Shaft cutting problems

jokers_wild96

jokers_wild96

AzB Silver Member
Silver Member
I'm having problems cutting a straight dowel on my cuesmith midsize. The dowel is 30" long and just under 1" in diameter. The ends are within .004" which I'm fine with, its the center that's got me confused. It's almost .030" smaller in the middle. I'm applying very minimal pressure on my tail stock, slowest speed on my lathe and 6 on the power feed. The finish of the cut is clean and smooth but can't figure out why it's cutting so small in the middle. Anyone have any thoughts.
 
Stick a 30" rod in there and indicate it end to end.
I'm thinking your taper bar is off at the middle.
 
Im not using the bars. Im just trying to cut a straight dowel to fine tune things but I noticed it's cutting weird in the middle. I need this problem addressed before I start cutting shaft tapers or I'm going to make a lot of garbage.
 
jokers_wild96 said:
Im not using the bars. Im just trying to cut a straight dowel to fine tune things but I noticed it's cutting weird in the middle. I need this problem addressed before I start cutting shaft tapers or I'm going to make a lot of garbage.
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Bed might be twisted.
Get a machinist level and level it dead nuts.

I'm assuming the cutter does not lift above the centerline going to the middle .

You still need to indicate a 30" rod . If it's .015 off in the middle while the rod is not even spinning, then the bed is off .
 
jokers_wild96 said:
I'm having problems cutting a straight dowel on my cuesmith midsize. The dowel is 30" long and just under 1" in diameter. The ends are within .004" which I'm fine with, its the center that's got me confused. It's almost .030" smaller in the middle. I'm applying very minimal pressure on my tail stock, slowest speed on my lathe and 6 on the power feed. The finish of the cut is clean and smooth but can't figure out why it's cutting so small in the middle. Anyone have any thoughts.
Click to expand...

First, did you buy from Chris Hightower brand new or did you buy it second hand? If from Chris,this is something to ask him about in a phone call, as he knows these machines better than anyone else. Slower feedrate may be the answer, plus it is possible to have the bed slightly tweaked at the same time, also, how tight are the gibs, and if it was used when you bought it, it may be possible that the bed is worn in that area, not likely, but possible. Roll the carriage by the hand wheel the whole length of the bed, from headstock to tailstock, it should be the same effort the whole way, if not, then the bed maybe worn or the gibs are not adjusted correctly, or, there is debris on the bed/stuck under the carriage.
Hopefully you get it running right and very soon. Happy building.
Dave
 
Can you post a picture of your setup please?
Then more accurate advice can be given.
What type of cutter are you using etc etc etc.
Neil
 
Usually the dowel will be fat in the middle because of tool push off..... small in the middle seems to be mechanical. Lay a straight edge on the lathe bed .... it may be low in the middle..... try the straight edge on the sides of the lathe bed.... it may not be straight.............

Kim
 
whammo57 said:
Usually the dowel will be fat in the middle because of tool push off..... small in the middle seems to be mechanical. Lay a straight edge on the lathe bed .... it may be low in the middle..... try the straight edge on the sides of the lathe bed.... it may not be straight.............

Kim
Click to expand...

Small in the middle can occur if the cutter pulls the workpiece towards it.
That is why a photo showing the set up will avoid conjecture of the actual problem.
 
So I still don't know what's causing that problem in the middle. I tried using my other cross slide and cut from the back and got different results, still not a straight cut. Since the piece I was using was scrap wood I used the bars and cut the shaft taper almost to a finish size and it was damn near perfect cutting the shaft. I checked the bed on top and side with a 4ft t square and everything looked straight. I appreciate the suggestions to help try and figure this out.
 
jokers_wild96 said:
So I still don't know what's causing that problem in the middle. I tried using my other cross slide and cut from the back and got different results, still not a straight cut. Since the piece I was using was scrap wood I used the bars and cut the shaft taper almost to a finish size and it was damn near perfect cutting the shaft. I checked the bed on top and side with a 4ft t square and everything looked straight. I appreciate the suggestions to help try and figure this out.
Click to expand...

