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| ...mount this threaded backplate on my non threaded lathe? |
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| lesterhawksby:
I'm (still) a total rookie and I'm pretty sure I'm just being a numpty and missing some obvious method, so apologies if this is a daft question, but... I have a lathe of the mini-lathe type: my chucks don't screw on to a central thread, they are bolted on to a flat plate (with a central register) at three places around the rim. I have recently acquired a backplate for another piece of equipment, which I want to turn a register onto to match that on my lathe - so I can use my lathe chucks on it. The backplate is (and needs to stay) threaded and registered to fit a Myford spindle nose: it seems the makers assume that a Myford lathe will also be available to fit it to... Because it has this fitting, the back of the backplate is quite small diameter compared to the front. (It's nowhere near as glossy as http://www.chronos.ltd.uk/acatalog/myfback.jpg but it's basically that shape, if you see what I mean.) Since any inaccuracy I inflict on the backplate register will affect everything I ever do using it, I am rather concerned to get this right! I understand that what's really important is keeping the register I put on the outside front concentric and parallel with the register that exists on the inside back. However, I'm really struggling to mount the plate concentrically enough on my lathe. I tried holding the outside of the smaller diameter part of the backplate in a 4-jaw chuck... this doesn't seem to be the answer, as I struggled for ages and I can't seem to get it concentric and parallel at the same time. Part of the problem is that the "neck" of the backplate is heavily radiused and that really restricts the way I can grip it. My 4-jaw doesn't seem to open large enough to comfortably grip the outside of the plate. I also can't figure out to indicate off the backplate's existing register while I do this, because the front's in the way from one side and the chuck from the other, so my measurement is doubful in any case. I've also failed at the "clamp to the faceplate" approach, because the only place I can see to hold it is around the outer rim which I want to turn. Therefore I thought I'd seek the collective wisdom of the Mad Modders! I'd appreciate opinions on whether I should... 1 - try the 4-jaw again and see if I can improve my admittedly weak 4-jaw skills to the point where I can successfully mount this thing (I can't imagine how) 2 - Build a temporary, chucked, adaptor: chuck a chunk of spare metal in the 3-jaw, turn it to the size of the backplate's register, and fit the backplate to that (will it be rigid enough? I don't think I can get a 12tpi thread out of my metric lathe, either. Ceases to be concentric as soon as I dismount it.) 3 - Build a less temporary, taper fitting, adaptor: like plan 2 but turn the chunk of spare metal to MT3 at one end first, turn it round and put the taper in the lathe spindle, then turn the Myford-type register on the front. (Rather daunting. Would I need to harden this? Would it need a space to fit a drive dog? Still the threading problem. But at least it's reusable.) 4 - Buy some pre-existing adaptor with the right thread (but I can't find one - maybe it has a proper name I don't know) Advice would be greatly appreciated. (apart from "buy a real Myford" ;-) that'll just make me scream) Thanks very much in advance! - Lost of London |
| philf:
Lester, I'm sure that the easiest option for you is to find someone local to you who has a Myford. It sounds a very easy job to modify your backplate once it's screwed onto a Myford spindle nose. I have a Myford adaptor for my D1-3 spindle lathe but I'm up in Cheshire so not of much use to you. Cheers. Phil. |
| andyf:
Hi Lester, If you want to have a chuck+backplate which will both bolt on to a minilathe and screw on to a Myford, that big spigot sticking out of the face of the backplate which you want to bolt to your spindle flange will be the problem, unless it will fit inside the MT3 taper in your spindle. Even if it does, you will need to be very careful not to damage the inside of the taper each time you fit the chuck + backplate. Assuming the spigot will pass down the taper, the job could be done by first drilling and tapping the three or four holes for the mounting studs. The pitch circle diameter (and the register recess diameter) for a C3 minilathe with a 3" spindle flange can be found here http://www.arceurotrade.co.uk/Catalogue/Chucks/Lathe-Chucks (look for the * in the first table on that page). You will probably find that the holes in your spindle flange allow ample clearance for the studs which go through them, because it is the register diameter which locates the chuck, and wriggle room for the studs prevents them pushing it out of line. Thus, ultra precision in the location of the studs isn't needed. Insert the studs temporarily so they stick out on the non-spigot side of the backplate and bolt it to the spindle flange, spigot outwards. Put equal thickness spacers on each stud, so tightening up the nuts doesnt bend the backplate over the shallow boss on the flange. Tap the backplate around gently until a DTI shows the internal Myford register in the spigot is running true. Then turn/bore the shallow register recess to the dimension shown in Arc Euro's table. This really needs to be spot on first time; removing for trial fits and refitting for further work is bound to cause trouble. Remove the job, refit the studs so they stick out of the spigot side, and do whatever needs to be done to attach your Myford chuck. If it is a scroll chuck, some precision will be needed here, to make sure a round bar gripped in its jaws will run as true as you can get it to the Myford Register diameter, and that will be difficult. Anway, if you can manage all that, you should end up with a chuck on a backplate which will screw on to a Myford, and has studs sticking out of it so it will also bolt on to a mini-lathe. Hope this makes sense! Andy |
| lesterhawksby:
Andy - Thank you, "spigot" is a word I should have known and used in describing this problem! As is "spindle flange". Even knowing the words is incredibly helpful... Your description of the problem is much better than mine: the spigot gets in the way of fitting the plate to the spindle flange. (I find knowing the right words is half the battle! I'll have to try searching again now...) I don't really feel the need to regularly fit this backplate to the lathe, I just want my flange-fitting chucks to fit the backplate - but since I need to use the lathe to do that, the problem's the same. Unfortunately the spigot is far bigger than MT3. You're right that the spindle flange holes don't seem to be providing any critical alignment so I think your suggestion of fitting the stud holes is an excellent point, I should definitely do that. Beyond that, I'm sorry to say, I'm not sure I understand your directions. Since the register I need to turn is on the non-spigot side of the plate, if I bolt the plate to the spindle flange spigot outwards, I can't reach the register... can I? I must have not explained very well. I'll try to take some pictures. (The aim is to be able to fit my existing chucks to a dividing head rather than have to buy new Myford-thread chucks. No, I'm not an experienced enough machinist yet to really justify a dividing head... but I'd better learn to be!) Thanks so much though. PhilF - good point, if I could borrow the use of a suitable lathe, it gets a lot easier. I don't know anyone with one. |
| andyf:
Ah, I thought you wanted to end up with a chuck which would could be swapped between a Myford and a mini-lathe, whereas you want to use your mini-lathe chucks on a Myford-nosed dividing head. I suspect, though others may put me right, that you will need to chuck up a length of round stock on your mini-lathe and make a dummy Myford nose on the end of it. The thread should be screwcut on the lathe, and the register diameter outside it should be as accurate as possibl as accurately as you can. Leave the dummy nose in the chuck to preserve concentricity, and screw on the backplate so that you can work on the side which will mate with the back of your chuck to produce the same raised register diameter as the one on your spindle flange. Finally, add (ample) clearance holes for the studs on the back of your chuck. As a guide, the studs on my 4" chucks are M8, but the clearance holes in my spindle flange are 9mm. Andy |
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