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View Full Version : Acme Tap, help required ***Warning machining content



DFMiller
10-16-2011, 06:41 PM
I am trying to make an Acme Tap to make some Delrin nuts. Yes I have read about the alternate methods. But I want to try this one out for comparison.

Using some leadscrew material. 1/2" Acme. I have a 10 degree taper on the lead-in
http://i560.photobucket.com/albums/ss47/ve7pke/Delrin%20Nuts/Acme%20Tap/IMG-20111016-00002.jpg

I have made it into 3 flutes using a 1/8? Ball nosed mill
http://i560.photobucket.com/albums/ss47/ve7pke/Delrin%20Nuts/Acme%20Tap/IMG-20111016-00003.jpg

When I try to tap it will not feed into the material. It will go for a bit then will not feed. I am tapping by hand in lathe with tap in tailstock and winding by hand. Some firm pressure to start.

I assume the issue is that I need to make the grooves bigger as I think there is too much material left on the tap.

I would have made the grooves bigger but could not find a larger Ball nosed mill handy.

Or I guess I could put some relief on the tap with a grinder.

I have tried a couple of different diameter holes and and at 0.401" and still no better

Thanks
Dave

DR
10-16-2011, 06:47 PM
You need to relieve behind the cutting edges.

As it is, all you're doing is rubbing.

Take a very close look at a commercially made tap and you'll see what needs to be done.

Tel
10-16-2011, 06:52 PM
Yes, the flutes need to be bigger in relation to the size of the tap - be worth buying a cheap ball nose to do the job. This is an M8 tap I did

http://richontools.com/index.php?main_page=index&cPath=15_59



http://inlinethumb51.webshots.com/46706/2200736980105506259S500x500Q85.jpg (http://good-times.webshots.com/photo/2200736980105506259niCJfz)

http://inlinethumb62.webshots.com/47997/2365852090105506259S500x500Q85.jpg (http://good-times.webshots.com/photo/2365852090105506259HGIYOe)

DFMiller
10-16-2011, 07:12 PM
OK,I do need to find or buy a bigger Ball mill. None of my tooling suppliers are open today. :-(

How many flutes is yours?
Did you have to grind any relief?
What material did you tap?

Thanks
Dave

John Stevenson
10-16-2011, 07:13 PM
Dave,

take a look at this picture and see how you need to position your cutter.

http://homepage.ntlworld.com/stevenson.engineers/lsteve/files/tapflute.BMP

Please not this is for a Left Hand tap so you need the cutter to the right of centre.

DFMiller
10-16-2011, 07:15 PM
Thanks John,
That make sense.
Mark 2 coming up soon.
Dave

wierdscience
10-16-2011, 07:27 PM
Also even in plastic you may find out as I did that it is possible to twist a homemade Acme tap trying to tap more than a couple thread diameters.

You might consider making your first tap into a roughing tap,and the new one the finishing tap.

DFMiller
10-16-2011, 07:46 PM
OK,
I will bite.
What do you propose in geometry to make a roughing and finishing tap?

Thanks
Dave

terry_g
10-16-2011, 08:18 PM
I made this tap to cut threads in a new compound feed nut for my metric Southbend 10k lathe. If I remember correctly I used 3 degrees for the taper. I should have made the flutes wider because it rubbed hard enough to twist the tap cutting in brass. It is 10mm x 2mm made from drill rod and hardened and tempered.

Terry

http://farm5.static.flickr.com/4033/4591762325_dc13c331f3_z.jpg

vpt
10-16-2011, 08:25 PM
When I built mine I turned the acme threads down to more of a pointed thread. It helped the tap dig in easier and pull itself into the nut being tapped.

http://img251.imageshack.us/img251/7664/nut037.jpg

http://img532.imageshack.us/img532/9537/tap005.jpg

http://img532.imageshack.us/img532/4828/tap013.jpg

http://img87.imageshack.us/img87/7369/nut2021.jpg

wierdscience
10-16-2011, 08:35 PM
OK,
I will bite.
What do you propose in geometry to make a roughing and finishing tap?

Thanks
Dave
Redo the flutes and turn the OD down so the tap is only cutting maybe 60% of the full profile for the roughing pass.

Tanto
10-17-2011, 01:20 AM
Dave,

take a look at this picture and see how you need to position your cutter.


John thanks very much for posting that. I have saved the image as I have a similar job coming up to go and that was simply gold!

