| Gallery, Projects and General > How do I?? |
| Press Fits and other questions |
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| fluxcored:
Hi Guys, I'm spending more and more hours on doing lathe work and am getting better finishes and accuracy all the time. I'm becoming faster as well, as I gain more confidence. I'm grinding my own tools and seems to improving on that front also but still have a long ways to go. I'm becoming more optimistic that the lathe will survive me! Where I need some help and advice is on press fits. I want to turn a bush for a pulley with a 18.56 mm diameter hole. Just to fly by my research and math - from Machinery's Handbook I calculate the diameter of the bush required for a press fit to be between 18.54 to 18.52 mm. It may be wrong. Now for the million dollar question. How the heck can a newbie like me with an old Boxford lathe turn to 0.01mm tolerances? I have got a good micrometer and a Chinese dial gauge so I know I'll be able to measure in that range. I guess even preceding that question is - my lathe is imperial and I sometimes get confused a lot. My general rule of thumb is that 10 graduations on the dial remove 0.4mm of material. So a single unit of graduation removes 0.04 mm of material. Does that sound correct? Also backlash - I still do'nt know how to counter that? I try to not to reverse the feed direction when I start cutting but that is not always possible. Also polishing/finishing - I will need to polish the bush with sandpaper to get it really smooth. How much allowance should I make for finishing. I'll be using ordinary mild steel that's why sanding will be necessary. I know I'll be able to get the bush to 18.6 easily. The last 0.06mm is going to be a tough one for me. Sorry for the long post!!! Regards. |
| sbwhart:
Hi I can only assume your machines in reasonable condition so:- First you want to make the shaft fit the hole its easyer to get an accurate OD than a bore. Second good sharp tool. Back lash you wind the tool back away from the job then wind it towards the job, as long as you keep going in the same direction it will be ok but if you have to change direction wind the tool well clear then start winding in again, hope this makes sense. Turn the job down close to size (+ 0.2mm ish) take an accurate measurement, work out the final cut put the cut on and Bobs your uncle, you should have the size you want. Metric/Imperial you're roughly correct in 0.1mm = .004 " Its actualy 3.937 " but for most jobs 0.004 " will be accurate enough. I've got metric machine and measuring kit, but use a lot of imperial drawings so what I do is convert the imperial size to metric I just find it easyer that way. Hope this helps Stew |
| Stilldrillin:
Hi Flux! What works for me...... Mostly! Always machine bush o/d to fit bore. Rough the bush o/d to +.1mm oversize, leaving 3mm or so extra on it`s length. Machining in the 3mm extra length area only. Take very light cuts, (half a graduation), until the pulley will just slide on....... This gives you your dial reading for a sliding fit! :thumbup The interference fit setting is "half a graduation" out from that. Handwheel, 1/2 turn out, to eliminate backlash & then approach "half a graduation out" in 2 or 3 passes. I often leave the 3mm portion on for alignment purposes, for when fitting the bush. Much easier to do, than to explain...... ::) Good luck! David D |
| fluxcored:
Guys, thanx. Stew you confirmed that my thinking's basically correct and that I'm on the right track. Dave, your method makes perfect sense!! Thank you both for taking the time to point me in the right direction. I am truly having fun and what's so good about the experience is knowing that I can fall back on you guys and this board for guidance. I have got a brand new 4 jaw chuck to fit but feel the time is not right for me tackle that job. Maybe after I'm done with the pulleys. :D Regards. |
| ieezitin:
Flux Regarding your backlash. Every machine has it so you have to get used to it. I look at the problem this way. Its all about knowing where you are in measurement terms. If every machine had a DRO on it then you would never really have to think about backlash again but I am assuming you do not so go to the next best thing your dial indicator. Example here. If you were turning a shaft and you had the indicator connected to the tool holder cross slide if you needed to pull back you could as the dial in not lying to you where it is in space. As it has been already said make sure you pull back quite a lot and get a good bite back in and if no pressure is let off you are good to go. OK now your bushing problem. I read it this way, you want to turn down a bushing insert to fit an existing hole of a pulley of 18.56mm diameter. Well your lathe is more than capable of turning down .004 and so are you. By being slow and determined while watching and noting everything what’s happening while you are turning down to your finial size of your bush. Here is how to do it. Get close to your finished size lets say .020 ( note down measurement of od of bush now) then stop. Take off your tool bit and make sure its sharp if not dress it up. So! Nudge the tool tip into the job so it just kisses it then take a pass. Take note of measurement if it took some off . Now slide to tool in .004 by your dial and make a pass get to the end and stop the tool and the machine do not run the tool back!. Note the diameter reading! Did it cut .004? If it did good if it did not make a note!. Now reverse your tool back on the back cut and let it finish then stop and take another measurement. By doing this you have established what your machine does or as I like to say “ she shows her personality she’s telling you what she likes.” By using these two measurement tests you will be getting a fare idea on what’s happening on the cuts plus your confidence level rises on making the cuts. Always make notes and jot down sizes often, this way you will eliminate a lot of silly mistakes and in turn get to know your machine. If you a .001 away then hone with emery cloth, always wet with oil I use WD40. Try also when machining build in little comforts like offering up the fit to the piece in the chuck. I work this way. When I am making something I always think of if I screw up what would be the best way out of it? Example your bush!. I would go for final size with confidence but if its wrong I would rather it be undersize than oversize. If its undersize and it’s a little bit slippery in the hole you can always center punch lots of little dots on the outside of the bush and beat in with a hammer : ) its not the space shuttle right!. Hope this has been a help to you. One more piece of advice machining is easy, it’s the little tricks that are hard. All the best. Anthony |
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