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Bookpress 5tpi Square thread help please!

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Martin King 223/05/2019 13:23:43
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Hi all,

Just got this old but knackered bookbinders nipping press in need of a new spindle.

We get a lot of these and they are usually trouble free to restore but this one was seized solid as the spindle was bent and rusted up VERY badly.

Used lots of heat, WD40 etc and short scaffold pole levers to at least get it turning. Cut off the screws to the bottom retainer and finally cut the spindle to remove it. The usual hassle of the very large securing nuts for the 'arch' being seized was for some reason not a problem, they came off easily.

Removing the longer stub of the spindle from the handle proved beyond me as it has a driven in pin locling the thread. Fortunately a pal used his large flypress and it came of like putting a finger in butter!

bpjob 1.jpg

bpjob 2.jpg

bpjob 3.jpg

I have salvaged the end piece of the short stub for reuse, turned the stub down to 1/2" and will bore the end of the new spindle to take it, then loctite and a pin.

I will do all the other work apart from the main thread first.

I have a nice piece of bronze bar for the new spindle that is just the right diameter but have never cut a square profile thread and am very wary of how to go about this.

My Super 7 has a gearbox but there is no 5 tpi setting, Am I out of luck or is there a work around please? Do I have to make a cutter or can I buy one?

Any help will be most welcome.

Cheers. Martin

JasonB23/05/2019 13:29:55
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Has the look of a double start to me.

Grind up a tool like a parting tool but with clearance so it does not rub on the helix of the thread. First thread I ever screwcut was a 1/4" x 16 sq

vintage engineer23/05/2019 13:32:57
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You will need to grind your own tool to cut the thread. I am not familiar with Myfords but it's all down to getting the right gear train to drive the leadscrew.

Martin King 223/05/2019 13:34:00
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202 photos

Thanks Jason, do I use the standard 30 deg on the compound which is all I have ever done before? How do I get 5tpi on the myford gear box please?

Martin

Martin King 223/05/2019 13:34:57
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I should add that this will be only the second thread I have ever cut!

M

Hopper23/05/2019 13:45:49
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To cut a 5tpi thread on your Super 7 with gearbox, you basically set the gearbox to cut 10tpi then alter the change gear train between mandrel and gearbox to double the revolutions of the gearbox input gear. Brian Wood's excellent book "Gearing of lathes for Screwcutting" has a chart showing that for the ML7 and variants with gearbox, 5tpi can be cut by setting gearbox to 10 tpi and fitting a 48T gear to the mandrel gear position. Brian posts on here regularly so I am sure he can confirm/deny the S7 would be the same. Presumably it would be.

As said already you would grind a square ended tool bit with extra clearance on the leading flank to accommodate teh helix angle of the thread. 5tpi thread has a pitch of .200" so tool needs to be .100" wide and thread depth is .100" Tool is plunged straight in, no angled topslide.

If your piece of bronze turns out to be a bit tough to machine, as it sometimes can be in certain grades, an old dodge is to rough the thread out as a V thread, taking care not to exceed the width or depth specs of the finished thread, then finish it off with the square tool.

You might cut a few practice square threads in some scrap material before tackling the main job.

Edited By Hopper on 23/05/2019 13:46:48

Edited By Hopper on 23/05/2019 13:48:03

Simon Williams 323/05/2019 13:52:28
385 forum posts
65 photos

Am I right in thinking those tool dimensions are right for a single start thread, but need modifying for a two start thread?

If this is a two start thread, the thread depth and width will be halved?

Rgds Simon

Andrew Johnston23/05/2019 13:56:27
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Here's one I made earlier:

square_thread.jpg

It's 8 tpi and about 1" diameter, cut as an exercise before I made the proper brake screw shafts.

The (HSS) cutting tool was simply a square parting off shape with the leading edge relieved to account for the helix angle. I made my tool a couple of thou less than the nominal 1/16" width. The thread is square, so you need to plunge straight in as the flanks are perpendicular to the shaft. Leave the top slide parallel to the lathe axis.