DR
10-17-2011, 06:51 AM
Google on something like "tap sharpening relieving fixture". Note all the references to axial and radial relief in the results of the Googling. It's too difficult to explain without detailed pictures which I don't have.

You NEED relief behind the cutting edges, otherwise it won't cut easily if at all. There is absolutely no doubt about the need for relief. Even what John S showed is not right since it has no relief. But, using the largest possible ball end mill without relief will help some.

For proper tap making/sharpening the relief must be ground in a precision fixture so each flute is ground the exact same amount. For the type of results (accuracy) that can be expected with a homemade tap the reliefs could be done carefully by hand on a bench grinder.

Now stop all this nonsense, do some research, and do it right.

J Tiers
10-17-2011, 09:52 AM
Google on something like "tap sharpening relieving fixture". Note all the references to axial and radial relief in the results of the Googling. It's too difficult to explain without detailed pictures which I don't have.

You NEED relief behind the cutting edges, otherwise it won't cut easily if at all. There is absolutely no doubt about the need for relief. Even what John S showed is not right since it has no relief. But, using the largest possible ball end mill without relief will help some.

For proper tap making/sharpening the relief must be ground in a precision fixture so each flute is ground the exact same amount. For the type of results (accuracy) that can be expected with a homemade tap the reliefs could be done carefully by hand on a bench grinder.

Now stop all this nonsense, do some research, and do it right.

I've measured several acme taps here, and near as I can tell to tenths, there isn't any actual relief on them...... not on OD, anyhow, and apparently not on width.

Since a tap is totally un-guided otherwise, it actually makes sense that there isn't relief. With much relief the tap might not cut a good thread.

Yah.. and for this application, asking the OP to do the work of $10 grand worth of equipment on the tap is unrealistic....

The MOST LIKELY reason for problems here is that the "hook" is not present, as JS showed, and in consequence the plastic material is not cutting clean. Plastic does that.... it gets "plowed" a bit, instead of clean-cut, and then springs back to grab.

Also swarf room is minimal on the tap as shown...... Plastic swarf you'd swear expanded.... it gets jammed in and you can't go forward, (sometimes hardly backward either)

Even relief is not very helpful in that case.

best thing is likely to be a rough and finish cut, AND backing out to clear swarf more than you;d expect.

DR
10-17-2011, 12:12 PM
I've measured several acme taps here, and near as I can tell to tenths, there isn't any actual relief on them...... not on OD, anyhow, and apparently not on width.

.................................................. .................................................. ....




I just checked the only ACME tap I had readily available to measure. Keeping in mind it's difficult to measure relief without a fixture and an indicator. I used a dial caliper.

The relief at the cutting end (taps only cut at the end, except maybe taper pipe taps) appears to be about .050" across two flutes of a four flute 3/4-8 Acme tap. So, that would mean each of the four flutes is relieved approx .025" behind the cutting edge.

I also checked a 1/2-20 conventional thread tap. It's relief across two of the the four flutes is approx .025".

Verify this yourself. Measure the diameter across the cutting edges at the smallest part of the end chamfer. Then rotate tap CCW and measure the diameter (right hand tap looking at the cutting end).


Again, why would the sharpening fixtures found by Googling as I suggested have so much mention of relief if taps were not relieved?

lakeside53
10-17-2011, 12:19 PM
If it was me... I'd make one tap with two parts - a section for roughing followed by a section for finishing. I have several commercial acme taps "tandem taps" made just like that.


Here' a link with examples : http://www.rntap.com/Categories/Miscellaneous-Taps-and-Tools/Duo-Acme-Tandem-Style-Taps.aspx

Boucher
10-17-2011, 05:40 PM
Ridgid makes special threading tools for cutting male threads in Sch 80 PVC the difference in performance is significant with very subtle differences in configuration. First the tools need to be very sharp and have a relief similar to that suggested by John Stevenson. The tools are marked for plastic and one does not cut steel with them. They will cut a proper thread in one pass. The other method is with a quick opening tool where the thread is roughed in several passes with the tool first set to cut oversized threads.
I have never seen special tooling for cutting internal threads but if you are making taps those principles would apply. The multiple tap roughing approach will probably give the best results.

