Multi Start Threading on Manual Lathe Question

[Rickyb]

I’m seeing a strange similarity to the side profile of the insert anew the profile of the thread groove you cut with it. The plane of your insert is vertical in the cut
correct?

 

[eKretz]

The guys are right if you're using that insert straight out of the box - it's ground with clearance up to 8 TPI, but you're cutting a lead equivalent to 4 TPI. Without grinding in some extra clearance it's probably rubbing pretty heavily.

 

[LexD]

The Dorian insert and toolholder is good for helix angles up to about 2 degrees, the thread you are trying to cut is many times more than that. The only way to get that tool/toolholder combination cutting properly would be to fit a 7 degree packer under the toolholder to tilt it over to the actual lead angle.

 

[guythatbrews]

The Dorian insert and toolholder is good for helix angles up to about 2 degrees, the thread you are trying to cut is many times more than that. The only way to get that tool/toolholder combination cutting properly would be to fit a 7 degree packer under the toolholder to tilt it over to the actual lead angle.

Put another way the vertical centerline should be parallel to the helix of the thread. For a single start thread your tool will work. As the number of thread starts increases the helix angle must get progressively steeper and the tool has to tilt more and more to remain parallel to the thread.

The helix angle is Arctan(lead/pitch diameter circumference) so roughly 6.5 degrees.

As LexD says make a shim or you can mill the bottom of the toolholder and maybe the top as well if the shank needs to be parallel.

Aloris make a holder that makes this easy. The HSS insert tilts.


I'm not crazy about these tools but one would work for your situation.

 

[Cole2534]

Put another way the vertical centerline should be parallel to the helix of the thread. For a single start thread your tool will work. As the number of thread starts increases the helix angle must get progressively steeper and the tool has to tilt more and more to remain parallel to the thread.

The helix angle is Arctan(lead/pitch diameter circumference) so roughly 6.5 degrees.

As LexD says make a shim or you can mill the bottom of the toolholder and maybe the top as well if the shank needs to be parallel.

Aloris make a holder that makes this easy. The HSS insert tilts.

View attachment 385860
I'm not crazy about these tools but one would work for your situation.

You're right, not sure where I got 20. Are you calculating from the major or minor D?

 

[guythatbrews]

You're right, not sure where I got 20. Are you calculating from the major or minor D?

Use the pitch diameter.

 

[mitchapalooza88]

Put another way the vertical centerline should be parallel to the helix of the thread. For a single start thread your tool will work. As the number of thread starts increases the helix angle must get progressively steeper and the tool has to tilt more and more to remain parallel to the thread.

The helix angle is Arctan(lead/pitch diameter circumference) so roughly 6.5 degrees.

As LexD says make a shim or you can mill the bottom of the toolholder and maybe the top as well if the shank needs to be parallel.

Aloris make a holder that makes this easy. The HSS insert tilts.

View attachment 385860
I'm not crazy about these tools but one would work for your situation.

Haha it’s funny you sent this. I actually have the tool but with the P-10 blade so I have the p-4 blade on order that should cover 4tpi

 

[mitchapalooza88]

Many thanks to everyone on the forum. I learned a bunch so far. I have the correct cutter on order but I wanted to try some of the advice tonight. I did not achieve perfect results but I think now I am on the right path.
The biggest change was I set up a drop indicator to track my compound movements more precisely.

I also tightened the gibs at each setting on the compound. I think this is maybe a work around of just dealing with an older machine (slowly working on making the lathe back to its glory days)

I ground my own HSS cutter with better clearance for relief angle. My lack of experience I ground maybe too much cause the tip (.004) did chip off. The tool did not show signs of being pushed as with my Dorian insert though so that was promising.

I also think my depth of cut was a little too much I went to .108. I took that number off my fishtail, I should have done more research.

 

[Cole2534]

Excellent improvement, well done.

As far as DOC, measured vs expected, mine NEVER agree.

 

[mitchapalooza88]

Excellent improvement, well done.

As far as DOC, measured vs expected, mine NEVER agree.

Ya for single point measurement my go to method has been using 3 wire and getting between the min-max for pitch dia. Kinda slow but it seems to work.

 

[john.k]

As an aside,currently available Chinese HSS is so crumbly ,you cant cut with a ground point ,as in threading........look in your glory box for a piece of Fagersta or Poldi Stahl and your threadcutting will be enabled.