A square thread can bind on the crest, on the root and on the flanks. These are independent; fixing one has no effect on the others. Assuming you already have the matching internal thread the crest and root interference can be sorted by calculation, ie, select the correct OD for the blank and cut deep enough to provide some clearance at the root. That just leaves the flanks. That's why I made my tool slightly narrower. Having cut the thread you can advance the tool a known distance (a few thou) with the top slide and shave that amount off one flank. Try the nut for fit and repeat as required.

I can't help with the lathe settings (my lathe will cut 5 tpi) but you might be able to select 10 tpi and fudge the drive train by a factor of 2?

Andrew

Hopper23/05/2019 14:09:50
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Posted by Simon Williams 3 on 23/05/2019 13:52:28:

Am I right in thinking those tool dimensions are right for a single start thread, but need modifying for a two start thread?

If this is a two start thread, the thread depth and width will be halved?

Rgds Simon

Yes, single start. Does not look like a double start in the picture.

But if it were a double start with all the same dimensions, what you would have is a 5tpi pitch, (ie 5 threads in one inch as shown against the ruler) with a 2.5tpi lead. You would set your lathe to cut 2.5tpi. Tool width and thread depth stay same as for 5tpi. You cut one thread first, which leaves a wide blank area in between each "groove".

You then either rotate the mandrel by 180 degrees by marking the mandrel gear and rotating the mandrel then reengaging the mandrel gear. Or do it the easy way by advancing the tool along the lathe axis by using the topslide by one pitch. (.200" ) then cutting the second thread.

Whether you could get a Myford with 8tpi leadscrew to cut 2.5tpi is a tough question. You would have to set the gearbox to 10tpi and speed the gearbox input gear up by a factor of 4, which could be tough to do with the available gears and space constraints.  Quite a bit of stress on the gears too. Maybe possible with compound gearing?

Again, Brian Wood may be able to shed more light.

 

Edited By Hopper on 23/05/2019 14:20:35

Nigel McBurney 123/05/2019 14:35:02
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I was informed during my apprenticeship that the crest clearance on all pitches and any tpi of square threads is a constant ten thou (0.010 ins), Jasons comment could be correct that does look like a two start square thread,which would be difficult and likely to damage the Myford gears,thats why lathes srewcutting charts start at a pitch that the lathe gearing can cope with.One way round this is to apply the drive to the leadscrew rather than the Headstock spindle. A similar approach is taken when milling threads on a universal mill, a short lead attachment was used.Again during my early days there was a regularly need to mill acme threaded shafts,so a temporary arrangement was fitted to the mill table,consisting of a slow speed motor driving via a vee belt to a pulley on the worm handle on a dividing head,the index plunger was removed. the cutter was about 5 inches dia teeth ground to acme angle and mounted on an arbour in a universal head, thread cut at one depth setting,quicker and more constant than screwcutting on a lathe.One way to make a screwcutting tool is use a round toolbit,grind the bit to the required thread angle then mount the tool in the tool post so that the tool can be rotated to the correct clearance by trial and error,then mark the tool so that the top cutting face can be ground at the correct angle.

Hopper23/05/2019 14:42:30
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Yes, .100" thread depth and width is nominal, no allowance for clearance. Depends too on the state of the nut it will screw into. If old and worn, there might be plenty of clearance there!

I have cut coarse square threads on my old Drummond by cranking the leadscrew handwheel to drive the headstock spindle via the gear train. Hard work.

 

Edited By Hopper on 23/05/2019 14:45:32

Andrew Johnston23/05/2019 14:48:50
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I'm in the single start thread camp.

Bookbinding presses are intended to apply, and hold, high pressures. A multistart thread would mitigate against that.

Andrew

Hopper23/05/2019 22:52:25
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3618 forum posts
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Also, helix angle does not look great enough for a double start thread with a 2.5tpi lead. But it's always tricky to tell details from photos.

Simon Williams 324/05/2019 09:19:58
385 forum posts
65 photos

So, (at risk of teaching my grandmother) to bottom out the one start/two start debate:

With a Sharpie follow the crest of the thread around the undamaged bit for one full turn. As you come back round it will be completely obvious whether you are one thread or two threads (or more!) along the work.