Duffy
10-17-2011, 07:08 PM
I did EXACTLYy what you are trying to do. The only differences were I was cutting a 7/16 LH nut and I used a 1/2" end mill to cut the flutes on my tap. (I did not have a ball end mill and did not know any better!) It worked JUST FINE. There was one problem and that was it was VERY difficult to turn the tap. I ended up using a big pair of visegrips and I could barely get 1 1/2 turns before backing out to clear chips.
The finished product worked very well as a replacement cross-feed nut on my Standard Modern lathe with virtually NO backlash.
Evans method using a hot air gun would have been A LOT easier but I was finished by then.

DFMiller
10-17-2011, 07:13 PM
Duffy,
How many flutes did yours have?
Got a picture.
I am just getting around to figuring out some dimensions using Johns drawing.
Thanks
Dave

gwilson
10-17-2011, 07:16 PM
The suggestions to relieve the tap are the correct ones. It won't start to cut unless you do. Making the flutes larger will also help the chips to have somewhere to go,but to get the tap to start to cut,grind a relief on the leading edges.

I had a gunsmith bring me a 3/4" tap. He had done a fine job of making the tap,but it would not cut at all. He hadn't ground any relief on it. I hit it on the belt grinder,carefully bringing the relief right up to the cutting edges. Then,it cut normally.

J Tiers
10-18-2011, 01:31 AM
The suggestions to relieve the tap are the correct ones. It won't start to cut unless you do.

there may be two different kinds of "relief" being discussed.....

You DO need to work on the front cutting portion.....

But from what at least one poster was saying, the tap should be relieved like a gear-cutter on the entire length of the tap, and I cannot measure any such thing on the couple acme taps I have, nor on the regular V-thread type I checked (1"-8).

darryl
10-18-2011, 02:27 AM
I'm wondering what happens when a tap with relief is backed out- do you hope to heck that no swarf gets to ride up that shallow narrowing gap and prevent the tap from coming out-

I think, and from my experience making similar taps, that the flutes need to be opened up more, which will leave less meat on the threads to jam in the hole.

What Sir John shows to give positive rake to the leading edges makes sense, but I would also like to see positive rake on the trailing edges. The tap shouldn't have to cut in reverse, ie when backing it out, but a negative angle there would act to bind crap in the hole rather than 'scoop' it out.

You should be able to go in deep enough with a ball end mill to get that positive rake on both sides of the teeth. You may not get a positive rake at the very beginning of the tap, but that soon passes after a couple or three threads.

The other thing about a tap that I wanted to suggest is that the profile in the flutes acts to roll up the cuttings and keeps them in larger, but compact pieces. These are less likely to interfere with the tapping action, reversing etc- but they need room to occupy so you don't have to back out so often. Another reason to take out a fair amount of meat when making the flutes.

Just looked at several commercially made taps I have- they do seem to be ground in such a way that the leading edges have positive rake, but the trailing edges do not have negative rake. At the point where the diameter is full size, there does appear to be a slight positive rake on the trailing edges as well.

John Stevenson
10-18-2011, 03:27 AM
Just off to the show so no time but I have a tap I made for doing delrin M24 x 4 pitch nuts.

I did have to tune it a tad to work better.
I'll post pics tonight when i get back.

DFMiller
10-19-2011, 02:46 AM
John,
Any reason why you chose to illustrate a 5 flute tap?. When I was messing around in the cad program I finally noticed that.

I was successful today tapping my block of Delrin today with the tap I pictured earlier.
I hacked it up terribly by taking the Foredom to it with a cut off wheel. I gave it some relief on the OD. It was easy to do and with the leadscrew material with a black coating it was easy to see where I had not removed material. Once I put some relief in it it cut pretty nicely.

When I do the left hand nuts I will try John's suggestion.

Thanks for everyone's help.
Dave

Paul Alciatore
10-19-2011, 04:14 AM
I just checked the only ACME tap I had readily available to measure. Keeping in mind it's difficult to measure relief without a fixture and an indicator. I used a dial caliper.

The relief at the cutting end (taps only cut at the end, except maybe taper pipe taps) appears to be about .050" across two flutes of a four flute 3/4-8 Acme tap. So, that would mean each of the four flutes is relieved approx .025" behind the cutting edge.

I also checked a 1/2-20 conventional thread tap. It's relief across two of the the four flutes is approx .025".

Verify this yourself. Measure the diameter across the cutting edges at the smallest part of the end chamfer. Then rotate tap CCW and measure the diameter (right hand tap looking at the cutting end).


Again, why would the sharpening fixtures found by Googling as I suggested have so much mention of relief if taps were not relieved?

