Help for DIY lathe build.

[moogieParticipant @moogie]

Hi I’m new here on the forum.  I have always wanted a lathe but could never afford one, my only option was to build one myself, or try to at least.

My question is, what would be the common mod/pitch size for a geared rack ?

A little information about what I am building, the bed is 1290mm long and came from some sort of shaft grinding machine.  For a gearbox I’ve got one from an old dumper. It will use a 3 phase electric motor around 1.1 to 2.2 kw with VFD and various pulleys to bring the rpm down to a usable speed. It will have a 200mm chuck. The cross slide will run on linear bearings. Below is a cad drawing of the lathe and the parts I have so far .

I have just received the steel to make the head stock/ gearbox housing and a cabinet to set it on, so hope to make progress over the holidays.

Thanks for any reply, Noel.

[JasonBModerator @jasonb]

At that sort of size 2MOD would do.

However as you are going a bit away from convention with your linear guides consider an ELS(electronic lead screw) which is driven by a motor at whatever rate you enter be that for fine feed or a thread.

[David SeniorParticipant @davidsenior29320]

Noel

Interesting project! I like the use of the ready-made bed.

If I’m understanding your scheme correctly, I think you will find the headstock spindle a weakness. I assume you are effectively mounting the chuck on the gearbox output flange. Is that output shaft constrained well enough in the gearbox to give you the stiffness you need for a headstock spindle? Also you may find having no spindle through hole a limitation.

Also consider the torsional stiffness of your bottom support channel. I would be tempted to see whether the headstock can be mounted directly on the bed, which should give you a more stable connection.

I agree with Jason about considering an ELS system (though that may be outside your budget). Assuming you are wanting to be able to do thread cutting that will be the simplest route.

I also think that a 3 phase motor and vfd will be sufficient without any gearbox (especially if the motor is powerful enough to give sufficient torque at low speed). I have a Raglan 5″ from which I have stripped all the variable speed drive and layshafts, and just run a 1hp motor and vfd connected directly to the spindle with a belt drive giving motor speed about twice the spindle speed, and so far that seems fine. I’ve also just removed all the gearbox and drop gears and fitted a 2-axis ELS. I have literally just done that so have very limited testing so far, but there are no apparent problems.

Best of luck with your project.

Dave

[SillyOldDufferModerator @sillyoldduffer]

Excellent, most start by buying a lathe and learning to use it by making a few simple hand tools.   For this Moogie gets a Gold Star with bonus brownie points.  Well done him!

Here’s a few suggestions though. I’ve annotated Moogie’s CAD screen-shot as a guide.

 

The U-channel ‘A’ used as a base is a potential weakness in the design, because it’s less rigid.

Lack of rigidity in the base ‘A’ will show up at ‘B’, where the headstock sits on the U-channel, disconnected from the stiff Union bed.  The headstock transfers power from the motor to the cutter and it’s important it can’t flex relative to the bed.  Moogie proposes a motor in the 1.1 to 2.2kW range, so the headstock and base have to cope with significant torque.  This isn’t a toy lathe!  One cure is to mount the headstock directly on the Union bed, another is to make sure base ‘A’ is rigid, perhaps a box section, or internally cross-braced, or fixed to a hefty bench – many variations available.

Although I’ve suggested mounting the headstock on the bed, leaving it on base ‘A’ allows another opportunity.  ‘C’ shows Moogie plans to have the headstock butting close to the bed.  However, it’s also possible to create a useful gap by moving the headstock away on a longer bed.  Gaps between headstock and bed are useful because they allow large diameter work to be turned, provided the gap doesn’t reduce rigidity too much.

At ‘D’, consider:

• Three feet rather than 4 might be an advantage because 3 feet can’t rock – the machine is more stable. (I’d probably stick with 4.)
• Adding screw adjustable feet to ‘D’ will make it easier to “level” the lathe.   To be pedantic, levelling in the sense of getting the lathe at a right angle to the ground isn’t important, the goal is to remove any twist between the bed and the headstock.  Test alignment with ‘Rollies Dad‘, then remove any twist found by tweaking the screw feet, which act as jacks.

At ‘E’ I was going to recommend simplifying the tailstock, less work, but I see Moogie has already made a pretty one!  Whether or not the tailstock needs to be shaped depends on the saddle, cross-slide and tool-post.  My lathe’s saddle has protruding ears front and back that improve stability.   The tailstock fits between the rear ears, and is shaped to leave  space for the compound slide and it’s hand-crank.  It allows the tailstock to get closer to the headstock, even though the saddle assembly is in the way:

Having CAD that can model Assemblies is helpful here.  Makes it possible to see what happens when the tailstock and saddle get close together – at what point do they collide, and can the design be changed to create a closer engagement.

Despite all that I advise Moogie to keep it simple.   His approach is sound, and I’m sure his lathe as described so far will perform.  The many refinements that are possible could add years to his project, and may not help much.   A lot of good Model Engineering is done on simple equipment, and with machines in poor condition.

I’m impressed!

Dave

 

 

[noel shelleyParticipant @noelshelley55608]

Welcome to the forum Noel. I admire your idea to make your own lathe, I had often thought of doing the same, but a wood lathe, using a large girder, but the cutting forces and the accuracy are nothing as to those in metal cutting. I take it you have little experience of using a lathe ?  Looking at your plan there are numerous weak points, the bed/ headstock and the spindle bearings come to mind, your gearbox shaft is made to take an axial loading NOT a radial one and will NOT have the accuracy that is needed. Then there is the saddle/ tool holding, all will require good accuracy to work to an acceptable degree. Might I suggest that you start with an old or worn lathe of some sort. It will give you sound basis to build on. I have over the years for very little money acquired 2 old myford that were worn and needed work or a few parts but both were usable. They are out there !  VFDs and 3 Phase motors, ELS are fine, but they are not what you need now.

I will send you a private message, look in messages at the top of the page,I may be able to help you.

Best wishes from Another Noel.

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