How to accurately bore on the lathe?

[macardoso]

Nothing like a 350 degree oven and air in a can to freeze the shaft. a 400 degree differential in parts , with a16mm bore will add an extra .0016" or .04mm.Once it cools off it may be difficult to remove!

So I may or may not have been screwing around doing just that and got my pulley stuck halfway on the shaft (motor not yet installed)

 

[P. Waller]

0.004mm is 0.000159”. Measured with a telescoping bore gage and mitutoyo half tenths mic. Admittedly this tolerance is rather unnecessary, but the exercise in hitting repeatable tight tolerance dimensions on a lathe is not.

Old machine shop joke, add a 0 to the right of the period, add a 0 to the cost left of the period (-:

 

[cascao]

For this kind of job I have grind a boring bar from a carbide rod. Very sharp and way less deflection than any steel.
What is the each shaft real dimension? At this tolerance maybe there some variation on shaft too and each shaft will match one specific pulley.

 

[macardoso]

Shafts are 16.000mm +.008/-.003 so that easily could play into these fits! I'll see if they actually vary and maybe match pulley to shaft. I'm going to try to finish the rest tonight and see how they turn out. Thanks again everyone

 

[higgite]

.004 MM is 0.00000159", how did you measure this?

Somebody needs to check their calculator battery. .004 mm = .000157".

Tom

 

[petertha]

My close tolerance boring experience is confined to model engine liners in various kinds of steel & cast iron. The target has to be within 0.0005" but more importantly, of a specific finish at that final bore. That's not exactly an easy combo to hit. I have a super stiff boring bar that has a shank diameter probably 70% of the bore ID. I tried HSS but preferred sharp inserts for consistency. Part of that was likely my inability to dress & set up HSS properly & maybe cheapo import HSS boring bar. They just aren't as common anymore.

What I think you will discover if you get into lapping after turning is bore gages are measuring an average of whatever hill tops they are residing on. That may be fine for your purpose, in fact maybe even desirable if you are using retainer type adhesive. Lapping its quite revealing. Your beautiful boring bar finish at 0.0005" tolerance inevitably turns into 'micro-stripes' where the lapping has flattened the hill tops with a matt finish but the troughs are still shiny. My pics don't do a great job of showing this properly but I'd recommend you try yourself. This is why I was recommending you change up your traverse feed selection on final spring passes. If you don't, you are just spring passing the bottom of the vee. But that has little to do with the dimension across the crests (think of it as threading which is essentially what is happening). Yes a more rounded tool tip minimizes this effect, but I feel also at resisting DOC so even more need for spring pass. Lapping also reveals other effects like ever so slight bore difference between the opening of the hole & closer to the chuck. Lapping (if done properly) doesn't lie, its visually quite apparent until the whole surface is entirely matt & now removing native material.

Of course all this is very material + cutting tool + machine rigidity dependant. The recommendation to stabilize temperature prior to measurement is very important at these tolerance levels. Re measurement, I like snap gauges too but I would recommend a good quality bore gage set against a known standard. Not only can you get good consistent direct readings without in-between-device chances of error but you can repeat this many times & at different positions down the bore, 'mapping it' so to speak.

If you can get to within 0.0001" using lathe tools & no grinding/lapping secondary finishing, you are a better man than me. The way I think about it, if every commercial 'cylinder bore making' company could hit the appropriate tolerance /finish with cutting tools alone & mitigate grinding / lapping / honing operations, why would they go through all that added expense?

 

[pacifica]

My close tolerance boring experience is confined to model engine liners in various kinds of steel & cast iron. The target has to be within 0.0005" but more importantly, of a specific finish at that final bore. That's not exactly an easy combo to hit. I have a super stiff boring bar that has a shank diameter probably 70% of the bore ID. I tried HSS but preferred sharp inserts for consistency. Part of that was likely my inability to dress & set up HSS properly & maybe cheapo import HSS boring bar. They just aren't as common anymore.

What I think you will discover if you get into lapping after turning is bore gages are measuring an average of whatever hill tops they are residing on. That may be fine for your purpose, in fact maybe even desirable if you are using retainer type adhesive. Lapping its quite revealing. Your beautiful boring bar finish at 0.0005" tolerance inevitably turns into 'micro-stripes' where the lapping has flattened the hill tops with a matt finish but the troughs are still shiny. My pics don't do a great job of showing this properly but I'd recommend you try yourself. This is why I was recommending you change up your traverse feed selection on final spring passes. If you don't, you are just spring passing the bottom of the vee. But that has little to do with the dimension across the crests (think of it as threading which is essentially what is happening). Yes a more rounded tool tip minimizes this effect, but I feel also at resisting DOC so even more need for spring pass. Lapping also reveals other effects like ever so slight bore difference between the opening of the hole & closer to the chuck. Lapping (if done properly) doesn't lie, its visually quite apparent until the whole surface is entirely matt & now removing native material.

Of course all this is very material + cutting tool + machine rigidity dependant. The recommendation to stabilize temperature prior to measurement is very important at these tolerance levels. Re measurement, I like snap gauges too but I would recommend a good quality bore gage set against a known standard. Not only can you get good consistent direct readings without in-between-device chances of error but you can repeat this many times & at different positions down the bore, 'mapping it' so to speak.

If you can get to within 0.0001" using lathe tools & no grinding/lapping secondary finishing, you are a better man than me. The way I think about it, if every commercial 'cylinder bore making' company could hit the appropriate tolerance /finish with cutting tools alone & mitigate grinding / lapping / honing operations, why would they go through all that added expense?

Beautiful work!

 

[pacifica]

I've been using dpmt (positive rake) inserted,size 32.52 and they give an excellent finish,only 2 edges,though.but worth it.finnish is better than my hand ground HSS!I use a belt grinder with an angled,clamped holder for accuracy of the angles.

 

[petertha]

Beautiful work!

Thank you. And just for completeness, here is a reamer finish (6061-T6 Al). Lathe bored about 0.005"under size, then in goes the reamer. They are not exactly cheap in larger diameters, but if you are doing a bunch of similar parts, they may be worth it.

 

[macardoso]

Petertha, beautiful work! I have some more practice to go before I could hope to hit .0001"!

The good news is that all 4 pulleys are done, 2 are a light press fit, I think I will do a thermal shrink, and 2 are a very tight slip fit, only about .0005" difference between the bore sizes! I'm very pleased that I could hold +/-.0003" on my target without too much trouble. I'm sure with a better insert selection and a stiffer (larger) boring bar I could do even better. Thanks for all the advice.

The last step is turning some 1/8" x 1/8" brass slugs and inserting them into the tips of the setscrews on the pulleys. I got one installed on my motor and it is not going anywhere!