How to match existing taper pin holes

The cross slide stop for my lathe was lost by the previous owner, and I’d like to replace it.
The hole pattern is 12mm x 30mm as closely as I can measure it. The owner’s manual says it’s secured by two M6 35mm socket head cap screws, and two 6mm x 40mm taper pins.

Matching existing taper pin holes seems challenging. My plan is to:
1) Insert and lightly set the two taper pins. Cut the stop block to that height.
2) *Assume* that the hole pattern is exactly 12mm x 30mm
3) Drill and counterbore for the socket head cap screws (shoulder bolts, maybe?)
4) Drill the taper pin holes at 6mm diameter
5) Hand ream the blocks so that the taper pins sit about 20 thou above flush
6) Mount the block with the screws
7) Use the block like a jig, and hand ream again until the pins are flush
8) Drive the pins home below flush, and stake

Is that likely to work? Is there a better way to match existing taper pin holes? Should I just drill and ream the block pin holes to .250”, use that as a jig, and ream the cross slide out? I could then use .250 dowel pins instead of the 6mm taper pins.
Thanks for any suggestions!
Dan

The only way to even have a chance is to drill through the cross slide casting using the existing pin holes as a drill guide and even then if you get it right you will still have to ream the holes slightly larger, so be sure to get longer taper pins and trim to size.

forget the pin holes. bolt it down. apply a little locktite to the flat surfaces if you ever thought you might want to bang into it really hard, but why would you? Why would you ever want to hit it harder than the 2 bolts will clamp it?

You have 3 options. Leave the pins out. Ream new pin holes in a new location and abandon the old pin hole. Ream for 1 or 2 sizes larger pins in the same location.

You should be able to hit one of the pin holes perfectly. The other one is the problem

I'd make an attempt to measure the exact distance between the existing holes, measuring across two pins installed. That measurement is itself quite challenging to do properly and it may go easier if you grow 2 more hands

You'd then expect to have to lightly ream the entire hole a little bit larger once you have the block in place. Don't forget you have permission to do a trial block or two to determine the proper spacing.

Use new taper pins with threaded tops so you can extract the pins at will. Don't drive them in and stake them without provision for removal. Murphy's law says you'll have to take it apart if you make it permanent.

Measure center to center very avcurately by putting a slightly oversize ball bearing on top each hole... leave the rest of the block oversize and correct it after the pins are fit.

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If you decide to use the taper pin holes, you should verify they were reamed vertically. They might be. But maybe not. If you determine they are vertical, their center to center distance could be determined by making up a couple of pins with ends tapered 1:50 (assuming metric tapers) or .010 per inch on the radius and the a straight section of any radius above the level of the bed. Add the outside to outside and inside to inside dimensions and divide by two. The other question is how the taper holes relate to the bolt holes. Similar measurements and a CAD program or old school compass and scale should allow the to be laid out. Happily, the bolt clearance holes can be relatively loose.

Denis

I don't see any real reason for using tapered pins in this application. This is a stop. It's position is relative only to the adjustment of the screw or pin that strikes it to stop the carriage motion. Frankly, I really don't think that pins are even needed as the cap screws should hold it. But if you are doing many parts and therefore bumping into this stop many times, you may want the extra insurance of the pins. If so, then so be it. But the pins are there only to prevent the stop block from moving after a setting has been made, as when successive parts are being made to the same dimension.

In my humble opinion, straight pins would serve the function of preventing the block from moving when it is hit over and over again. There is no real reason for them to be tapered. In this case, I would consider using roll pins (spring pins). They will insure a tight fit in a straight hole so there would be no room for motion.

I would find some straight pins (roll pins if possible) that are larger than the existing holes. Then,

1. Drill and ream out one hole for one of those pins in both the saddle and the stop block. You can do the stop block first and use it as a guide for reaming the hole in the saddle.

2. Drill the holes for the cap screws now. Their locations can be marked with center buttons while the block is held in position by the first pin.

3. Drill but do not ream the hole for the other pin in the saddle only.

3. Install the first pin in the first hole and a center button in the second hole in the saddle. Install the stop block on the first pin, with the cap screws loosely tightened and tap it down on the center button to mark the location of the second pin.

5. Now drill the second hole in the stop block.

6. Install the block again on the first pin with the cap screws tight and ream for the second pin.

This should produce an almost perfect fit and the straight pins will hold it in place just as well as the taper pins did.

