORKING. see pics

You can clearly see in the third picture that the angle of the topslide is not the same as the angle in the V of the fishtail that is pressed up against the job. The idea is for the right hand edge of the point of the tool to slide down the side or flank of the thread.. The left hand edge of the tool does all the cutting. You need to understand and feel what is sensible by looking at it not following instructions otherwise you will have no chance of getting a Whitworth 55 degree thread angle set right.

Try making a cardboard tool shape, and move it around on a bit of paper with a 'thread' drawn on it.

While we're at it; you've way too much overhang on that toolbit too. Pull it back into the tool holder so the amount of tool sticking out is about 1 or 1.5 times the thickness of the toolbit. Gives a way better finish due to less flex and therefore less chatter.

I'm with Ian C on not bothering to set the topslide at an angle for smaller threads.

Another way of doing it is to leave the topside in the standard position (0 degrees) and then use the topslide handle to move the tool forwards (toward headstock) a half of whatever depth of cut you are taking. This then actually moves the tool at the required 30 degree angle to do most of the cutting on the one side of the tool bit. So, if you take a 10 thou deep cut on the cross slide handle, move the top slide handle forwards by 5 thou. Tool movement forms a 1:2:3 triangle , 5 thou along, 10 thou in and 15 thou of actual tool movement along the hypotenuse. Angles of such triangle being 30, 60 and 90 degrees.

This method gives the advantage of cutting mostly on the one side of the toolbit, but saves all the faffing about changing in the topslide angle setting. Plus, it has the advantage over the angled topslide method, that when you get to the correct thread depth (which is easily read directly from the cross slide collar), you can then take another cut or two at the same depth but use the top slide to move the tool a thou or two each way each time, thus taking a very light clean up cut on one thread face at a time, giving a nice smooth finish and allowing you to fit the thread precisely to its matching nut.

Top of the tool could also do with some rake grinding onto it, the flat top you have would be OK for brass but not ideal for steel.