HTH Simon

Brian Wood24/05/2019 09:58:01
1895 forum posts
37 photos

Hello Martin,

I have actually made a 5 tpi square thread in steel for a machine vice, the first one I had ever tackled so take heart, it isn't daunting!

As Hopper has said [I am liking his praise for my book by the way!] you will need a driver gear of 48 teeth coupled to the 10 tpi setting in the screwcutting gearbox on your Myford lathe to get the gearing right for the job. When I cut my example I made a cutting tool from a piece of round tool steel, ground to give a tip width of 2.5 mm, tapering back a little on the two sides to give clearance and with about 5 degrees of top rake..

I mounted this up in a split clamp like tool holder so that the cutting tool could be rotated to the helix angle of the thread, that is important so that you don't take off side material in the cutting, The angle was established by sighting from the failed screw, it doesn't have to be that precise either as long as the tool clears both sides while the tip is doing it's work

I ran the first few passes with a standard Vee thread cutting tool to get some of the 'meat' away first and prove the set up, before changing the tool. Be sure you don't go so deep that you take material away from what will be the sharp corners of the finished thread.

I also swapped to the mandrel handle at this stage, I suggest you use yours as well, it is much more controllable.

The square cutting followed the Vee into the material with a light feed and coolant and keep cutting until you get to the bottom of the thread. The geometry is easy with a square thread, it will be 2.54 mm wide and the same in depth

As to whether the thread is single start or double, look at the broken one end on, the difference is obvious. I agree with Andrew Johnson here, it is more likely to be single start to get maximum pressure on the book plattern

As you reach depth, offer the embryo thread up to the nut part and try the fit, It is very likely to need opening slightly in which case shift the tool laterally by no more than a few thou and take another shaving off the side of your new thread. Keep checking until the thread enters the nut and then run a tired abrasive pad along the top of the thread to take off burrs and such, after this it should fit smoothly

Best of luck

Brian

Martin King 224/05/2019 15:02:27
583 forum posts
202 photos

Hi All,

Had a very nice chat with Brian Wood today and the spindle is certainly a twin start job as proved by the "Simon Method" with a Sharpie.

This is not really viable on my Myford and cannot use a single start as the "nut" in ths case is the very large chunk of cast iron 'arch' to the press. No way I can rework that.

Also to use the 48 tooth change gear involves altering the banjo assembly as per Brian's book (now ordered!).

I am going to try an easier route and get some Acme thread allscrew and suitable nut, then attempt to set the arch on the mill and bore out to fit the nut. After that fette the ends to fit the handle and retained bottom piece.

Bit of a shame that I had already cut up my nice long bronze rod and half made the spindle but all good practise!

My grateful thanks to all who have replied here, much appreciated and I will be back wit the next installment when I have my ducks in a row!

Cheers, Martin

JasonB24/05/2019 15:37:56
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You need someone with a 4th axis CNC which should be able to mill a 2.5 pitch 2 start even if they don't know what one looks likedevil

Can be done good enough on a manual mill without gearing the table but very time consuming .

Andrew Johnston24/05/2019 16:44:59
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That's not very nice. sad

But I wouldn't use the CNC mill, I'd screwcut it on the lathe - it'd be quicker.

Andrew

JasonB24/05/2019 17:39:50
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Ah yes, you can go to 2tpi and no worries about stopping at the end as I should think things happen quite fast at those sort of pitchessmiley

Andrew Johnston24/05/2019 21:00:12
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4649 forum posts
522 photos

The 2.5 tpi setting requires me to transpose a couple of gears in the drive train. I did a quick experiment with the correct gearbox settings but not transposing the gears, which equates to 10 tpi. At 40 rpm it seemed pretty sloooow. So 40 rpm and 2.5tpi would be fine using the conventional half nuts. With the Ainjest I'd probably be up at 85rpm or more. The problem isn't stopping but starting, as you need to get the drive fully engaged before the carriage starts to move significantly. I suspect one may well get away without needing a travelling steady.

Andrew

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