I find this discussion very interesting. From what I know about cutting edges, I can see no reason why a tap would need relief, either on the tapered section or on the full section behind it. Relief is ground on most cutting edges to allow them to penetrate deeper into the material being cut, not to allow them to cut in the first place. Consider a work piece with a completely flat surface on it's top. Now, consider a HSS tool that has a completely flat bottom surface that is placed in contact with the flat top of the work piece. It also has a "cutting" edge ground onto one end, with a positive, zero, or negative rake angle. Now move that tool across the flat work piece. What you effectively have is a bearing, not a cutting tool because that flat bottom of the tool will guide the cutting along the flat top of the work piece but will not allow that cutting edge to penetrate downwards into the work. So you have a zero relief angle and it does not allow any cutting to be made.

Or does it? Imagine that same, basically flat work piece has a high spot in a small area. Now when the same tool, with no relief, comes up to that high spot, it WILL shave it off. It will cut into the high spot and make it level with the rest of the flat surface. So a tool with no relief angle can indeed cut, but only in a limited manner. It will remove high spots, but will not allow you to reduce the level of the overall, flat surface. Even better, is the high spot is actually a step up to a higher level that extends for a considerable distance, this tool, with no relief, should be able to continue cutting that higher step off for a considerable distance, even forever. This is because it is already at the lower level and it simply stays at that lower level for the duration of the cut.

Now look at the cutting action of a tap. The tap is designed to cut only a small amount of material to produce a thread. It is cutting a set of grooves in the sides of the hole. The flat surface of the above example is replaced by the cylindrical surface of the inside of the hole. But, you do not want it to cut sideways which would move the hole sideways and increase it's basic size. Cutting sideways for the tap is the equivalent of the flat cutter I spoke of above cutting downwards into the flat surface. So it would seen that a tap with no relief should cut a thread just fine just as the flat tool will cut off high spots on a flat surface just fine.

I am neglecting friction here and a plastic material may produce a considerable amount of friction because it may expand a bit at the point where the cutting is being performed and then contract a bit on the the non cutting threads on the tap that serve to guide it into the work. The positive rake angle would serve to minimize this effect as it will tend to draw the sides of the hole toward the center of the tap as it cuts (just as a tool with positive rake will be drawn into soft materials like brass).

As to why commercial taps have relief ground into the tapered cutting edges, I would suggest this: Most commercial taps have the tapered section formed by grinding a "flat" at an appropriate angle. Flats are easier to grind than a conical surface. But, flat surfaces must have a relief angle at the cutting edge if they are going to clear the work at the back edge of the flute. This rear edge must be kept below the diameter being cut by the cutting edge. In a production situation, it will probably be ground with a slight amount of extra clearance to be sure the back edge will clear. I would suggest that this is the more likely reason why clearance is observed on commercial taps. This would also explain why sharpening fixtures would speak of clearance angles. They are only necessary if you are grinding flats, not if you use a conical, tapered surface.

One other comment, the best taps are ground, not milled. Grinding will produce a better, sharper cutting edge, just as a knife edge or a lathe tool that is stoned will be a lot sharper than one that is coarse ground or filed to an edge. A sharper edge will cut better, with less drag. I suspect the real secrets are dead sharp cutting edges and larger flute spaces for the chips. Even a commercial tap will get hard to turn if the flutes fill up with chips.

I am not an expert on this and if anybody can explain where the above is wrong, I will welcome it.

J Tiers
10-19-2011, 09:30 AM
Paul, on a plastic material, a relief is one way to actually get it to cut.....

With steel the effect is on a micro level, with plastic it is on an almost macro level.

No need for relief on the main cylindrical portion..... all that does is guide, and hopefully not drag too much. But for the cutting portion, it is needed.

All materials are springy. The cutter will not quite cut a full shape if the material is well supported at the full cutter section right behind the edge, because the slight spring of the material away from the cutter causes it tio cut slightly undersize, similar to a springy lathe workpieces.

The lathe will then cut OVERSIZE on an OD, but with an ID, the effect is to cut UNDERSIZED. Similarly with a tap, if the material springs away, it cannot be cut to the full profile, it will close down after the cutter edge passes, and will then be supported, holding material away from the edge. That leads to a "bog down" as the closing down becomes tighter and tighter, and more area is "clamping" the tap.

