Making an ACME tap or bore single point.

Bassdoctor · Oct 18, 2011

Well I took the cross slide off my Yuassa 13"x40" lathe and the lead screw looks like a "v" thread LOL. :bawling:

I was wanting to replace the screw and the half nuts too. I can thread the screw with no problem but the nuts will be a different story. Should I try to make a tap out of 01 and heat treat it or try treading with a bar. The thread is 7/8" - 8 TPI LH.
I am planning on using preheatreated 4140 for the screw and CDA 954 for the half nuts.
Can I get some advice and input from you guys please.
Thanks , Mark.

kpotter · Oct 18, 2011

I have made many acme taps from mild steel acme rod. I used them to make new nuts out of bronze. Some taps are tandem meaning that they do a light cut then a finish. I made two taps one with a steep angle and the other with a more traditional profile. I would use the steep one first to get half the threads and then the finish tap. They hold up fine in bronze and might work ok in iron if you are carefull.

sfriedberg · Oct 18, 2011

It's too bad it wasn't ACME 7/8-6 LH (or ACME 7/8-5 LH), because $43 will get you a bronze nut for same from Roton.

kpotter, do you case harden such taps?

kpotter · Oct 18, 2011

I have used them unhardend and I have hardened them by heating and quenching. The stuff is mild steel that I used so not much hardening if any but they still worked fine and were much cheaper than acme taps. I used a ball mill to mill the grooves I did four grooves, I have done this dozens of times and it has worked fine but not nearly as good as an acme tap.

Kevin Barrett · Oct 18, 2011

If you say cross slide then it isn't really half nuts you need to make. It is pretty easy to do an internal acme thread however.

Grind a small HSS tool with the required 29 degree included angle then establish the proper width of flat on the tool tip.

You can make a simple boring bar to hold this tool,your bearing bronze will thread easily
and the acme thread form data is easily available.

This is fairly straight forward stuff,the only concern i would have is the overall length of the thread affecting the rigidity of my bar.Obviously a real long(deep) bore is tricky.

I would be carefull with the external thread as well, you will have to use the travelling steady rest and perhaps 4140 prehard could give trouble with chatter.

I did a lead screw for a Series 2 Bridgeport and used 1045 ground shafting,it cut very nicely,although admittedly not as "good" as 4140 or a factory part it still worked daily for many years.

Bruce Nelson · Oct 19, 2011

This internal acme thread that you need to cut is well within the skills of an average machinist to machine it with a single point threading tool in a boring bar. The minor diameter for a 7/8-8tpi internal thread is plenty large enough for a boring bar. Being it is a left-hand thread, it is actually easier and safer to cut, as the threading tool goes from left to right. I would machine the external thread on the cross-feed screw first, and verify the accuracy of that thread with the 3-wire thread system. If you don't have access to the 3-wire system, or cannot perform the calculations for the measurement over the wires, you can do a very close substitute be obtaining a measurement on an unworn area of your old thread with three wires of any size that fit in the thread grooves, and bear on the flanks of the thread, but still project above the thread. You can then machine your new cross feed screw to the identical micrometer measurement over the same wires. After you have machined the external thread, you can use it as a gage for machining the internal thread.

My recommendation for material for the cross feed screw is 1144 Stressproof steel. It has free machining qualities, better than 4140 yet has good wearing properties. Certainly better than original equipment. I also recommend using a follower rest.

Lord Byron

FishTaco · Oct 19, 2011

Bruce Nelson said:
This internal acme thread that you need to cut is well within the skills of an average machinist to machine it with a single point threading tool in a boring bar. The minor diameter for a 7/8-8tpi internal thread is plenty large enough for a boring bar. Being it is a left-hand thread, it is actually easier and safer to cut, as the threading tool goes from left to right. I would machine the external thread on the cross-feed screw first, and verify the accuracy of that thread with the 3-wire thread system. If you don't have access to the 3-wire system, or cannot perform the calculations for the measurement over the wires, you can do a very close substitute be obtaining a measurement on an unworn area of your old thread with three wires of any size that fit in the thread grooves, and bear on the flanks of the thread, but still project above the thread. You can then machine your new cross feed screw to the identical micrometer measurement over the same wires. After you have machined the external thread, you can use it as a gage for machining the internal thread.

My recommendation for material for the cross feed screw is 1144 Stressproof steel. It has free machining qualities, better than 4140 yet has good wearing properties. Certainly better than original equipment. I also recommend using a follower rest.

Lord Byron

Just make sure your wires aren't touching the thread root or it will give you a false reading. Which will make getting the correct measurements on the new screw impossible. You can probably buy a Top Notch threading insert that has the correct Acme pitch ground in it.

Bassdoctor · Oct 19, 2011

yeah

Thanks for all the assistance and help.

I'm not familiar with some new metal compositions as it has been years as I worked as a journeyman machinist. I found a 7/8-8 LH ACME tap on Ebay.uk for $62.00 including shipping. :willy_nilly:

I'm going to try this C1144 stress proof steel for the screw.

I did find a place today that sells scrap or drop pieces of tool steel but the pound. :drool5:

Thanks. Mark.
:typing:

WizardOfBoz · Oct 19, 2011

E-taps has a "finisher tap" for $139: http://www.e-taps.com/ofertapage 118.htm
item GLARGP0875X08F

Also, to check your external threading work you can use this 7/8-8 ring no-go gage:

7/8 8 ACME 2G LEFT HAND NO GO RING GAGE GAUGE MACHINE | eBay

Amazing the stuff you find.

dgfoster · May 27, 2017

I am resurrecting this old thread since the question of tapping an acme thread has come up several times over the last several years. Interestingly, I was picking up a rotab from a guy the other day and he brought out a tap he had picked up in a bucket of tools but was uncertain of its use.

Regal Cutting Tools

Looks like an interesting solution to the problem of just trying to tap a plain hole by cutting the acme thread straight-away. This special cuts a v-groove thread first and the the second, acme portion, finishes the job. Mentioning this as I do not think this type of combination tap has been mentioned. (Won't be amazed if somebody proves that wrong. Oh well.)

Denis

L Vanice · May 27, 2017

I don't know if tandem acme taps have been mentioned, but they have certainly been available in the catalogs for decades. MSC page 282 has a bunch.

Larry

gwilson · May 28, 2017

It would be useful for anyone cutting Acme taps to get a round Acme thread gauge. It will give you cut outs of the width of the tip required,and a gauge to compare your tap to while grinding it on each side.
I've made quite a few Acme taps over the years.

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