 

[cyanidekid]

As an aside,currently available Chinese HSS is so crumbly ,you cant cut with a ground point ,as in threading........look in your glory box for a piece of Fagersta or Poldi Stahl and your threadcutting will be enabled.

what's wrong with Mo-Max or Vasco Supreme?

 

[Cole2534]

what's wrong with Mo-Max or Vasco Supreme?

Couple years ago I found some 1/4 x 1/2 x 6 Mo-Max on Amazon, 10 pieces for like $35. I have no idea how I stumbled across it but it was a heck of a deal and I now use that stuff for everything.

 

[wood2steel]

what's wrong with Mo-Max or Vasco Supreme?

Mo-Max and Rex toolbits have my vote!
I've got a box of bits marked USAAF; Don't have a clue what grade tool steel; but will definitely hold a fine edge!! I cut European T-1 grade tool steel almost daily with vitrified 46-K (blue), or 46/60 Dual grit (purple) stones (under coolant); amazing to watch those plow through these high grade steels!

 

[guythatbrews]

Haha it’s funny you sent this. I actually have the tool but with the P-10 blade so I have the p-4 blade on order that should cover 4tpi

You really need a blade for 12 TPI. Remember pitch is the number of threads per inch, or 12 in your case. Lead is the distance the screw advances in one turn, or 3/12"=1/4" in your case.

 

[guythatbrews]

Unless you have a disc mike you won't be able to measure over wires until you get all three threads going, but you can use one wire with a regular mike. Calculate your measurement over wires then cut to (MOW + Actual Major dia.)/2. Just measure from the back side of the blank. It's probsbly prudent to leave a few thousandths finish allowance on all three threads then go back and take a finish pass.

It was a good move to use a DI to index your tool. Once you get the right tool and it is set correctly, index error will be your challenge. Almost certainly you will have to cut your PD a little small to allow for index error.

Those numbers on your fishtail are probably for a sharp vee thread, and you will not have a sharp tool. I never worry about the exact number but just cut until the thread looks like it is getting close, then start measuring. However, it's not easy to see that on a three start. One way to figure your total infeed for a particular tool is to plunge a groove on a test piece to the correct measured PD and note that amount. You can also use an optical comparator.

 

[michiganbuck]

Here find a pretty good multi-thread video.
Interesting one-wire thread measuring@13;21
(I know one wire measuring has some conditions/cautions.)

 

[michiganbuck]

 

[eKretz]

You really need a blade for 12 TPI. Remember pitch is the number of threads per inch, or 12 in your case. Lead is the distance the screw advances in one turn, or 3/12"=1/4" in your case.

Not so much for the clearance, but yes for the nose radius. If he's going to be tilting the tool, I'm with you.

 

[guythatbrews]

Not so much for the clearance, but yes for the nose radius. If he's going to be tilting the tool, I'm with you.

Yes the nose radius, which I assumed would be too large for 12TPI.

Here is the deal that came with my 10EE for indexing stuff in the chuck.

It adds about an inch of overhang to what is already plenty of overhang with the 8" 3 jaw adjust true. I've never used it or even cleaned it. It looks painfully slow to index but I will bet it is precise.

One thing that happens when you tilt the tool is the thread angle in the transverse plane changes, just like when you add rake to the tool. The thread angle is no longer 60 degrees. It's very small for low helix single start threads and everyone ignores it, but I suspect it may become noticeable for hi-helix or multi-start threads. I do not know if the top of the aloris tool should be ground flat or normal to the tilted cutter. Which has the most error? If it's ground normal then the rake on the leading and trailing tool edge is 0. If flat, then the leading edge rake is positive and the trailing edge is negative.

I'm a 29.5 degree compound guy most of the time. I learned this way and had rationalized 29.5* because it seems the leading edge chip has somewhere to go instead of crashing into the trailing edge chip, as happens for straight-in threading. But it just occurred to me that tap chip formation works just like a straight-in single pointing chip formation. Good enough for a tap, good enough for single pointing. Right? Maybe the real reason is for a flat top tool, 29.5* causes most of the chip to be taken on the leading positive rake edge with just a scrape on the negative rake edge. This becomes more of a consideration with high helix threads when the trailing edge can be pretty negative. If you are indexing the starts with the compound then straight in it is, and it seems the top ground normal to the tilted tool will give the best results with neutral rake on both flanks instead of a big negative rake on the trailing edge. Coincidentally the way I've done it in the past. It just seemed more "right" at the time.