If the cutter edge is relieved, the material is NOT supported behind the edge, and the edge develops a full force into the material. All the force is ON the edge, so the tendency is to cut much more nearly to full size.

With plastic material, these tendencies are very obvious. With other materials, they may be so small as to be far less important.

it is not uncommon for the main cylindrical portion of the tool to be tapered down in diameter very slightly, as you measure farther away from the edge... drills, reamers, and perhaps even taps may be made that way.

But for a tap, as far as I know, the only area with actual relief is the front cutting area. I cannot measure any even on a 1" tap in the cylindrical portion.

DFMiller
10-19-2011, 01:07 PM
I will try to get a picture to show what I did. I think the main issue is that I had to much material on the OD that was touching the whole. By removing most of that material the "cutting edge" was able to scrap away the material. The change was much more dramatic than I expected.

The bottom line is it seemed to work well. Once I get a little farther I can measure the backlash in the screw. I eventually expect to replace the Acme screws with some ball screws but need to survey the rest of the old Barker mill more before spending much money.

Thanks

I have learned some.

Dave

beveledegg
10-19-2011, 02:44 PM
I recently had to make a replacement nut for a 3/16"-20 left hand acme screw out of bronze...one of those gotta have it now situations. I single pointed the shank of a drill and cut four reliefs to make a tap. The tap cut the first few threads and spun just as yours did. I remember my old shop teacher telling that acme taps like to have lots of pressure when tapping. The pressure helped and it made the tap easier to turn (still had to back up several times). If I were to do this again, however, I would make a roughing tap with either short or skinny teeth.

DFMiller
10-19-2011, 04:01 PM
Welcome to the Forum,
You may be right. I was applying all the pressure I could on my lathe.
Here is a picture of what I did.
http://i560.photobucket.com/albums/ss47/ve7pke/Delrin%20Nuts/Acme%20Tap/IMG_0241.jpg

Dave

DR
10-20-2011, 09:51 PM
Dave,

It looks like you put relief on the non-cutting part of the tap. That part of the tap has no function other than to guide the tap at the correct pitch.

Put relief on front chamfer area right up to the cutting edge and you should find pressure won't need to be so high, but torque required will still be high. Even a factory made 1/2" Acme tap will be hard to turn by hand, that's why they make roughing and finishing versions.

Needing high pressure implies the tap is forming threads rather than cutting.

gzig5
10-21-2011, 05:26 PM
Welcome to the Forum,
You may be right. I was applying all the pressure I could on my lathe.
Here is a picture of what I did.



Dave

If it is a RH tap, you relieved it right, but you should get right up next to the cutting edge. 1mm back from the edge is plenty, .5mm is better. It looks like you have only one flute.

I would suggest two things. Double the amount of tapered lead and cut at least three flutes (each relieved up to the cutting edge) in the manner John Stevenson described or a flat end mill will work too. The longer taper will allow the tap to start easier. The additional flutes will keep it centered in the hole and cutting on diameter. I can't see a single flute tap like that cutting correctly because there is nothing to support the back side. It think the thread would end up undersized.

rohart
10-21-2011, 06:02 PM
I agree with the requirement that apart from the rake, especially important with delrin, relief (top relief) should be there.

But, and this is not so important with delrin, I tap two stage with acme, and my next acme tap will be three stage.

By that I mean that the major diameter of tap 1 and tap 2 do not get to the major diameter of the screw. Only the third tap 3 cuts to the full depth. Tap 1 will cut to slightly more that 1/3 depth.

Cutting steel or brass to an acme or trapeziod thread form, there's just too much material to be removed by a short tap.

Sure, a foot long tap could do it, but it wouldn't fit in my drawer.

I have to admit I only did it once, but it worked, albeit a bit stiffly, in brass, as a two stage tap, and this was on a small diameter. It was an 8mm diameter 2mm pitch. I cut the relief with an ordinary rectangulat x-section end mill, positioned over axis so the teeth had an acute angle to them. I would cut the relief deeper next time. I did not provide any top relief.

DFMiller
10-21-2011, 06:17 PM
Gzig5
Yes that is the RH one. I need to do a LH one soon.
Its actually a 3 flute tap. I thought I posted a picture earlier that showed that.
This one was not flutes per Johns suggestions. It was just three cuts with a 1/8" Ball nosed. I will take all suggestion into considerations on the LH one. Including the relieving closer to the cutting edge. It works so I am happy.
Thank you for your suggestions.
Dave