Help with compound rest travel error

I'm running an SC4 type lathe. As I am turning down a grade 5 bolt into a small part, I find that by using the carriage wheel I can get a wonderful finish. When I use the compound rest to run the tool parallel to the ways I get a 1 thou variation with my indicator every turn of the wheel... So every 50 thou. that produces an rather unpleasant finish. I have the compound rest and tool post locked down. How can I remove this error? The wheel is a bit tight to turn. Do I need to allow more play in the wheel or is there another issue?

Thanks

What is a mini lathe doing on this forum?
If you get 1 thou variation in diameter every turn of the dial the compound is not parallel with the bed. The variation in finish every 50 thou is probably due to changing your hand position on the crank every turn and resulting change in feedrate causes the variation in the finish.

JW,is that you ?

Illinoyance said:

What is a mini lathe doing on this forum?
If you get 1 thou variation in diameter every turn of the dial the compound is not parallel with the bed. The variation in finish every 50 thou is probably due to changing your hand position on the crank every turn and resulting change in feedrate causes the variation in the finish.

Click to expand...

It's not a CUMULATIVE 1 thou variation. It varies between dead on and 1 thou off with each revolution of the compound rest wheel. It's parallel. But, yes, due to the tightness of the wheel, it is difficult to keep a consistent hold. I take it that loosening this will allow a smoother one-handed turn and remove the variation.

And a mini-lathe is what I have. If there is a section meant for this I will keep my eye out for it next time.

you are not going to hand crank that compound without introducing some flex and inconsistency...does the pattern follow the tension and release of your own hand motions?
most would probably be happy with .001" on that lathe.

you may be expecting too much.

Loose gibs? Maybe a bent leadscrew on the compound? Is variation in hand pressure causing the deflection? I have to admit I don't know much about the mini lathes.

Illinoyance said:

Loose gibs? Maybe a bent leadscrew on the compound? Is variation in hand pressure causing the deflection? I have to admit I don't know much about the mini lathes.

Click to expand...

Perhaps just set to the compound dial numbers and not indicated...with being under some load. Perhaps turning a taper at set.

iwananew10K said:

you are not going to hand crank that compound without introducing some flex and inconsistency...does the pattern follow the tension and release of your own hand motions?
most would probably be happy with .001" on that lathe.

you may be expecting too much.

Click to expand...

The pattern follows not my hand motion, rather it follows the absolute position of the wheel and the tightness. Turning the wheel I feel a smooth easy turn and at a point in the turn there is more resistance... past there is loosens again. So, the pattern exists even when slowly and carefully turning the wheel two-handed. Now I don't care about the 1 thou. But there is a visible ridged pattern in the finish that is not present if I use the carriage wheel to advance the work towards the spindle.

Illinoyance said:

Loose gibs? Maybe a bent leadscrew on the compound? Is variation in hand pressure causing the deflection? I have to admit I don't know much about the mini lathes.

Click to expand...

something like a bent lead screw is what I'm concerned about. I may have to take the thing apart completely

QT: [When I use the compound rest to run the tool parallel to the ways I get a...\
why are you traveling with the compound? are you trying to turn a taper?

QT: [something like a bent lead screw is what I'm concerned about.] if true you may need to make a new lead screw with tuning it between centers.

Perhaps with the nut on straighten it? Do you see any run out in the screw?

Is the part that you are running at center and well supported?

Grade 4 bolt cold drawn low carbon steel is crap steel...

sicesp said:

And a mini-lathe is what I have. If there is a section meant for this I will keep my eye out for it next time.

Click to expand...

https://www.practicalmachinist.com/vb/general/machinery-discussion-guidelines-137724/

There is:
Home Page - Projects and Articles on Our Forum! | The Hobby-Machinist

On a given part, if the carriage feed produces a mirror finish (nearly) and the compound feed produces a horrible finish I would say that eliminates a lot of variables. It's a grade 5 bolt, but material variable is eliminated.

I turned a 2 inch part held in a properly sized ER32 collet.

I the point about Gibs mentioned above is the solution. Thanks Illinoying. The Gibs were not loose but were not set evenly I believe. The movement of the compound is now reduced markedly.

I appreciate the helpful replies.

BTW, I note where I should have seen this is the wrong forum. Chalk that up to my lack of familiarity with this board and please accept my apologies. I will post more carefully next time in the correct section. Maybe the mod can move this.

sicesp said:

I'm running an SC4 type lathe. As I am turning down a grade 5 bolt into a small part, I find that by using the carriage wheel I can get a wonderful finish. When I use the compound rest to run the tool parallel to the ways I get a 1 thou variation with my indicator every turn of the wheel... So every 50 thou. that produces an rather unpleasant finish. I have the compound rest and tool post locked down. How can I remove this error? The wheel is a bit tight to turn. Do I need to allow more play in the wheel or is there another issue?

Click to expand...

What are you using as an "indicator"? (micrometer, dial test indicator, something else). How is the indicator set up?

The way grade 5 bolts are made, and most any bolt or fastener of the size that fit in your lathe, is by a cold forming process. The heads are cold forged and the threads are rolled in. This introduces stresses on the surface of the material that go even deeper than the minor diameter of the threads. After cold forming, most are heat treated which relieves some stresses but introduces others. These stresses vary over the length and width of the bolt. A bolt is a finished product and meant to be used as-is. For this reason it can be difficult to get a nice finish on any brand or size of machine with just about any kind of cutter. Grinding can give very nice results but it is a slow and messy process on a lathe.
Unless there is some compelling reason to use a grade 5 bolt then you will be better served by using some material that is more homogeneous to begin with.

The compound is not the best way to turn a parallel cylinder on a lathe. The compound can do short tapers, chamfers, and transitions or shoulders. For straight turning you have the carriage with a nice power feed to keep things smooth and consistent. If you are trying to use it for turning a precise length you will get better results using longitudinal feed (hand or power) and carriage stops, DRO, or a long range dial indicator.

In other words... without discussing the particular brand or size of machine you have, your problems are a combination of choice of material and method. You could well get the similar results on a Hardinge HLV using the very best tooling as you are getting on your particular machine and tooling.

-DU-

sicesp said:

BTW, I note where I should have seen this is the wrong forum. Chalk that up to my lack of familiarity with this board and please accept my apologies. I will post more carefully next time in the correct section. Maybe the mod can move this.

Click to expand...

Glad you found the problem with Illinoyance's help


Small hobby lathes are not allowed any place on PM...

You can mention a problem with not adding the machine name, but likely some one will ask the name.....

michiganbuck said:

Glad you found the problem with Illinoyance's help


Small hobby lathes are not allowed any place on PM...

You can mention a problem with not adding the machine name, but likely some one will ask the name.....

Click to expand...

The problem with this is simple.
We spend countless hours trying to help a newbie with a mini lathe, and all
methods suggested to help, fail, because of the mini lathe.
You can't polish a turd.

Plain-and-simple.

So the Op simply need to ask at the above linked site, and will then get the
help he/she needs.