So, when the cross slide is butted to the taper bar, it's ok.
But, not when it's on the bed in the middle .
So, it's the bed.

A square won't tell you much.
 
You wont see a 2 thou bend in a lathe bed extrusion, but it will make a 4 thou on diameter change. Getting them to be straight and twist free is the real answer for setting up any lathe bed. Leveling a machine tool is the term used to get it to be as true as it was when made in the factory and working perfectly, before it was packaged and shipped. It does not mean that the spirit level is level everywhere. But if it was, it would also be good.
So there are things you can do, like make things that clamp it to get the twist out of it or to twist it to get it true. You may need center blocks to push it forward or back to get it dead straight. Its a topic in itself setting up the bed correct. Then the head stock needs to be correctly aligned to the newly leveled bed. Lastly the tail stocks need to be centered and so do the steady rests. The fixed ones are what I start with, and then adjust all the adjustable ones.
A taper bar for the most part negates the importance of the leveling of the bed, but it will be different in it's alignment where ever the centre is repositioned to.
Neil
 
As others have requested, we need to see pictures of your setup in order to help diagnose your problem. Please post pictures of your lathe with a dowel mounted in it. Then a close-up of the headsrock, cross-slide with cutter, and the realistic. Finally, rig up a mount to hold a dial indicator at the same height as your cutter. Then you can watch the indicator needle as you move the cross-slide down the ways. This will help you see what the cutter is doing while it turns your dowel.
As Joey noted, the fact that it cut better when using the taper bar indicates where the problem is. The above procedure will help define the problem better.
My 2 cents
Gary
 
It sounds like the bed may need the middle foot pad screws adjusted if you are using a router. If you have taper bars the bed being off should not come into play and shafts should cut fine.
 
cueman said:
It sounds like the bed may need the middle foot pad screws adjusted if you are using a router. If you have taper bars the bed being off should not come into play and shafts should cut fine.
Click to expand...

Yeah, once I engaged the taper bars it cut fine. Once my new lathe is set up this one will only do tapering, points/inlays and finishing.
 
whammo57 said:
Usually the dowel will be fat in the middle because of tool push off..... small in the middle seems to be mechanical. Lay a straight edge on the lathe bed .... it may be low in the middle..... try the straight edge on the sides of the lathe bed.... it may not be straight.............

Kim
Click to expand...

This got me thinking about my setup. After scrutinizing my bed I decided it was not where it should be. I spent a whole day getting it as good as I could, and what a difference it made. Thank all of you for sharing your knowledge!!

Cheers
 
Hi,

Try 6 min travel time for 30 inch distance. This is critical parameter is you want a shaft that barely requires sanding after turning. If you travel too fast the rms finish will be bad. You will also shock the wood.

Lathe speed between 120 and 150 rpm. Find the sweet spot for your equipment. If your lowest lathe speed cant get that slow, modify the lead screw to run direct drive with a DC Motor and controller. If your travel speed is too fast your not going to get a good result.

Use a 6 wing cutter on a lathe mounted Router. Not a 2 fluke.

Make sure your router is mounted rock solid to your tool post set up mechanically. No hose clamps or bullshit rigging.

Set elevation of cutter above or below center line of dowel so just the edge of cutter is engaged in the kerf cut.

Never take less than a .010 pass . At a minimum of .010 kerf embedment, the shaft won't oscillate in the center with the minimum tailstock pressure. When you think you have the lightest pressure, lick the center hole and back off a little more. Again find that sweet spot. Of course you should have a live center with a bearing on the tailstock side.

If you are not using a metal lathe with mass, make sure any lightweight set up is securely mounted to your bench.

If you get all the above parameters in the range described above and the results a still bad then your lathe needs to be replaced with one that will be repeatable with precision results to do this job.

JMO,

Rick
 
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