If you really want to stay with the tapered pins, you can use a sacrificial pin with a center turned on it as a center punch to locate the new holes in the block. I would do them one at a time and after the two cap screws were installed. Insert the first taper pin while you are marking the location for the second one.

Of course, you should use a taper reamer on the combination of the two holes (saddle and stop block) as a final cut to ensure that the holes are aligned both laterally and axially. This makes me wonder if the hole in the saddle is blind or if it goes through. And if it goes through, what is behind it? Is there clearance for reaming it a bit larger?

Thanks, guys for all the great ideas. I suspect that Hu recognizes the machine in question, and he’s exactly right that there’s not a lot of room there for measuring and working. 
Using precision balls to find the exact dimension is a good idea. The assumption here is that the holes were reamed to the same diameter (depth) on the top of the cross slide, but I will have to do a bit of geometry to see what difference a few thousandths makes. I suspect not very much…
It seems the consensus is leaning towards replacing the taper pins, and simplifying the problem significantly. As a few have pointed out, it’s just a stop, and not likely to get hammered (at least not while I own the lathe) so the bolts should suffice. To increase the holding power a bit, I might take some much-longer partially threaded bolts, and make a sort of custom shoulder bolt, too.
Again, thanks for the ideas… I’ll keep reading, and post an update when I get it installed.

dgfoster wins the prize. They aren't even *close* to being square. So, on to plan B: Use the block as a jig, drill, hand ream, and pin.

Why even bother with the pins?

You would have to be insane to hit the block so hard that it would move with two m6 screws holding it down. It wouldn't move with two m3 screws.

John, I'm tempted to agree with you and the others that suggested something similar. Slightly overdrilling an angled hole, by hand, even with a jig is going to be a trick, as the drill is going to follow the hole.

You could plug the two holes with soft taper pins.. cut near flush and file, then hone to make them disappear. Then drill new hole 1/4 (or so) off that location. Mostly any way you try to match will come out poor.

dgfoster nailed one problem ... now check to see the bolt holes are straight also.

this makes me think about tapered pins. what is the benefit of using them as opposed to straight pins?

dian said:

this makes me think about tapered pins. what is the benefit of using them as opposed to straight pins?

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Remembering that steel is rubber in disguise, taper pins get stuffed into their "sockets" filling them with essentially zero clearance. Straight dowels just slide into place---more or less snug fit, but still can't be as tight as a driven taper pin which compresses into its socket.

An an additional advantage of taper pins is that they are remarkably self-retaining but, at the same time, easily removed with a firm tap on the small end.

Aside: Steam locomotive builders knew knew of the fixation advantage of taper pins as they locked the left and right main castings of the engines together with taper pins, not dowels. You can imagine the enormous forces in play trying to cause one half to "work" relative to the other.

Denis

Another way
Drill the holes for the pins oversized in the new block Like 2 -3 mm or so
Bolt the block down, insert pins and fill the gap between pins and block with a silicon carbide filled epoxy made for such a purpose

But I would simply make pin and hole oversized

Peter

dian said:

what is the benefit of using them as opposed to straight pins?

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Apart from being easier to pull than cylindrical pins, taper pins are for ream-as-you-go hand fits. A well equipped fitter will have the correct length tapered reamer to match each pin to a perfectly centered location. The hand reamers have spiral flues. Somehow I've acquired many kilograms of 20D tapered pins of various sizes. Kinda long unless they're cut.

Another method might be to attach with stripper bolts and not have taper pins or dowels. Might need a reamer and drill for this. Spotting location an off angle condition would make over sizeing the pins a tough job.

Taper pins also provide a high degree of location retaining with a method that requires less precision tools such as close size reamers and exact spindle alignment set up.

well, first of all tapered pins come hardened or soft. that will make a difference. as to working loose, at least the straight pin will maintain horizontal alignment never the less. also whenever the contraption will see disassembly you might get some dirt in between or a slight bow or one part has grown more from temp. change that the other. then the tapered pin will not provide firm location any more. so for parts permanently fixed together the tapered pin is a kind of crossover between a pin and a rivet? yes, i see how the are removed more easyly.

edit: also i wonder if there is the possibility of the tapered pin raising a burr between the surfaces.